Add fine-grained C# AST debug steps

Record AST transform groups and mutation steps through the C# pipeline, replay selected steps with the stepper, and carry modified-node ranges through output so the Debug Steps pane can highlight the selected mutation without replacing its full step tree.

Assisted-by: CodeAlta:gpt-5.5:CodeAlta

ICSharpCode.Decompiler.Tests/TestCases/Pretty/OutVariables.cs

@@ -81,5 +81,16 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			func();
 			func2();
 		}
+
+		public static void CapturedBoolResult(Dictionary<int, int> d, int key)
+		{
+			// The boolean result of the out-returning call is captured into a local, yet the out
+			// parameter is still promoted to an inline 'out var' rather than a separate declaration.
+			bool value = d.TryGetValue(key, out var value2);
+			Console.WriteLine(value);
+			Console.WriteLine(value);
+			Console.WriteLine(value2);
+			Console.WriteLine(value2);
+		}
 	}
 }

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -176,6 +176,8 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		List<IAstTransform> astTransforms = GetAstTransforms();
 
+		public Stepper Stepper { get; set; } = new Stepper();
+
 		/// <summary>
 		/// Returns all built-in transforms of the C# AST pipeline.
 		/// </summary>
@@ -714,17 +716,27 @@ namespace ICSharpCode.Decompiler.CSharp
 		void RunTransforms(AstNode rootNode, DecompileRun decompileRun, ITypeResolveContext decompilationContext)
 		{
 			var typeSystemAstBuilder = CreateAstBuilder(decompileRun.Settings);
-			var context = new TransformContext(typeSystem, decompileRun, decompilationContext, typeSystemAstBuilder);
+			var context = new TransformContext(typeSystem, decompileRun, decompilationContext, typeSystemAstBuilder) {
+				Stepper = Stepper
+			};
 			// The tree handed to the pipeline must already be well-formed; check it once up front so a
 			// malformed builder output is caught here rather than blamed on the first transform (DEBUG only).
 			rootNode.CheckInvariant();
-			foreach (var transform in astTransforms)
+			try
+			{
+				foreach (var transform in astTransforms)
+				{
+					CancellationToken.ThrowIfCancellationRequested();
+					context.StepStartGroup(transform.GetType().Name);
+					transform.Run(rootNode, context);
+					// Verify the slot structure survived the transform (DEBUG only); mirrors the IL
+					// pipeline's per-transform ILInstruction.CheckInvariant.
+					rootNode.CheckInvariant();
+					context.StepEndGroup(keepIfEmpty: true);
+				}
+			}
+			catch (StepLimitReachedException)
 			{
-				CancellationToken.ThrowIfCancellationRequested();
-				transform.Run(rootNode, context);
-				// Verify the slot structure survived the transform (DEBUG only); mirrors the IL
-				// pipeline's per-transform ILInstruction.CheckInvariant.
-				rootNode.CheckInvariant();
 			}
 			CancellationToken.ThrowIfCancellationRequested();
 			rootNode.AcceptVisitor(new InsertParenthesesVisitor { InsertParenthesesForReadability = true });

ICSharpCode.Decompiler/CSharp/Transforms/AddCheckedBlocks.cs

@@ -155,7 +155,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return new InsertedNodeList(a, b);
 			}
 
-			public abstract void Insert();
+			public abstract void Insert(TransformContext context);
 		}
 
 		class InsertedNodeList : InsertedNode
@@ -168,10 +168,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				this.child2 = child2;
 			}
 
-			public override void Insert()
+			public override void Insert(TransformContext context)
 			{
-				child1.Insert();
+				child1.Insert(context);
-				child2.Insert();
+				child2.Insert(context);
 			}
 		}
 
@@ -186,12 +186,15 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				this.isChecked = isChecked;
 			}
 
-			public override void Insert()
+			public override void Insert(TransformContext context)
 			{
+				context.Step(isChecked ? "Add checked expression" : "Add unchecked expression", expression);
+				Expression? replacement;
 				if (isChecked)
-					expression.ReplaceWith(e => new CheckedExpression { Expression = e });
+					replacement = expression.ReplaceWith(e => new CheckedExpression { Expression = e });
 				else
-					expression.ReplaceWith(e => new UncheckedExpression { Expression = e });
+					replacement = expression.ReplaceWith(e => new UncheckedExpression { Expression = e });
+				context.EndStep(replacement);
 			}
 		}
 
