Enable nullable reference types across the remaining C# Syntax files

Completes the nullable migration for ICSharpCode.Decompiler/CSharp/Syntax: every
hand-written file now carries #nullable enable. The pattern-matching API
(INode/Pattern/PatternExtensions DoMatch/Match/IsMatch) takes a nullable 'other',
since matching legitimately compares a pattern against a missing child. Members
that genuinely return or accept null are annotated to match (AttributeSection's
target token, IAnnotatable.Annotation<T>, Statement/Expression.ReplaceWith,
SyntaxExtensions.GetNextStatement), and a latent null dereference in
Backreference.DoMatch is fixed.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -764,8 +764,8 @@ namespace ICSharpCode.Decompiler.CSharp
 			Statement firstStatement = foreachBody.Statements.First();
 			if (firstStatement is LabelStatement)
 			{
-				// skip the entry-point label, if any
+				// skip the entry-point label, if any; the assignment statement always follows it
-				firstStatement = firstStatement.GetNextStatement();
+				firstStatement = firstStatement.GetNextStatement()!;
 			}
 			Debug.Assert(firstStatement is ExpressionStatement);
 			firstStatement.Remove();

ICSharpCode.Decompiler/CSharp/Syntax/AstType.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.Collections.Generic;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
@@ -39,7 +41,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		public bool IsVar()
 		{
-			SimpleType st = this as SimpleType;
+			SimpleType? st = this as SimpleType;
 			return st != null && st.Identifier == "var" && st.TypeArguments.Count == 0;
 		}
 

ICSharpCode.Decompiler/CSharp/Syntax/DepthFirstAstVisitor.cs

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -711,7 +713,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			{
 				child.AcceptVisitor(this);
 			}
-			return default(T);
+			// Unconstrained T: the default-returning visitor methods are the no-op base behavior
+			// for node kinds a derived visitor does not override.
+			return default(T)!;
 		}
 
 		public virtual T VisitSyntaxTree(SyntaxTree unit)
@@ -1384,7 +1388,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			{
 				child.AcceptVisitor(this, data);
 			}
-			return default(S);
+			// Unconstrained S: the default-returning visitor methods are the no-op base behavior
+			// for node kinds a derived visitor does not override.
+			return default(S)!;
 		}
 
 		public virtual S VisitSyntaxTree(SyntaxTree unit, T data)

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax

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

@@ -22,6 +22,8 @@
 
 
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	class DecompilerAstNodeAttribute : System.Attribute

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/Expression.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;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -35,11 +37,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return (Expression)base.Clone();
 		}
 
-		public Expression ReplaceWith(Func<Expression, Expression> replaceFunction)
+		public Expression? ReplaceWith(Func<Expression, Expression> replaceFunction)
 		{
 			if (replaceFunction == null)
 				throw new ArgumentNullException(nameof(replaceFunction));
-			return (Expression)base.ReplaceWith(node => replaceFunction((Expression)node));
+			return (Expression?)base.ReplaceWith(node => replaceFunction((Expression)node));
 		}
 	}
 }

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -42,9 +44,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 
 		// DoMatch compares the name string; exclude the token slot to avoid matching it twice.
+		// Optional: most attribute sections have no target (e.g. [Foo]); only [return:]/[assembly:]/...
+		// carry one, so the backing token slot may be empty.
 		[ExcludeFromMatch]
 		[Slot("Identifier")]
-		public partial Identifier AttributeTargetToken { get; set; }
+		public partial Identifier? AttributeTargetToken { get; set; }
 
 		[Slot("Attribute")]
 		public partial AstNodeCollection<Attribute> Attributes { get; }

ICSharpCode.Decompiler/CSharp/Syntax/GeneralScope/Trivia.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
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>

ICSharpCode.Decompiler/CSharp/Syntax/IAnnotatable.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.CodeAnalysis;
@@ -43,7 +45,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// <typeparam name='T'>
 		/// The type of the annotation.
 		/// </typeparam>
-		T Annotation<T>() where T : class;
+		T? Annotation<T>() where T : class;
 
 		/// <summary>
 		/// Gets the first annotation of the specified type.
@@ -52,7 +54,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// <param name='type'>
 		/// The type of the annotation.
 		/// </param>
-		object Annotation(Type type);
+		object? Annotation(Type type);
 
