Merge branch 'nullable'

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -543,7 +543,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				argumentList.Arguments.ToList(), argumentList.ArgumentNames);
 
 			if (((AssignmentExpression)assignment).Left is IndexerExpression indexer && !indexer.Target.IsNull)
-				indexer.Target.ReplaceWith(n => null);
+				indexer.Target.Remove();
 
 			if (value != null)
 				return assignment;

ICSharpCode.Decompiler/CSharp/Resolver/CSharpOperators.cs

@@ -128,9 +128,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				get { return null; }
 			}
 
-			IType IEntity.DeclaringType {
-				get { return null; }
-			}
+			public IType DeclaringType => SpecialType.UnknownType;
 
 			IMember IMember.MemberDefinition {
 				get { return this; }

ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs

@@ -1,4 +1,5 @@
-// 
+#nullable enable
+// 
 // AstNode.cs
 //
 // Author:
@@ -39,7 +40,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	public abstract class AstNode : AbstractAnnotatable, IFreezable, INode, ICloneable
 	{
 		// the Root role must be available when creating the null nodes, so we can't put it in the Roles class
-		internal static readonly Role<AstNode> RootRole = new Role<AstNode>("Root");
+		internal static readonly Role<AstNode?> RootRole = new Role<AstNode?>("Root", null);
 
 		#region Null
 		public static readonly AstNode Null = new NullAstNode();
@@ -73,7 +74,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				return visitor.VisitNullNode(this, data);
 			}
 
-			protected internal override bool DoMatch(AstNode other, PatternMatching.Match match)
+			protected internal override bool DoMatch(AstNode? other, PatternMatching.Match match)
 			{
 				return other == null || other.IsNull;
 			}
@@ -81,7 +82,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		#endregion
 
 		#region PatternPlaceholder
-		public static implicit operator AstNode(PatternMatching.Pattern pattern)
+		public static implicit operator AstNode?(PatternMatching.Pattern? pattern)
 		{
 			return pattern != null ? new PatternPlaceholder(pattern) : null;
 		}
@@ -114,23 +115,23 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				return visitor.VisitPatternPlaceholder(this, child, data);
 			}
 
-			protected internal override bool DoMatch(AstNode other, PatternMatching.Match match)
+			protected internal override bool DoMatch(AstNode? other, PatternMatching.Match match)
 			{
 				return child.DoMatch(other, match);
 			}
 
-			bool PatternMatching.INode.DoMatchCollection(Role role, PatternMatching.INode pos, PatternMatching.Match match, PatternMatching.BacktrackingInfo backtrackingInfo)
+			bool PatternMatching.INode.DoMatchCollection(Role? role, PatternMatching.INode? pos, PatternMatching.Match match, PatternMatching.BacktrackingInfo backtrackingInfo)
 			{
 				return child.DoMatchCollection(role, pos, match, backtrackingInfo);
 			}
 		}
 		#endregion
 
-		AstNode parent;
-		AstNode prevSibling;
-		AstNode nextSibling;
-		AstNode firstChild;
-		AstNode lastChild;
+		AstNode? parent;
+		AstNode? prevSibling;
+		AstNode? nextSibling;
+		AstNode? firstChild;
+		AstNode? lastChild;
 
 		// Flags, from least significant to most significant bits:
 		// - Role.RoleIndexBits: role index
@@ -157,7 +158,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		{
 			if (!IsFrozen)
 			{
-				for (AstNode child = firstChild; child != null; child = child.nextSibling)
+				for (AstNode? child = firstChild; child != null; child = child.nextSibling)
 					child.Freeze();
 				flags |= frozenBit;
 			}
@@ -197,7 +198,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		public AstNode Parent {
+		public AstNode? Parent {
 			get { return parent; }
 		}
 
@@ -224,19 +225,19 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			flags = (flags & ~roleIndexMask) | role.Index;
 		}
 
-		public AstNode NextSibling {
+		public AstNode? NextSibling {
 			get { return nextSibling; }
 		}
 
-		public AstNode PrevSibling {
+		public AstNode? PrevSibling {
 			get { return prevSibling; }
 		}
 
-		public AstNode FirstChild {
+		public AstNode? FirstChild {
 			get { return firstChild; }
 		}
 
-		public AstNode LastChild {
+		public AstNode? LastChild {
 			get { return lastChild; }
 		}
 
@@ -248,8 +249,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public IEnumerable<AstNode> Children {
 			get {
-				AstNode next;
-				for (AstNode cur = firstChild; cur != null; cur = next)
+				AstNode? next;
+				for (AstNode? cur = firstChild; cur != null; cur = next)
 				{
 					Debug.Assert(cur.parent == this);
 					// Remember next before yielding cur.
@@ -265,7 +266,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		public IEnumerable<AstNode> Ancestors {
 			get {
-				for (AstNode cur = parent; cur != null; cur = cur.parent)
+				for (AstNode? cur = parent; cur != null; cur = cur.parent)
 				{
 					yield return cur;
 				}
@@ -277,7 +278,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		public IEnumerable<AstNode> AncestorsAndSelf {
 			get {
-				for (AstNode cur = this; cur != null; cur = cur.parent)
+				for (AstNode? cur = this; cur != null; cur = cur.parent)
 				{
 					yield return cur;
 				}
@@ -298,18 +299,18 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			get { return GetDescendantsImpl(true); }
 		}
 
-		public IEnumerable<AstNode> DescendantNodes(Func<AstNode, bool> descendIntoChildren = null)
+		public IEnumerable<AstNode> DescendantNodes(Func<AstNode, bool>? descendIntoChildren = null)
 		{
 			return GetDescendantsImpl(false, descendIntoChildren);
 		}
 
-		public IEnumerable<AstNode> DescendantNodesAndSelf(Func<AstNode, bool> descendIntoChildren = null)
+		public IEnumerable<AstNode> DescendantNodesAndSelf(Func<AstNode, bool>? descendIntoChildren = null)
 		{
 			return GetDescendantsImpl(true, descendIntoChildren);
 		}
 
 
-		IEnumerable<AstNode> GetDescendantsImpl(bool includeSelf, Func<AstNode, bool> descendIntoChildren = null)
+		IEnumerable<AstNode> GetDescendantsImpl(bool includeSelf, Func<AstNode, bool>? descendIntoChildren = null)
 		{
 			if (includeSelf)
 			{
@@ -318,9 +319,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					yield break;
 			}
 
-			Stack<AstNode> nextStack = new Stack<AstNode>();
+			Stack<AstNode?> nextStack = new Stack<AstNode?>();
 			nextStack.Push(null);
-			AstNode pos = firstChild;
+			AstNode? pos = firstChild;
 			while (pos != null)
 			{
 				// Remember next before yielding pos.
@@ -339,7 +340,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// Gets the first child with the specified role.
 		/// Returns the role's null object if the child is not found.
 		/// </summary>
-		public T GetChildByRole<T>(Role<T> role) where T : AstNode
+		public T GetChildByRole<T>(Role<T> role) where T : AstNode?
 		{
 			if (role == null)
 				throw new ArgumentNullException(nameof(role));
@@ -352,12 +353,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return role.NullObject;
 		}
 
-		public T GetParent<T>() where T : AstNode
+		public T? GetParent<T>() where T : AstNode
 		{
 			return Ancestors.OfType<T>().FirstOrDefault();
 		}
 
-		public AstNode GetParent(Func<AstNode, bool> pred)
+		public AstNode GetParent(Func<AstNode, bool>? pred)
 		{
 			return Ancestors.FirstOrDefault(pred);
 		}
@@ -392,7 +393,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			AddChildUnsafe(child, role);
 		}
 
-		public void AddChildWithExistingRole(AstNode child)
+		public void AddChildWithExistingRole(AstNode? child)
 		{
 			if (child == null || child.IsNull)
 				return;
@@ -419,13 +420,13 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 			else
 			{
-				lastChild.nextSibling = child;
+				lastChild!.nextSibling = child;
 				child.prevSibling = lastChild;
 				lastChild = child;
 			}
 		}
 
-		public void InsertChildBefore<T>(AstNode nextSibling, T child, Role<T> role) where T : AstNode
+		public void InsertChildBefore<T>(AstNode? nextSibling, T child, Role<T> role) where T : AstNode
 		{
 			if (role == null)
 				throw new ArgumentNullException(nameof(role));
@@ -469,7 +470,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			nextSibling.prevSibling = child;
 		}
 
-		public void InsertChildAfter<T>(AstNode prevSibling, T child, Role<T> role) where T : AstNode
+		public void InsertChildAfter<T>(AstNode? prevSibling, T child, Role<T> role) where T : AstNode
 		{
 			InsertChildBefore((prevSibling == null || prevSibling.IsNull) ? firstChild : prevSibling.nextSibling, child, role);
 		}
@@ -511,7 +512,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// <summary>
 		/// Replaces this node with the new node.
 		/// </summary>
-		public void ReplaceWith(AstNode newNode)
+		public void ReplaceWith(AstNode? newNode)
 		{
 			if (newNode == null || newNode.IsNull)
 			{
@@ -578,7 +579,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			nextSibling = null;
 		}
 
-		public AstNode ReplaceWith(Func<AstNode, AstNode> replaceFunction)
+		public AstNode? ReplaceWith(Func<AstNode, AstNode?> replaceFunction)
 		{
 			if (replaceFunction == null)
 				throw new ArgumentNullException(nameof(replaceFunction));
@@ -587,10 +588,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				throw new InvalidOperationException(this.IsNull ? "Cannot replace the null nodes" : "Cannot replace the root node");
 			}
 			AstNode oldParent = parent;
-			AstNode oldSuccessor = nextSibling;
+			AstNode? oldSuccessor = nextSibling;
 			Role oldRole = this.Role;
 			Remove();
-			AstNode replacement = replaceFunction(this);
+			AstNode? replacement = replaceFunction(this);
 			if (oldSuccessor != null && oldSuccessor.parent != oldParent)
 				throw new InvalidOperationException("replace function changed nextSibling of node being replaced?");
 			if (!(replacement == null || replacement.IsNull))
@@ -626,7 +627,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			copy.flags &= ~frozenBit; // unfreeze the copy
 
 			// Then perform a deep copy:
-			for (AstNode cur = firstChild; cur != null; cur = cur.nextSibling)
+			for (AstNode? cur = firstChild; cur != null; cur = cur.nextSibling)
 			{
 				copy.AddChildUnsafe(cur.Clone(), cur.Role);
 			}
@@ -649,37 +650,37 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public abstract S AcceptVisitor<T, S>(IAstVisitor<T, S> visitor, T data);
 
 		#region Pattern Matching
-		protected static bool MatchString(string pattern, string text)
+		protected static bool MatchString(string? pattern, string? text)
 		{
 			return PatternMatching.Pattern.MatchString(pattern, text);
 		}
 
-		protected internal abstract bool DoMatch(AstNode other, PatternMatching.Match match);
+		protected internal abstract bool DoMatch(AstNode? other, PatternMatching.Match match);
 
-		bool PatternMatching.INode.DoMatch(PatternMatching.INode other, PatternMatching.Match match)
+		bool PatternMatching.INode.DoMatch(PatternMatching.INode? other, PatternMatching.Match match)
 		{
-			AstNode o = other as AstNode;
+			AstNode? o = other as AstNode;
 			// try matching if other is null, or if other is an AstNode
 			return (other == null || o != null) && DoMatch(o, match);
 		}
 
-		bool PatternMatching.INode.DoMatchCollection(Role role, PatternMatching.INode pos, PatternMatching.Match match, PatternMatching.BacktrackingInfo backtrackingInfo)
+		bool PatternMatching.INode.DoMatchCollection(Role? role, PatternMatching.INode? pos, PatternMatching.Match match, PatternMatching.BacktrackingInfo? backtrackingInfo)
 		{
-			AstNode o = pos as AstNode;
+			AstNode? o = pos as AstNode;
 			return (pos == null || o != null) && DoMatch(o, match);
 		}
 
-		PatternMatching.INode PatternMatching.INode.NextSibling {
+		PatternMatching.INode? PatternMatching.INode.NextSibling {
 			get { return nextSibling; }
 		}
 
-		PatternMatching.INode PatternMatching.INode.FirstChild {
+		PatternMatching.INode? PatternMatching.INode.FirstChild {
 			get { return firstChild; }
 		}
 
 		#endregion
 
-		public AstNode GetNextNode()
+		public AstNode? GetNextNode()
 		{
 			if (NextSibling != null)
 				return NextSibling;
@@ -693,7 +694,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		/// <returns>The next node.</returns>
 		/// <param name="pred">The predicate.</param>
-		public AstNode GetNextNode(Func<AstNode, bool> pred)
+		public AstNode? GetNextNode(Func<AstNode, bool> pred)
 		{
 			var next = GetNextNode();
 			while (next != null && !pred(next))
@@ -701,7 +702,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return next;
 		}
 
-		public AstNode GetPrevNode()
+		public AstNode? GetPrevNode()
 		{
 			if (PrevSibling != null)
 				return PrevSibling;
@@ -715,7 +716,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		/// <returns>The next node.</returns>
 		/// <param name="pred">The predicate.</param>
-		public AstNode GetPrevNode(Func<AstNode, bool> pred)
+		public AstNode? GetPrevNode(Func<AstNode, bool> pred)
 		{
 			var prev = GetPrevNode();
 			while (prev != null && !pred(prev))
@@ -723,7 +724,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return prev;
 		}
 		// filters all non c# nodes (comments, white spaces or pre processor directives)
-		public AstNode GetCSharpNodeBefore(AstNode node)
+		public AstNode? GetCSharpNodeBefore(AstNode node)
 		{
 			var n = node.PrevSibling;
 			while (n != null)
@@ -740,7 +741,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		/// <returns>The next node.</returns>
 		/// <param name="pred">The predicate.</param>
-		public AstNode GetNextSibling(Func<AstNode, bool> pred)
+		public AstNode? GetNextSibling(Func<AstNode, bool> pred)
 		{
 			var next = NextSibling;
 			while (next != null && !pred(next))
@@ -753,7 +754,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		/// <returns>The next node.</returns>
 		/// <param name="pred">The predicate.</param>
-		public AstNode GetPrevSibling(Func<AstNode, bool> pred)
+		public AstNode? GetPrevSibling(Func<AstNode, bool> pred)
 		{
 			var prev = PrevSibling;
 			while (prev != null && !pred(prev))
@@ -767,7 +768,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// the current method declaration.
 		/// (End exclusive)
 		/// </summary>
-		public AstNode GetNodeAt(int line, int column, Predicate<AstNode> pred = null)
+		public AstNode? GetNodeAt(int line, int column, Predicate<AstNode>? pred = null)
 		{
 			return GetNodeAt(new TextLocation(line, column), pred);
 		}
@@ -777,9 +778,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// the current method declaration.
 		/// (End exclusive)
 		/// </summary>
-		public AstNode GetNodeAt(TextLocation location, Predicate<AstNode> pred = null)
+		public AstNode? GetNodeAt(TextLocation location, Predicate<AstNode>? pred = null)
 		{
-			AstNode result = null;
+			AstNode? result = null;
 			AstNode node = this;
 			while (node.LastChild != null)
 			{
@@ -806,7 +807,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// the current method declaration.
 		/// (End exclusive)
 		/// </summary>
-		public T GetNodeAt<T>(int line, int column) where T : AstNode
+		public T? GetNodeAt<T>(int line, int column) where T : AstNode
 		{
 			return GetNodeAt<T>(new TextLocation(line, column));
 		}
@@ -816,9 +817,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// the current method declaration.
 		/// (End exclusive)
 		/// </summary>
-		public T GetNodeAt<T>(TextLocation location) where T : AstNode
+		public T? GetNodeAt<T>(TextLocation location) where T : AstNode
 		{
-			T result = null;
+			T? result = null;
 			AstNode node = this;
 			while (node.LastChild != null)
 			{
@@ -848,7 +849,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// the current method declaration.
 		/// (End inclusive)
 		/// </summary>
-		public AstNode GetAdjacentNodeAt(int line, int column, Predicate<AstNode> pred = null)
+		public AstNode? GetAdjacentNodeAt(int line, int column, Predicate<AstNode>? pred = null)
 		{
 			return GetAdjacentNodeAt(new TextLocation(line, column), pred);
 		}
@@ -858,9 +859,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// the current method declaration.
 		/// (End inclusive)
 		/// </summary>
-		public AstNode GetAdjacentNodeAt(TextLocation location, Predicate<AstNode> pred = null)
+		public AstNode? GetAdjacentNodeAt(TextLocation location, Predicate<AstNode>? pred = null)
 		{
-			AstNode result = null;
+			AstNode? result = null;
 			AstNode node = this;
 			while (node.LastChild != null)
 			{
@@ -887,7 +888,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// the current method declaration.
 		/// (End inclusive)
 		/// </summary>
-		public T GetAdjacentNodeAt<T>(int line, int column) where T : AstNode
+		public T? GetAdjacentNodeAt<T>(int line, int column) where T : AstNode
 		{
 			return GetAdjacentNodeAt<T>(new TextLocation(line, column));
 		}
@@ -897,9 +898,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// the current method declaration.
 		/// (End inclusive)
 		/// </summary>
-		public T GetAdjacentNodeAt<T>(TextLocation location) where T : AstNode
+		public T? GetAdjacentNodeAt<T>(TextLocation location) where T : AstNode
 		{
-			T result = null;
+			T? result = null;
 			AstNode node = this;
 			while (node.LastChild != null)
 			{
@@ -908,8 +909,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					child = child.prevSibling;
 				if (child != null && location <= child.EndLocation)
 				{
-					if (child is T)
-						result = (T)child;
+					if (child is T t)
+						result = t;
 					node = child;
 				}
 				else
@@ -928,7 +929,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		public AstNode GetNodeContaining(TextLocation startLocation, TextLocation endLocation)
 		{
-			for (AstNode child = firstChild; child != null; child = child.nextSibling)
+			for (AstNode? child = firstChild; child != null; child = child.nextSibling)
 			{
 				if (child.StartLocation <= startLocation && endLocation <= child.EndLocation)
 					return child.GetNodeContaining(startLocation, endLocation);
@@ -949,10 +950,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		public IEnumerable<AstNode> GetNodesBetween(TextLocation start, TextLocation end)
 		{
-			AstNode node = this;
+			AstNode? node = this;
 			while (node != null)
 			{
-				AstNode next;
+				AstNode? next;
 				if (start <= node.StartLocation && node.EndLocation <= end)
 				{
 					// Remember next before yielding node.
@@ -984,7 +985,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// <param name='formattingOptions'>
 		/// Formatting options.
 		/// </param>
-		public virtual string ToString(CSharpFormattingOptions formattingOptions)
+		public virtual string ToString(CSharpFormattingOptions? formattingOptions)
 		{
 			if (IsNull)
 				return "";