@@ -208,11 +211,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				this.isChecked = isChecked;
 			}
 
-			public override void Insert()
+			public override void Insert(TransformContext context)
 			{
 				// 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);
+				context.Step(isChecked ? "Add checked block" : "Add unchecked block", firstStatement);
 				BlockStatement newBlock = new BlockStatement();
 				// Move all statements except for the first
 				Statement? next;
@@ -222,12 +226,13 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					newBlock.Add(stmt.Detach());
 				}
 				// Replace the first statement with the new (un)checked block
-				if (isChecked)
+				Statement checkedBlock = isChecked
-					firstStatement.ReplaceWith(new CheckedStatement { Body = newBlock });
+					? new CheckedStatement { Body = newBlock }
-				else
+					: new UncheckedStatement { Body = newBlock };
-					firstStatement.ReplaceWith(new UncheckedStatement { Body = newBlock });
+				firstStatement.ReplaceWith(checkedBlock);
 				// now also move the first node into the new block
 				newBlock.Statements.InsertAfter(null, firstStatement);
+				context.EndStep(checkedBlock);
 			}
 		}
 		#endregion
@@ -260,11 +265,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				Result r = GetResultFromBlock(block);
 				if (context.DecompileRun.Settings.CheckForOverflowUnderflow)
 				{
-					r.NodesToInsertInCheckedContext?.Insert();
+					r.NodesToInsertInCheckedContext?.Insert(context);
 				}
 				else
 				{
-					r.NodesToInsertInUncheckedContext?.Insert();
+					r.NodesToInsertInUncheckedContext?.Insert(context);
 				}
 			}
 		}

ICSharpCode.Decompiler/CSharp/Transforms/AddXmlDocumentationTransform.cs

@@ -49,6 +49,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					string doc = provider.GetDocumentation(entity);
 					if (doc != null)
 					{
+						context.Step("Add XML documentation", entityDecl);
 						InsertXmlDocumentation(entityDecl, new StringReader(doc));
 					}
 				}

ICSharpCode.Decompiler/CSharp/Transforms/CombineQueryExpressions.cs

@@ -19,6 +19,7 @@
 #nullable enable
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
@@ -32,11 +33,21 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	/// </summary>
 	public class CombineQueryExpressions : IAstTransform
 	{
+		[AllowNull] TransformContext context;
+
 		public void Run(AstNode rootNode, TransformContext context)
 		{
 			if (!context.Settings.QueryExpressions)
 				return;
-			CombineQueries(rootNode, new Dictionary<string, object?>());
+			this.context = context;
+			try
+			{
+				CombineQueries(rootNode, new Dictionary<string, object?>());
+			}
+			finally
+			{
+				this.context = null;
+			}
 		}
 
 		static readonly InvocationExpression castPattern = new InvocationExpression {
@@ -68,10 +79,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					else
 					{
 						QueryContinuationClause continuation = new QueryContinuationClause();
+						context.Step("Introduce query continuation", fromClause);
 						continuation.PrecedingQuery = innerQuery.Detach();
 						continuation.Identifier = fromClause.Identifier;
 						continuation.CopyAnnotationsFrom(fromClause);
 						fromClause.ReplaceWith(continuation);
+						context.EndStep(continuation);
 					}
 				}
 				else
@@ -79,6 +92,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					Match m = castPattern.Match(fromClause.Expression);
 					if (m.Success)
 					{
+						context.Step("Move Cast type into from clause", fromClause);
 						fromClause.Type = m.Get<AstType>("targetType").Single().Detach();
 						fromClause.Expression = m.Get<Expression>("inExpr").Single().Detach();
 					}
@@ -117,6 +131,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			// from * in (from x in ... select new { members of anonymous type }) ...
 			// =>
 			// from x in ... { let x = ... } ...
+			context.Step("Remove transparent query identifier", fromClause);
 			fromClause.Remove();
 			selectClause.Remove();
 			// Move clauses from innerQuery to query
@@ -125,6 +140,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				query.Clauses.InsertAfter(insertionPos, insertionPos = clause.Detach());
 			}
+			context.EndStep(query.Clauses.First());
 
