Enable nullable reference types across the C# transforms

Turn on #nullable enable across the AST transform pipeline, ahead of
annotating the slot properties themselves. TransformContext now exposes the
nullable CurrentMember/CurrentTypeDefinition/CurrentModule contract already
declared by ITypeResolveContext, and the generated pattern-to-node conversion
returns a non-null node so patterns can be used in collection initializers
without warnings. No IL changes.
Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler.Generators/DecompilerSyntaxTreeGenerator.cs

@@ -197,10 +197,18 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 
 		if (source.NeedsPatternPlaceholder)
 		{
+			// The placeholder conversion is part of the pattern-construction DSL, where a non-null
+			// pattern is the invariant; the result is therefore non-nullable for the specific node
+			// types. AstNode (the base) and ParameterDeclaration keep the nullable contract, and only
+			// AstNode also accepts a nullable pattern.
+			bool nullableReturn = source.NodeName is "AstNode" or "ParameterDeclaration";
+			string returnQ = nullableReturn ? "?" : "";
+			string paramQ = source.NodeName == "AstNode" ? "?" : "";
+			string forgive = nullableReturn ? "" : "!";
 			builder.Append(
-		$@"		public static implicit operator {source.NodeName}?(PatternMatching.Pattern? pattern)
+		$@"		public static implicit operator {source.NodeName}{returnQ}(PatternMatching.Pattern{paramQ} pattern)
 	{{
-		return pattern != null ? new PatternPlaceholder(pattern) : null;
+		return pattern != null ? new PatternPlaceholder(pattern) : null{forgive};
 	}}
 
 	sealed class PatternPlaceholder : {source.NodeName}, INode

ICSharpCode.Decompiler/CSharp/Syntax/AstNodeCollection.cs

@@ -304,9 +304,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return Pattern.DoMatchCollection(AsNodeList(), other.AsNodeList(), match);
 		}
 
-		public void InsertAfter(T existingItem, T newItem)
+		public void InsertAfter(T? existingItem, T newItem)
 		{
-			Insert(IndexOf(existingItem) + 1, newItem);
+			// A null existingItem yields IndexOf == -1, so the new item is inserted at the front.
+			Insert(IndexOf(existingItem!) + 1, newItem);
 		}
 
 		public void InsertBefore(T existingItem, T newItem)

ICSharpCode.Decompiler/CSharp/Transforms/AddCheckedBlocks.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;
 using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
@@ -143,7 +146,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		/// </summary>
 		abstract class InsertedNode
 		{
-			public static InsertedNode operator +(InsertedNode a, InsertedNode b)
+			public static InsertedNode? operator +(InsertedNode? a, InsertedNode? b)
 			{
 				if (a == null)
 					return b;
@@ -194,11 +197,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		class InsertedBlock : InsertedNode
 		{
-			readonly Statement firstStatement; // inclusive
+			readonly Statement? firstStatement; // inclusive
-			readonly Statement lastStatement; // exclusive
+			readonly Statement? lastStatement; // exclusive
 			readonly bool isChecked;
 
-			public InsertedBlock(Statement firstStatement, Statement lastStatement, bool isChecked)
+			public InsertedBlock(Statement? firstStatement, Statement? lastStatement, bool isChecked)
 			{
 				this.firstStatement = firstStatement;
 				this.lastStatement = lastStatement;
@@ -207,10 +210,13 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 			public override void Insert()
 			{
+				// An InsertedBlock with a null start has infinite cost in the search and is never
+				// selected for insertion, so by the time Insert runs firstStatement is non-null.
+				Debug.Assert(firstStatement != null);
 				BlockStatement newBlock = new BlockStatement();
 				// Move all statements except for the first
-				Statement next;
+				Statement? next;
-				for (Statement stmt = firstStatement.GetNextStatement(); stmt != lastStatement; stmt = next)
+				for (Statement? stmt = firstStatement.GetNextStatement(); stmt != null && stmt != lastStatement; stmt = next)
 				{
 					next = stmt.GetNextStatement();
 					newBlock.Add(stmt.Detach());
@@ -233,18 +239,18 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		class Result
 		{
 			public Cost CostInCheckedContext;
-			public InsertedNode NodesToInsertInCheckedContext;
+			public InsertedNode? NodesToInsertInCheckedContext;
 			public Cost CostInUncheckedContext;
-			public InsertedNode NodesToInsertInUncheckedContext;
+			public InsertedNode? NodesToInsertInUncheckedContext;
 		}
 		#endregion
 
 		public void Run(AstNode node, TransformContext context)
 		{
-			BlockStatement block = node as BlockStatement;
+			BlockStatement? block = node as BlockStatement;
 			if (block == null)
 			{
-				for (AstNode child = node.FirstChild; child != null; child = child.NextSibling)
+				for (AstNode? child = node.FirstChild; child != null; child = child.NextSibling)
 				{
 					Run(child, context);
 				}
@@ -268,20 +274,20 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			// For a block, we are tracking 4 possibilities:
 			// a) context is checked, no unchecked block open
 			Cost costCheckedContext = new Cost(0, 0);
-			InsertedNode nodesCheckedContext = null;
+			InsertedNode? nodesCheckedContext = null;
 			// b) context is checked, an unchecked block is open
 			Cost costCheckedContextUncheckedBlockOpen = Cost.Infinite;
-			InsertedNode nodesCheckedContextUncheckedBlockOpen = null;
+			InsertedNode? nodesCheckedContextUncheckedBlockOpen = null;
-			Statement uncheckedBlockStart = null;
+			Statement? uncheckedBlockStart = null;
 			// c) context is unchecked, no checked block open
 			Cost costUncheckedContext = new Cost(0, 0);
-			InsertedNode nodesUncheckedContext = null;
+			InsertedNode? nodesUncheckedContext = null;
 			// d) context is unchecked, a checked block is open
 			Cost costUncheckedContextCheckedBlockOpen = Cost.Infinite;
-			InsertedNode nodesUncheckedContextCheckedBlockOpen = null;
+			InsertedNode? nodesUncheckedContextCheckedBlockOpen = null;
-			Statement checkedBlockStart = null;
+			Statement? checkedBlockStart = null;
 