ICSharpCode.Decompiler/CSharp/Syntax/ComposedType.cs

@@ -41,7 +41,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public static readonly TokenRole ReadonlyRole = new TokenRole("readonly");
 		public static readonly TokenRole NullableRole = new TokenRole("?");
 		public static readonly TokenRole PointerRole = new TokenRole("*");
-		public static readonly Role<ArraySpecifier> ArraySpecifierRole = new Role<ArraySpecifier>("ArraySpecifier");
+		public static readonly Role<ArraySpecifier> ArraySpecifierRole = new Role<ArraySpecifier>("ArraySpecifier", null);
 		public AstNodeCollection<AttributeSection> Attributes {
 			get { return base.GetChildrenByRole(AttributeRole); }
 		}
@@ -299,4 +299,3 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 	}
 }
-

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/ArrayCreateExpression.cs

@@ -25,7 +25,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	public class ArrayCreateExpression : Expression
 	{
 		public readonly static TokenRole NewKeywordRole = new TokenRole("new");
-		public readonly static Role<ArraySpecifier> AdditionalArraySpecifierRole = new Role<ArraySpecifier>("AdditionalArraySpecifier");
+		public readonly static Role<ArraySpecifier> AdditionalArraySpecifierRole = new Role<ArraySpecifier>("AdditionalArraySpecifier", null);
 		public readonly static Role<ArrayInitializerExpression> InitializerRole = new Role<ArrayInitializerExpression>("Initializer", ArrayInitializerExpression.Null);
 
 		public CSharpTokenNode NewToken {

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/QueryExpression.cs

@@ -16,13 +16,11 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System.Collections.Generic;
-
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	public class QueryExpression : Expression
 	{
-		public static readonly Role<QueryClause> ClauseRole = new Role<QueryClause>("Clause");
+		public static readonly Role<QueryClause> ClauseRole = new Role<QueryClause>("Clause", null);
 
 		#region Null
 		public new static readonly QueryExpression Null = new NullQueryExpression();
@@ -420,7 +418,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	public class QueryOrderClause : QueryClause
 	{
 		public static readonly TokenRole OrderbyKeywordRole = new TokenRole("orderby");
-		public static readonly Role<QueryOrdering> OrderingRole = new Role<QueryOrdering>("Ordering");
+		public static readonly Role<QueryOrdering> OrderingRole = new Role<QueryOrdering>("Ordering", null);
 
 		public CSharpTokenNode OrderbyToken {
 			get { return GetChildByRole(OrderbyKeywordRole); }
@@ -585,4 +583,4 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return o != null && this.Projection.DoMatch(o.Projection, match) && this.Key.DoMatch(o.Key, match);
 		}
 	}
-}
+}

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/SwitchExpression.cs

@@ -25,7 +25,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	public class SwitchExpression : Expression
 	{
 		public static readonly TokenRole SwitchKeywordRole = new TokenRole("switch");
-		public static readonly Role<SwitchExpressionSection> SwitchSectionRole = new Role<SwitchExpressionSection>("SwitchSection");
+		public static readonly Role<SwitchExpressionSection> SwitchSectionRole = new Role<SwitchExpressionSection>("SwitchSection", null);
 
 		public Expression Expression {
 			get { return GetChildByRole(Roles.Expression); }

ICSharpCode.Decompiler/CSharp/Syntax/GeneralScope/PreProcessorDirective.cs

@@ -69,7 +69,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 	public class PragmaWarningPreprocessorDirective : PreProcessorDirective
 	{
-		public static readonly Role<PrimitiveExpression> WarningRole = new Role<PrimitiveExpression>("Warning");
+		public static readonly Role<PrimitiveExpression> WarningRole = new Role<PrimitiveExpression>("Warning", null);
 
 		public static readonly TokenRole PragmaKeywordRole = new TokenRole("#pragma");
 		public static readonly TokenRole WarningKeywordRole = new TokenRole("warning");
@@ -200,4 +200,3 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 	}
 }
-

ICSharpCode.Decompiler/CSharp/Syntax/Role.cs

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Threading;
 
@@ -64,7 +66,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	/// Represents the role a node plays within its parent.
 	/// All nodes with this role have type T.
 	/// </summary>
-	public class Role<T> : Role where T : class
+	public class Role<T> : Role where T : class?
 	{
 		readonly string name; // helps with debugging the AST
 		readonly T nullObject;
@@ -86,21 +88,19 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return node is T;
 		}
 
+		[Obsolete("Use the other overload explicitly specifying the nullObject.")]
 		public Role(string name)
 		{
 			if (name == null)
 				throw new ArgumentNullException(nameof(name));
 			this.name = name;
+			this.nullObject = null!;
 		}
 
 		public Role(string name, T nullObject)
 		{
-			if (name == null)
-				throw new ArgumentNullException(nameof(name));
-			if (nullObject == null)
-				throw new ArgumentNullException(nameof(nullObject));
+			this.name = name ?? throw new ArgumentNullException(nameof(name));
 			this.nullObject = nullObject;
-			this.name = name;
 		}
 
 		public override string ToString()

ICSharpCode.Decompiler/CSharp/Syntax/Roles.cs

@@ -34,18 +34,18 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		// some pre defined constants for common roles
 		public static readonly Role<Identifier> Identifier = new Role<Identifier>("Identifier", Syntax.Identifier.Null);
 		public static readonly Role<BlockStatement> Body = new Role<BlockStatement>("Body", BlockStatement.Null);
-		public static readonly Role<ParameterDeclaration> Parameter = new Role<ParameterDeclaration>("Parameter");
+		public static readonly Role<ParameterDeclaration> Parameter = new Role<ParameterDeclaration>("Parameter", null);
 		public static readonly Role<Expression> Argument = new Role<Expression>("Argument", Syntax.Expression.Null);
 		public static readonly Role<AstType> Type = new Role<AstType>("Type", AstType.Null);
 		public static readonly Role<Expression> Expression = new Role<Expression>("Expression", Syntax.Expression.Null);
 		public static readonly Role<Expression> TargetExpression = new Role<Expression>("Target", Syntax.Expression.Null);
 		public readonly static Role<Expression> Condition = new Role<Expression>("Condition", Syntax.Expression.Null);
-		public static readonly Role<TypeParameterDeclaration> TypeParameter = new Role<TypeParameterDeclaration>("TypeParameter");
+		public static readonly Role<TypeParameterDeclaration> TypeParameter = new Role<TypeParameterDeclaration>("TypeParameter", null);
 		public static readonly Role<AstType> TypeArgument = new Role<AstType>("TypeArgument", AstType.Null);
-		public readonly static Role<Constraint> Constraint = new Role<Constraint>("Constraint");
+		public readonly static Role<Constraint> Constraint = new Role<Constraint>("Constraint", null);
 		public static readonly Role<VariableInitializer> Variable = new Role<VariableInitializer>("Variable", VariableInitializer.Null);
 		public static readonly Role<Statement> EmbeddedStatement = new Role<Statement>("EmbeddedStatement", Statement.Null);
-		public readonly static Role<EntityDeclaration> TypeMemberRole = new Role<EntityDeclaration>("TypeMember");
+		public readonly static Role<EntityDeclaration> TypeMemberRole = new Role<EntityDeclaration>("TypeMember", null);
 
 		public static readonly Role<VariableDesignation> VariableDesignationRole = new Role<VariableDesignation>("VariableDesignation", VariableDesignation.Null);
 
@@ -68,12 +68,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public static readonly TokenRole Colon = new TokenRole(":");
 		public static readonly TokenRole DoubleColon = new TokenRole("::");
 		public static readonly TokenRole Arrow = new TokenRole("=>");
-		public static readonly Role<Comment> Comment = new Role<Comment>("Comment");
-		public static readonly Role<PreProcessorDirective> PreProcessorDirective = new Role<PreProcessorDirective>("PreProcessorDirective");
+		public static readonly Role<Comment> Comment = new Role<Comment>("Comment", null);
+		public static readonly Role<PreProcessorDirective> PreProcessorDirective = new Role<PreProcessorDirective>("PreProcessorDirective", null);
 
 		public readonly static Role<AstType> BaseType = new Role<AstType>("BaseType", AstType.Null);
 
-		public static readonly Role<Attribute> Attribute = new Role<Attribute>("Attribute");
+		public static readonly Role<Attribute> Attribute = new Role<Attribute>("Attribute", null);
 		public static readonly Role<CSharpTokenNode> AttributeTargetRole = new Role<CSharpTokenNode>("AttributeTarget", CSharpTokenNode.Null);
 
 		public readonly static TokenRole WhereKeyword = new TokenRole("where");

ICSharpCode.Decompiler/CSharp/Syntax/Statements/LocalFunctionDeclarationStatement.cs

@@ -22,7 +22,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	public class LocalFunctionDeclarationStatement : Statement
 	{
-		public static readonly Role<MethodDeclaration> MethodDeclarationRole = new Role<MethodDeclaration>("Method");
+		public static readonly Role<MethodDeclaration> MethodDeclarationRole = new Role<MethodDeclaration>("Method", null);
 
 		public MethodDeclaration Declaration {
 			get { return GetChildByRole(MethodDeclarationRole); }

ICSharpCode.Decompiler/CSharp/Syntax/Statements/SwitchStatement.cs

@@ -33,7 +33,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	public class SwitchStatement : Statement
 	{
 		public static readonly TokenRole SwitchKeywordRole = new TokenRole("switch");
-		public static readonly Role<SwitchSection> SwitchSectionRole = new Role<SwitchSection>("SwitchSection");
+		public static readonly Role<SwitchSection> SwitchSectionRole = new Role<SwitchSection>("SwitchSection", null);
 
 		public CSharpTokenNode SwitchToken {
 			get { return GetChildByRole(SwitchKeywordRole); }
@@ -134,7 +134,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 		#endregion
 
-		public static readonly Role<CaseLabel> CaseLabelRole = new Role<CaseLabel>("CaseLabel");
+		public static readonly Role<CaseLabel> CaseLabelRole = new Role<CaseLabel>("CaseLabel", null);
 
 		public override NodeType NodeType {
 			get {

ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/EntityDeclaration.cs

@@ -25,8 +25,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	public abstract class EntityDeclaration : AstNode
 	{
-		public static readonly Role<AttributeSection> AttributeRole = new Role<AttributeSection>("Attribute");
-		public static readonly Role<CSharpModifierToken> ModifierRole = new Role<CSharpModifierToken>("Modifier");
+		public static readonly Role<AttributeSection> AttributeRole = new Role<AttributeSection>("Attribute", null);
+		public static readonly Role<CSharpModifierToken> ModifierRole = new Role<CSharpModifierToken>("Modifier", null);
 		public static readonly Role<AstType> PrivateImplementationTypeRole = new Role<AstType>("PrivateImplementationType", AstType.Null);
 
 		public override NodeType NodeType {

ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/FixedFieldDeclaration.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	public class FixedFieldDeclaration : EntityDeclaration
 	{
 		public static readonly TokenRole FixedKeywordRole = new TokenRole("fixed");
-		public static readonly Role<FixedVariableInitializer> VariableRole = new Role<FixedVariableInitializer>("FixedVariable");
+		public static readonly Role<FixedVariableInitializer> VariableRole = new Role<FixedVariableInitializer>("FixedVariable", null);
 
 		public override SymbolKind SymbolKind {
 			get { return SymbolKind.Field; }
@@ -67,4 +67,3 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 	}
 }
-

ICSharpCode.Decompiler/IL/ILAstWritingOptions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2017 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -84,7 +85,7 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
 		{
 			OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
 		}
@@ -94,6 +95,6 @@ namespace ICSharpCode.Decompiler.IL
 			PropertyChanged?.Invoke(this, e);
 		}
 
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 	}
 }

ICSharpCode.Decompiler/IL/ILInstructionExtensions.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Generic;
 using System.Text;
 
@@ -20,7 +21,7 @@ namespace ICSharpCode.Decompiler.IL
 			return target;
 		}
 
-		public static ILInstruction GetNextSibling(this ILInstruction instruction)
+		public static ILInstruction? GetNextSibling(this ILInstruction? instruction)
 		{
 			if (instruction?.Parent == null)
 				return null;

ICSharpCode.Decompiler/IL/ILTypeExtensions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -145,12 +146,12 @@ namespace ICSharpCode.Decompiler.IL
 		/// 
 		/// Returns SpecialType.UnknownType for unsupported instructions.
 		/// </summary>
-		public static IType InferType(this ILInstruction inst, ICompilation compilation)
+		public static IType InferType(this ILInstruction inst, ICompilation? compilation)
 		{
 			switch (inst)
 			{
 				case NewObj newObj:
-					return newObj.Method.DeclaringType;
+					return newObj.Method.DeclaringType ?? SpecialType.UnknownType;
 				case NewArr newArr:
 					if (compilation != null)
 						return new ArrayType(compilation, newArr.Type, newArr.Indices.Count);

ICSharpCode.Decompiler/IL/ILVariable.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -19,7 +20,6 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Linq;
 
 using ICSharpCode.Decompiler.TypeSystem;
 
@@ -192,7 +192,7 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		public string Name { get; set; }
+		public string? Name { get; set; }
 
 		public bool HasGeneratedName { get; set; }
 
@@ -202,7 +202,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <remarks>
 		/// This property is set automatically when the variable is added to the <c>ILFunction.Variables</c> collection.
 		/// </remarks>
-		public ILFunction Function { get; internal set; }
+		public ILFunction? Function { get; internal set; }
 
 		/// <summary>
 		/// Gets the block container in which this variable is captured.
@@ -212,7 +212,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <remarks>
 		/// This property returns null for variables that are not captured.
 		/// </remarks>
-		public BlockContainer CaptureScope { get; internal set; }
+		public BlockContainer? CaptureScope { get; internal set; }
 
 		/// <summary>
 		/// Gets the index of this variable within the <c>Function.Variables</c> collection.
@@ -309,7 +309,7 @@ namespace ICSharpCode.Decompiler.IL
 			return list.Count - 1;
 		}
 
-		void RemoveInstruction<T>(List<T> list, int index, T inst) where T : class, IInstructionWithVariableOperand
+		void RemoveInstruction<T>(List<T> list, int index, T? inst) where T : class, IInstructionWithVariableOperand
 		{
 			Debug.Assert(list[index] == inst);
 			int indexToMove = list.Count - 1;
@@ -370,7 +370,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// The field which was converted to a local variable.
 		/// Set when the variable is from a 'yield return' or 'async' state machine.
 		/// </summary>
-		public IField StateMachineField;
+		public IField? StateMachineField;
 
 		public ILVariable(VariableKind kind, IType type, int? index = null)
 		{
@@ -398,7 +398,7 @@ namespace ICSharpCode.Decompiler.IL
 			CheckInvariant();
 		}
 
-		public override string ToString()
+		public override string? ToString()
 		{
 			return Name;
 		}
@@ -472,7 +472,7 @@ namespace ICSharpCode.Decompiler.IL
 			if (CaptureScope != null)
 			{
 				output.Write(" captured in ");
-				output.WriteLocalReference(CaptureScope.EntryPoint.Label, CaptureScope);
+				output.WriteLocalReference(CaptureScope.EntryPoint?.Label, CaptureScope);
 			}
 			if (StateMachineField != null)
 			{
@@ -528,7 +528,7 @@ namespace ICSharpCode.Decompiler.IL
 	{
 		public static readonly ILVariableEqualityComparer Instance = new ILVariableEqualityComparer();
 
-		public bool Equals(ILVariable x, ILVariable y)
+		public bool Equals(ILVariable? x, ILVariable? y)
 		{
 			if (x == y)
 				return true;

ICSharpCode.Decompiler/IL/InstructionFlags.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/InstructionOutputExtensions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions.tt