 			foreach (var expr in match.Get<Expression>("expr"))
 			{
@@ -176,7 +192,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				newIdent.RemoveAnnotations<Semantics.MemberResolveResult>(); // remove the reference to the property of the anonymous type
 				if (fromOrLetIdentifiers.TryGetValue(mre.MemberName, out var annotation) && annotation != null)
 					newIdent.AddAnnotation(annotation);
+				context.Step("Replace transparent query identifier reference", mre);
 				mre.ReplaceWith(newIdent);
+				context.EndStep(newIdent);
 				return;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -213,6 +213,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				if (stmt.Expression is DirectionExpression dir && IsValidInStatementExpression(dir.Expression))
 				{
+					context.Step("Unwrap direction expression statement", stmt);
 					stmt.Expression = dir.Expression.Detach();
 				}
 				else if (!IsValidInStatementExpression(stmt.Expression))
@@ -222,12 +223,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					// if possible use C# 7.0 discard-assignment
 					if (context.Settings.Discards && !ExpressionBuilder.HidesVariableWithName(function, "_"))
 					{
+						context.Step("Assign invalid expression statement to discard", stmt);
 						stmt.Expression = new AssignmentExpression(
 							new IdentifierExpression("_"), // no ResolveResult
 							stmt.Expression.Detach());
 					}
 					else
 					{
+						context.Step("Assign invalid expression statement to temporary", stmt);
 						// assign result to dummy variable
 						var type = stmt.Expression.GetResolveResult().Type;
 						var v = function.RegisterVariable(
@@ -542,7 +545,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					continue;
 
 				var designation = StatementBuilder.TranslateDeconstructionDesignation(deconstruct, isForeach: false);
-				left.ReplaceWith(new DeclarationExpression { Type = new SimpleType("var"), Designation = designation });
+				context.Step("Declare deconstruction variables", left);
+				var declarationExpression = new DeclarationExpression { Type = new SimpleType("var"), Designation = designation };
+				left.ReplaceWith(declarationExpression);
+				context.EndStep(declarationExpression);
 