-			Statement statement = block.Statements.FirstOrDefault();
+			Statement? statement = block.Statements.FirstOrDefault();
 			while (true)
 			{
 				// Blocks can be closed 'for free'. We use '<=' so that blocks are closed as late as possible (goal 4b)
@@ -346,7 +352,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (node is BlockStatement)
 				return GetResultFromBlock((BlockStatement)node);
 			Result result = new Result();
-			for (AstNode child = node.FirstChild; child != null; child = child.NextSibling)
+			for (AstNode? child = node.FirstChild; child != null; child = child.NextSibling)
 			{
 				Result childResult = GetResult(child);
 				result.CostInCheckedContext += childResult.CostInCheckedContext;
@@ -354,10 +360,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				result.CostInUncheckedContext += childResult.CostInUncheckedContext;
 				result.NodesToInsertInUncheckedContext += childResult.NodesToInsertInUncheckedContext;
 			}
-			Expression expr = node as Expression;
+			Expression? expr = node as Expression;
 			if (expr != null)
 			{
-				CheckedUncheckedAnnotation annotation = expr.Annotation<CheckedUncheckedAnnotation>();
+				CheckedUncheckedAnnotation? annotation = expr.Annotation<CheckedUncheckedAnnotation>();
 				if (annotation != null)
 				{
 					if (annotation.IsExplicit)

ICSharpCode.Decompiler/CSharp/Transforms/AddXmlDocumentationTransform.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.IO;
 using System.Linq;
@@ -62,7 +64,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		static void InsertXmlDocumentation(AstNode node, StringReader r)
 		{
 			// Find the first non-empty line:
-			string firstLine;
+			string? firstLine;
 			do
 			{
 				firstLine = r.ReadLine();
@@ -70,7 +72,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					return;
 			} while (string.IsNullOrWhiteSpace(firstLine));
 			string indentation = firstLine.Substring(0, firstLine.Length - firstLine.TrimStart().Length);
-			string line = firstLine;
+			string? line = firstLine;
 			int skippedWhitespaceLines = 0;
 			// Copy all lines from input to output, except for empty lines at the end.
 			while (line != null)

ICSharpCode.Decompiler/CSharp/Transforms/CombineQueryExpressions.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;
 
@@ -47,8 +49,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		void CombineQueries(AstNode node, Dictionary<string, object> fromOrLetIdentifiers)
 		{
-			AstNode next;
+			AstNode? next;
-			for (AstNode child = node.FirstChild; child != null; child = next)
+			for (AstNode? child = node.FirstChild; child != null; child = next)
 			{
 				// store reference to next child before transformation
 				next = child.NextSibling;
@@ -105,7 +107,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			if (!CSharpDecompiler.IsTransparentIdentifier(fromClause.Identifier))
 				return false;
-			QuerySelectClause selectClause = innerQuery.Clauses.Last() as QuerySelectClause;
+			QuerySelectClause? selectClause = innerQuery.Clauses.Last() as QuerySelectClause;
+			if (selectClause == null)
+				return false;
 			Match match = selectTransparentIdentifierPattern.Match(selectClause);
 			if (!match.Success)
 				return false;
@@ -116,7 +120,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			fromClause.Remove();
 			selectClause.Remove();
 			// Move clauses from innerQuery to query
-			QueryClause insertionPos = null;
+			QueryClause? insertionPos = null;
 			foreach (var clause in innerQuery.Clauses)
 			{
 				query.Clauses.InsertAfter(insertionPos, insertionPos = clause.Detach());

ICSharpCode.Decompiler/CSharp/Transforms/ContextTrackingVisitor.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.Diagnostics;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
@@ -28,8 +30,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	/// </summary>
 	public abstract class ContextTrackingVisitor<TResult> : DepthFirstAstVisitor<TResult>
 	{
-		protected ITypeDefinition currentTypeDefinition;
+		protected ITypeDefinition? currentTypeDefinition;
-		protected IMethod currentMethod;
+		protected IMethod? currentMethod;
 
 		protected void Initialize(TransformContext context)
 		{
@@ -45,7 +47,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		public override TResult VisitTypeDeclaration(TypeDeclaration typeDeclaration)
 		{
-			ITypeDefinition oldType = currentTypeDefinition;
+			ITypeDefinition? oldType = currentTypeDefinition;
 			try
 			{
 				currentTypeDefinition = typeDeclaration.GetSymbol() as ITypeDefinition;

ICSharpCode.Decompiler/CSharp/Transforms/CustomPatterns.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;
 
@@ -27,7 +29,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 {
 	sealed class TypePattern : Pattern
 	{
-		readonly string ns;
+		readonly string? ns;
 		readonly string name;
 
 		public TypePattern(Type type)
@@ -38,8 +40,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		public override bool DoMatch(INode other, Match match)
 		{
-			ComposedType ct = other as ComposedType;
+			ComposedType? ct = other as ComposedType;
-			AstType o;
+			AstType? o;
 			if (ct != null && !ct.HasRefSpecifier && !ct.HasNullableSpecifier && ct.PointerRank == 0 && !ct.ArraySpecifiers.Any())
 			{
 				// Special case: ILSpy sometimes produces a ComposedType but then removed all array specifiers
@@ -73,7 +75,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		public override bool DoMatch(INode other, Match match)
 		{
-			InvocationExpression ie = other as InvocationExpression;
+			InvocationExpression? ie = other as InvocationExpression;
 			if (ie != null && ie.Annotation<LdTokenAnnotation>() != null && ie.Arguments.Count == 1)
 			{
 				return childNode.DoMatch(ie.Arguments.Single(), match);

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -16,9 +16,12 @@
 // 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;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
@@ -54,7 +57,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				return new InsertionPoint {
 					level = level - 1,
-					nextNode = nextNode.Parent
+					// Insertion points live inside a method body, so walking up always finds a parent.
+					nextNode = nextNode.Parent!
 				};
 			}
 
@@ -63,7 +67,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				InsertionPoint result = this;
 				while (result.level > targetLevel)
 				{
-					result.nextNode = result.nextNode.Parent;
+					result.nextNode = result.nextNode.Parent!;
 					result.level -= 1;
 				}
 				return result;
@@ -82,7 +86,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			public readonly ILVariable ILVariable;
 			public IType Type => ILVariable.Type;
-			public string Name => ILVariable.Name;
+			// Variables reaching this transform have already been assigned a name.
+			public string Name => ILVariable.Name!;
 
 			/// <summary>
 			/// Whether the variable needs to be default-initialized.
@@ -108,7 +113,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			/// </summary>
 			public IdentifierExpression FirstUse;
 