 		/// <summary>
 		/// Adds an annotation to this instance.
@@ -90,7 +92,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		// or to an AnnotationList.
 		// Once it is pointed at an AnnotationList, it will never change (this allows thread-safety support by locking the list)
 
-		object annotations;
+		object? annotations;
 
 		/// <summary>
 		/// Clones all annotations.
@@ -102,7 +104,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		protected void CloneAnnotations()
 		{
-			ICloneable cloneable = annotations as ICloneable;
+			ICloneable? cloneable = annotations as ICloneable;
 			if (cloneable != null)
 				annotations = cloneable.Clone();
 		}
@@ -126,8 +128,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					for (int i = 0; i < this.Count; i++)
 					{
 						object obj = this[i];
-						ICloneable c = obj as ICloneable;
+						ICloneable? c = obj as ICloneable;
-						copy.Add(c != null ? c.Clone() : obj);
+						copy.Add(c != null ? c.Clone()! : obj);
 					}
 					return copy;
 				}
@@ -139,12 +141,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			if (annotation == null)
 				throw new ArgumentNullException(nameof(annotation));
 			retry: // Retry until successful
-			object oldAnnotation = Interlocked.CompareExchange(ref this.annotations, annotation, null);
+			object? oldAnnotation = Interlocked.CompareExchange(ref this.annotations, annotation, null);
 			if (oldAnnotation == null)
 			{
 				return; // we successfully added a single annotation
 			}
-			AnnotationList list = oldAnnotation as AnnotationList;
+			AnnotationList? list = oldAnnotation as AnnotationList;
 			if (list == null)
 			{
 				// we need to transform the old annotation into a list
@@ -170,8 +172,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public virtual void RemoveAnnotations<T>() where T : class
 		{
 			retry: // Retry until successful
-			object oldAnnotations = this.annotations;
+			object? oldAnnotations = this.annotations;
-			AnnotationList list = oldAnnotations as AnnotationList;
+			AnnotationList? list = oldAnnotations as AnnotationList;
 			if (list != null)
 			{
 				lock (list)
@@ -192,8 +194,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			if (type == null)
 				throw new ArgumentNullException(nameof(type));
 			retry: // Retry until successful
-			object oldAnnotations = this.annotations;
+			object? oldAnnotations = this.annotations;
-			AnnotationList list = oldAnnotations as AnnotationList;
+			AnnotationList? list = oldAnnotations as AnnotationList;
 			if (list != null)
 			{
 				lock (list)
@@ -209,17 +211,17 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		public T Annotation<T>() where T : class
+		public T? Annotation<T>() where T : class
 		{
-			object annotations = this.annotations;
+			object? annotations = this.annotations;
-			AnnotationList list = annotations as AnnotationList;
+			AnnotationList? list = annotations as AnnotationList;
 			if (list != null)
 			{
 				lock (list)
 				{
 					foreach (object obj in list)
 					{
-						T t = obj as T;
+						T? t = obj as T;
 						if (t != null)
 							return t;
 					}
@@ -232,12 +234,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		public object Annotation(Type type)
+		public object? Annotation(Type type)
 		{
 			if (type == null)
 				throw new ArgumentNullException(nameof(type));
-			object annotations = this.annotations;
+			object? annotations = this.annotations;
-			AnnotationList list = annotations as AnnotationList;
+			AnnotationList? list = annotations as AnnotationList;
 			if (list != null)
 			{
 				lock (list)
@@ -262,8 +264,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// </summary>
 		public IEnumerable<object> Annotations {
 			get {
-				object annotations = this.annotations;
+				object? annotations = this.annotations;
-				AnnotationList list = annotations as AnnotationList;
+				AnnotationList? list = annotations as AnnotationList;
 				if (list != null)
 				{
 					lock (list)

ICSharpCode.Decompiler/CSharp/Syntax/Modifiers.cs

@@ -26,6 +26,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable enable
+
 using System;
 using System.Collections.Immutable;
 

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/AnyNode.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
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 {
 	/// <summary>
@@ -24,18 +26,18 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 	/// <remarks>Does not match null nodes.</remarks>
 	public class AnyNode : Pattern
 	{
-		readonly string groupName;
+		readonly string? groupName;
 
-		public string GroupName {
+		public string? GroupName {
 			get { return groupName; }
 		}
 