 				foreach (var v in usedVariables)
 				{
@@ -595,7 +601,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		void InsertVariableDeclarations(TransformContext context)
 		{
-			var replacements = new List<(AstNode, AstNode)>();
+			var replacements = new List<(AstNode OldNode, Func<AstNode> CreateNewNode, string StepDescription)>();
 			foreach (var (ilVariable, v) in variableDict)
 			{
 				if (v.RemovedDueToCollision || v.DeclaredInDeconstruction)
@@ -621,17 +627,19 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						type.AddTrailingTrivia(new Comment("pinned", CommentType.MultiLine));
 					}
-					var vds = new VariableDeclarationStatement(type, v.Name, assignment.Right.Detach());
+					replacements.Add((v.InsertionPoint.nextNode, () => {
-					var init = vds.Variables.Single();
+						var vds = new VariableDeclarationStatement(type, v.Name, assignment.Right.Detach());
-					init.AddAnnotation(assignment.Left.GetResolveResult());
+						var init = vds.Variables.Single();
-					foreach (object annotation in assignment.Left.Annotations.Concat(assignment.Annotations))
+						init.AddAnnotation(assignment.Left.GetResolveResult());
-					{
+						foreach (object annotation in assignment.Left.Annotations.Concat(assignment.Annotations))
-						if (!(annotation is ResolveResult))
 						{
-							init.AddAnnotation(annotation);
+							if (!(annotation is ResolveResult))
+							{
+								init.AddAnnotation(annotation);
+							}
 						}
-					}
+						return vds;
-					replacements.Add((v.InsertionPoint.nextNode, vds));
+					}, "Combine variable declaration with initializer"));
 				}
 				else if (CanBeDeclaredAsOutVariable(v, out var dirExpr))
 				{
@@ -675,7 +683,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						ovd.RemoveAnnotations<ResolveResult>();
 						ovd.AddAnnotation(new OutVarResolveResult(v.Type));
 					}
-					replacements.Add((dirExpr, ovd));
+					replacements.Add((dirExpr, () => ovd, "Declare out variable"));
 				}
 				else
 				{
@@ -688,6 +696,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					}
 					var vds = new VariableDeclarationStatement(type, v.Name, initializer);
 					vds.Variables.Single().AddAnnotation(new ILVariableResolveResult(ilVariable));
+					context.Step("Insert variable declaration", v.InsertionPoint.nextNode);
 					if (v.InsertionPoint.nextNode.Parent is LambdaExpression lambda)
 					{
 						Debug.Assert(lambda.Body is not BlockStatement);
@@ -708,24 +717,27 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						AstType unsafeType = context.TypeSystemAstBuilder.ConvertType(
 							context.TypeSystem.FindType(KnownTypeCode.Unsafe));
+						AstNode insertedNode;
 						if (context.Settings.OutVariables)
 						{
 							var outVarDecl = new OutVarDeclarationExpression(type.Clone(), v.Name);
 							outVarDecl.Variable.AddAnnotation(new ILVariableResolveResult(ilVariable));
+							var skipInitStatement = new ExpressionStatement {
+								Expression = new InvocationExpression {
+									Target = new MemberReferenceExpression {
+										Target = new TypeReferenceExpression(unsafeType),
+										MemberName = "SkipInit"
+									},
+									Arguments = {
+										outVarDecl
+									}
+								}
+							};
 							insertionParent.InsertChildBefore(
 								v.InsertionPoint.nextNode,
-								new ExpressionStatement {
+								skipInitStatement,
-									Expression = new InvocationExpression {
-										Target = new MemberReferenceExpression {
-											Target = new TypeReferenceExpression(unsafeType),
-											MemberName = "SkipInit"
-										},
-										Arguments = {
-											outVarDecl
-										}
-									}
-								},
 								Slots.Statement);
+							insertedNode = skipInitStatement;
 						}
 						else
 						{
@@ -733,25 +745,28 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 								v.InsertionPoint.nextNode,
 								vds,
 								Slots.Statement);
+							insertedNode = vds;
+							var skipInitStatement = new ExpressionStatement {
+								Expression = new InvocationExpression {
+									Target = new MemberReferenceExpression {
+										Target = new TypeReferenceExpression(unsafeType),
+										MemberName = "SkipInit"
+									},
+									Arguments = {
+										new DirectionExpression(
+											FieldDirection.Out,
+											new IdentifierExpression(v.Name)
+												.WithRR(new ILVariableResolveResult(ilVariable))
+										)
+									}
+								}
+							};
 							insertionParent.InsertChildBefore(
 								v.InsertionPoint.nextNode,
-								new ExpressionStatement {
+								skipInitStatement,
-									Expression = new InvocationExpression {
-										Target = new MemberReferenceExpression {
-											Target = new TypeReferenceExpression(unsafeType),
-											MemberName = "SkipInit"
-										},
-										Arguments = {
-											new DirectionExpression(
-												FieldDirection.Out,
-												new IdentifierExpression(v.Name)
-													.WithRR(new ILVariableResolveResult(ilVariable))
-											)
-										}
-									}
-								},
 								Slots.Statement);
 						}
+						context.EndStep(insertedNode);
 					}
 					else
 					{
@@ -759,13 +774,17 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 							v.InsertionPoint.nextNode,
 							vds,
 							Slots.Statement);
+						context.EndStep(vds);
 					}
 				}
 			}
 			// perform replacements at end, so that we don't replace a node while it is still referenced by a VariableToDeclare
-			foreach (var (oldNode, newNode) in replacements)
+			foreach (var (oldNode, createNewNode, stepDescription) in replacements)
 			{
+				context.Step(stepDescription, oldNode);
+				var newNode = createNewNode();
 				oldNode.ReplaceWith(newNode);
+				context.EndStep(newNode);
 			}
 		}
 

ICSharpCode.Decompiler/CSharp/Transforms/EscapeInvalidIdentifiers.cs

@@ -56,7 +56,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			foreach (var ident in rootNode.DescendantsAndSelf.OfType<Identifier>())
 			{
-				ident.Name = ReplaceInvalid(ident.Name);
+				string newName = ReplaceInvalid(ident.Name);
+				if (newName != ident.Name)
+				{
+					context.Step($"Escape identifier '{ident.Name}'", ident);
+					ident.Name = newName;
+				}
 			}
 		}
 	}