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 <#@ template debug="false" hostspecific="false" language="C#" #>
 <#@ assembly name="System.Core" #>
 <#@ import namespace="System.Linq" #>
@@ -32,7 +34,7 @@
 			AbstractBaseClass, CustomArguments(("left", null), ("right", null)), HasFlag("InstructionFlags.None")),
 		new OpCode("CallInstruction", "Instruction with a list of arguments.",
 			AbstractBaseClass, CustomChildren(new []{ new ArgumentInfo("arguments") { IsCollection = true }}),
-			CustomWriteTo, MayThrow, SideEffect),
+			CustomConstructor, CustomWriteTo, MayThrow, SideEffect),
 		new OpCode("PatternInstruction", "Base class for pattern matching in ILAst.", AbstractBaseClass, ResultType("Unknown")) { Namespace = "ICSharpCode.Decompiler.IL.Patterns" },
 		new OpCode("CompoundAssignmentInstruction", "Common instruction for compound assignments.",
 			AbstractBaseClass, CustomConstructor, CustomArguments(("target", null), ("value", null))),
@@ -68,7 +70,7 @@
 				new ChildInfo("init") { CanInlineInto = true },
 				new ChildInfo("body")
 			}),
-			CustomInvariant("Debug.Assert(Variable.Kind == VariableKind.PinnedRegionLocal);")),
+			CustomInvariant("DebugAssert(Variable.Kind == VariableKind.PinnedRegionLocal);")),
 		new OpCode("binary", "Common instruction for add, sub, mul, div, rem, bit.and, bit.or, bit.xor, shl and shr.",
 			CustomClassName("BinaryNumericInstruction"), Binary, CustomWriteTo, CustomConstructor, CustomComputeFlags,
 			MatchCondition("CheckForOverflow == o.CheckForOverflow && Sign == o.Sign && Operator == o.Operator && IsLifted == o.IsLifted")),
@@ -215,11 +217,11 @@
 		new OpCode("ldnull", "Loads the null reference.",
 			CustomClassName("LdNull"), NoArguments, ResultType("O")),
 		new OpCode("ldftn", "Load method pointer",
-			CustomClassName("LdFtn"), NoArguments, HasMethodOperand, ResultType("I")),
+			CustomClassName("LdFtn"), NoArguments, HasMethodOperand(), ResultType("I")),
 		new OpCode("ldvirtftn", "Load method pointer",
-			CustomClassName("LdVirtFtn"), Unary, HasMethodOperand, MayThrow, ResultType("I")),
+			CustomClassName("LdVirtFtn"), Unary, HasMethodOperand(), MayThrow, ResultType("I")),
 		new OpCode("ldvirtdelegate", "Virtual delegate construction",
-			CustomClassName("LdVirtDelegate"), Unary, HasTypeOperand, HasMethodOperand,
+			CustomClassName("LdVirtDelegate"), Unary, HasTypeOperand, HasMethodOperand(),
 			MayThrow, ResultType("O")),
 		new OpCode("ldtypetoken", "Loads runtime representation of metadata token",
 			CustomClassName("LdTypeToken"), NoArguments, HasTypeOperand, ResultType("O")),
@@ -287,7 +289,7 @@
 				+ "The input must be an object reference (O)." + Environment.NewLine
 				+ "If the input is an array/string, evaluates to a reference to the first element/character, or to a null reference if the array is null or empty." + Environment.NewLine
 				+ "Otherwise, uses the GetPinnableReference method to get the reference, or evaluates to a null reference if the input is null." + Environment.NewLine,
-			CustomArguments(("argument", new[] { "O" })), ResultType("Ref"), HasMethodOperand),
+			CustomArguments(("argument", new[] { "O" })), ResultType("Ref"), HasMethodOperand(nullable: true)),
 		new OpCode("string.to.int", "Maps a string value to an integer. This is used in switch(string).",
 			CustomArguments(("argument", new[] { "O" })), CustomConstructor, CustomWriteTo, ResultType("I4")),
 
@@ -297,7 +299,7 @@
 
 		new OpCode("user.logic.operator", "Use of user-defined &amp;&amp; or || operator.",
 			CustomClassName("UserDefinedLogicOperator"),
-			HasMethodOperand, ResultType("O"),
+			HasMethodOperand(), ResultType("O"),
 			CustomChildren(new []{
 				new ChildInfo("left") { CanInlineInto = true },
 				new ChildInfo("right") { CanInlineInto = false } // only executed depending on value of left
@@ -337,7 +339,7 @@
 			CustomClassName("DynamicIsEventInstruction"), Dynamic, CustomArguments(("argument", new[] { "O" })), CustomWriteTo),
 
 		new OpCode("match", "ILAst representation of C# patterns",
-			CustomClassName("MatchInstruction"), HasVariableOperand("Store"), HasMethodOperand,
+			CustomClassName("MatchInstruction"), HasVariableOperand("Store"), HasMethodOperand(nullable: true),
 			BoolFlag("IsDeconstructCall"), BoolFlag("IsDeconstructTuple"), BoolFlag("CheckType"), BoolFlag("CheckNotNull"),
 			CustomChildren(new []{
 				new ChildInfo("testedOperand") { CanInlineInto = true },
@@ -371,8 +373,8 @@
 	};
 #>
 using System;
-using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using ICSharpCode.Decompiler.TypeSystem;
 
@@ -437,7 +439,7 @@ namespace <#=opCode.Namespace#>
 		}
 <#	} #>
 <#		if (opCode.GeneratePerformMatch) { #>
-		protected internal override bool PerformMatch(ILInstruction other, ref Patterns.Match match)
+		protected internal override bool PerformMatch(ILInstruction? other, ref Patterns.Match match)
 		{
 			var o = other as <#=opCode.Name#>;
 			return <#=string.Join(" && ", opCode.PerformMatchConditions)#>;
@@ -637,10 +639,15 @@ namespace ICSharpCode.Decompiler.IL
 		public string TypeName;
 		public string Name;
 		public string FieldName;
+		public bool IsReferenceType = true;
 		
 		public override string ToString()
 		{
-			return "out " + TypeName + " " + Name;
+			if (IsReferenceType) {
+				return "[NotNullWhen(true)] out " + TypeName + "? " + Name;
+			} else {
+				return "out " + TypeName + " " + Name;
+			}
 		}
 	}
 	
@@ -866,7 +873,7 @@ namespace ICSharpCode.Decompiler.IL
 						opCode.WriteArguments.Add("output.Write(\", \");");
 					opCode.WriteArguments.Add("this." + arg + ".WriteTo(output, options);");
 					opCode.Members.Add("public static readonly SlotInfo " + children[i].SlotName + " = " + children[i].GetSlotInit() + ";");
-					opCode.Members.Add("ILInstruction " + arg + ";");
+					opCode.Members.Add("ILInstruction " + arg + " = null!;");
 					opCode.Members.Add("public ILInstruction " + argProp + " {" + Environment.NewLine
 						+ "\tget { return this." + arg + "; }" + Environment.NewLine
 						+ "\tset {" + Environment.NewLine
@@ -885,7 +892,7 @@ namespace ICSharpCode.Decompiler.IL
 							checkString += " || ";
 						checkString += arg + ".ResultType == " + expectedTypeCode;
 					}
-					opCode.Invariants.Add("Debug.Assert(" + checkString + ");");
+					opCode.Invariants.Add("DebugAssert(" + checkString + ");");
 				}
 			}
 			opCode.WriteArguments.Add("output.Write(')');");
@@ -1008,8 +1015,7 @@ namespace ICSharpCode.Decompiler.IL
 		Action<OpCode> action = opCode => {
 			opCode.ConstructorParameters.Add("ILVariable variable");
 			opCode.Members.Add("ILVariable variable;");
-			opCode.ConstructorBody.Add("Debug.Assert(variable != null);");
-			opCode.ConstructorBody.Add("this.variable = variable;");
+			opCode.ConstructorBody.Add("this.variable = variable ?? throw new ArgumentNullException(nameof(variable));");
 			opCode.MatchParameters.Add(new MatchParamInfo { TypeName = "ILVariable", Name = "variable", FieldName = "Variable" });
 			opCode.PerformMatchConditions.Add("variable == o.variable");
 			opCode.GenerateWriteTo = true;
@@ -1019,7 +1025,7 @@ namespace ICSharpCode.Decompiler.IL
 			opCode.Members.Add(@"public ILVariable Variable {
 	get { return variable; }
 	set {
-		Debug.Assert(value != null);
+		DebugAssert(value != null);
 		if (IsConnected)
 			variable.RemoveAccessInstruction(this);
 		variable = value;
@@ -1048,8 +1054,8 @@ protected override void Disconnected()
 }
 ".Replace("Access", accessType));
 			if (generateCheckInvariant) {
-				opCode.Invariants.Add("Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(variable.Function));");
-				opCode.Invariants.Add("Debug.Assert(phase <= ILPhase.InILReader || variable.Function.Variables[variable.IndexInFunction] == variable);");
+				opCode.Invariants.Add("DebugAssert(phase <= ILPhase.InILReader || this.IsDescendantOf(variable.Function!));");
+				opCode.Invariants.Add("DebugAssert(phase <= ILPhase.InILReader || variable.Function!.Variables[variable.IndexInFunction] == variable);");
 			}
 		};
 		if (accessType == "Load") {
@@ -1093,22 +1099,26 @@ protected override void Disconnected()
 		opCode.WriteOperand.Add("type.WriteTo(output);");
 	};
 
-	static Action<OpCode> HasMethodOperand = opCode => {
-		opCode.ConstructorParameters.Add("IMethod method");
-		opCode.Members.Add("readonly IMethod method;");
-		opCode.ConstructorBody.Add("this.method = method;");
-		opCode.MatchParameters.Add(new MatchParamInfo { TypeName = "IMethod", Name = "method", FieldName = "Method" });
-		opCode.PerformMatchConditions.Add("object.Equals(method, o.method)");
-		opCode.Members.Add("/// <summary>Returns the method operand.</summary>" + Environment.NewLine
-						+ "public IMethod Method { get { return method; } }");
-		opCode.GenerateWriteTo = true;
-		opCode.WriteOperand.Add("if (method != null) {");
-		opCode.WriteOperand.Add("\toutput.Write(' ');");
-		opCode.WriteOperand.Add("\tmethod.WriteTo(output);");
-		opCode.WriteOperand.Add("}");
-		opCode.Interfaces.Add("IInstructionWithMethodOperand");
-	};
-	
+	static Action<OpCode> HasMethodOperand(bool nullable = false)
+	{
+		return opCode => {
+			string n = nullable ? "?" : "";
+			opCode.ConstructorParameters.Add($"IMethod{n} method");
+			opCode.Members.Add($"readonly IMethod{n} method;");
+			opCode.ConstructorBody.Add("this.method = method;");
+			opCode.MatchParameters.Add(new MatchParamInfo { TypeName = $"IMethod{n}", Name = "method", FieldName = "Method", IsReferenceType = !nullable });
+			opCode.PerformMatchConditions.Add("object.Equals(method, o.method)");
+			opCode.Members.Add("/// <summary>Returns the method operand.</summary>" + Environment.NewLine
+							+ $"public IMethod{n} Method => method;");
+			opCode.GenerateWriteTo = true;
+			opCode.WriteOperand.Add("if (method != null) {");
+			opCode.WriteOperand.Add("\toutput.Write(' ');");
+			opCode.WriteOperand.Add("\tmethod.WriteTo(output);");
+			opCode.WriteOperand.Add("}");
+			opCode.Interfaces.Add("IInstructionWithMethodOperand");
+		};
+	}
+
 	static Action<OpCode> HasMemberOperand = opCode => {
 		opCode.ConstructorParameters.Add("IMember member");
 		opCode.Members.Add("readonly IMember member;");
@@ -1139,7 +1149,12 @@ protected override void Disconnected()
 		return opCode => {
 			NoArguments(opCode);
 			opCode.ConstructorParameters.Add(operandType + " value");
-			opCode.MatchParameters.Add(new MatchParamInfo { TypeName = operandType, Name = "value", FieldName = "Value" });
+			opCode.MatchParameters.Add(new MatchParamInfo {
+				TypeName = operandType,
+				Name = "value",
+				FieldName = "Value",
+				IsReferenceType = operandType == "string"
+			});
 			opCode.PerformMatchConditions.Add("this.Value == o.Value");
 			opCode.Members.Add("public readonly " + operandType + " Value;");
 			opCode.ConstructorBody.Add("this.Value = value;");

ICSharpCode.Decompiler/IL/Instructions/Await.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2017 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -22,7 +23,7 @@ namespace ICSharpCode.Decompiler.IL
 {
 	partial class Await
 	{
-		public IMethod GetAwaiterMethod;
-		public IMethod GetResultMethod;
+		public IMethod? GetAwaiterMethod;
+		public IMethod? GetResultMethod;
 	}
 }

ICSharpCode.Decompiler/IL/Instructions/BinaryNumericInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -214,5 +215,3 @@ namespace ICSharpCode.Decompiler.IL
 		}
 	}
 }
-
-

ICSharpCode.Decompiler/IL/Instructions/Block.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014-2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014-2016 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -16,14 +17,13 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.TypeSystem;
-
 namespace ICSharpCode.Decompiler.IL
 {
 	/// <summary>
@@ -48,7 +48,7 @@ namespace ICSharpCode.Decompiler.IL
 
 		public readonly BlockKind Kind;
 		public readonly InstructionCollection<ILInstruction> Instructions;
-		ILInstruction finalInstruction;
+		ILInstruction finalInstruction = null!;
 
 		/// <summary>
 		/// For blocks in a block container, this field holds
@@ -124,10 +124,10 @@ namespace ICSharpCode.Decompiler.IL
 					foreach (var inst in Instructions)
 					{
 						var stloc = inst as StLoc;
-						Debug.Assert(stloc != null, "Instructions in CallWithNamedArgs must be assignments");
-						Debug.Assert(stloc.Variable.Kind == VariableKind.NamedArgument);
-						Debug.Assert(stloc.Variable.IsSingleDefinition && stloc.Variable.LoadCount == 1);
-						Debug.Assert(stloc.Variable.LoadInstructions.Single().Parent == finalInstruction);
+						DebugAssert(stloc != null, "Instructions in CallWithNamedArgs must be assignments");
+						DebugAssert(stloc.Variable.Kind == VariableKind.NamedArgument);
+						DebugAssert(stloc.Variable.IsSingleDefinition && stloc.Variable.LoadCount == 1);
+						DebugAssert(stloc.Variable.LoadInstructions.Single().Parent == finalInstruction);
 					}
 					var call = (CallInstruction)finalInstruction;
 					if (call.IsInstanceCall)
@@ -140,22 +140,22 @@ namespace ICSharpCode.Decompiler.IL
 				case BlockKind.ArrayInitializer:
 					var final = finalInstruction as LdLoc;
 					Debug.Assert(final != null && final.Variable.IsSingleDefinition && final.Variable.Kind == VariableKind.InitializerTarget);
-					IType type = null;
-					Debug.Assert(Instructions[0].MatchStLoc(final.Variable, out var init) && init.MatchNewArr(out type));
+					IType? type = null;
+					Debug.Assert(Instructions[0].MatchStLoc(final!.Variable, out var init) && init.MatchNewArr(out type));
 					for (int i = 1; i < Instructions.Count; i++)
 					{
-						Debug.Assert(Instructions[i].MatchStObj(out ILInstruction target, out _, out var t) && type != null && type.Equals(t));
-						Debug.Assert(target.MatchLdElema(out t, out ILInstruction array) && type.Equals(t));
-						Debug.Assert(array.MatchLdLoc(out ILVariable v) && v == final.Variable);
+						DebugAssert(Instructions[i].MatchStObj(out ILInstruction? target, out _, out var t) && type != null && type.Equals(t));
+						DebugAssert(target.MatchLdElema(out t, out ILInstruction? array) && type.Equals(t));
+						DebugAssert(array.MatchLdLoc(out ILVariable? v) && v == final.Variable);
 					}
 					break;
 				case BlockKind.CollectionInitializer:
 				case BlockKind.ObjectInitializer:
 					var final2 = finalInstruction as LdLoc;
 					Debug.Assert(final2 != null);
-					var initVar2 = final2.Variable;
+					var initVar2 = final2!.Variable;
 					Debug.Assert(initVar2.StoreCount == 1 && initVar2.Kind == VariableKind.InitializerTarget);
-					IType type2 = null;
+					IType? type2 = null;
 					bool condition = Instructions[0].MatchStLoc(final2.Variable, out var init2);
 					Debug.Assert(condition);
 					Debug.Assert(init2 is NewObj
@@ -300,7 +300,7 @@ namespace ICSharpCode.Decompiler.IL
 		public void Remove()
 		{
 			Debug.Assert(ChildIndex > 0);
-			var container = (BlockContainer)Parent;
+			var container = (BlockContainer)Parent!;
 			Debug.Assert(container.Blocks[ChildIndex] == this);
 			container.Blocks.SwapRemoveAt(ChildIndex);
 		}
@@ -325,7 +325,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the predecessor of the given instruction.
 		/// Returns null if inst.Parent is not a block.
 		/// </summary>
-		public static ILInstruction GetPredecessor(ILInstruction inst)
+		public static ILInstruction? GetPredecessor(ILInstruction inst)
 		{
 			if (inst.Parent is Block block && inst.ChildIndex > 0)
 			{
@@ -341,7 +341,8 @@ namespace ICSharpCode.Decompiler.IL
 		/// If inst is a block consisting of a single instruction, returns that instruction.
 		/// Otherwise, returns the input instruction.
 		/// </summary>
-		public static ILInstruction Unwrap(ILInstruction inst)
+		[return: NotNullIfNotNull("inst")]
+		public static ILInstruction? Unwrap(ILInstruction? inst)
 		{
 			if (inst is Block block)
 			{
@@ -355,19 +356,19 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the closest parent Block.
 		/// Returns null, if the instruction is not a descendant of a Block.
 		/// </summary>
-		public static Block FindClosestBlock(ILInstruction inst)
+		public static Block? FindClosestBlock(ILInstruction? inst)
 		{
 			var curr = inst;
 			while (curr != null)
 			{
-				if (curr is Block)
-					return (Block)curr;
+				if (curr is Block b)
+					return b;
 				curr = curr.Parent;
 			}
 			return null;
 		}
 
-		public bool MatchInlineAssignBlock(out CallInstruction call, out ILInstruction value)
+		public bool MatchInlineAssignBlock([NotNullWhen(true)] out CallInstruction? call, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			call = null;
 			value = null;
@@ -385,7 +386,7 @@ namespace ICSharpCode.Decompiler.IL
 			return this.FinalInstruction.MatchLdLoc(tmp);
 		}
 
-		public bool MatchIfAtEndOfBlock(out ILInstruction condition, out ILInstruction trueInst, out ILInstruction falseInst)
+		public bool MatchIfAtEndOfBlock([NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst, [NotNullWhen(true)] out ILInstruction? falseInst)
 		{
 			condition = null;
 			trueInst = null;

ICSharpCode.Decompiler/IL/Instructions/BlockContainer.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -19,11 +20,11 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.Util;
-
 namespace ICSharpCode.Decompiler.IL
 {
 	/// <summary>
@@ -57,13 +58,18 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		Block entryPoint;
+		Block? entryPoint;
 
 		/// <summary>
 		/// Gets the container's entry point. This is the first block in the Blocks collection.
 		/// </summary>
 		public Block EntryPoint {
-			get { return entryPoint; }
+			get {
+				// HACK: While it's possible to have BlockContainers without entry point,
+				// normally every container must have an entry point according to its invariant.
+				// Thus it's easier on the transforms if this property returns a non-nullable EntryPoint.
+				return entryPoint!;
+			}
 			private set {
 				if (entryPoint != null && IsConnected)
 					entryPoint.IncomingEdgeCount--;
@@ -102,7 +108,7 @@ namespace ICSharpCode.Decompiler.IL
 		protected internal override void InstructionCollectionUpdateComplete()
 		{
 			base.InstructionCollectionUpdateComplete();
-			this.EntryPoint = this.Blocks.FirstOrDefault();
+			this.EntryPoint = this.Blocks.FirstOrDefault()!;
 		}
 
 		protected override void Connected()
@@ -174,7 +180,7 @@ namespace ICSharpCode.Decompiler.IL
 			return Blocks[index];
 		}
 