-			public VariableToDeclare ReplacementDueToCollision;
+			public VariableToDeclare? ReplacementDueToCollision;
 			public bool InvolvedInCollision;
 			public bool RemovedDueToCollision => ReplacementDueToCollision != null;
 			public bool DeclaredInDeconstruction;
@@ -138,6 +143,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		}
 
 		readonly Dictionary<ILVariable, VariableToDeclare> variableDict = new Dictionary<ILVariable, VariableToDeclare>();
+		[AllowNull]
 		TransformContext context;
 
 		public void Run(AstNode rootNode, TransformContext context)
@@ -284,7 +290,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		/// </remarks>
 		void FindInsertionPoints(AstNode node, int nodeLevel)
 		{
-			BlockContainer scope = node.Annotation<BlockContainer>();
+			BlockContainer? scope = node.Annotation<BlockContainer>();
 			if (scope != null && IsRelevantScope(scope))
 			{
 				// track loops and function bodies as scopes, for comparison with CaptureScope.
@@ -305,7 +311,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 			try
 			{
-				for (AstNode child = node.FirstChild; child != null; child = child.NextSibling)
+				for (AstNode? child = node.FirstChild; child != null; child = child.NextSibling)
 				{
 					FindInsertionPoints(child, nodeLevel + 1);
 				}
@@ -329,7 +335,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						InsertionPoint newPoint;
 						int startIndex = scopeTracking.Count - 1;
-						BlockContainer captureScope = variable.CaptureScope;
+						BlockContainer? captureScope = variable.CaptureScope;
 						while (captureScope != null && !IsRelevantScope(captureScope))
 						{
 							captureScope = BlockContainer.FindClosestContainer(captureScope.Parent);
@@ -364,7 +370,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 								}
 							}
 						}
-						if (variableDict.TryGetValue(variable, out VariableToDeclare v))
+						if (variableDict.TryGetValue(variable, out var v))
 						{
 							v.InsertionPoint = FindCommonParent(v.InsertionPoint, newPoint);
 						}
@@ -561,7 +567,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		}
 
-		bool IsMatchingAssignment(VariableToDeclare v, out AssignmentExpression assignment)
+		bool IsMatchingAssignment(VariableToDeclare v, [NotNullWhen(true)] out AssignmentExpression? assignment)
 		{
 			assignment = v.InsertionPoint.nextNode as AssignmentExpression;
 			if (assignment == null)
@@ -595,7 +601,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (v.RemovedDueToCollision || v.DeclaredInDeconstruction)
 					continue;
 
-				if (CombineDeclarationAndInitializer(v, context) && IsMatchingAssignment(v, out AssignmentExpression assignment))
+				if (CombineDeclarationAndInitializer(v, context) && IsMatchingAssignment(v, out var assignment))
 				{
 					// 'int v; v = expr;' can be combined to 'int v = expr;'
 					AstType type;
@@ -674,7 +680,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				else
 				{
 					// Insert a separate declaration statement.
-					Expression initializer = null;
+					Expression? initializer = null;
 					AstType type = context.TypeSystemAstBuilder.ConvertType(v.Type);
 					if (v.DefaultInitialization == VariableInitKind.NeedsDefaultValue)
 					{
@@ -694,6 +700,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						v.InsertionPoint = v.InsertionPoint.Up();
 					}
 					Debug.Assert(v.InsertionPoint.nextNode.Slot?.Kind == SlotKind.Statement);
+					// The insertion point is a statement within a block, so it always has a parent.
+					AstNode insertionNode = v.InsertionPoint.nextNode;
+					AstNode insertionParent = insertionNode.Parent
+						?? throw new InvalidOperationException("Variable insertion point has no parent.");
 					if (v.DefaultInitialization == VariableInitKind.NeedsSkipInit)
 					{
 						AstType unsafeType = context.TypeSystemAstBuilder.ConvertType(
@@ -702,7 +712,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						{
 							var outVarDecl = new OutVarDeclarationExpression(type.Clone(), v.Name);
 							outVarDecl.Variable.AddAnnotation(new ILVariableResolveResult(ilVariable));
-							v.InsertionPoint.nextNode.Parent.InsertChildBefore(
+							insertionParent.InsertChildBefore(
 								v.InsertionPoint.nextNode,
 								new ExpressionStatement {
 									Expression = new InvocationExpression {
@@ -719,11 +729,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						}
 						else
 						{
-							v.InsertionPoint.nextNode.Parent.InsertChildBefore(
+							insertionParent.InsertChildBefore(
 								v.InsertionPoint.nextNode,
 								vds,
 								BlockStatement.StatementRole);
-							v.InsertionPoint.nextNode.Parent.InsertChildBefore(
+							insertionParent.InsertChildBefore(
 								v.InsertionPoint.nextNode,
 								new ExpressionStatement {
 									Expression = new InvocationExpression {
@@ -745,7 +755,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					}
 					else
 					{
-						v.InsertionPoint.nextNode.Parent.InsertChildBefore(
+						insertionParent.InsertChildBefore(
 							v.InsertionPoint.nextNode,
 							vds,
 							BlockStatement.StatementRole);
@@ -759,7 +769,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		}
 
-		private bool CanBeDeclaredAsOutVariable(VariableToDeclare v, out DirectionExpression dirExpr)
+		private bool CanBeDeclaredAsOutVariable(VariableToDeclare v, [NotNullWhen(true)] out DirectionExpression? dirExpr)
 		{
 			dirExpr = v.FirstUse.Parent as DirectionExpression;
 			if (dirExpr == null || dirExpr.FieldDirection != FieldDirection.Out)
@@ -768,7 +778,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return false;
 			if (v.DefaultInitialization != VariableInitKind.None)
 				return false;
-			for (AstNode node = v.FirstUse; node != null; node = node.Parent)
+			for (AstNode? node = v.FirstUse; node != null; node = node.Parent)
 			{
 				if (node.Slot?.Kind == SlotKind.EmbeddedStatement)
 				{
@@ -812,9 +822,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				var v = variableDict[ilVar];
 				if (!v.RemovedDueToCollision)
 					continue;
-				while (v.RemovedDueToCollision)
+				while (v.ReplacementDueToCollision is { } replacement)
 				{
-					v = v.ReplacementDueToCollision;
+					v = replacement;
 				}
 				node.RemoveAnnotations<ILVariableResolveResult>();
 				node.AddAnnotation(new ILVariableResolveResult(v.ILVariable, v.Type));

ICSharpCode.Decompiler/CSharp/Transforms/EscapeInvalidIdentifiers.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/FixNameCollisions.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;
 
@@ -68,7 +70,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (node is IdentifierExpression || node is MemberReferenceExpression)
 				{
 					ISymbol symbol = node.GetSymbol();
-					if (symbol != null && renamedSymbols.TryGetValue(symbol, out string newName))
+					if (symbol != null && renamedSymbols.TryGetValue(symbol, out string? newName))
 					{
 						node.GetChildByRole(Roles.Identifier).Name = newName;
 					}

ICSharpCode.Decompiler/CSharp/Transforms/FlattenSwitchBlocks.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;