-		public AnyNode(string groupName = null)
+		public AnyNode(string? groupName = null)
 		{
 			this.groupName = groupName;
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			match.Add(this.groupName, other);
 			return other != null;

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/AnyNodeOrNull.cs

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 {
 	/// <summary>
@@ -32,18 +34,18 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 	/// <remarks>Does not match null nodes.</remarks>
 	public class AnyNodeOrNull : Pattern
 	{
-		readonly string groupName;
+		readonly string? groupName;
 
-		public string GroupName {
+		public string? GroupName {
 			get { return groupName; }
 		}
 
-		public AnyNodeOrNull(string groupName = null)
+		public AnyNodeOrNull(string? groupName = null)
 		{
 			this.groupName = groupName;
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			if (other == null)
 			{

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/Backreference.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.Linq;
 
@@ -39,11 +41,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			this.referencedGroupName = referencedGroupName;
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			var last = match.Get(referencedGroupName).LastOrDefault();
-			if (last == null && other == null)
+			if (last == null)
-				return true;
+				return other == null;
 			return last.IsMatch(other);
 		}
 	}

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/BacktrackingInfo.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.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/Choice.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;
 using System.Collections.Generic;
@@ -43,7 +45,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			alternatives.Add(alternative);
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			var checkPoint = match.CheckPoint();
 			foreach (INode alt in alternatives)

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/INode.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;
 
@@ -26,7 +28,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 	/// </summary>
 	public interface INode
 	{
-		bool DoMatch(INode other, Match match);
+		bool DoMatch(INode? other, Match match);
 
 		/// <summary>
 		/// Matches this pattern node against the collection element at <paramref name="pos"/>.
@@ -51,7 +53,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 		/// However, it is also possible to match two ASTs without any pattern nodes -
 		/// doing so will produce a successful match if the two ASTs are structurally identical.
 		/// </remarks>
-		public static Match Match(this INode pattern, INode other)
+		public static Match Match(this INode pattern, INode? other)
 		{
 			if (pattern == null)
 				throw new ArgumentNullException(nameof(pattern));
@@ -62,7 +64,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 				return default(PatternMatching.Match);
 		}
 
-		public static bool IsMatch(this INode pattern, INode other)
+		public static bool IsMatch(this INode pattern, INode? other)
 		{
 			if (pattern == null)
 				throw new ArgumentNullException(nameof(pattern));

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/IPatternPlaceholder.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
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 {
 	/// <summary>

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/IdentifierExpressionBackreference.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.Linq;
 
@@ -39,7 +41,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			this.referencedGroupName = referencedGroupName;
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			var ident = other as IdentifierExpression;
 			if (ident == null || ident.TypeArguments.Any())

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/Match.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.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
@@ -27,7 +29,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 	{
 		// TODO: maybe we should add an implicit Match->bool conversion? (implicit operator bool(Match m) { return m != null; })
 
-		List<KeyValuePair<string, INode>> results;
+		List<KeyValuePair<string, INode?>> results;
 
 		public bool Success {
 			get { return results != null; }
@@ -36,7 +38,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 		internal static Match CreateNew()
 		{
 			Match m;
-			m.results = new List<KeyValuePair<string, INode>>();
+			m.results = new List<KeyValuePair<string, INode?>>();
 			return m;
 		}
 
@@ -50,7 +52,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			results.RemoveRange(checkPoint, results.Count - checkPoint);
 		}
 
-		public IEnumerable<INode> Get(string groupName)
+		public IEnumerable<INode?> Get(string groupName)
 		{
 			if (results == null)
 				yield break;
@@ -68,7 +70,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			foreach (var pair in results)
 			{
 				if (pair.Key == groupName)
-					yield return (T)pair.Value;
+					yield return (T)pair.Value!;
 			}
 		}
 
@@ -84,19 +86,19 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			return false;
 		}
 
-		public void Add(string groupName, INode node)
+		public void Add(string? groupName, INode? node)
 		{
 			if (groupName != null && node != null)
 			{
-				results.Add(new KeyValuePair<string, INode>(groupName, node));
+				results.Add(new KeyValuePair<string, INode?>(groupName, node));
 			}
 		}
 
-		internal void AddNull(string groupName)
+		internal void AddNull(string? groupName)
 		{
 			if (groupName != null)
 			{
-				results.Add(new KeyValuePair<string, INode>(groupName, null));
+				results.Add(new KeyValuePair<string, INode?>(groupName, null));
 			}
 		}
 	}