-		protected override void SetChild(int index, ILInstruction value)
+		protected override void SetChild(int index, ILInstruction? value)
 		{
 			if (Blocks[index] != value)
 				throw new InvalidOperationException("Cannot replace blocks in BlockContainer");
@@ -189,12 +195,12 @@ namespace ICSharpCode.Decompiler.IL
 		{
 			base.CheckInvariant(phase);
 			Debug.Assert(Blocks.Count > 0 && EntryPoint == Blocks[0]);
-			Debug.Assert(!IsConnected || EntryPoint?.IncomingEdgeCount >= 1);
-			Debug.Assert(EntryPoint == null || Parent is ILFunction || !ILRangeIsEmpty);
+			Debug.Assert(!IsConnected || EntryPoint.IncomingEdgeCount >= 1);
+			Debug.Assert(Parent is ILFunction || !ILRangeIsEmpty);
 			Debug.Assert(Blocks.All(b => b.HasFlag(InstructionFlags.EndPointUnreachable)));
 			Debug.Assert(Blocks.All(b => b.Kind == BlockKind.ControlFlow)); // this also implies that the blocks don't use FinalInstruction
 			Debug.Assert(TopologicalSort(deleteUnreachableBlocks: true).Count == Blocks.Count, "Container should not have any unreachable blocks");
-			Block bodyStartBlock;
+			Block? bodyStartBlock;
 			switch (Kind)
 			{
 				case ContainerKind.Normal:
@@ -319,7 +325,7 @@ namespace ICSharpCode.Decompiler.IL
 			Blocks.ReplaceList(newOrder);
 		}
 
-		public static BlockContainer FindClosestContainer(ILInstruction inst)
+		public static BlockContainer? FindClosestContainer(ILInstruction? inst)
 		{
 			while (inst != null)
 			{
@@ -330,18 +336,18 @@ namespace ICSharpCode.Decompiler.IL
 			return null;
 		}
 
-		public static BlockContainer FindClosestSwitchContainer(ILInstruction inst)
+		public static BlockContainer? FindClosestSwitchContainer(ILInstruction? inst)
 		{
 			while (inst != null)
 			{
-				if (inst is BlockContainer bc && bc.entryPoint.Instructions.FirstOrDefault() is SwitchInstruction)
+				if (inst is BlockContainer { Kind: ContainerKind.Switch } bc)
 					return bc;
 				inst = inst.Parent;
 			}
 			return null;
 		}
 
-		public bool MatchConditionBlock(Block block, out ILInstruction condition, out Block bodyStartBlock)
+		public bool MatchConditionBlock(Block block, [NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out Block? bodyStartBlock)
 		{
 			condition = null;
 			bodyStartBlock = null;

ICSharpCode.Decompiler/IL/Instructions/Branch.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -30,7 +31,7 @@ namespace ICSharpCode.Decompiler.IL
 	partial class Branch : SimpleInstruction, IBranchOrLeaveInstruction
 	{
 		readonly int targetILOffset;
-		Block targetBlock;
+		Block? targetBlock;
 
 		public Branch(int targetILOffset) : base(OpCode.Branch)
 		{
@@ -48,7 +49,11 @@ namespace ICSharpCode.Decompiler.IL
 		}
 
 		public Block TargetBlock {
-			get { return targetBlock; }
+			get {
+				// HACK: We treat TargetBlock as non-nullable publicly, because it's only null inside
+				// the ILReader, and becomes non-null once the BlockBuilder has run.
+				return targetBlock!;
+			}
 			set {
 				if (targetBlock != null && IsConnected)
 					targetBlock.IncomingEdgeCount--;
@@ -62,7 +67,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the BlockContainer that contains the target block.
 		/// </summary>
 		public BlockContainer TargetContainer {
-			get { return (BlockContainer)targetBlock?.Parent; }
+			get { return (BlockContainer)targetBlock?.Parent!; }
 		}
 
 		protected override void Connected()
@@ -92,9 +97,9 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		internal static bool GetExecutesFinallyBlock(ILInstruction inst, BlockContainer container)
+		internal static bool GetExecutesFinallyBlock(ILInstruction? inst, BlockContainer? container)
 		{
-			for (; inst != container; inst = inst.Parent)
+			for (; inst != container && inst != null; inst = inst.Parent)
 			{
 				if (inst.Parent is TryFinally && inst.SlotInfo == TryFinally.TryBlockSlot)
 					return true;
@@ -107,9 +112,9 @@ namespace ICSharpCode.Decompiler.IL
 			base.CheckInvariant(phase);
 			if (phase > ILPhase.InILReader)
 			{
-				Debug.Assert(targetBlock.Parent is BlockContainer);
-				Debug.Assert(this.IsDescendantOf(targetBlock.Parent));
-				Debug.Assert(targetBlock.Parent.Children[targetBlock.ChildIndex] == targetBlock);
+				Debug.Assert(targetBlock?.Parent is BlockContainer);
+				Debug.Assert(this.IsDescendantOf(targetBlock!.Parent!));
+				Debug.Assert(targetBlock!.Parent!.Children[targetBlock.ChildIndex] == targetBlock);
 			}
 		}
 
@@ -118,7 +123,7 @@ namespace ICSharpCode.Decompiler.IL
 			WriteILRange(output, options);
 			output.Write(OpCode);
 			output.Write(' ');
-			output.WriteLocalReference(TargetLabel, (object)targetBlock ?? TargetILOffset);
+			output.WriteLocalReference(TargetLabel, (object?)targetBlock ?? TargetILOffset);
 		}
 	}
 

ICSharpCode.Decompiler/IL/Instructions/CallIndirect.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2017 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -31,7 +32,7 @@ namespace ICSharpCode.Decompiler.IL
 		public static readonly SlotInfo FunctionPointerSlot = new SlotInfo("FunctionPointer", canInlineInto: true);
 		public static readonly SlotInfo ArgumentSlot = new SlotInfo("Argument", canInlineInto: true, isCollection: true);
 
-		ILInstruction functionPointer;
+		ILInstruction functionPointer = null!;
 		public readonly InstructionCollection<ILInstruction> Arguments;
 		public bool IsInstance { get; }
 		public bool HasExplicitThis { get; }

ICSharpCode.Decompiler/IL/Instructions/CallInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -19,7 +20,6 @@
 using System;
 using System.Diagnostics;
 
-using ICSharpCode.Decompiler.Disassembler;
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.IL
@@ -52,7 +52,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets/Sets the type specified in the 'constrained.' prefix.
 		/// Returns null if no 'constrained.' prefix exists for this call.
 		/// </summary>
-		public IType ConstrainedTo;
+		public IType? ConstrainedTo;
 
 		/// <summary>
 		/// Gets whether the IL stack was empty at the point of this call.
@@ -62,8 +62,7 @@ namespace ICSharpCode.Decompiler.IL
 
 		protected CallInstruction(OpCode opCode, IMethod method) : base(opCode)
 		{
-			Debug.Assert(method != null);
-			this.Method = method;
+			this.Method = method ?? throw new ArgumentNullException(nameof(method));
 			this.Arguments = new InstructionCollection<ILInstruction>(this, 0);
 		}
 
@@ -78,7 +77,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the parameter for the argument with the specified index.
 		/// Returns null for the <c>this</c> parameter.
 		/// </summary>
-		public IParameter GetParameter(int argumentIndex)
+		public IParameter? GetParameter(int argumentIndex)
 		{
 			int firstParamIndex = (Method.IsStatic || OpCode == OpCode.NewObj) ? 0 : 1;
 			if (argumentIndex < firstParamIndex)
@@ -160,9 +159,9 @@ namespace ICSharpCode.Decompiler.IL
 			output.Write(')');
 		}
 
-		protected internal sealed override bool PerformMatch(ILInstruction other, ref Patterns.Match match)
+		protected internal sealed override bool PerformMatch(ILInstruction? other, ref Patterns.Match match)
 		{
-			CallInstruction o = other as CallInstruction;
+			CallInstruction? o = other as CallInstruction;
 			return o != null && this.OpCode == o.OpCode && this.Method.Equals(o.Method) && this.IsTail == o.IsTail
 				&& object.Equals(this.ConstrainedTo, o.ConstrainedTo)
 				&& Patterns.ListMatch.DoMatch(this.Arguments, o.Arguments, ref match);

ICSharpCode.Decompiler/IL/Instructions/Comp.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -21,7 +22,6 @@ using System.Diagnostics;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.TypeSystem;
-using ICSharpCode.Decompiler.Util;
 
 namespace ICSharpCode.Decompiler.IL
 {
@@ -231,5 +231,3 @@ namespace ICSharpCode.Decompiler.IL
 		}
 	}
 }
-
-

ICSharpCode.Decompiler/IL/Instructions/CompoundAssignmentInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2016 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -172,7 +173,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Gets whether the specific binary instruction is compatible with a compound operation on the specified type.
 		/// </summary>
-		internal static bool IsBinaryCompatibleWithType(BinaryNumericInstruction binary, IType type, DecompilerSettings settings)
+		internal static bool IsBinaryCompatibleWithType(BinaryNumericInstruction binary, IType type, DecompilerSettings? settings)
 		{
 			if (binary.IsLifted)
 			{

ICSharpCode.Decompiler/IL/Instructions/Conv.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/DeconstructResultInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2020 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2020 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -47,9 +48,9 @@ namespace ICSharpCode.Decompiler.IL
 			output.Write(')');
 		}
 
-		MatchInstruction FindMatch()
+		MatchInstruction? FindMatch()
 		{
-			for (ILInstruction inst = this; inst != null; inst = inst.Parent)
+			for (ILInstruction? inst = this; inst != null; inst = inst.Parent)
 			{
 				if (inst.Parent is MatchInstruction match && inst != match.TestedOperand)
 					return match;
@@ -60,10 +61,10 @@ namespace ICSharpCode.Decompiler.IL
 		void AdditionalInvariants()
 		{
 			var matchInst = FindMatch();
-			Debug.Assert(matchInst != null && (matchInst.IsDeconstructCall || matchInst.IsDeconstructTuple));
-			Debug.Assert(Argument.MatchLdLoc(matchInst.Variable));
+			DebugAssert(matchInst != null && (matchInst.IsDeconstructCall || matchInst.IsDeconstructTuple));
+			DebugAssert(Argument.MatchLdLoc(matchInst.Variable));
 			var outParamType = matchInst.GetDeconstructResultType(this.Index);
-			Debug.Assert(outParamType.GetStackType() == ResultType);
+			DebugAssert(outParamType.GetStackType() == ResultType);
 		}
 	}
 }

ICSharpCode.Decompiler/IL/Instructions/DefaultValue.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/DynamicInstructions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2018 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2018 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -56,7 +57,7 @@ namespace ICSharpCode.Decompiler.IL
 
 	public struct CSharpArgumentInfo
 	{
-		public string Name { get; set; }
+		public string? Name { get; set; }
 		public CSharpArgumentInfoFlags Flags { get; set; }
 		public IType CompileTimeType { get; set; }
 
@@ -66,9 +67,9 @@ namespace ICSharpCode.Decompiler.IL
 	partial class DynamicInstruction
 	{
 		public CSharpBinderFlags BinderFlags { get; }
-		public IType CallingContext { get; }
+		public IType? CallingContext { get; }
 
-		protected DynamicInstruction(OpCode opCode, CSharpBinderFlags binderFlags, IType context)
+		protected DynamicInstruction(OpCode opCode, CSharpBinderFlags binderFlags, IType? context)
 			: base(opCode)
 		{
 			BinderFlags = binderFlags;
@@ -141,10 +142,10 @@ namespace ICSharpCode.Decompiler.IL
 			output.Write(')');
 		}
 
-		public DynamicConvertInstruction(CSharpBinderFlags binderFlags, IType type, IType context, ILInstruction argument)
+		public DynamicConvertInstruction(CSharpBinderFlags binderFlags, IType type, IType? context, ILInstruction argument)
 			: base(OpCode.DynamicConvertInstruction, binderFlags, context)
 		{
-			Type = type;
+			this.type = type;
 			Argument = argument;
 		}
 
@@ -171,7 +172,7 @@ namespace ICSharpCode.Decompiler.IL
 		public IReadOnlyList<IType> TypeArguments { get; }
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicInvokeMemberInstruction(CSharpBinderFlags binderFlags, string name, IType[] typeArguments, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicInvokeMemberInstruction(CSharpBinderFlags binderFlags, string name, IType[]? typeArguments, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicInvokeMemberInstruction, binderFlags, context)
 		{
 			Name = name;
@@ -216,10 +217,10 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class DynamicGetMemberInstruction
 	{
-		public string Name { get; }
+		public string? Name { get; }
 		public CSharpArgumentInfo TargetArgumentInfo { get; }
 
-		public DynamicGetMemberInstruction(CSharpBinderFlags binderFlags, string name, IType context, CSharpArgumentInfo targetArgumentInfo, ILInstruction target)
+		public DynamicGetMemberInstruction(CSharpBinderFlags binderFlags, string? name, IType? context, CSharpArgumentInfo targetArgumentInfo, ILInstruction target)
 			: base(OpCode.DynamicGetMemberInstruction, binderFlags, context)
 		{
 			Name = name;
@@ -249,11 +250,11 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class DynamicSetMemberInstruction
 	{
-		public string Name { get; }
+		public string? Name { get; }
 		public CSharpArgumentInfo TargetArgumentInfo { get; }
 		public CSharpArgumentInfo ValueArgumentInfo { get; }
 
-		public DynamicSetMemberInstruction(CSharpBinderFlags binderFlags, string name, IType context, CSharpArgumentInfo targetArgumentInfo, ILInstruction target, CSharpArgumentInfo valueArgumentInfo, ILInstruction value)
+		public DynamicSetMemberInstruction(CSharpBinderFlags binderFlags, string? name, IType? context, CSharpArgumentInfo targetArgumentInfo, ILInstruction target, CSharpArgumentInfo valueArgumentInfo, ILInstruction value)
 			: base(OpCode.DynamicSetMemberInstruction, binderFlags, context)
 		{
 			Name = name;
@@ -293,7 +294,7 @@ namespace ICSharpCode.Decompiler.IL
 	{
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicGetIndexInstruction(CSharpBinderFlags binderFlags, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicGetIndexInstruction(CSharpBinderFlags binderFlags, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicGetIndexInstruction, binderFlags, context)
 		{
 			ArgumentInfo = argumentInfo;
@@ -325,7 +326,7 @@ namespace ICSharpCode.Decompiler.IL
 	{
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicSetIndexInstruction(CSharpBinderFlags binderFlags, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicSetIndexInstruction(CSharpBinderFlags binderFlags, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicSetIndexInstruction, binderFlags, context)
 		{
 			ArgumentInfo = argumentInfo;
@@ -355,11 +356,11 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class DynamicInvokeConstructorInstruction
 	{
-		readonly IType resultType;
+		readonly IType? resultType;
 
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicInvokeConstructorInstruction(CSharpBinderFlags binderFlags, IType type, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicInvokeConstructorInstruction(CSharpBinderFlags binderFlags, IType? type, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicInvokeConstructorInstruction, binderFlags, context)
 		{
 			ArgumentInfo = argumentInfo;
@@ -395,7 +396,7 @@ namespace ICSharpCode.Decompiler.IL
 		public CSharpArgumentInfo RightArgumentInfo { get; }
 		public ExpressionType Operation { get; }
 
-		public DynamicBinaryOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType context, CSharpArgumentInfo leftArgumentInfo, ILInstruction left, CSharpArgumentInfo rightArgumentInfo, ILInstruction right)
+		public DynamicBinaryOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType? context, CSharpArgumentInfo leftArgumentInfo, ILInstruction left, CSharpArgumentInfo rightArgumentInfo, ILInstruction right)
 			: base(OpCode.DynamicBinaryOperatorInstruction, binderFlags, context)
 		{
 			Operation = operation;
@@ -437,7 +438,7 @@ namespace ICSharpCode.Decompiler.IL
 		public CSharpArgumentInfo RightArgumentInfo { get; }
 		public ExpressionType Operation { get; }
 
-		public DynamicLogicOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType context, CSharpArgumentInfo leftArgumentInfo, ILInstruction left, CSharpArgumentInfo rightArgumentInfo, ILInstruction right)
+		public DynamicLogicOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType? context, CSharpArgumentInfo leftArgumentInfo, ILInstruction left, CSharpArgumentInfo rightArgumentInfo, ILInstruction right)
 			: base(OpCode.DynamicLogicOperatorInstruction, binderFlags, context)
 		{
 			Operation = operation;
@@ -486,7 +487,7 @@ namespace ICSharpCode.Decompiler.IL
 		public CSharpArgumentInfo OperandArgumentInfo { get; }
 		public ExpressionType Operation { get; }
 
-		public DynamicUnaryOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType context, CSharpArgumentInfo operandArgumentInfo, ILInstruction operand)
+		public DynamicUnaryOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType? context, CSharpArgumentInfo operandArgumentInfo, ILInstruction operand)
 			: base(OpCode.DynamicUnaryOperatorInstruction, binderFlags, context)
 		{
 			Operation = operation;
@@ -533,7 +534,7 @@ namespace ICSharpCode.Decompiler.IL
 	{
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicInvokeInstruction(CSharpBinderFlags binderFlags, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicInvokeInstruction(CSharpBinderFlags binderFlags, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicInvokeInstruction, binderFlags, context)
 		{
 			ArgumentInfo = argumentInfo;
@@ -562,9 +563,9 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class DynamicIsEventInstruction
 	{
-		public string Name { get; }
+		public string? Name { get; }
 
-		public DynamicIsEventInstruction(CSharpBinderFlags binderFlags, string name, IType context, ILInstruction argument)
+		public DynamicIsEventInstruction(CSharpBinderFlags binderFlags, string? name, IType? context, ILInstruction argument)
 			: base(OpCode.DynamicIsEventInstruction, binderFlags, context)
 		{
 			Name = name;