ICSharpCode.Decompiler/CSharp/Transforms/IAstTransform.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 ICSharpCode.Decompiler.CSharp.Syntax;
 
 namespace ICSharpCode.Decompiler.CSharp.Transforms

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceExtensionMethods.cs

@@ -16,8 +16,11 @@
 // 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;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
@@ -34,8 +37,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	/// </summary>
 	public class IntroduceExtensionMethods : DepthFirstAstVisitor, IAstTransform
 	{
+		[AllowNull]
 		TransformContext context;
+		[AllowNull]
 		CSharpResolver resolver;
+		[AllowNull]
 		CSharpConversions conversions;
 
 		public void Run(AstNode rootNode, TransformContext context)
@@ -122,7 +128,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 			else
 			{
-				memberRefExpr.Target = firstArgument.Detach();
+				// The target is not an IdentifierExpression, so CanTransformToExtensionMethodCall
+				// matched the MemberReferenceExpression case and memberRefExpr is non-null.
+				memberRefExpr!.Target = firstArgument.Detach();
 			}
 			if (invocationExpression.GetResolveResult() is CSharpInvocationResolveResult irr)
 			{
@@ -137,9 +145,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		}
 
 		static bool CanTransformToExtensionMethodCall(CSharpResolver resolver,
-			InvocationExpression invocationExpression, out MemberReferenceExpression memberRefExpr,
+			InvocationExpression invocationExpression, out MemberReferenceExpression? memberRefExpr,
-			out ResolveResult target,
+			[NotNullWhen(true)] out ResolveResult? target,
-			out Expression firstArgument)
+			[NotNullWhen(true)] out Expression? firstArgument)
 		{
 			var method = invocationExpression.GetSymbol() as IMethod;
 			memberRefExpr = null;
@@ -176,7 +184,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 			Debug.Assert(target != null);
 			ResolveResult[] args = new ResolveResult[invocationExpression.Arguments.Count - 1];
-			string[] argNames = null;
+			string[]? argNames = null;
 			int pos = 0;
 			foreach (var arg in invocationExpression.Arguments.Skip(1))
 			{

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs

@@ -16,7 +16,11 @@
 // 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;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
@@ -30,6 +34,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	/// </summary>
 	public class IntroduceQueryExpressions : IAstTransform
 	{
+		[AllowNull]
 		TransformContext context;
 
 		public void Run(AstNode rootNode, TransformContext context)
@@ -50,15 +55,15 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				}
 				// See if the data source of this query is a degenerate query,
 				// and combine the queries if possible.
-				QueryExpression innerQuery = fromClause.Expression as QueryExpression;
+				QueryExpression? innerQuery = fromClause.Expression as QueryExpression;
-				while (IsDegenerateQuery(innerQuery))
+				while (innerQuery != null && IsDegenerateQuery(innerQuery))
 				{
 					QueryFromClause innerFromClause = (QueryFromClause)innerQuery.Clauses.First();
-					ILVariable innerVariable = innerFromClause.Annotation<ILVariableResolveResult>()?.Variable;
+					ILVariable? innerVariable = innerFromClause.Annotation<ILVariableResolveResult>()?.Variable;
-					ILVariable rangeVariable = fromClause.Annotation<ILVariableResolveResult>()?.Variable;
+					ILVariable? rangeVariable = fromClause.Annotation<ILVariableResolveResult>()?.Variable;
 					// Replace the fromClause with all clauses from the inner query
 					fromClause.Remove();
-					QueryClause insertionPos = null;
+					QueryClause? insertionPos = null;
 					foreach (var clause in innerQuery.Clauses)
 					{
 						CombineRangeVariables(clause, innerVariable, rangeVariable);
@@ -70,21 +75,26 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		}
 
-		private void CombineRangeVariables(QueryClause clause, ILVariable oldVariable, ILVariable newVariable)
+		private void CombineRangeVariables(QueryClause clause, ILVariable? oldVariable, ILVariable? newVariable)
 		{
+			if (oldVariable == null || newVariable == null)
+				return;
 			foreach (var identifier in clause.DescendantNodes().OfType<Identifier>())
 			{
-				var variable = identifier.Parent.Annotation<ILVariableResolveResult>()?.Variable;
+				var parent = identifier.Parent;
+				if (parent == null)
+					continue;
+				var variable = parent.Annotation<ILVariableResolveResult>()?.Variable;
 				if (variable == oldVariable)
 				{
-					identifier.Parent.RemoveAnnotations<ILVariableResolveResult>();
+					parent.RemoveAnnotations<ILVariableResolveResult>();
-					identifier.Parent.AddAnnotation(new ILVariableResolveResult(newVariable));
+					parent.AddAnnotation(new ILVariableResolveResult(newVariable));
 					identifier.ReplaceWith(Identifier.Create(newVariable.Name));
 				}
 			}
 		}
 
-		bool IsDegenerateQuery(QueryExpression query)
+		bool IsDegenerateQuery(QueryExpression? query)
 		{
 			if (query == null)
 				return false;
@@ -94,7 +104,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		void DecompileQueries(AstNode node)
 		{
-			Expression query = DecompileQuery(node as InvocationExpression);
+			Expression? query = DecompileQuery(node as InvocationExpression);
 			if (query != null)
 			{
 				if (node.Parent is ExpressionStatement && CanUseDiscardAssignment())
@@ -102,8 +112,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				node.ReplaceWith(query);
 			}
 
-			AstNode next;
+			AstNode? next;
-			for (AstNode child = (query ?? node).FirstChild; child != null; child = next)
+			for (AstNode? child = (query ?? node).FirstChild; child != null; child = next)
 			{
 				// store reference to next child before transformation
 				next = child.NextSibling;
@@ -117,11 +127,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return context.Settings.Discards;
 		}
 