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/NamedNode.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;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
@@ -44,7 +46,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			this.childNode = childNode;
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			match.Add(this.groupName, other);
 			return childNode.DoMatch(other, match);

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/OptionalNode.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;
 
@@ -48,7 +50,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			return childNode.DoMatch(pos < other.Count ? other[pos] : null, match);
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			if (other == null)
 				return true;

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/Pattern.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.Collections.Generic;
 using System.Diagnostics;
 
@@ -31,7 +33,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 		/// </summary>
 		public static readonly string AnyString = "$any$";
 
-		public static bool MatchString(string pattern, string text)
+		public static bool MatchString(string? pattern, string? text)
 		{
 			return pattern == AnyString || pattern == text;
 		}
@@ -48,7 +50,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			}
 		}
 
-		public abstract bool DoMatch(INode other, Match match);
+		public abstract bool DoMatch(INode? other, Match match);
 
 		public virtual bool DoMatchCollection(IReadOnlyList<INode> other, int pos, Match match, BacktrackingInfo backtrackingInfo)
 		{

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/Repeat.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;
 
@@ -60,7 +62,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 			return false; // never do a normal (single-element) match; always make the caller look at the results on the back-tracking stack.
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			if (other == null)
 				return this.MinCount <= 0;

ICSharpCode.Decompiler/CSharp/Syntax/Roles.cs

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	public static class Roles

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax

ICSharpCode.Decompiler/CSharp/Syntax/Statements/Statement.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;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -35,11 +37,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return (Statement)base.Clone();
 		}
 
-		public Statement ReplaceWith(Func<Statement, Statement> replaceFunction)
+		public Statement? ReplaceWith(Func<Statement, Statement> replaceFunction)
 		{
 			if (replaceFunction == null)
 				throw new ArgumentNullException(nameof(replaceFunction));
-			return (Statement)base.ReplaceWith(node => replaceFunction((Statement)node));
+			return (Statement?)base.ReplaceWith(node => replaceFunction((Statement)node));
 		}
 	}
 }

ICSharpCode.Decompiler/CSharp/Syntax/SyntaxExtensions.cs

@@ -18,6 +18,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -51,15 +53,15 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				|| operatorType == BinaryOperatorType.ExclusiveOr;
 		}
 
-		public static Statement GetNextStatement(this Statement statement)
+		public static Statement? GetNextStatement(this Statement statement)
 		{
-			AstNode next = statement.NextSibling;
+			AstNode? next = statement.NextSibling;
 			while (next != null && !(next is Statement))
 				next = next.NextSibling;
-			return (Statement)next;
+			return (Statement?)next;
 		}
 
-		public static bool IsArgList(this AstType type)
+		public static bool IsArgList(this AstType? type)
 		{
 			var simpleType = type as SimpleType;
 			return simpleType != null && simpleType.Identifier == "__arglist";

ICSharpCode.Decompiler/CSharp/Syntax/TextLocation.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.ComponentModel;
 using System.Globalization;
@@ -98,7 +100,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// <summary>
 		/// Equality test.
 		/// </summary>
-		public override bool Equals(object obj)
+		public override bool Equals(object? obj)
 		{
 			if (!(obj is TextLocation))
 				return false;
@@ -187,17 +189,17 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 	public class TextLocationConverter : TypeConverter
 	{
-		public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
+		public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
 		{
 			return sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
 		}
 
-		public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
+		public override bool CanConvertTo(ITypeDescriptorContext? context, Type? destinationType)
 		{
 			return destinationType == typeof(TextLocation) || base.CanConvertTo(context, destinationType);
 		}
 
-		public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
+		public override object? ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
 		{
 			if (value is string)
 			{
@@ -210,7 +212,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return base.ConvertFrom(context, culture, value);
 		}
 
-		public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
+		public override object? ConvertTo(ITypeDescriptorContext? context, CultureInfo? culture, object? value, Type destinationType)
 		{
 			if (value is TextLocation)
 			{

ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/EntityDeclaration.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.Collections.Generic;
 using System.Linq;
 

ICSharpCode.Decompiler/CSharp/Transforms/CombineQueryExpressions.cs

@@ -36,7 +36,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			if (!context.Settings.QueryExpressions)
 				return;
-			CombineQueries(rootNode, new Dictionary<string, object>());
+			CombineQueries(rootNode, new Dictionary<string, object?>());
 		}
 
 		static readonly InvocationExpression castPattern = new InvocationExpression {
@@ -47,7 +47,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		};
 