ICSharpCode.Decompiler/IL/Instructions/ExpressionTreeCast.cs

@@ -1,7 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
-
+#nullable enable
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.IL
@@ -13,7 +10,7 @@ namespace ICSharpCode.Decompiler.IL
 		public ExpressionTreeCast(IType type, ILInstruction argument, bool isChecked)
 			: base(OpCode.ExpressionTreeCast, argument)
 		{
-			this.Type = type;
+			this.type = type;
 			this.IsChecked = isChecked;
 		}
 

ICSharpCode.Decompiler/IL/Instructions/ILFunction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -18,7 +19,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
 
@@ -35,7 +35,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// May be null for functions that were not constructed from metadata,
 		/// e.g., expression trees.
 		/// </summary>
-		public readonly IMethod Method;
+		public readonly IMethod? Method;
 
 		/// <summary>
 		/// Gets the generic context of this function.
@@ -54,7 +54,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// has no effect, as the name should not be used in the final AST construction.
 		/// </para>
 		/// </summary>
-		public string Name;
+		public string? Name;
 
 		/// <summary>
 		/// Size of the IL code in this function.
@@ -71,7 +71,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the scope in which the local function is declared.
 		/// Returns null, if this is not a local function.
 		/// </summary>
-		public BlockContainer DeclarationScope { get; internal set; }
+		public BlockContainer? DeclarationScope { get; internal set; }
 
 		/// <summary>
 		/// Gets the set of captured variables by this ILFunction.
@@ -108,17 +108,17 @@ namespace ICSharpCode.Decompiler.IL
 		/// Return element type -- if the async method returns Task{T}, this field stores T.
 		/// If the async method returns Task or void, this field stores void.
 		/// </summary>
-		public IType AsyncReturnType;
+		public IType? AsyncReturnType;
 
 		/// <summary>
 		/// If this function is an iterator/async, this field stores the compiler-generated MoveNext() method.
 		/// </summary>
-		public IMethod MoveNextMethod;
+		public IMethod? MoveNextMethod;
 
 		/// <summary>
 		/// If this function is a local function, this field stores the reduced version of the function.
 		/// </summary>
-		internal TypeSystem.Implementation.LocalFunctionMethod ReducedMethod;
+		internal TypeSystem.Implementation.LocalFunctionMethod? ReducedMethod;
 
 		public DebugInfo.AsyncDebugInfo AsyncDebugInfo;
 
@@ -131,8 +131,10 @@ namespace ICSharpCode.Decompiler.IL
 			get {
 				if (ctorCallStart == int.MinValue)
 				{
-					if (!this.Method.IsConstructor || this.Method.IsStatic)
+					if (this.Method == null || !this.Method.IsConstructor || this.Method.IsStatic)
+					{
 						ctorCallStart = -1;
+					}
 					else
 					{
 						ctorCallStart = this.Descendants.FirstOrDefault(d => d is CallInstruction call && !(call is NewObj)
@@ -150,7 +152,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// T is the delegate type that matches the signature of this method.
 		/// Otherwise this must be null.
 		/// </summary>
-		public IType DelegateType;
+		public IType? DelegateType;
 
 		ILFunctionKind kind;
 
@@ -181,7 +183,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// where the stack is empty, nop instructions, and the instruction following
 		/// a call instruction
 		/// </summary>
-		public List<int> SequencePointCandidates { get; set; }
+		public List<int>? SequencePointCandidates { get; set; }
 
 		/// <summary>
 		/// Constructs a new ILFunction from the given metadata and with the given ILAst body.
@@ -408,20 +410,9 @@ namespace ICSharpCode.Decompiler.IL
 
 		int helperVariableCount;
 
-		public ILVariable RegisterVariable(VariableKind kind, IType type, string name = null)
-		{
-			return RegisterVariable(kind, type, type.GetStackType(), name);
-		}
-
-		public ILVariable RegisterVariable(VariableKind kind, StackType stackType, string name = null)
-		{
-			var type = Method.Compilation.FindType(stackType.ToKnownTypeCode());
-			return RegisterVariable(kind, type, stackType, name);
-		}
-
-		ILVariable RegisterVariable(VariableKind kind, IType type, StackType stackType, string name = null)
+		public ILVariable RegisterVariable(VariableKind kind, IType type, string? name = null)
 		{
-			var variable = new ILVariable(kind, type, stackType);
+			var variable = new ILVariable(kind, type);
 			if (string.IsNullOrWhiteSpace(name))
 			{
 				name = "I_" + (helperVariableCount++);

ICSharpCode.Decompiler/IL/Instructions/ILInstruction.cs

@@ -21,9 +21,11 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Threading;
 
 using ICSharpCode.Decompiler.IL.Patterns;
+using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.Util;
 
@@ -60,7 +62,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.OpCode = opCode;
 		}
 
-		protected void ValidateChild(ILInstruction inst)
+		protected void ValidateChild(ILInstruction? inst)
 		{
 			if (inst == null)
 				throw new ArgumentNullException(nameof(inst));
@@ -69,6 +71,16 @@ namespace ICSharpCode.Decompiler.IL
 			// make sure to read the remarks on the ReplaceWith() method.
 		}
 
+		internal static void DebugAssert([DoesNotReturnIf(false)] bool b)
+		{
+			Debug.Assert(b);
+		}
+
+		internal static void DebugAssert([DoesNotReturnIf(false)] bool b, string msg)
+		{
+			Debug.Assert(b, msg);
+		}
+
 		[Conditional("DEBUG")]
 		internal virtual void CheckInvariant(ILPhase phase)
 		{
@@ -493,7 +505,7 @@ namespace ICSharpCode.Decompiler.IL
 
 			internal ChildrenEnumerator(ILInstruction inst)
 			{
-				Debug.Assert(inst != null);
+				DebugAssert(inst != null);
 				this.inst = inst;
 				this.pos = -1;
 				this.end = inst!.GetChildCount();
@@ -899,9 +911,9 @@ namespace ICSharpCode.Decompiler.IL
 		/// If extraction is not possible, the ILAst is left unmodified and the function returns null.
 		/// May return null if extraction is not possible.
 		/// </summary>
-		public ILVariable Extract()
+		public ILVariable Extract(ILTransformContext context)
 		{
-			return Transforms.ExtractionContext.Extract(this);
+			return Transforms.ExtractionContext.Extract(this, context);
 		}
 
 		/// <summary>
@@ -950,7 +962,7 @@ namespace ICSharpCode.Decompiler.IL
 
 	public interface IInstructionWithMethodOperand
 	{
-		IMethod Method { get; }
+		IMethod? Method { get; }
 	}
 
 	public interface ILiftableInstruction

ICSharpCode.Decompiler/IL/Instructions/ILVariableCollection.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2016 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -128,7 +129,7 @@ namespace ICSharpCode.Decompiler.IL
 				{
 					// However, remove unused this-parameters of delegates, expression trees, etc.
 					// These will be replaced with the top-level function's this-parameter.
-					if (v.Index == -1 && v.Function.Kind != ILFunctionKind.TopLevelFunction)
+					if (v.Index == -1 && v.Function!.Kind != ILFunctionKind.TopLevelFunction)
 						return true;
 					return false;
 				}

ICSharpCode.Decompiler/IL/Instructions/IfInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -35,7 +36,7 @@ namespace ICSharpCode.Decompiler.IL
 	/// </remarks>
 	partial class IfInstruction : ILInstruction
 	{
-		public IfInstruction(ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst = null) : base(OpCode.IfInstruction)
+		public IfInstruction(ILInstruction condition, ILInstruction trueInst, ILInstruction? falseInst = null) : base(OpCode.IfInstruction)
 		{
 			this.Condition = condition;
 			this.TrueInst = trueInst;
@@ -47,7 +48,7 @@ namespace ICSharpCode.Decompiler.IL
 			return new IfInstruction(lhs, rhs, new LdcI4(0));
 		}
 
-		public static IfInstruction LogicOr(ILInstruction lhs, ILInstruction rhs)
+		public static IfInstruction LogicOr(ILInstruction lhs, ILInstruction? rhs)
 		{
 			return new IfInstruction(lhs, new LdcI4(1), rhs);
 		}
@@ -126,7 +127,7 @@ namespace ICSharpCode.Decompiler.IL
 			if (slot == IfInstruction.ConditionSlot)
 				return true;
 			if (slot == IfInstruction.TrueInstSlot || slot == IfInstruction.FalseInstSlot || slot == NullCoalescingInstruction.FallbackInstSlot)
-				return IsInConditionSlot(inst.Parent);
+				return IsInConditionSlot(inst.Parent!);
 			if (inst.Parent is Comp comp)
 			{
 				if (comp.Left == inst && comp.Right.MatchLdcI4(0))

ICSharpCode.Decompiler/IL/Instructions/LdFlda.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/LdLen.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/Leave.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -32,9 +33,9 @@ namespace ICSharpCode.Decompiler.IL
 	/// </remarks>
 	partial class Leave : ILInstruction, IBranchOrLeaveInstruction
 	{
-		BlockContainer targetContainer;
+		BlockContainer? targetContainer;
 
-		public Leave(BlockContainer targetContainer, ILInstruction value = null) : base(OpCode.Leave)
+		public Leave(BlockContainer? targetContainer, ILInstruction? value = null) : base(OpCode.Leave)
 		{
 			// Note: ILReader will create Leave instructions with targetContainer==null to represent 'endfinally',
 			// the targetContainer will then be filled in by BlockBuilder
@@ -54,7 +55,7 @@ namespace ICSharpCode.Decompiler.IL
 		}
 
 		public BlockContainer TargetContainer {
-			get { return targetContainer; }
+			get { return targetContainer!; }
 			set {
 				if (targetContainer != null && IsConnected)
 					targetContainer.LeaveCount--;
@@ -109,8 +110,8 @@ namespace ICSharpCode.Decompiler.IL
 		internal override void CheckInvariant(ILPhase phase)
 		{
 			base.CheckInvariant(phase);
-			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(targetContainer));
-			Debug.Assert(phase <= ILPhase.InILReader || phase == ILPhase.InAsyncAwait || value.ResultType == targetContainer.ResultType);
+			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(targetContainer!));
+			Debug.Assert(phase <= ILPhase.InILReader || phase == ILPhase.InAsyncAwait || value.ResultType == targetContainer!.ResultType);
 		}
 
 		public override void WriteTo(ITextOutput output, ILAstWritingOptions options)

ICSharpCode.Decompiler/IL/Instructions/LockInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/LogicInstructions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2017 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/MatchInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2020 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2020 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -18,10 +19,9 @@
 
 using System;
 using System.Diagnostics;
-using System.Linq;
+using System.Diagnostics.CodeAnalysis;
 
 using ICSharpCode.Decompiler.TypeSystem;
-
 namespace ICSharpCode.Decompiler.IL
 {
 	partial class MatchInstruction : ILInstruction
@@ -96,7 +96,7 @@ namespace ICSharpCode.Decompiler.IL
 		public int NumPositionalPatterns {
 			get {
 				if (IsDeconstructCall)
-					return method.Parameters.Count - (method.IsStatic ? 1 : 0);
+					return method!.Parameters.Count - (method.IsStatic ? 1 : 0);
 				else if (IsDeconstructTuple)
 					return TupleType.GetTupleElementTypes(variable.Type).Length;
 				else
@@ -118,7 +118,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// (even if the pattern fails to match!).
 		/// The pattern matching instruction evaluates to 1 (as I4) if the pattern matches, or 0 otherwise.
 		/// </summary>
-		public static bool IsPatternMatch(ILInstruction inst, out ILInstruction testedOperand)
+		public static bool IsPatternMatch(ILInstruction? inst, [NotNullWhen(true)] out ILInstruction? testedOperand)
 		{
 			switch (inst)
 			{
@@ -153,9 +153,9 @@ namespace ICSharpCode.Decompiler.IL
 		{
 			if (this.IsDeconstructCall)
 			{
-				int firstOutParam = (method.IsStatic ? 1 : 0);
-				var outParamType = this.Method.Parameters[firstOutParam + index].Type;
-				if (!(outParamType is ByReferenceType brt))
+				int firstOutParam = (method!.IsStatic ? 1 : 0);
+				var outParamType = method.Parameters[firstOutParam + index].Type;
+				if (outParamType is not ByReferenceType brt)
 					throw new InvalidOperationException("deconstruct out param must be by reference");
 				return brt.ElementType;
 			}
@@ -185,8 +185,8 @@ namespace ICSharpCode.Decompiler.IL
 			Debug.Assert(SubPatterns.Count >= NumPositionalPatterns);
 			foreach (var subPattern in SubPatterns)
 			{
-				if (!IsPatternMatch(subPattern, out ILInstruction operand))
-					Debug.Fail("Sub-Pattern must be a valid pattern");
+				if (!IsPatternMatch(subPattern, out ILInstruction? operand))
+					throw new InvalidOperationException("Sub-Pattern must be a valid pattern");
 				// the first child is TestedOperand
 				int subPatternIndex = subPattern.ChildIndex - 1;
 				if (subPatternIndex < NumPositionalPatterns)
@@ -210,8 +210,11 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		internal static bool IsDeconstructMethod(IMethod method)
+		internal static bool IsDeconstructMethod(IMethod? method)
 		{
+			if (method == null
+				)
+				return false;
 			if (method.Name != "Deconstruct")
 				return false;
 			if (method.ReturnType.Kind != TypeKind.Void)
@@ -260,7 +263,10 @@ namespace ICSharpCode.Decompiler.IL
 			if (IsDeconstructCall)
 			{
 				output.Write(".deconstruct[");
-				method.WriteTo(output);
+				if (method == null)
+					output.Write("<null>");
+				else
+					method.WriteTo(output);
 				output.Write(']');
 			}
 			if (IsDeconstructTuple)

ICSharpCode.Decompiler/IL/Instructions/MemoryInstructions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/NullCoalescingInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/NullableInstructions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2018 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2018 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/PatternMatching.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -16,8 +17,9 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using ICSharpCode.Decompiler.TypeSystem;
+using System.Diagnostics.CodeAnalysis;
 
+using ICSharpCode.Decompiler.TypeSystem;
 namespace ICSharpCode.Decompiler.IL
 {
 	partial class ILInstruction
@@ -73,13 +75,13 @@ namespace ICSharpCode.Decompiler.IL
 			return MatchLdcI(out long v) && v == val;
 		}
 
-		public bool MatchLdLoc(ILVariable variable)
+		public bool MatchLdLoc(ILVariable? variable)
 		{
 			var inst = this as LdLoc;
 			return inst != null && inst.Variable == variable;
 		}
 
-		public bool MatchLdLoca(ILVariable variable)
+		public bool MatchLdLoca(ILVariable? variable)
 		{
 			var inst = this as LdLoca;
 			return inst != null && inst.Variable == variable;
@@ -88,7 +90,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches either ldloc (if the variable is a reference type), or ldloca (otherwise).
 		/// </summary>
-		public bool MatchLdLocRef(ILVariable variable)
+		public bool MatchLdLocRef(ILVariable? variable)
 		{
 			return MatchLdLocRef(out var v) && v == variable;
 		}
@@ -96,7 +98,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches either ldloc (if the variable is a reference type), or ldloca (otherwise).
 		/// </summary>
-		public bool MatchLdLocRef(out ILVariable variable)
+		public bool MatchLdLocRef([NotNullWhen(true)] out ILVariable? variable)
 		{
 			switch (this)
 			{
@@ -118,7 +120,7 @@ namespace ICSharpCode.Decompiler.IL
 			return inst != null && inst.Variable.Kind == VariableKind.Parameter && inst.Variable.Index < 0;
 		}
 
-		public bool MatchStLoc(out ILVariable variable)
+		public bool MatchStLoc([NotNullWhen(true)] out ILVariable? variable)
 		{
 			var inst = this as StLoc;
 			if (inst != null)
@@ -130,7 +132,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchStLoc(ILVariable variable, out ILInstruction value)
+		public bool MatchStLoc(ILVariable? variable, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			var inst = this as StLoc;
 			if (inst != null && inst.Variable == variable)
@@ -142,7 +144,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLdLen(StackType type, out ILInstruction array)
+		public bool MatchLdLen(StackType type, [NotNullWhen(true)] out ILInstruction? array)
 		{
 			var inst = this as LdLen;
 			if (inst != null && inst.ResultType == type)
@@ -154,7 +156,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchReturn(out ILInstruction value)
+		public bool MatchReturn([NotNullWhen(true)] out ILInstruction? value)
 		{
 			var inst = this as Leave;
 			if (inst != null && inst.IsLeavingFunction)
@@ -166,7 +168,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchBranch(out Block targetBlock)
+		public bool MatchBranch([NotNullWhen(true)] out Block? targetBlock)
 		{
 			var inst = this as Branch;
 			if (inst != null)
@@ -178,13 +180,13 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchBranch(Block targetBlock)
+		public bool MatchBranch(Block? targetBlock)
 		{
 			var inst = this as Branch;
 			return inst != null && inst.TargetBlock == targetBlock;
 		}
 
-		public bool MatchLeave(out BlockContainer targetContainer, out ILInstruction value)
+		public bool MatchLeave([NotNullWhen(true)] out BlockContainer? targetContainer, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			var inst = this as Leave;
 			if (inst != null)
@@ -198,7 +200,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLeave(BlockContainer targetContainer, out ILInstruction value)
+		public bool MatchLeave(BlockContainer? targetContainer, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			var inst = this as Leave;
 			if (inst != null && targetContainer == inst.TargetContainer)
@@ -210,7 +212,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLeave(out BlockContainer targetContainer)
+		public bool MatchLeave([NotNullWhen(true)] out BlockContainer? targetContainer)
 		{
 			var inst = this as Leave;
 			if (inst != null && inst.Value.MatchNop())
@@ -222,16 +224,15 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLeave(BlockContainer targetContainer)
+		public bool MatchLeave(BlockContainer? targetContainer)
 		{
 			var inst = this as Leave;
 			return inst != null && inst.TargetContainer == targetContainer && inst.Value.MatchNop();
 		}
 