-		QueryExpression DecompileQuery(InvocationExpression invocation)
+		QueryExpression? DecompileQuery(InvocationExpression? invocation)
 		{
 			if (invocation == null)
 				return null;
-			MemberReferenceExpression mre = invocation.Target as MemberReferenceExpression;
+			MemberReferenceExpression? mre = invocation.Target as MemberReferenceExpression;
 			if (mre == null || IsNullConditional(mre.Target))
 				return null;
 			switch (mre.MemberName)
@@ -133,7 +143,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (!IsComplexQuery(mre))
 						return null;
 					Expression expr = invocation.Arguments.Single();
-					if (MatchSimpleLambda(expr, out ParameterDeclaration parameter, out Expression body))
+					if (MatchSimpleLambda(expr, out var parameter, out var body))
 					{
 						QueryExpression query = new QueryExpression();
 						query.Clauses.Add(MakeFromClause(parameter, mre.Target.Detach()));
@@ -148,8 +158,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						Expression keyLambda = invocation.Arguments.ElementAt(0);
 						Expression projectionLambda = invocation.Arguments.ElementAt(1);
-						if (MatchSimpleLambda(keyLambda, out ParameterDeclaration parameter1, out Expression keySelector)
+						if (MatchSimpleLambda(keyLambda, out var parameter1, out var keySelector)
-							&& MatchSimpleLambda(projectionLambda, out ParameterDeclaration parameter2, out Expression elementSelector)
+							&& MatchSimpleLambda(projectionLambda, out var parameter2, out var elementSelector)
 							&& parameter1.Name == parameter2.Name)
 						{
 							QueryExpression query = new QueryExpression();
@@ -166,7 +176,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					else if (invocation.Arguments.Count == 1)
 					{
 						Expression lambda = invocation.Arguments.Single();
-						if (MatchSimpleLambda(lambda, out ParameterDeclaration parameter, out Expression keySelector))
+						if (MatchSimpleLambda(lambda, out var parameter, out var keySelector))
 						{
 							QueryExpression query = new QueryExpression();
 							query.Clauses.Add(MakeFromClause(parameter, mre.Target.Detach()));
@@ -181,11 +191,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (invocation.Arguments.Count != 2)
 						return null;
 					var fromExpressionLambda = invocation.Arguments.ElementAt(0);
-					if (!MatchSimpleLambda(fromExpressionLambda, out ParameterDeclaration parameter, out Expression collectionSelector))
+					if (!MatchSimpleLambda(fromExpressionLambda, out var parameter, out var collectionSelector))
 						return null;
 					if (IsNullConditional(collectionSelector))
 						return null;
-					LambdaExpression lambda = invocation.Arguments.ElementAt(1) as LambdaExpression;
+					LambdaExpression? lambda = invocation.Arguments.ElementAt(1) as LambdaExpression;
 					if (lambda != null && lambda.Parameters.Count == 2 && lambda.Body is Expression)
 					{
 						ParameterDeclaration p1 = lambda.Parameters.ElementAt(0);
@@ -208,7 +218,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (!IsComplexQuery(mre))
 						return null;
 					Expression expr = invocation.Arguments.Single();
-					if (MatchSimpleLambda(expr, out ParameterDeclaration parameter, out Expression body))
+					if (MatchSimpleLambda(expr, out var parameter, out var body))
 					{
 						QueryExpression query = new QueryExpression();
 						query.Clauses.Add(MakeFromClause(parameter, mre.Target.Detach()));
@@ -227,7 +237,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (!IsComplexQuery(mre))
 						return null;
 					var lambda = invocation.Arguments.Single();
-					if (MatchSimpleLambda(lambda, out ParameterDeclaration parameter, out Expression orderExpression))
+					if (MatchSimpleLambda(lambda, out var parameter, out var orderExpression))
 					{
 						if (ValidateThenByChain(invocation, parameter.Name))
 						{
@@ -244,8 +254,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 								InvocationExpression tmp = (InvocationExpression)mre.Target;
 								mre = (MemberReferenceExpression)tmp.Target;
 								lambda = tmp.Arguments.Single();
-								MatchSimpleLambda(lambda, out parameter, out orderExpression);
+								// ValidateThenByChain already confirmed every clause in the chain is a simple
+								// lambda, so this match always succeeds.
+								bool matched = MatchSimpleLambda(lambda, out parameter, out orderExpression);
+								Debug.Assert(matched);
 							}
+							// The chain was validated and every iteration re-matched a simple lambda,
+							// so the final range-variable parameter is set.
+							Debug.Assert(parameter != null);
 							// insert new ordering at beginning
 							orderClause.Orderings.InsertAfter(
 								null, new QueryOrdering {
@@ -271,12 +287,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (IsNullConditional(source2))
 						return null;
 					Expression outerLambda = invocation.Arguments.ElementAt(1);
-					if (!MatchSimpleLambda(outerLambda, out ParameterDeclaration element1, out Expression key1))
+					if (!MatchSimpleLambda(outerLambda, out var element1, out var key1))
 						return null;
 					Expression innerLambda = invocation.Arguments.ElementAt(2);
-					if (!MatchSimpleLambda(innerLambda, out ParameterDeclaration element2, out Expression key2))
+					if (!MatchSimpleLambda(innerLambda, out var element2, out var key2))
 						return null;
-					LambdaExpression lambda = invocation.Arguments.ElementAt(3) as LambdaExpression;
+					LambdaExpression? lambda = invocation.Arguments.ElementAt(3) as LambdaExpression;
 					if (lambda != null && lambda.Parameters.Count == 2 && lambda.Body is Expression)
 					{
 						ParameterDeclaration p1 = lambda.Parameters.ElementAt(0);
@@ -365,13 +381,13 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		/// <summary>
 		/// Ensure that all ThenBy's are correct, and that the list of ThenBy's is terminated by an 'OrderBy' invocation.
 		/// </summary>
-		bool ValidateThenByChain(InvocationExpression invocation, string expectedParameterName)
+		bool ValidateThenByChain(InvocationExpression? invocation, string expectedParameterName)
 		{
 			if (invocation == null || invocation.Arguments.Count != 1)
 				return false;
 			if (!(invocation.Target is MemberReferenceExpression mre))
 				return false;
-			if (!MatchSimpleLambda(invocation.Arguments.Single(), out ParameterDeclaration parameter, out _))
+			if (!MatchSimpleLambda(invocation.Arguments.Single(), out var parameter, out _))
 				return false;
 			if (parameter.Name != expectedParameterName)
 				return false;
@@ -385,7 +401,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		}
 
 		/// <summary>Matches simple lambdas of the form "a => b"</summary>
-		bool MatchSimpleLambda(Expression expr, out ParameterDeclaration parameter, out Expression body)
+		bool MatchSimpleLambda(Expression expr, [NotNullWhen(true)] out ParameterDeclaration? parameter, [NotNullWhen(true)] out Expression? body)
 		{
 			if (expr is LambdaExpression lambda && lambda.Parameters.Count == 1 && lambda.Body is Expression)
 			{

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUnsafeModifier.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.CSharp.Resolver;
@@ -40,8 +42,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		protected override bool VisitChildren(AstNode node)
 		{
 			bool result = false;
-			AstNode next;
+			AstNode? next;
-			for (AstNode child = node.FirstChild; child != null; child = next)
+			for (AstNode? child = node.FirstChild; child != null; child = next)
 			{
 				// Store next to allow the loop to continue
 				// if the visitor removes/replaces child.
@@ -116,7 +118,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		public override bool VisitMemberReferenceExpression(MemberReferenceExpression memberReferenceExpression)
 		{
 			bool result = base.VisitMemberReferenceExpression(memberReferenceExpression);
-			UnaryOperatorExpression uoe = memberReferenceExpression.Target as UnaryOperatorExpression;
+			UnaryOperatorExpression? uoe = memberReferenceExpression.Target as UnaryOperatorExpression;
 			if (uoe != null && uoe.Operator == UnaryOperatorType.Dereference)
 			{
 				PointerReferenceExpression pre = new PointerReferenceExpression();

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUsingDeclarations.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.Collections.Immutable;
@@ -49,7 +51,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				// #define directives are leading trivia on the SyntaxTree, not children, so using
 				// declarations go at the very start of the member list.
-				AstNode insertionPoint = null;
+				AstNode? insertionPoint = null;
 