-		void CombineQueries(AstNode node, Dictionary<string, object> fromOrLetIdentifiers)
+		void CombineQueries(AstNode node, Dictionary<string, object?> fromOrLetIdentifiers)
 		{
 			AstNode? next;
 			for (AstNode? child = node.FirstChild; child != null; child = next)
@@ -103,7 +103,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		};
 
-		bool TryRemoveTransparentIdentifier(QueryExpression query, QueryFromClause fromClause, QueryExpression innerQuery, Dictionary<string, object> letClauses)
+		bool TryRemoveTransparentIdentifier(QueryExpression query, QueryFromClause fromClause, QueryExpression innerQuery, Dictionary<string, object?> letClauses)
 		{
 			if (!CSharpDecompiler.IsTransparentIdentifier(fromClause.Identifier))
 				return false;
@@ -161,7 +161,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		/// <summary>
 		/// Removes all occurrences of transparent identifiers
 		/// </summary>
-		void RemoveTransparentIdentifierReferences(AstNode node, Dictionary<string, object> fromOrLetIdentifiers)
+		void RemoveTransparentIdentifierReferences(AstNode node, Dictionary<string, object?> fromOrLetIdentifiers)
 		{
 			foreach (AstNode child in node.Children)
 			{
@@ -174,7 +174,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				mre.TypeArguments.MoveTo(newIdent.TypeArguments);
 				newIdent.CopyAnnotationsFrom(mre);
 				newIdent.RemoveAnnotations<Semantics.MemberResolveResult>(); // remove the reference to the property of the anonymous type
-				if (fromOrLetIdentifiers.TryGetValue(mre.MemberName, out var annotation))
+				if (fromOrLetIdentifiers.TryGetValue(mre.MemberName, out var annotation) && annotation != null)
 					newIdent.AddAnnotation(annotation);
 				mre.ReplaceWith(newIdent);
 				return;

ICSharpCode.Decompiler/CSharp/Transforms/CustomPatterns.cs

@@ -38,7 +38,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			this.name = type.Name;
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			ComposedType? ct = other as ComposedType;
 			AstType? o;
@@ -73,7 +73,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			this.childNode = new AnyNode(groupName);
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			InvocationExpression? ie = other as InvocationExpression;
 			if (ie != null && ie.Annotation<LdTokenAnnotation>() != null && ie.Arguments.Count == 1)
@@ -107,7 +107,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				), "TypeHandle");
 		}
 
-		public override bool DoMatch(INode other, Match match)
+		public override bool DoMatch(INode? other, Match match)
 		{
 			return childNode.DoMatch(other, match);
 		}

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceExtensionMethods.cs

@@ -48,7 +48,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			this.context = context;
 			this.conversions = CSharpConversions.Get(context.TypeSystem);
-			InitializeContext(rootNode.Annotation<UsingScope>());
+			// The decompiler attaches a UsingScope annotation to the syntax-tree root.
+			InitializeContext(rootNode.Annotation<UsingScope>()!);
 			rootNode.AcceptVisitor(this);
 		}
 
@@ -111,14 +112,16 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				if (!context.Settings.RefExtensionMethods || dirExpr.FieldDirection == FieldDirection.Out)
 					return;
-				firstArgument = dirExpr.Expression;
+				// A ref/out direction expression always wraps an operand.
+				firstArgument = dirExpr.Expression!;
 				target = firstArgument.GetResolveResult();
 				dirExpr.Detach();
 			}
 			else if (firstArgument is NullReferenceExpression)
 			{
 				Debug.Assert(context.RequiredNamespacesSuperset.Contains(method.Parameters[0].Type.Namespace));
-				firstArgument = firstArgument.ReplaceWith(expr => new CastExpression(context.TypeSystemAstBuilder.ConvertType(method.Parameters[0].Type), expr.Detach()));
+				// The replacement is a freshly created CastExpression, so the result is non-null.
+				firstArgument = firstArgument.ReplaceWith(expr => new CastExpression(context.TypeSystemAstBuilder.ConvertType(method.Parameters[0].Type), expr.Detach()))!;
 			}
 			if (invocationExpression.Target is IdentifierExpression identifierExpression)
 			{