ICSharpCode.Decompiler/CSharp/Transforms/FixNameCollisions.cs

@@ -56,6 +56,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (memberNames.Contains(oldName) && symbol is IField { Accessibility: Accessibility.Private })
 					{
 						string newName = PickNewName(memberNames, oldName);
+						context.Step($"Rename field '{oldName}' to '{newName}'", fieldDecl);
 						fieldDecl.Variables.Single().Name = newName;
 						renamedSymbols[symbol] = newName;
 					}
@@ -70,6 +71,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (symbol != null && renamedSymbols.TryGetValue(symbol, out string? newName))
 					{
 						// An IdentifierExpression / MemberReferenceExpression always carries its name identifier.
+						context.Step($"Rename field reference to '{newName}'", node);
 						node.GetChild(Slots.Identifier)!.Name = newName;
 					}
 				}

ICSharpCode.Decompiler/CSharp/Transforms/FlattenSwitchBlocks.cs

@@ -22,6 +22,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (blockStatement == null || blockStatement.Statements.Any(ContainsLocalDeclaration))
 					continue;
 
+				context.Step("Flatten switch section block", blockStatement);
 				blockStatement.Remove();
 				blockStatement.Statements.MoveTo(switchSection.Statements);
 			}

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceExtensionMethods.cs

@@ -108,10 +108,13 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return;
 			}
 			var method = (IMethod)invocationExpression.GetSymbol()!;
+			bool stepped = false;
 			if (firstArgument is DirectionExpression dirExpr)
 			{
 				if (!context.Settings.RefExtensionMethods || dirExpr.FieldDirection == FieldDirection.Out)
 					return;
+				context.Step("Introduce extension method call", invocationExpression);
+				stepped = true;
 				// A ref/out direction expression always wraps an operand.
 				firstArgument = dirExpr.Expression!;
 				target = firstArgument.GetResolveResult();
@@ -120,17 +123,23 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			else if (firstArgument is NullReferenceExpression)
 			{
 				Debug.Assert(context.RequiredNamespacesSuperset.Contains(method.Parameters[0].Type.Namespace));
+				context.Step("Introduce extension method call", invocationExpression);
+				stepped = true;
 				// 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)
 			{
+				if (!stepped)
+					context.Step("Introduce extension method call", invocationExpression);
 				identifierExpression.Detach();
 				memberRefExpr = new MemberReferenceExpression(firstArgument.Detach(), method.Name, identifierExpression.TypeArguments.Detach());
 				invocationExpression.Target = memberRefExpr;
 			}
 			else
 			{
+				if (!stepped)
+					context.Step("Introduce extension method call", invocationExpression);
 				// The target is not an IdentifierExpression, so CanTransformToExtensionMethodCall
 				// matched the MemberReferenceExpression case and memberRefExpr is non-null.
 				memberRefExpr!.Target = firstArgument.Detach();

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs

@@ -51,6 +51,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (IsDegenerateQuery(query))
 				{
 					// introduce select for degenerate query
+					context.Step("Add degenerate query select clause", query);
 					query.Clauses.Add(new QuerySelectClause { Expression = new IdentifierExpression(fromClause.Identifier).CopyAnnotationsFrom(fromClause) });
 				}
 				// See if the data source of this query is a degenerate query,
@@ -61,6 +62,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					QueryFromClause innerFromClause = (QueryFromClause)innerQuery.Clauses.First();
 					ILVariable? innerVariable = innerFromClause.Annotation<ILVariableResolveResult>()?.Variable;
 					ILVariable? rangeVariable = fromClause.Annotation<ILVariableResolveResult>()?.Variable;
+					context.Step("Combine nested query clauses", fromClause);
 					// Replace the fromClause with all clauses from the inner query
 					fromClause.Remove();
 					QueryClause? insertionPos = null;
@@ -69,6 +71,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						CombineRangeVariables(clause, innerVariable, rangeVariable);
 						query.Clauses.InsertAfter(insertionPos, insertionPos = clause.Detach());
 					}
+					context.EndStep(innerFromClause);
 					fromClause = innerFromClause;
 					innerQuery = fromClause.Expression as QueryExpression;
 				}
@@ -87,9 +90,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				var variable = parent.Annotation<ILVariableResolveResult>()?.Variable;
 				if (variable == oldVariable)
 				{
+					context.Step("Combine query range variables", identifier);
 					parent.RemoveAnnotations<ILVariableResolveResult>();
 					parent.AddAnnotation(new ILVariableResolveResult(newVariable));
-					identifier.ReplaceWith(Identifier.Create(newVariable.Name!));
+					var newIdentifier = Identifier.Create(newVariable.Name!);
+					identifier.ReplaceWith(newIdentifier);
+					context.EndStep(newIdentifier);
 				}
 			}
 		}
@@ -110,6 +116,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (node.Parent is ExpressionStatement && CanUseDiscardAssignment())
 					query = new AssignmentExpression(new IdentifierExpression("_"), query);
 				node.ReplaceWith(query);
+				context.EndStep(query);
 			}
 