 				// Now add using declarations for those namespaces:
 				IOrderedEnumerable<string> sortedImports = requiredImports.ImportedNamespaces
@@ -119,7 +121,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				base.VisitSimpleType(simpleType); // also visit type arguments
 			}
 
-			private void AddImportedNamespace(IType type)
+			private void AddImportedNamespace(IType? type)
 			{
 				if (type != null && !IsParentOfCurrentNamespace(type.Namespace))
 				{
@@ -214,7 +216,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				this.astBuilder = CreateAstBuilder(resolver);
 			}
 
-			TypeSystemAstBuilder CreateAstBuilder(CSharpResolver resolver, IL.ILFunction function = null)
+			TypeSystemAstBuilder CreateAstBuilder(CSharpResolver resolver, IL.ILFunction? function = null)
 			{
 				if (function != null)
 				{

ICSharpCode.Decompiler/CSharp/Transforms/NormalizeBlockStatements.cs

@@ -1,8 +1,12 @@
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 
+using System.Diagnostics.CodeAnalysis;
+
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching;
 
@@ -10,9 +14,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 {
 	class NormalizeBlockStatements : DepthFirstAstVisitor, IAstTransform
 	{
+		[AllowNull]
 		TransformContext context;
 		bool hasNamespace;
-		NamespaceDeclaration singleNamespaceDeclaration;
+		NamespaceDeclaration? singleNamespaceDeclaration;
 
 		public override void VisitSyntaxTree(SyntaxTree syntaxTree)
 		{
@@ -196,7 +201,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			Attributes = { new Repeat(new AnyNode()) },
 			Modifiers = Modifiers.Any,
 			PrivateImplementationType = new AnyNodeOrNull(),
-			Parameters = { new Repeat(new AnyNode()) },
+			Parameters = { new Repeat(new AnyNode())! },
 			ReturnType = new AnyNode(),
 			Getter = new Accessor() {
 				Modifiers = Modifiers.Any,

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -16,8 +16,11 @@
 // 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;
 using System.Diagnostics;
 using System.Linq;
 using System.Reflection.Metadata;
@@ -35,6 +38,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	public sealed class PatternStatementTransform : ContextTrackingVisitor<AstNode>, IAstTransform
 	{
 		readonly DeclareVariables declareVariables = new DeclareVariables();
+		[AllowNull]
 		TransformContext context;
 
 		public void Run(AstNode rootNode, TransformContext context)
@@ -62,7 +66,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			// Go through the children, and keep visiting a node as long as it changes.
 			// Because some transforms delete/replace nodes before and after the node being transformed, we rely
 			// on the transform's return value to know where we need to keep iterating.
-			for (AstNode child = node.FirstChild; child != null; child = child.NextSibling)
+			for (AstNode? child = node.FirstChild; child != null; child = child.NextSibling)
 			{
 				AstNode oldChild;
 				do
@@ -77,7 +81,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		public override AstNode VisitExpressionStatement(ExpressionStatement expressionStatement)
 		{
-			AstNode result = TransformForeachOnMultiDimArray(expressionStatement);
+			AstNode? result = TransformForeachOnMultiDimArray(expressionStatement);
 			if (result != null)
 				return result;
 			result = TransformFor(expressionStatement);
@@ -88,7 +92,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		public override AstNode VisitForStatement(ForStatement forStatement)
 		{
-			AstNode result = TransformForeachOnArray(forStatement);
+			AstNode? result = TransformForeachOnArray(forStatement);
 			if (result != null)
 				return result;
 			return base.VisitForStatement(forStatement);
@@ -96,7 +100,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		public override AstNode VisitIfElseStatement(IfElseStatement ifElseStatement)
 		{
-			AstNode simplifiedIfElse = SimplifyCascadingIfElseStatements(ifElseStatement);
+			AstNode? simplifiedIfElse = SimplifyCascadingIfElseStatements(ifElseStatement);
 			if (simplifiedIfElse != null)
 				return simplifiedIfElse;
 			return base.VisitIfElseStatement(ifElseStatement);
@@ -107,7 +111,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (context.Settings.AutomaticProperties
 				&& (!propertyDeclaration.Setter.IsNull || context.Settings.GetterOnlyAutomaticProperties))
 			{
-				AstNode result = TransformAutomaticProperty(propertyDeclaration);
+				AstNode? result = TransformAutomaticProperty(propertyDeclaration);
 				if (result != null)
 					return result;
 			}
@@ -120,7 +124,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			base.VisitCustomEventDeclaration(eventDeclaration);
 			if (context.Settings.AutomaticEvents)
 			{
-				AstNode result = TransformAutomaticEvents(eventDeclaration);
+				AstNode? result = TransformAutomaticEvents(eventDeclaration);
 				if (result != null)
 					return result;
 			}
@@ -174,7 +178,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		};
 
-		public ForStatement TransformFor(ExpressionStatement node)
+		public ForStatement? TransformFor(ExpressionStatement node)
 		{
 			if (!context.Settings.ForStatement)
 				return null;
@@ -182,7 +186,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (!m1.Success)
 				return null;
 			var variable = m1.Get<IdentifierExpression>("variable").Single().GetILVariable();
-			AstNode next = node.NextSibling;
+			AstNode? next = node.NextSibling;
+			if (next == null)
+				return null;
 			if (next is ForStatement forStatement && ForStatementUsesVariable(forStatement, variable))
 			{
 				node.Remove();
@@ -339,7 +345,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return false;
 		}
 
-		Statement TransformForeachOnArray(ForStatement forStatement)
+		Statement? TransformForeachOnArray(ForStatement forStatement)
 		{
 			if (!context.Settings.ForEachStatement)
 				return null;
@@ -432,7 +438,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				)
 			)));
 
-		bool MatchLowerBound(int indexNum, out IL.ILVariable index, IL.ILVariable collection, Statement statement)
+		bool MatchLowerBound(int indexNum, [NotNullWhen(true)] out IL.ILVariable? index, IL.ILVariable collection, Statement statement)
 		{
 			index = null;
 			var m = variableAssignLowerBoundPattern.Match(statement);
@@ -444,7 +450,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return m.Get<IdentifierExpression>("collection").Single().GetILVariable() == collection;
 		}
 