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs

@@ -168,7 +168,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 								Projection = elementSelector.Detach(),
 								Key = keySelector.Detach()
 							};
-							queryGroupClause.AddAnnotation(new QueryGroupClauseAnnotation(keyLambda.Annotation<IL.ILFunction>(), projectionLambda.Annotation<IL.ILFunction>()));
+							// The matched group-by lambdas always carry their ILFunction annotation.
+							queryGroupClause.AddAnnotation(new QueryGroupClauseAnnotation(keyLambda.Annotation<IL.ILFunction>()!, projectionLambda.Annotation<IL.ILFunction>()!));
 							query.Clauses.Add(queryGroupClause);
 							return query;
 						}
@@ -314,7 +315,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 								joinClause.IntoIdentifier = p2.Name; // into p2.Name
 								joinClause.IntoIdentifierToken!.CopyAnnotationsFrom(p2);
 							}
-							joinClause.AddAnnotation(new QueryJoinClauseAnnotation(outerLambda.Annotation<IL.ILFunction>(), innerLambda.Annotation<IL.ILFunction>()));
+							// The matched join key-selector lambdas always carry their ILFunction annotation.
+							joinClause.AddAnnotation(new QueryJoinClauseAnnotation(outerLambda.Annotation<IL.ILFunction>()!, innerLambda.Annotation<IL.ILFunction>()!));
 							query.Clauses.Add(joinClause);
 							query.Clauses.Add(new QuerySelectClause { Expression = ((Expression)lambda.Body).Detach() }.CopyAnnotationsFrom(lambda));
 							return query;

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUsingDeclarations.cs

@@ -379,7 +379,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 			public override void VisitSimpleType(SimpleType simpleType)
 			{
-				TypeResolveResult rr;
+				TypeResolveResult? rr;
 				if ((rr = simpleType.Annotation<TypeResolveResult>()) == null)
 				{
 					base.VisitSimpleType(simpleType);

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -323,7 +323,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return declPoint.Ancestors.Contains(loop) && !declareVariables.WasMerged(itemVar);
 		}
 
-		static bool AddressUsedForSingleCall(IL.ILVariable v, IL.BlockContainer loop)
+		static bool AddressUsedForSingleCall(IL.ILVariable v, IL.BlockContainer? loop)
 		{
 			if (v.StoreCount == 1 && v.AddressCount == 1 && v.LoadCount == 0 && v.Type.IsReferenceType == false)
 			{
@@ -525,7 +525,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (!MatchForeachOnMultiDimArray(upperBounds, collection, stmt, out var foreachVariable, out var statements, out var lowerBounds))
 				return null;
 			statementsToDelete.Add(stmt);
-			statementsToDelete.Add(stmt.GetNextStatement());
+			// The matched multi-dimensional foreach pattern guarantees a statement after stmt.
+			statementsToDelete.Add(stmt.GetNextStatement()!);
 			var itemVariable = foreachVariable.GetILVariable();
 			if (itemVariable == null || !itemVariable.IsSingleDefinition
 				|| (itemVariable.Kind != IL.VariableKind.Local && itemVariable.Kind != IL.VariableKind.StackSlot)
@@ -1098,7 +1099,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				dd.CopyAnnotationsFrom(methodDef);
 				dd.Modifiers = methodDef.Modifiers & ~(Modifiers.Protected | Modifiers.Override);
 				dd.Body = m.Get<BlockStatement>("body").Single().Detach();
-				dd.Name = currentTypeDefinition?.Name;
+				// A destructor only appears inside a type declaration, so the context tracker
+				// has an enclosing type at this point.
+				dd.Name = currentTypeDefinition!.Name;
 				methodDef.ReplaceWith(dd);
 				return dd;
 			}
@@ -1188,9 +1191,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					var bAndC = expr.Right as BinaryOperatorExpression;
 					if (bAndC != null && bAndC.Operator == expr.Operator)
 					{
-						// make bAndC the parent and expr the child
+						// make bAndC the parent and expr the child.
-						var b = bAndC.Left.Detach();
+						// A conditional-and/or operator always has both operands present.
-						var c = bAndC.Right.Detach();
+						var b = bAndC.Left!.Detach();
+						var c = bAndC.Right!.Detach();
 						expr.ReplaceWith(bAndC.Detach());
 						bAndC.Left = expr;
 						bAndC.Right = c;