-		public bool MatchIfInstruction(out ILInstruction condition, out ILInstruction trueInst, out ILInstruction falseInst)
+		public bool MatchIfInstruction([NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst, [NotNullWhen(true)] out ILInstruction? falseInst)
 		{
-			var inst = this as IfInstruction;
-			if (inst != null)
+			if (this is IfInstruction inst)
 			{
 				condition = inst.Condition;
 				trueInst = inst.TrueInst;
@@ -244,7 +245,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchIfInstructionPositiveCondition(out ILInstruction condition, out ILInstruction trueInst, out ILInstruction falseInst)
+		public bool MatchIfInstructionPositiveCondition([NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst, [NotNullWhen(true)] out ILInstruction? falseInst)
 		{
 			if (MatchIfInstruction(out condition, out trueInst, out falseInst))
 			{
@@ -252,7 +253,7 @@ namespace ICSharpCode.Decompiler.IL
 				while (condition.MatchLogicNot(out var arg))
 				{
 					condition = arg;
-					ILInstruction tmp = trueInst;
+					ILInstruction? tmp = trueInst;
 					trueInst = falseInst;
 					falseInst = tmp;
 				}
@@ -264,7 +265,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches an if instruction where the false instruction is a nop.
 		/// </summary>
-		public bool MatchIfInstruction(out ILInstruction condition, out ILInstruction trueInst)
+		public bool MatchIfInstruction([NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst)
 		{
 			var inst = this as IfInstruction;
 			if (inst != null && inst.FalseInst.MatchNop())
@@ -283,7 +284,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Note: unlike C# '&amp;&amp;', this instruction is not limited to booleans,
 		/// but allows passing through arbitrary I4 values on the rhs (but not on the lhs).
 		/// </summary>
-		public bool MatchLogicAnd(out ILInstruction lhs, out ILInstruction rhs)
+		public bool MatchLogicAnd([NotNullWhen(true)] out ILInstruction? lhs, [NotNullWhen(true)] out ILInstruction? rhs)
 		{
 			var inst = this as IfInstruction;
 			if (inst != null && inst.FalseInst.MatchLdcI4(0))
@@ -302,7 +303,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Note: unlike C# '||', this instruction is not limited to booleans,
 		/// but allows passing through arbitrary I4 values on the rhs (but not on the lhs).
 		/// </summary>
-		public bool MatchLogicOr(out ILInstruction lhs, out ILInstruction rhs)
+		public bool MatchLogicOr([NotNullWhen(true)] out ILInstruction? lhs, [NotNullWhen(true)] out ILInstruction? rhs)
 		{
 			var inst = this as IfInstruction;
 			if (inst != null && inst.TrueInst.MatchLdcI4(1))
@@ -319,7 +320,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches an logical negation.
 		/// </summary>
-		public bool MatchLogicNot(out ILInstruction arg)
+		public bool MatchLogicNot([NotNullWhen(true)] out ILInstruction? arg)
 		{
 			if (this is Comp comp && comp.Kind == ComparisonKind.Equality
 				&& comp.LiftingKind == ComparisonLiftingKind.None
@@ -332,7 +333,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchTryCatchHandler(out ILVariable variable)
+		public bool MatchTryCatchHandler([NotNullWhen(true)] out ILVariable? variable)
 		{
 			var inst = this as TryCatchHandler;
 			if (inst != null)
@@ -347,7 +348,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches comp(left == right) or logic.not(comp(left != right)).
 		/// </summary>
-		public bool MatchCompEquals(out ILInstruction left, out ILInstruction right)
+		public bool MatchCompEquals([NotNullWhen(true)] out ILInstruction? left, [NotNullWhen(true)] out ILInstruction? right)
 		{
 			ILInstruction thisInst = this;
 			var compKind = ComparisonKind.Equality;
@@ -376,7 +377,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches 'comp(arg == ldnull)'
 		/// </summary>
-		public bool MatchCompEqualsNull(out ILInstruction arg)
+		public bool MatchCompEqualsNull([NotNullWhen(true)] out ILInstruction? arg)
 		{
 			if (!MatchCompEquals(out var left, out var right))
 			{
@@ -403,7 +404,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches 'comp(arg != ldnull)'
 		/// </summary>
-		public bool MatchCompNotEqualsNull(out ILInstruction arg)
+		public bool MatchCompNotEqualsNull([NotNullWhen(true)] out ILInstruction? arg)
 		{
 			if (!MatchCompNotEquals(out var left, out var right))
 			{
@@ -430,7 +431,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches comp(left != right) or logic.not(comp(left == right)).
 		/// </summary>
-		public bool MatchCompNotEquals(out ILInstruction left, out ILInstruction right)
+		public bool MatchCompNotEquals([NotNullWhen(true)] out ILInstruction? left, [NotNullWhen(true)] out ILInstruction? right)
 		{
 			ILInstruction thisInst = this;
 			var compKind = ComparisonKind.Inequality;
@@ -456,7 +457,7 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		public bool MatchLdFld(out ILInstruction target, out IField field)
+		public bool MatchLdFld([NotNullWhen(true)] out ILInstruction? target, [NotNullWhen(true)] out IField? field)
 		{
 			if (this is LdObj ldobj && ldobj.Target is LdFlda ldflda && ldobj.UnalignedPrefix == 0 && !ldobj.IsVolatile)
 			{
@@ -472,7 +473,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLdsFld(out IField field)
+		public bool MatchLdsFld([NotNullWhen(true)] out IField? field)
 		{
 			if (this is LdObj ldobj && ldobj.Target is LdsFlda ldsflda && ldobj.UnalignedPrefix == 0 && !ldobj.IsVolatile)
 			{
@@ -483,12 +484,12 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLdsFld(IField field)
+		public bool MatchLdsFld(IField? field)
 		{
 			return MatchLdsFld(out var f) && f.Equals(field);
 		}
 
-		public bool MatchStsFld(out IField field, out ILInstruction value)
+		public bool MatchStsFld([NotNullWhen(true)] out IField? field, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			if (this is StObj stobj && stobj.Target is LdsFlda ldsflda && stobj.UnalignedPrefix == 0 && !stobj.IsVolatile)
 			{
@@ -501,7 +502,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchStFld(out ILInstruction target, out IField field, out ILInstruction value)
+		public bool MatchStFld([NotNullWhen(true)] out ILInstruction? target, [NotNullWhen(true)] out IField? field, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			if (this is StObj stobj && stobj.Target is LdFlda ldflda && stobj.UnalignedPrefix == 0 && !stobj.IsVolatile)
 			{
@@ -522,7 +523,7 @@ namespace ICSharpCode.Decompiler.IL
 			return op != null && op.Operator == @operator;
 		}
 
-		public bool MatchBinaryNumericInstruction(BinaryNumericOperator @operator, out ILInstruction left, out ILInstruction right)
+		public bool MatchBinaryNumericInstruction(BinaryNumericOperator @operator, [NotNullWhen(true)] out ILInstruction? left, [NotNullWhen(true)] out ILInstruction? right)
 		{
 			var op = this as BinaryNumericInstruction;
 			if (op != null && op.Operator == @operator)
@@ -536,7 +537,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchBinaryNumericInstruction(out BinaryNumericOperator @operator, out ILInstruction left, out ILInstruction right)
+		public bool MatchBinaryNumericInstruction(out BinaryNumericOperator @operator, [NotNullWhen(true)] out ILInstruction? left, [NotNullWhen(true)] out ILInstruction? right)
 		{
 			var op = this as BinaryNumericInstruction;
 			if (op != null)

ICSharpCode.Decompiler/IL/Instructions/SimpleInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -40,7 +41,7 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class Nop
 	{
-		public string Comment;
+		public string? Comment;
 
 		public NopKind Kind;
 
@@ -61,10 +62,10 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class InvalidBranch : SimpleInstruction
 	{
-		public string Message;
+		public string? Message;
 		public StackType ExpectedResultType = StackType.Void;
 
-		public InvalidBranch(string message) : this()
+		public InvalidBranch(string? message) : this()
 		{
 			this.Message = message;
 		}
@@ -88,10 +89,10 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class InvalidExpression : SimpleInstruction
 	{
-		public string Message;
+		public string? Message;
 		public StackType ExpectedResultType = StackType.Unknown;
 
-		public InvalidExpression(string message) : this()
+		public InvalidExpression(string? message) : this()
 		{
 			this.Message = message;
 		}

ICSharpCode.Decompiler/IL/Instructions/StLoc.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -38,8 +39,8 @@ namespace ICSharpCode.Decompiler.IL
 		internal override void CheckInvariant(ILPhase phase)
 		{
 			base.CheckInvariant(phase);
-			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(variable.Function));
-			Debug.Assert(phase <= ILPhase.InILReader || variable.Function.Variables[variable.IndexInFunction] == variable);
+			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(variable.Function!));
+			Debug.Assert(phase <= ILPhase.InILReader || variable.Function!.Variables[variable.IndexInFunction] == variable);
 			Debug.Assert(value.ResultType == variable.StackType);
 		}
 	}

ICSharpCode.Decompiler/IL/Instructions/StringToInt.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -23,23 +24,23 @@ namespace ICSharpCode.Decompiler.IL
 {
 	partial class StringToInt
 	{
-		public List<(string Key, int Value)> Map { get; }
+		public List<(string? Key, int Value)> Map { get; }
 
-		public StringToInt(ILInstruction argument, List<(string Key, int Value)> map)
+		public StringToInt(ILInstruction argument, List<(string? Key, int Value)> map)
 			: base(OpCode.StringToInt)
 		{
 			this.Argument = argument;
 			this.Map = map;
 		}
 
-		public StringToInt(ILInstruction argument, string[] map)
+		public StringToInt(ILInstruction argument, string?[] map)
 			: this(argument, ArrayToDictionary(map))
 		{
 		}
 
-		static List<(string Key, int Value)> ArrayToDictionary(string[] map)
+		static List<(string? Key, int Value)> ArrayToDictionary(string?[] map)
 		{
-			var dict = new List<(string Key, int Value)>();
+			var dict = new List<(string? Key, int Value)>();
 			for (int i = 0; i < map.Length; i++)
 			{
 				dict.Add((map[i], i));

ICSharpCode.Decompiler/IL/Instructions/SwitchInstruction.cs

@@ -1,3 +1,4 @@
+#nullable enable
 // Copyright (c) 2014 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
@@ -16,7 +17,6 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
 using System.Diagnostics;
 using System.Linq;
 
@@ -49,7 +49,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.Sections = new InstructionCollection<SwitchSection>(this, 1);
 		}
 
-		ILInstruction value;
+		ILInstruction value = null!;
 		public ILInstruction Value {
 			get { return this.value; }
 			set {

ICSharpCode.Decompiler/IL/Instructions/TryInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -33,7 +34,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.TryBlock = tryBlock;
 		}
 
-		ILInstruction tryBlock;
+		ILInstruction tryBlock = null!;
 		public ILInstruction TryBlock {
 			get { return this.tryBlock; }
 			set {
@@ -144,7 +145,7 @@ namespace ICSharpCode.Decompiler.IL
 			base.CheckInvariant(phase);
 			Debug.Assert(Parent is TryCatch);
 			Debug.Assert(filter.ResultType == StackType.I4);
-			Debug.Assert(this.IsDescendantOf(variable.Function));
+			Debug.Assert(this.IsDescendantOf(variable.Function!));
 		}
 
 		public override StackType ResultType {
@@ -202,7 +203,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.FinallyBlock = finallyBlock;
 		}
 
-		ILInstruction finallyBlock;
+		ILInstruction finallyBlock = null!;
 		public ILInstruction FinallyBlock {
 			get { return this.finallyBlock; }
 			set {
@@ -299,7 +300,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.FaultBlock = faultBlock;
 		}
 
-		ILInstruction faultBlock;
+		ILInstruction faultBlock = null!;
 		public ILInstruction FaultBlock {
 			get { return this.faultBlock; }
 			set {

ICSharpCode.Decompiler/IL/Instructions/UnaryInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Instructions/UsingInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/Patterns/AnyNode.cs

@@ -1,8 +1,5 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
+#nullable enable
+using System;
 
 namespace ICSharpCode.Decompiler.IL.Patterns
 {
@@ -37,19 +34,20 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 
 	partial class AnyNode : PatternInstruction
 	{
-		CaptureGroup group;
+		CaptureGroup? group;
 
-		public AnyNode(CaptureGroup group = null)
+		public AnyNode(CaptureGroup? group = null)
 			: base(OpCode.AnyNode)
 		{
 			this.group = group;
 		}
 
-		protected internal override bool PerformMatch(ILInstruction other, ref Match match)
+		protected internal override bool PerformMatch(ILInstruction? other, ref Match match)
 		{
 			if (other == null)
 				return false;
-			match.Add(group, other);
+			if (group != null)
+				match.Add(group, other);
 			return true;
 		}
 	}

ICSharpCode.Decompiler/IL/Patterns/ListMatch.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2016 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -44,7 +45,7 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 		/// If the method returns true, it adds the capture groups (if any) to the match.
 		/// If the method returns false, the match object remains in a partially-updated state and needs to be restored
 		/// before it can be reused.</returns>
-		internal static bool DoMatch(IReadOnlyList<ILInstruction> patterns, IReadOnlyList<ILInstruction> syntaxList, ref Match match)
+		internal static bool DoMatch(IReadOnlyList<ILInstruction> patterns, IReadOnlyList<ILInstruction?> syntaxList, ref Match match)
 		{
 			ListMatch listMatch = new ListMatch(syntaxList);
 			do
@@ -107,14 +108,14 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 		/// <summary>
 		/// The syntax list we are matching against.
 		/// </summary>
-		internal readonly IReadOnlyList<ILInstruction> SyntaxList;
+		internal readonly IReadOnlyList<ILInstruction?> SyntaxList;
 
 		/// <summary>
 		/// The current index in the syntax list.
 		/// </summary>
 		internal int SyntaxIndex;
 
-		ListMatch(IReadOnlyList<ILInstruction> syntaxList)
+		ListMatch(IReadOnlyList<ILInstruction?> syntaxList)
 		{
 			this.SyntaxList = syntaxList;
 			this.SyntaxIndex = 0;
@@ -122,8 +123,8 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 			this.restoreStack = null;
 		}
 
-		List<SavePoint> backtrackingStack;
-		Stack<int> restoreStack;
+		List<SavePoint>? backtrackingStack;
+		Stack<int>? restoreStack;
 
 		void AddSavePoint(SavePoint savepoint)
 		{

ICSharpCode.Decompiler/IL/Patterns/Match.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2016 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -33,7 +34,7 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 	{
 		static readonly List<KeyValuePair<CaptureGroup, ILInstruction>> emptyResults = new List<KeyValuePair<CaptureGroup, ILInstruction>>();
 
-		List<KeyValuePair<CaptureGroup, ILInstruction>> results;
+		List<KeyValuePair<CaptureGroup, ILInstruction>>? results;
 
 		/// <summary>
 		/// Gets whether the match was successful.

ICSharpCode.Decompiler/IL/PrimitiveType.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/IL/SemanticHelper.cs

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System.Linq;
 
 using ICSharpCode.Decompiler.Util;
@@ -89,4 +91,4 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 	}
-}
+}

ICSharpCode.Decompiler/IL/SlotInfo.cs

@@ -16,6 +16,7 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
 
 namespace ICSharpCode.Decompiler.IL
 {

ICSharpCode.Decompiler/IL/StackType.cs

@@ -16,6 +16,7 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
 
 namespace ICSharpCode.Decompiler.IL
 {

ICSharpCode.Decompiler/IL/Transforms/FixLoneIsInst.cs

@@ -16,7 +16,6 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
 using System.Collections.Generic;
 using System.Linq;
 
@@ -63,7 +62,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			{
 				// Use extraction to turn isInst.Argument into a pure instruction, thus making the emulation possible
 				context.Step("FixLoneIsInst", isInst);
-				isInst.Argument.Extract();
+				isInst.Argument.Extract(context);
 			}
 		}
 	}

ICSharpCode.Decompiler/IL/Transforms/FixRemainingIncrements.cs

@@ -16,7 +16,6 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -73,7 +72,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				else
 				{
 					context.Step($"Fix {call.Method.Name} call at 0x{call.StartILOffset:x4} using new local", call);
-					var newVariable = call.Arguments[0].Extract();
+					var newVariable = call.Arguments[0].Extract(context);
 					if (newVariable == null)
 					{
 						Debug.Fail("Failed to extract argument of remaining increment/decrement");

ICSharpCode.Decompiler/IL/Transforms/ILExtraction.cs

@@ -21,6 +21,8 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 
+using ICSharpCode.Decompiler.TypeSystem;
+
 namespace ICSharpCode.Decompiler.IL.Transforms
 {
 	/// <summary>
@@ -33,6 +35,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// </summary>
 		readonly ILFunction Function;
 
+		readonly ILTransformContext context;
+
 		/// <summary>
 		/// Combined flags of all instructions being moved.
 		/// </summary>
@@ -47,17 +51,19 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// </summary>
 		readonly List<Func<ILInstruction>> MoveActions = new List<Func<ILInstruction>>();
 
-		ExtractionContext(ILFunction function)
+		ExtractionContext(ILFunction function, ILTransformContext context)
 		{
 			Debug.Assert(function != null);
 			this.Function = function;
+			this.context = context;
 		}
 
 		internal void RegisterMove(ILInstruction predecessor)
 		{
 			FlagsBeingMoved |= predecessor.Flags;
 			MoveActions.Add(delegate {
-				var v = Function.RegisterVariable(VariableKind.StackSlot, predecessor.ResultType);
+				var type = context.TypeSystem.FindType(predecessor.ResultType.ToKnownTypeCode());
+				var v = Function.RegisterVariable(VariableKind.StackSlot, type);
 				predecessor.ReplaceWith(new LdLoc(v));
 				return new StLoc(v, predecessor);
 			});
@@ -91,10 +97,10 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// 
 		/// May return null if extraction is not possible.
 		/// </summary>
-		public static ILVariable Extract(ILInstruction instToExtract)
+		public static ILVariable Extract(ILInstruction instToExtract, ILTransformContext context)
 		{
 			var function = instToExtract.Ancestors.OfType<ILFunction>().First();
-			ExtractionContext ctx = new ExtractionContext(function);
+			ExtractionContext ctx = new ExtractionContext(function, context);
 			ctx.FlagsBeingMoved = instToExtract.Flags;
 			ILInstruction inst = instToExtract;
 			while (inst != null)
@@ -113,8 +119,9 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				{
 					// We've reached the target block, and extraction is possible all the way.
 					int insertIndex = inst.ChildIndex;
+					var type = context.TypeSystem.FindType(instToExtract.ResultType.ToKnownTypeCode());
 					// Move instToExtract itself:
-					var v = function.RegisterVariable(VariableKind.StackSlot, instToExtract.ResultType);
+					var v = function.RegisterVariable(VariableKind.StackSlot, type);
 					instToExtract.ReplaceWith(new LdLoc(v));
 					block.Instructions.Insert(insertIndex, new StLoc(v, instToExtract));
 					// Apply the other move actions:

ICSharpCode.Decompiler/IL/Transforms/NamedArgumentTransform.cs

@@ -1,8 +1,5 @@
-using System;
-using System.Collections.Generic;
-using System.Diagnostics;
+using System.Diagnostics;
 using System.Linq;
-using System.Text;
 
 using ICSharpCode.Decompiler.TypeSystem;
 
@@ -90,7 +87,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		{
 			var call = (CallInstruction)arg.Parent;
 			Debug.Assert(context.Function == call.Ancestors.OfType<ILFunction>().First());
-			var v = context.Function.RegisterVariable(VariableKind.NamedArgument, arg.ResultType);
+			var type = context.TypeSystem.FindType(arg.ResultType.ToKnownTypeCode());
+			var v = context.Function.RegisterVariable(VariableKind.NamedArgument, type);
 			context.Step($"Introduce named argument '{v.Name}'", arg);
 			if (!(call.Parent is Block namedArgBlock) || namedArgBlock.Kind != BlockKind.CallWithNamedArgs)
 			{

ICSharpCode.Decompiler/IL/Transforms/TransformArrayInitializers.cs

@@ -257,7 +257,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				var otherLoadOfV = v.LoadInstructions.FirstOrDefault(l => !(l.Parent is Cpblk));
 				if (otherLoadOfV == null)
 					return false;
-				finalStore = otherLoadOfV.Parent.Extract();
+				finalStore = otherLoadOfV.Parent.Extract(context);
 				if (finalStore == null)
 					return false;
 				value = ((StLoc)finalStore.StoreInstructions[0]).Value;

ICSharpCode.Decompiler/TypeSystem/IMember.cs

@@ -62,6 +62,12 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// </summary>
 		IType ReturnType { get; }
 
+		/// <summary>
+		/// Gets/Sets the declaring type (incl. type arguments, if any).
+		/// If this is not a specialized member, the value returned is equal to <see cref="IEntity.DeclaringTypeDefinition"/>.
+		/// </summary>
+		new IType DeclaringType { get; }
+
 		/// <summary>
 		/// Gets the interface members explicitly implemented by this member.
 		/// </summary>

ICSharpCode.Decompiler/TypeSystem/Implementation/SyntheticRangeIndexer.cs

@@ -92,7 +92,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		TypeParameterSubstitution IMember.Substitution => underlyingMethod.Substitution;
 		EntityHandle IEntity.MetadataToken => underlyingMethod.MetadataToken;
 		public string Name => underlyingMethod.Name;
-		IType IEntity.DeclaringType => underlyingMethod.DeclaringType;
+		public IType DeclaringType => underlyingMethod.DeclaringType;
 		ITypeDefinition IEntity.DeclaringTypeDefinition => underlyingMethod.DeclaringTypeDefinition;
 		IModule IEntity.ParentModule => underlyingMethod.ParentModule;
 		Accessibility IEntity.Accessibility => underlyingMethod.Accessibility;

ICSharpCode.Decompiler/Util/BitSet.cs

@@ -15,6 +15,7 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+#nullable enable
 
 using System;
 using System.Diagnostics;

ICSharpCode.Decompiler/Util/BusyManager.cs

@@ -15,6 +15,7 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+#nullable enable
 
 using System;
 using System.Collections.Generic;
@@ -36,9 +37,9 @@ namespace ICSharpCode.Decompiler.Util
 		{
 			public static readonly BusyLock Failed = new BusyLock(null);
 
-			readonly List<object> objectList;
+			readonly List<object?>? objectList;
 
-			internal BusyLock(List<object> objectList)
+			internal BusyLock(List<object?>? objectList)
 			{
 				this.objectList = objectList;
 			}
@@ -56,13 +57,13 @@ namespace ICSharpCode.Decompiler.Util
 			}
 		}
 
-		[ThreadStatic] static List<object> _activeObjects;
+		[ThreadStatic] static List<object?>? _activeObjects;
 
-		public static BusyLock Enter(object obj)
+		public static BusyLock Enter(object? obj)
 		{
-			List<object> activeObjects = _activeObjects;
+			List<object?>? activeObjects = _activeObjects;
 			if (activeObjects == null)
-				activeObjects = _activeObjects = new List<object>();
+				activeObjects = _activeObjects = new List<object?>();
 			for (int i = 0; i < activeObjects.Count; i++)
 			{
 				if (activeObjects[i] == obj)

ICSharpCode.Decompiler/Util/CacheManager.cs

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Collections.Concurrent;
 

ICSharpCode.Decompiler/Util/CallbackOnDispose.cs

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Threading;
 
@@ -30,7 +32,7 @@ namespace ICSharpCode.Decompiler.Util
 	/// </remarks>
 	public sealed class CallbackOnDispose : IDisposable
 	{
-		Action action;
+		Action? action;
 
 		public CallbackOnDispose(Action action)
 		{
@@ -41,7 +43,7 @@ namespace ICSharpCode.Decompiler.Util
 
 		public void Dispose()
 		{
-			Action a = Interlocked.Exchange(ref action, null);
+			Action? a = Interlocked.Exchange(ref action, null);
 			if (a != null)
 			{
 				a();

ICSharpCode.Decompiler/Util/CollectionExtensions.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -14,13 +15,13 @@ namespace ICSharpCode.Decompiler.Util
 		}
 
 #if !NETCORE
-		public static IEnumerable<(A, B)> Zip<A, B>(this IEnumerable<A> input1, IEnumerable<B> input2)
+		public static IEnumerable<(A, B)> Zip<A, B>(this IEnumerable<A>? input1, IEnumerable<B>? input2)
 		{
 			return input1.Zip(input2, (a, b) => (a, b));
 		}
 #endif
 
-		public static IEnumerable<(A, B)> ZipLongest<A, B>(this IEnumerable<A> input1, IEnumerable<B> input2)
+		public static IEnumerable<(A?, B?)> ZipLongest<A, B>(this IEnumerable<A> input1, IEnumerable<B> input2)
 		{
 			using (var it1 = input1.GetEnumerator())
 			{
@@ -60,7 +61,7 @@ namespace ICSharpCode.Decompiler.Util
 		}
 
 #if !NETCORE
-		public static HashSet<T> ToHashSet<T>(this IEnumerable<T> input)
+		public static HashSet<T> ToHashSet<T>(this IEnumerable<T>? input)
 		{
 			return new HashSet<T>(input);
 		}
@@ -76,14 +77,14 @@ namespace ICSharpCode.Decompiler.Util
 			return input.Skip(input.Count - count);
 		}
 
-		public static T PopOrDefault<T>(this Stack<T> stack)
+		public static T? PopOrDefault<T>(this Stack<T> stack)
 		{
 			if (stack.Count == 0)
 				return default(T);
 			return stack.Pop();
 		}
 
-		public static T PeekOrDefault<T>(this Stack<T> stack)
+		public static T? PeekOrDefault<T>(this Stack<T> stack)
 		{
 			if (stack.Count == 0)
 				return default(T);
@@ -277,7 +278,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// Returns the minimum element.
 		/// </summary>
 		/// <exception cref="InvalidOperationException">The input sequence is empty</exception>
-		public static T MinBy<T, K>(this IEnumerable<T> source, Func<T, K> keySelector, IComparer<K> keyComparer)
+		public static T MinBy<T, K>(this IEnumerable<T> source, Func<T, K> keySelector, IComparer<K>? keyComparer)
 		{
 			if (source == null)
 				throw new ArgumentNullException(nameof(source));
@@ -318,7 +319,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// Returns the maximum element.
 		/// </summary>
 		/// <exception cref="InvalidOperationException">The input sequence is empty</exception>
-		public static T MaxBy<T, K>(this IEnumerable<T> source, Func<T, K> keySelector, IComparer<K> keyComparer)
+		public static T MaxBy<T, K>(this IEnumerable<T> source, Func<T, K> keySelector, IComparer<K>? keyComparer)
 		{
 			if (source == null)
 				throw new ArgumentNullException(nameof(source));
@@ -353,12 +354,12 @@ namespace ICSharpCode.Decompiler.Util
 			list.RemoveAt(list.Count - 1);
 		}
 
-		public static T OnlyOrDefault<T>(this IEnumerable<T> source, Func<T, bool> predicate) => OnlyOrDefault(source.Where(predicate));
+		public static T? OnlyOrDefault<T>(this IEnumerable<T>? source, Func<T, bool>? predicate) => OnlyOrDefault(source.Where(predicate));
 
-		public static T OnlyOrDefault<T>(this IEnumerable<T> source)
+		public static T? OnlyOrDefault<T>(this IEnumerable<T> source)
 		{
 			bool any = false;
-			T first = default;
+			T? first = default;
 			foreach (var t in source)
 			{
 				if (any)
@@ -372,14 +373,14 @@ namespace ICSharpCode.Decompiler.Util
 
 		#region Aliases/shortcuts for Enumerable extension methods
 		public static bool Any<T>(this ICollection<T> list) => list.Count > 0;
-		public static bool Any<T>(this T[] array, Predicate<T> match) => Array.Exists(array, match);
-		public static bool Any<T>(this List<T> list, Predicate<T> match) => list.Exists(match);
+		public static bool Any<T>(this T[]? array, Predicate<T>? match) => Array.Exists(array, match);
+		public static bool Any<T>(this List<T> list, Predicate<T>? match) => list.Exists(match);
 
-		public static bool All<T>(this T[] array, Predicate<T> match) => Array.TrueForAll(array, match);
-		public static bool All<T>(this List<T> list, Predicate<T> match) => list.TrueForAll(match);
+		public static bool All<T>(this T[]? array, Predicate<T>? match) => Array.TrueForAll(array, match);
+		public static bool All<T>(this List<T> list, Predicate<T>? match) => list.TrueForAll(match);
 
-		public static T FirstOrDefault<T>(this T[] array, Predicate<T> predicate) => Array.Find(array, predicate);
-		public static T FirstOrDefault<T>(this List<T> list, Predicate<T> predicate) => list.Find(predicate);
+		public static T FirstOrDefault<T>(this T[]? array, Predicate<T>? predicate) => Array.Find(array, predicate);
+		public static T FirstOrDefault<T>(this List<T> list, Predicate<T>? predicate) => list.Find(predicate);
 
 		public static T Last<T>(this IList<T> list) => list[list.Count - 1];
 		#endregion

ICSharpCode.Decompiler/Util/EmptyList.cs

@@ -15,6 +15,7 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+#nullable enable
 
 using System;
 using System.Collections;
@@ -91,11 +92,11 @@ namespace ICSharpCode.Decompiler.Util
 		}
 
 		T IEnumerator<T>.Current {
-			get { return default(T); }
+			get { throw new NotSupportedException(); }
 		}
 
 		object IEnumerator.Current {
-			get { return default(T); }
+			get { throw new NotSupportedException(); }
 		}
 
 		void IDisposable.Dispose()

ICSharpCode.Decompiler/Util/ExtensionMethods.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
+#nullable enable
+// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -26,7 +27,7 @@ namespace ICSharpCode.Decompiler.Util
 	/// </summary>
 	static class ExtensionMethods
 	{
-		public static Predicate<T> And<T>(this Predicate<T> filter1, Predicate<T> filter2)
+		public static Predicate<T>? And<T>(this Predicate<T>? filter1, Predicate<T>? filter2)
 		{
 			if (filter1 == null)
 				return filter2;

ICSharpCode.Decompiler/Util/FileUtility.cs

@@ -16,7 +16,10 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Text;
 
@@ -28,9 +31,10 @@ namespace ICSharpCode.Decompiler.Util
 		/// Gets the normalized version of fileName.
 		/// Slashes are replaced with backslashes, backreferences "." and ".." are 'evaluated'.
 		/// </summary>
-		public static string NormalizePath(string fileName)
+		[return: NotNullIfNotNull("fileName")]
+		public static string? NormalizePath(string? fileName)
 		{
-			if (string.IsNullOrEmpty(fileName))
+			if (fileName == null || fileName.Length == 0)
 				return fileName;
 
 			int i;
@@ -169,14 +173,14 @@ namespace ICSharpCode.Decompiler.Util
 				&& (fileName[1] == '\\' || fileName[1] == '/');
 		}
 
-		public static bool IsEqualFileName(string fileName1, string fileName2)
+		public static bool IsEqualFileName(string? fileName1, string? fileName2)
 		{
 			return string.Equals(NormalizePath(fileName1),
 								 NormalizePath(fileName2),
 								 StringComparison.OrdinalIgnoreCase);
 		}
 
-		public static bool IsBaseDirectory(string baseDirectory, string testDirectory)
+		public static bool IsBaseDirectory(string? baseDirectory, string? testDirectory)
 		{
 			if (baseDirectory == null || testDirectory == null)
 				return false;
@@ -189,9 +193,10 @@ namespace ICSharpCode.Decompiler.Util
 			return testDirectory.StartsWith(baseDirectory, StringComparison.OrdinalIgnoreCase);
 		}
 
-		static string AddTrailingSeparator(string input)
+		[return: NotNullIfNotNull("input")]
+		static string? AddTrailingSeparator(string? input)
 		{
-			if (string.IsNullOrEmpty(input))
+			if (input == null || input.Length == 0)
 				return input;
 			if (input[input.Length - 1] == Path.DirectorySeparatorChar || input[input.Length - 1] == Path.AltDirectorySeparatorChar)
 				return input;
@@ -212,9 +217,9 @@ namespace ICSharpCode.Decompiler.Util
 		/// Converts a given absolute path and a given base path to a path that leads
 		/// from the base path to the absoulte path. (as a relative path)
 		/// </summary>
-		public static string GetRelativePath(string baseDirectoryPath, string absPath)
+		public static string GetRelativePath(string? baseDirectoryPath, string absPath)
 		{
-			if (string.IsNullOrEmpty(baseDirectoryPath))
+			if (baseDirectoryPath == null || baseDirectoryPath.Length == 0)
 			{
 				return absPath;
 			}
@@ -252,7 +257,8 @@ namespace ICSharpCode.Decompiler.Util
 			return erg.ToString();
 		}
 
-		public static string TrimPath(string path, int max_chars)
+		[return: NotNullIfNotNull("path")]
+		public static string? TrimPath(string? path, int max_chars)
 		{
 			const char ellipsis = '\u2026'; // HORIZONTAL ELLIPSIS
 			const int ellipsisLength = 2;

ICSharpCode.Decompiler/Util/GraphVizGraph.cs

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -34,8 +36,8 @@ namespace ICSharpCode.Decompiler.Util
 		List<GraphVizNode> nodes = new List<GraphVizNode>();
 		List<GraphVizEdge> edges = new List<GraphVizEdge>();
 
-		public string rankdir;
-		public string Title;
+		public string? rankdir;
+		public string? Title;
 
 		public void AddEdge(GraphVizEdge edge)
 		{
@@ -58,7 +60,7 @@ namespace ICSharpCode.Decompiler.Util
 			Show(null);
 		}
 
-		public void Show(string name)
+		public void Show(string? name)
 		{
 			if (name == null)
 				name = Title;
@@ -83,7 +85,7 @@ namespace ICSharpCode.Decompiler.Util
 			}
 		}
 
-		static void WriteGraphAttribute(TextWriter writer, string name, string value)
+		static void WriteGraphAttribute(TextWriter writer, string name, string? value)
 		{
 			if (value != null)
 				writer.WriteLine("{0}={1};", name, Escape(value));
@@ -105,7 +107,7 @@ namespace ICSharpCode.Decompiler.Util
 			}
 		}
 
-		internal static void WriteAttribute(TextWriter writer, string name, string value, ref bool isFirst)
+		internal static void WriteAttribute(TextWriter writer, string name, string? value, ref bool isFirst)
 		{
 			if (value != null)
 			{
@@ -141,13 +143,13 @@ namespace ICSharpCode.Decompiler.Util
 		public readonly string Source, Target;
 
 		/// <summary>edge stroke color</summary>
-		public string color;
+		public string? color;
 		/// <summary>use edge to affect node ranking</summary>
 		public bool? constraint;
 
-		public string label;
+		public string? label;
 
-		public string style;
+		public string? style;
 
 		/// <summary>point size of label</summary>
 		public int? fontsize;
@@ -184,9 +186,9 @@ namespace ICSharpCode.Decompiler.Util
 	sealed class GraphVizNode
 	{
 		public readonly string ID;
-		public string label;
+		public string? label;
 
-		public string labelloc;
+		public string? labelloc;
 
 		/// <summary>point size of label</summary>
 		public int? fontsize;
@@ -195,10 +197,10 @@ namespace ICSharpCode.Decompiler.Util
 		public double? height;
 
 		/// <summary>space around label</summary>
-		public string margin;
+		public string? margin;
 
 		/// <summary>node shape</summary>
-		public string shape;
+		public string? shape;
 
 		public GraphVizNode(string id)
 		{

ICSharpCode.Decompiler/Util/Interval.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2016 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -111,7 +112,7 @@ namespace ICSharpCode.Decompiler.Util
 		}
 
 		#region Equals and GetHashCode implementation
-		public override bool Equals(object obj)
+		public override bool Equals(object? obj)
 		{
 			return (obj is Interval) && Equals((Interval)obj);
 		}
@@ -281,7 +282,7 @@ namespace ICSharpCode.Decompiler.Util
 		}
 
