diff --git a/ICSharpCode.Decompiler/CSharp/CallBuilder.cs b/ICSharpCode.Decompiler/CSharp/CallBuilder.cs
index ff5122676..7cf420ba7 100644
--- a/ICSharpCode.Decompiler/CSharp/CallBuilder.cs
+++ b/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;
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs b/ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs
index 91a05b3e2..b23183c6d 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs
+++ b/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 "";
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/ComposedType.cs b/ICSharpCode.Decompiler/CSharp/Syntax/ComposedType.cs
index 05532d89e..c042a2ca9 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/ComposedType.cs
+++ b/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
 		}
 	}
 }
-
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/ArrayCreateExpression.cs b/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/ArrayCreateExpression.cs
index eaad6ff6b..e4314f001 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/ArrayCreateExpression.cs
+++ b/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 {
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/QueryExpression.cs b/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/QueryExpression.cs
index 6b91ec02f..87315b01f 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/QueryExpression.cs
+++ b/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);
 		}
 	}
-}
\ No newline at end of file
+}
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/SwitchExpression.cs b/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/SwitchExpression.cs
index 9c9e166d6..ca66a0796 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/Expressions/SwitchExpression.cs
+++ b/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); }
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/GeneralScope/PreProcessorDirective.cs b/ICSharpCode.Decompiler/CSharp/Syntax/GeneralScope/PreProcessorDirective.cs
index 87005c003..a1b175d1c 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/GeneralScope/PreProcessorDirective.cs
+++ b/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
 		}
 	}
 }
-
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/Role.cs b/ICSharpCode.Decompiler/CSharp/Syntax/Role.cs
index 6d9afb8df..2411a2652 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/Role.cs
+++ b/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()
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/Roles.cs b/ICSharpCode.Decompiler/CSharp/Syntax/Roles.cs
index 10c9ab72c..f55c63542 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/Roles.cs
+++ b/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");
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/Statements/LocalFunctionDeclarationStatement.cs b/ICSharpCode.Decompiler/CSharp/Syntax/Statements/LocalFunctionDeclarationStatement.cs
index 105789bb3..da2d1550f 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/Statements/LocalFunctionDeclarationStatement.cs
+++ b/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); }
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/Statements/SwitchStatement.cs b/ICSharpCode.Decompiler/CSharp/Syntax/Statements/SwitchStatement.cs
index c6f3c5745..a3412d1c8 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/Statements/SwitchStatement.cs
+++ b/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 {
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/EntityDeclaration.cs b/ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/EntityDeclaration.cs
index d040f0483..28cfb0798 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/EntityDeclaration.cs
+++ b/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 {
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/FixedFieldDeclaration.cs b/ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/FixedFieldDeclaration.cs
index 32e8a7512..2d31d86ab 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/FixedFieldDeclaration.cs
+++ b/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
 		}
 	}
 }
-