-		bool MatchForeachOnMultiDimArray(IL.ILVariable[] upperBounds, IL.ILVariable collection, Statement firstInitializerStatement, out IdentifierExpression foreachVariable, out IList<Statement> statements, out IL.ILVariable[] lowerBounds)
+		bool MatchForeachOnMultiDimArray(IL.ILVariable[] upperBounds, IL.ILVariable collection, Statement firstInitializerStatement, [NotNullWhen(true)] out IdentifierExpression? foreachVariable, [NotNullWhen(true)] out IList<Statement>? statements, out IL.ILVariable[] lowerBounds)
 		{
 			int i = 0;
 			foreachVariable = null;
@@ -452,7 +458,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			lowerBounds = new IL.ILVariable[upperBounds.Length];
 			Statement stmt = firstInitializerStatement;
 			Match m = default(Match);
-			while (i < upperBounds.Length && MatchLowerBound(i, out IL.ILVariable indexVariable, collection, stmt))
+			while (i < upperBounds.Length && MatchLowerBound(i, out var indexVariable, collection, stmt))
 			{
 				m = forOnArrayMultiDimPattern.Match(stmt.GetNextStatement());
 				if (!m.Success)
@@ -477,14 +483,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		Statement TransformForeachOnMultiDimArray(ExpressionStatement expressionStatement)
+		Statement? TransformForeachOnMultiDimArray(ExpressionStatement expressionStatement)
 		{
 			if (!context.Settings.ForEachStatement)
 				return null;
 			Match m;
-			Statement stmt = expressionStatement;
+			Statement? stmt = expressionStatement;
-			IL.ILVariable collection = null;
+			IL.ILVariable? collection = null;
-			IL.ILVariable[] upperBounds = null;
+			IL.ILVariable[]? upperBounds = null;
 			List<Statement> statementsToDelete = new List<Statement>();
 			int i = 0;
 			// first we look for all the upper bound initializations
@@ -512,9 +518,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				upperBounds[i] = m.Get<IdentifierExpression>("variable").Single().GetILVariable();
 				stmt = stmt.GetNextStatement();
 				i++;
-			} while (stmt != null && i < upperBounds.Length);
+			} while (stmt != null && upperBounds != null && i < upperBounds.Length);
 
-			if (upperBounds?.LastOrDefault() == null || collection == null)
+			if (upperBounds?.LastOrDefault() == null || collection == null || stmt == null)
 				return null;
 			if (!MatchForeachOnMultiDimArray(upperBounds, collection, stmt, out var foreachVariable, out var statements, out var lowerBounds))
 				return null;
@@ -609,12 +615,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		PropertyDeclaration TransformAutomaticProperty(PropertyDeclaration propertyDeclaration)
+		PropertyDeclaration? TransformAutomaticProperty(PropertyDeclaration propertyDeclaration)
 		{
-			IProperty property = propertyDeclaration.GetSymbol() as IProperty;
+			IProperty? property = propertyDeclaration.GetSymbol() as IProperty;
-			if (!CanTransformToAutomaticProperty(property, !property.DeclaringTypeDefinition.Fields.Any(f => f.Name == "_" + property.Name && f.IsCompilerGenerated())))
+			if (property == null)
+				return null;
+			if (!CanTransformToAutomaticProperty(property, !(property.DeclaringTypeDefinition?.Fields.Any(f => f.Name == "_" + property.Name && f.IsCompilerGenerated()) ?? false)))
 				return null;
-			IField field = null;
+			IField? field = null;
 			Match m = automaticPropertyPattern.Match(propertyDeclaration);
 			if (m.Success)
 			{
@@ -636,8 +644,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				RemoveCompilerGeneratedAttribute(propertyDeclaration.Getter.Attributes);
 				RemoveCompilerGeneratedAttribute(propertyDeclaration.Setter.Attributes);
-				propertyDeclaration.Getter.Body = null;
+				// Clearing the accessor body turns it into an auto-property accessor; the slot setter
-				propertyDeclaration.Setter.Body = null;
+				// tolerates null until optional slots become nullable in the AST declarations.
+				propertyDeclaration.Getter.Body = null!;
+				propertyDeclaration.Setter.Body = null!;
 				propertyDeclaration.Modifiers &= ~Modifiers.Readonly;
 				propertyDeclaration.Getter.Modifiers &= ~Modifiers.Readonly;
 
@@ -703,14 +713,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return base.VisitIdentifier(identifier);
 		}
 
-		internal static bool IsBackingFieldOfAutomaticProperty(IField field, out IProperty property)
+		internal static bool IsBackingFieldOfAutomaticProperty(IField field, [NotNullWhen(true)] out IProperty? property)
 		{
 			property = null;
-			if (!NameCouldBeBackingFieldOfAutomaticProperty(field.Name, out string propertyName))
+			if (!NameCouldBeBackingFieldOfAutomaticProperty(field.Name, out var propertyName))
 				return false;
 			if (!field.IsCompilerGenerated())
 				return false;
-			property = field.DeclaringTypeDefinition
+			property = field.DeclaringTypeDefinition?
 				.GetProperties(p => p.Name == propertyName, GetMemberOptions.IgnoreInheritedMembers)
 				.FirstOrDefault();
 			return property != null;
@@ -726,7 +736,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		static readonly System.Text.RegularExpressions.Regex automaticPropertyBackingFieldNameRegex
 			= new System.Text.RegularExpressions.Regex(@"^(<(?<name>.+)>k__BackingField|_(?<name>.+))$");
 
-		static bool NameCouldBeBackingFieldOfAutomaticProperty(string name, out string propertyName)
+		static bool NameCouldBeBackingFieldOfAutomaticProperty(string name, [NotNullWhen(true)] out string? propertyName)
 		{
 			propertyName = null;
 			var m = automaticPropertyBackingFieldNameRegex.Match(name);
@@ -736,16 +746,17 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		Identifier ReplaceBackingFieldUsage(Identifier identifier)
+		Identifier? ReplaceBackingFieldUsage(Identifier identifier)
 		{
 			if (NameCouldBeBackingFieldOfAutomaticProperty(identifier.Name, out _))
 			{
 				var parent = identifier.Parent;
+				if (parent == null)
+					return null;
 				var mrr = parent.Annotation<MemberResolveResult>();
-				var field = mrr?.Member as IField;
+				if (mrr?.Member is IField field && IsBackingFieldOfAutomaticProperty(field, out var property)
-				if (field != null && IsBackingFieldOfAutomaticProperty(field, out var property)
 					&& CanTransformToAutomaticProperty(property, !(field.IsCompilerGenerated() && field.Name == "_" + property.Name))
-					&& currentMethod.AccessorOwner != property)
+					&& currentMethod?.AccessorOwner != property)
 				{
 					if (!property.CanSet && !context.Settings.GetterOnlyAutomaticProperties)
 						return null;
@@ -757,9 +768,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return null;
 		}
 