 		#region Equals and GetHashCode implementation
-		public override bool Equals(object obj)
+		public override bool Equals(object? obj)
 		{
 			return (obj is LongInterval) && Equals((LongInterval)obj);
 		}

ICSharpCode.Decompiler/Util/KeyComparer.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
+#nullable enable
+// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/Util/LongSet.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2016 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -82,7 +83,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// <summary>
 		/// Creates a new LongSet the contains the values from the specified intervals.
 		/// </summary>
-		public LongSet(IEnumerable<LongInterval> intervals)
+		public LongSet(IEnumerable<LongInterval>? intervals)
 			: this(MergeOverlapping(intervals.Where(i => !i.IsEmpty).OrderBy(i => i.Start)).ToImmutableArray())
 		{
 		}
@@ -350,7 +351,7 @@ namespace ICSharpCode.Decompiler.Util
 		}
 
 		#region Equals and GetHashCode implementation
-		public override bool Equals(object obj)
+		public override bool Equals(object? obj)
 		{
 			return obj is LongSet && SetEquals((LongSet)obj);
 		}

ICSharpCode.Decompiler/Util/MultiDictionary.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
+#nullable enable
+// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -33,7 +34,7 @@ namespace ICSharpCode.Decompiler.Util
 			dict = new Dictionary<TKey, List<TValue>>();
 		}
 
-		public MultiDictionary(IEqualityComparer<TKey> comparer)
+		public MultiDictionary(IEqualityComparer<TKey>? comparer)
 		{
 			dict = new Dictionary<TKey, List<TValue>>(comparer);
 		}

ICSharpCode.Decompiler/Util/NullAttributes.cs

@@ -25,5 +25,13 @@ namespace System.Diagnostics.CodeAnalysis
 
 		public bool ReturnValue { get; }
 	}
+
+	[AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+	internal sealed class DoesNotReturnIfAttribute : Attribute
+	{
+		public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+		public bool ParameterValue { get; }
+	}
 }
 #endif

ICSharpCode.Decompiler/Util/Platform.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
+#nullable enable
+// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software

ICSharpCode.Decompiler/Util/ProjectedList.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
+#nullable enable
+// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -25,7 +26,7 @@ namespace ICSharpCode.Decompiler.Util
 	{
 		readonly IList<TInput> input;
 		readonly Func<TInput, TOutput> projection;
-		readonly TOutput[] items;
+		readonly TOutput?[] items;
 
 		public ProjectedList(IList<TInput> input, Func<TInput, TOutput> projection)
 		{
@@ -35,12 +36,12 @@ namespace ICSharpCode.Decompiler.Util
 				throw new ArgumentNullException(nameof(projection));
 			this.input = input;
 			this.projection = projection;
-			this.items = new TOutput[input.Count];
+			this.items = new TOutput?[input.Count];
 		}
 
 		public TOutput this[int index] {
 			get {
-				TOutput output = LazyInit.VolatileRead(ref items[index]);
+				TOutput? output = LazyInit.VolatileRead(ref items[index]);
 				if (output != null)
 				{
 					return output;
@@ -72,7 +73,7 @@ namespace ICSharpCode.Decompiler.Util
 		readonly IList<TInput> input;
 		readonly TContext context;
 		readonly Func<TContext, TInput, TOutput> projection;
-		readonly TOutput[] items;
+		readonly TOutput?[] items;
 
 		public ProjectedList(TContext context, IList<TInput> input, Func<TContext, TInput, TOutput> projection)
 		{
@@ -83,12 +84,12 @@ namespace ICSharpCode.Decompiler.Util
 			this.input = input;
 			this.context = context;
 			this.projection = projection;
-			this.items = new TOutput[input.Count];
+			this.items = new TOutput?[input.Count];
 		}
 
 		public TOutput this[int index] {
 			get {
-				TOutput output = LazyInit.VolatileRead(ref items[index]);
+				TOutput? output = LazyInit.VolatileRead(ref items[index]);
 				if (output != null)
 				{
 					return output;

ICSharpCode.Decompiler/Util/ReferenceComparer.cs

@@ -15,22 +15,23 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+#nullable enable
 
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
 namespace ICSharpCode.Decompiler.Util
 {
-	public sealed class ReferenceComparer : IEqualityComparer<object>
+	public sealed class ReferenceComparer : IEqualityComparer<object?>
 	{
 		public readonly static ReferenceComparer Instance = new ReferenceComparer();
 
-		public new bool Equals(object x, object y)
+		public new bool Equals(object? x, object? y)
 		{
 			return x == y;
 		}
 
-		public int GetHashCode(object obj)
+		public int GetHashCode(object? obj)
 		{
 			return RuntimeHelpers.GetHashCode(obj);
 		}

ICSharpCode.Decompiler/Util/ResourcesFile.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2018 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2018 Daniel Grunwald
 // Based on the .NET Core ResourceReader; make available under the MIT license
 // by the .NET Foundation.
 // 
@@ -30,7 +31,7 @@ namespace ICSharpCode.Decompiler.Util
 	/// <summary>
 	/// .resources file.
 	/// </summary>
-	public class ResourcesFile : IEnumerable<KeyValuePair<string, object>>, IDisposable
+	public class ResourcesFile : IEnumerable<KeyValuePair<string, object?>>, IDisposable
 	{
 		sealed class MyBinaryReader : BinaryReader
 		{
@@ -87,7 +88,7 @@ namespace ICSharpCode.Decompiler.Util
 		readonly long fileStartPosition;
 		readonly long nameSectionPosition;
 		readonly long dataSectionPosition;
-		long[] startPositions;
+		long[]? startPositions;
 
 		/// <summary>
 		/// Creates a new ResourcesFile.
@@ -285,7 +286,7 @@ namespace ICSharpCode.Decompiler.Util
 			return true;
 		}
 
-		object LoadObject(int dataOffset)
+		object? LoadObject(int dataOffset)
 		{
 			try
 			{
@@ -318,7 +319,7 @@ namespace ICSharpCode.Decompiler.Util
 		// from that location.
 		// Anyone who calls LoadObject should make sure they take a lock so 
 		// no one can cause us to do a seek in here.
-		private object LoadObjectV1(int dataOffset)
+		private object? LoadObjectV1(int dataOffset)
 		{
 			Debug.Assert(System.Threading.Monitor.IsEntered(reader));
 			reader.Seek(dataSectionPosition + dataOffset, SeekOrigin.Begin);
@@ -372,7 +373,7 @@ namespace ICSharpCode.Decompiler.Util
 			}
 		}
 
-		private object LoadObjectV2(int dataOffset)
+		private object? LoadObjectV2(int dataOffset)
 		{
 			Debug.Assert(System.Threading.Monitor.IsEntered(reader));
 			reader.Seek(dataSectionPosition + dataOffset, SeekOrigin.Begin);
@@ -464,19 +465,19 @@ namespace ICSharpCode.Decompiler.Util
 			}
 		}
 
-		public object GetResourceValue(int index)
+		public object? GetResourceValue(int index)
 		{
 			GetResourceName(index, out int dataOffset);
 			return LoadObject(dataOffset);
 		}
 
-		public IEnumerator<KeyValuePair<string, object>> GetEnumerator()
+		public IEnumerator<KeyValuePair<string, object?>> GetEnumerator()
 		{
 			for (int i = 0; i < numResources; i++)
 			{
 				string name = GetResourceName(i, out int dataOffset);
-				object val = LoadObject(dataOffset);
-				yield return new KeyValuePair<string, object>(name, val);
+				object? val = LoadObject(dataOffset);
+				yield return new KeyValuePair<string, object?>(name, val);
 			}
 		}
 
@@ -487,7 +488,7 @@ namespace ICSharpCode.Decompiler.Util
 
 		long[] GetStartPositions()
 		{
-			long[] positions = LazyInit.VolatileRead(ref startPositions);
+			long[]? positions = LazyInit.VolatileRead(ref startPositions);
 			if (positions != null)
 				return positions;
 			lock (reader)
@@ -541,11 +542,11 @@ namespace ICSharpCode.Decompiler.Util
 
 	public class ResourceSerializedObject
 	{
-		public string TypeName { get; }
+		public string? TypeName { get; }
 		readonly ResourcesFile file;
 		readonly long position;
 
-		internal ResourceSerializedObject(string typeName, ResourcesFile file, long position)
+		internal ResourceSerializedObject(string? typeName, ResourcesFile file, long position)
 		{
 			this.TypeName = typeName;
 			this.file = file;

ICSharpCode.Decompiler/Util/TreeTraversal.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
+#nullable enable
+// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -32,7 +33,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// <param name="root">The root element of the tree.</param>
 		/// <param name="recursion">The function that gets the children of an element.</param>
 		/// <returns>Iterator that enumerates the tree structure in pre-order.</returns>
-		public static IEnumerable<T> PreOrder<T>(T root, Func<T, IEnumerable<T>> recursion)
+		public static IEnumerable<T> PreOrder<T>(T root, Func<T, IEnumerable<T>?> recursion)
 		{
 			return PreOrder(new T[] { root }, recursion);
 		}
@@ -43,7 +44,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// <param name="input">The root elements of the forest.</param>
 		/// <param name="recursion">The function that gets the children of an element.</param>
 		/// <returns>Iterator that enumerates the tree structure in pre-order.</returns>
-		public static IEnumerable<T> PreOrder<T>(IEnumerable<T> input, Func<T, IEnumerable<T>> recursion)
+		public static IEnumerable<T> PreOrder<T>(IEnumerable<T> input, Func<T, IEnumerable<T>?> recursion)
 		{
 			Stack<IEnumerator<T>> stack = new Stack<IEnumerator<T>>();
 			try
@@ -55,7 +56,7 @@ namespace ICSharpCode.Decompiler.Util
 					{
 						T element = stack.Peek().Current;
 						yield return element;
-						IEnumerable<T> children = recursion(element);
+						IEnumerable<T>? children = recursion(element);
 						if (children != null)
 						{
 							stack.Push(children.GetEnumerator());
@@ -79,7 +80,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// <param name="root">The root element of the tree.</param>
 		/// <param name="recursion">The function that gets the children of an element.</param>
 		/// <returns>Iterator that enumerates the tree structure in post-order.</returns>
-		public static IEnumerable<T> PostOrder<T>(T root, Func<T, IEnumerable<T>> recursion)
+		public static IEnumerable<T> PostOrder<T>(T root, Func<T, IEnumerable<T>?> recursion)
 		{
 			return PostOrder(new T[] { root }, recursion);
 		}
@@ -90,7 +91,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// <param name="input">The root elements of the forest.</param>
 		/// <param name="recursion">The function that gets the children of an element.</param>
 		/// <returns>Iterator that enumerates the tree structure in post-order.</returns>
-		public static IEnumerable<T> PostOrder<T>(IEnumerable<T> input, Func<T, IEnumerable<T>> recursion)
+		public static IEnumerable<T> PostOrder<T>(IEnumerable<T> input, Func<T, IEnumerable<T>?> recursion)
 		{
 			Stack<IEnumerator<T>> stack = new Stack<IEnumerator<T>>();
 			try
@@ -101,7 +102,7 @@ namespace ICSharpCode.Decompiler.Util
 					while (stack.Peek().MoveNext())
 					{
 						T element = stack.Peek().Current;
-						IEnumerable<T> children = recursion(element);
+						IEnumerable<T>? children = recursion(element);
 						if (children != null)
 						{
 							stack.Push(children.GetEnumerator());

ICSharpCode.Decompiler/Util/UnicodeNewline.cs

@@ -1,3 +1,4 @@
+#nullable enable
 //
 // UnicodeNewline.cs
 //
@@ -118,7 +119,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// <returns>0 == no new line, otherwise it returns either 1 or 2 depending of the length of the delimiter.</returns>
 		/// <param name="curChar">The current character.</param>
 		/// <param name="nextChar">A callback getting the next character (may be null).</param>
-		public static int GetDelimiterLength(char curChar, Func<char> nextChar = null)
+		public static int GetDelimiterLength(char curChar, Func<char>? nextChar = null)
 		{
 			if (curChar == CR)
 			{
@@ -161,7 +162,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// <param name = "length">The length of the delimiter</param>
 		/// <param name = "type">The type of the delimiter</param>
 		/// <param name="nextChar">A callback getting the next character (may be null).</param>
-		public static bool TryGetDelimiterLengthAndType(char curChar, out int length, out UnicodeNewline type, Func<char> nextChar = null)
+		public static bool TryGetDelimiterLengthAndType(char curChar, out int length, out UnicodeNewline type, Func<char>? nextChar = null)
 		{
 			if (curChar == CR)
 			{
@@ -275,7 +276,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// <returns>0 == no new line, otherwise it returns either 1 or 2 depending of the length of the delimiter.</returns>
 		/// <param name="curChar">The current character.</param>
 		/// <param name="nextChar">A callback getting the next character (may be null).</param>
-		public static UnicodeNewline GetDelimiterType(char curChar, Func<char> nextChar = null)
+		public static UnicodeNewline GetDelimiterType(char curChar, Func<char>? nextChar = null)
 		{
 			switch (curChar)
 			{

ICSharpCode.Decompiler/Util/UnionFind.cs

@@ -15,6 +15,7 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+#nullable enable
 
 using System.Collections.Generic;
 
@@ -32,6 +33,12 @@ namespace ICSharpCode.Decompiler.Util
 			public int rank;
 			public Node parent;
 			public T value;
+
+			internal Node(T value)
+			{
+				this.value = value;
+				this.parent = this;
+			}
 		}
 
 		public UnionFind()
@@ -44,10 +51,7 @@ namespace ICSharpCode.Decompiler.Util
 			Node node;
 			if (!mapping.TryGetValue(element, out node))
 			{
-				node = new Node {
-					value = element,
-					rank = 0
-				};
+				node = new Node(element);
 				node.parent = node;
 				mapping.Add(element, node);
 			}
@@ -84,5 +88,3 @@ namespace ICSharpCode.Decompiler.Util
 		}
 	}
 }
-
-

ICSharpCode.Decompiler/Util/Win32Resources.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Reflection.PortableExecutable;
@@ -15,21 +16,21 @@ namespace ICSharpCode.Decompiler.Util
 		/// </summary>
 		/// <param name="pe"></param>
 		/// <returns></returns>
-		public static unsafe Win32ResourceDirectory ReadWin32Resources(this PEReader pe)
+		public static unsafe Win32ResourceDirectory? ReadWin32Resources(this PEReader pe)
 		{
 			if (pe == null)
 			{
 				throw new ArgumentNullException(nameof(pe));
 			}
 
-			int rva = pe.PEHeaders.PEHeader.ResourceTableDirectory.RelativeVirtualAddress;
+			int rva = pe.PEHeaders.PEHeader?.ResourceTableDirectory.RelativeVirtualAddress ?? 0;
 			if (rva == 0)
 				return null;
 			byte* pRoot = pe.GetSectionData(rva).Pointer;
 			return new Win32ResourceDirectory(pe, pRoot, 0, new Win32ResourceName("Root"));
 		}
 
-		public static Win32ResourceDirectory Find(this Win32ResourceDirectory root, Win32ResourceName type)
+		public static Win32ResourceDirectory? Find(this Win32ResourceDirectory root, Win32ResourceName type)
 		{
 			if (root is null)
 				throw new ArgumentNullException(nameof(root));
@@ -41,7 +42,7 @@ namespace ICSharpCode.Decompiler.Util
 			return root.FindDirectory(type);
 		}
 
-		public static Win32ResourceDirectory Find(this Win32ResourceDirectory root, Win32ResourceName type, Win32ResourceName name)
+		public static Win32ResourceDirectory? Find(this Win32ResourceDirectory root, Win32ResourceName type, Win32ResourceName name)
 		{
 			if (root is null)
 				throw new ArgumentNullException(nameof(root));
@@ -55,7 +56,7 @@ namespace ICSharpCode.Decompiler.Util
 			return root.FindDirectory(type)?.FindDirectory(name);
 		}
 
-		public static Win32ResourceData Find(this Win32ResourceDirectory root, Win32ResourceName type, Win32ResourceName name, Win32ResourceName langId)
+		public static Win32ResourceData? Find(this Win32ResourceDirectory root, Win32ResourceName type, Win32ResourceName name, Win32ResourceName langId)
 		{
 			if (root is null)
 				throw new ArgumentNullException(nameof(root));
@@ -122,7 +123,7 @@ namespace ICSharpCode.Decompiler.Util
 			return new string(pString->NameString, 0, pString->Length);
 		}
 
-		public Win32ResourceDirectory FindDirectory(Win32ResourceName name)
+		public Win32ResourceDirectory? FindDirectory(Win32ResourceName name)
 		{
 			foreach (var directory in Directories)
 			{
@@ -132,7 +133,7 @@ namespace ICSharpCode.Decompiler.Util
 			return null;
 		}
 
-		public Win32ResourceData FindData(Win32ResourceName name)
+		public Win32ResourceData? FindData(Win32ResourceName name)
 		{
 			foreach (var data in Datas)
 			{
@@ -142,12 +143,12 @@ namespace ICSharpCode.Decompiler.Util
 			return null;
 		}
 
-		public Win32ResourceDirectory FirstDirectory()
+		public Win32ResourceDirectory? FirstDirectory()
 		{
 			return Directories.Count != 0 ? Directories[0] : null;
 		}
 
-		public Win32ResourceData FirstData()
+		public Win32ResourceData? FirstData()
 		{
 			return Datas.Count != 0 ? Datas[0] : null;
 		}
@@ -248,7 +249,7 @@ namespace ICSharpCode.Decompiler.Util
 			return _name.GetHashCode();
 		}
 
-		public override bool Equals(object obj)
+		public override bool Equals(object? obj)
 		{
 			if (!(obj is Win32ResourceName name))
 				return false;

ILSpy.BamlDecompiler.Tests/ILSpy.BamlDecompiler.Tests.csproj

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
 
   <PropertyGroup>
     <TargetFramework>net472</TargetFramework>
@@ -112,4 +112,4 @@
     <Page Include="Cases\Issue445.xaml" />
   </ItemGroup>
 
-</Project>
+</Project>