-		Identifier ReplaceEventFieldAnnotation(Identifier identifier)
+		Identifier? ReplaceEventFieldAnnotation(Identifier identifier)
 		{
 			var parent = identifier.Parent;
+			if (parent == null)
+				return null;
 			var mrr = parent.Annotation<MemberResolveResult>();
 			if (mrr?.Member is not IField field || field.Accessibility != Accessibility.Private)
 				return null;
@@ -769,7 +782,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (module.MetadataFile.PropertyAndEventBackingFieldLookup.IsEventBackingField((FieldDefinitionHandle)field.MetadataToken, out var eventHandle))
 			{
 				var eventDef = module.ResolveEntity(eventHandle) as IEvent;
-				if (eventDef != null && currentMethod.AccessorOwner != eventDef)
+				if (eventDef != null && currentMethod?.AccessorOwner != eventDef)
 				{
 					parent.RemoveAnnotations<MemberResolveResult>();
 					parent.AddAnnotation(new MemberResolveResult(mrr.TargetResult, eventDef));
@@ -914,7 +927,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (!m.Success)
 				return false;
 			Expression fieldExpression = m.Get<Expression>("field").Single();
-			IField eventField = fieldExpression.GetSymbol() as IField;
+			IField? eventField = fieldExpression.GetSymbol() as IField;
 			if (eventField == null)
 				return false;
 			var module = eventField.ParentModule as MetadataModule;
@@ -990,7 +1003,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		EventDeclaration TransformAutomaticEvents(CustomEventDeclaration ev)
+		EventDeclaration? TransformAutomaticEvents(CustomEventDeclaration ev)
 		{
 			if (!ev.PrivateImplementationType.IsNull)
 				return null;
@@ -1065,7 +1078,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			Body = destructorBodyPattern
 		};
 
-		DestructorDeclaration TransformDestructor(MethodDeclaration methodDef)
+		DestructorDeclaration? TransformDestructor(MethodDeclaration methodDef)
 		{
 			Match m = destructorPattern.Match(methodDef);
 			if (m.Success)
@@ -1082,7 +1095,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return null;
 		}
 
-		DestructorDeclaration TransformDestructorBody(DestructorDeclaration dtorDef)
+		DestructorDeclaration? TransformDestructorBody(DestructorDeclaration dtorDef)
 		{
 			Match m = destructorBodyPattern.Match(dtorDef.Body);
 			if (m.Success)
@@ -1109,7 +1122,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		/// Simplify nested 'try { try {} catch {} } finally {}'.
 		/// This transformation must run after the using/lock tranformations.
 		/// </summary>
-		TryCatchStatement TransformTryCatchFinally(TryCatchStatement tryFinally)
+		TryCatchStatement? TransformTryCatchFinally(TryCatchStatement tryFinally)
 		{
 			if (tryCatchFinallyPattern.IsMatch(tryFinally))
 			{
@@ -1140,7 +1153,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		};
 
-		AstNode SimplifyCascadingIfElseStatements(IfElseStatement node)
+		AstNode? SimplifyCascadingIfElseStatements(IfElseStatement node)
 		{
 			Match m = cascadingIfElsePattern.Match(node);
 			if (m.Success)

ICSharpCode.Decompiler/CSharp/Transforms/PrettifyAssignments.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.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
@@ -39,6 +42,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	/// </remarks>
 	class PrettifyAssignments : DepthFirstAstVisitor, IAstTransform
 	{
+		[AllowNull]
 		TransformContext context;
 
 		public override void VisitAssignmentExpression(AssignmentExpression assignment)
@@ -48,7 +52,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			// Also supports "x = (T)(x op y)" -> "x op= y", if x.GetType() == T
 			// and y is implicitly convertible to T.
 			Expression rhs = assignment.Right;
-			IType expectedType = null;
+			IType? expectedType = null;
 			if (assignment.Right is CastExpression { Type: var astType } cast)
 			{
 				rhs = cast.Expression;
@@ -89,7 +93,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				}
 			}
 
-			bool IsImplicitlyConvertible(Expression rhs, IType expectedType)
+			bool IsImplicitlyConvertible(Expression rhs, IType? expectedType)
 			{
 				if (expectedType == null)
 					return true;
@@ -132,13 +136,13 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		static bool CanConvertToCompoundAssignment(Expression left)
 		{
-			MemberReferenceExpression mre = left as MemberReferenceExpression;
+			MemberReferenceExpression? mre = left as MemberReferenceExpression;
 			if (mre != null)
 				return IsWithoutSideEffects(mre.Target);
-			IndexerExpression ie = left as IndexerExpression;
+			IndexerExpression? ie = left as IndexerExpression;
 			if (ie != null)
 				return IsWithoutSideEffects(ie.Target) && ie.Arguments.All(IsWithoutSideEffects);
-			UnaryOperatorExpression uoe = left as UnaryOperatorExpression;
+			UnaryOperatorExpression? uoe = left as UnaryOperatorExpression;
 			if (uoe != null && uoe.Operator == UnaryOperatorType.Dereference)
 				return IsWithoutSideEffects(uoe.Expression);
 			return IsWithoutSideEffects(left);

ICSharpCode.Decompiler/CSharp/Transforms/RemoveCLSCompliantAttribute.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.Linq;

ICSharpCode.Decompiler/CSharp/Transforms/ReplaceMethodCallsWithOperators.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.Linq;
 using System.Reflection;
 
@@ -41,6 +44,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			MemberName = "TypeHandle"
 		};
 
+		[AllowNull]
 		TransformContext context;
 
 		public override void VisitInvocationExpression(InvocationExpression invocationExpression)
@@ -508,7 +512,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			Match m = getMethodOrConstructorFromHandlePattern.Match(castExpression);
 			if (m.Success)
 			{
-				IMethod method = m.Get<AstNode>("method").Single().GetSymbol() as IMethod;
+				IMethod? method = m.Get<AstNode>("method").Single().GetSymbol() as IMethod;
 				if (m.Has("declaringType") && method != null)
 				{
 					Expression newNode = new MemberReferenceExpression(new TypeReferenceExpression(m.Get<AstType>("declaringType").Single().Detach()), method.Name);

ICSharpCode.Decompiler/CSharp/Transforms/TransformContext.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.Immutable;
 using System.Threading;
 
@@ -40,17 +42,17 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		/// <summary>
 		/// Returns the current member; or null if a whole type or module is being decompiled.
 		/// </summary>
-		public IMember CurrentMember => decompilationContext.CurrentMember;
+		public IMember? CurrentMember => decompilationContext.CurrentMember;
 
 		/// <summary>
 		/// Returns the current type definition; or null if a module is being decompiled.
 		/// </summary>
-		public ITypeDefinition CurrentTypeDefinition => decompilationContext.CurrentTypeDefinition;
+		public ITypeDefinition? CurrentTypeDefinition => decompilationContext.CurrentTypeDefinition;
 
 		/// <summary>
 		/// Returns the module that is being decompiled.
 		/// </summary>
-		public IModule CurrentModule => decompilationContext.CurrentModule;
+		public IModule? CurrentModule => decompilationContext.CurrentModule;
 
 		/// <summary>
 		/// Returns the max possible set of namespaces that will be used during decompilation.