 			AstNode? next;
@@ -145,6 +152,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					Expression expr = invocation.Arguments.Single();
 					if (MatchSimpleLambda(expr, out var parameter, out var body))
 					{
+						context.Step("Build select query", invocation);
 						QueryExpression query = new QueryExpression();
 						query.Clauses.Add(MakeFromClause(parameter, mre.Target.Detach()));
 						query.Clauses.Add(new QuerySelectClause { Expression = WrapExpressionInParenthesesIfNecessary(body.Detach(), parameter.Name!) }.CopyAnnotationsFrom(expr));
@@ -162,6 +170,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 							&& MatchSimpleLambda(projectionLambda, out var parameter2, out var elementSelector)
 							&& parameter1.Name == parameter2.Name)
 						{
+							context.Step("Build group query", invocation);
 							QueryExpression query = new QueryExpression();
 							query.Clauses.Add(MakeFromClause(parameter1, mre.Target.Detach()));
 							var queryGroupClause = new QueryGroupClause {
@@ -179,6 +188,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						Expression lambda = invocation.Arguments.Single();
 						if (MatchSimpleLambda(lambda, out var parameter, out var keySelector))
 						{
+							context.Step("Build group query", invocation);
 							QueryExpression query = new QueryExpression();
 							query.Clauses.Add(MakeFromClause(parameter, mre.Target.Detach()));
 							query.Clauses.Add(new QueryGroupClause { Projection = new IdentifierExpression(parameter.Name!).CopyAnnotationsFrom(parameter), Key = keySelector.Detach() });
@@ -203,6 +213,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						ParameterDeclaration p2 = lambda.Parameters.ElementAt(1);
 						if (p1.Name == parameter.Name)
 						{
+							context.Step("Build select-many query", invocation);
 							QueryExpression query = new QueryExpression();
 							query.Clauses.Add(MakeFromClause(p1, mre.Target.Detach()));
 							query.Clauses.Add(MakeFromClause(p2, collectionSelector.Detach()).CopyAnnotationsFrom(fromExpressionLambda));
@@ -221,6 +232,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					Expression expr = invocation.Arguments.Single();
 					if (MatchSimpleLambda(expr, out var parameter, out var body))
 					{
+						context.Step("Build where query", invocation);
 						QueryExpression query = new QueryExpression();
 						query.Clauses.Add(MakeFromClause(parameter, mre.Target.Detach()));
 						query.Clauses.Add(new QueryWhereClause { Condition = body.Detach() }.CopyAnnotationsFrom(expr));
@@ -242,6 +254,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						if (ValidateThenByChain(invocation, parameter.Name!))
 						{
+							context.Step("Build order query", invocation);
 							QueryOrderClause orderClause = new QueryOrderClause();
 							while (mre.MemberName == "ThenBy" || mre.MemberName == "ThenByDescending")
 							{
@@ -302,6 +315,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						if (ValidateParameter(p1) && ValidateParameter(p2)
 							&& p1.Name == element1.Name && (p2.Name == element2.Name || mre.MemberName == "GroupJoin"))
 						{
+							context.Step(mre.MemberName == "GroupJoin" ? "Build group join query" : "Build join query", invocation);
 							QueryExpression query = new QueryExpression();
 							query.Clauses.Add(MakeFromClause(element1, source1.Detach()));
 							QueryJoinClause joinClause = new QueryJoinClause();

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUnsafeModifier.cs

@@ -29,9 +29,19 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 {
 	public class IntroduceUnsafeModifier : DepthFirstAstVisitor<bool>, IAstTransform
 	{
+		TransformContext? context;
+
 		public void Run(AstNode compilationUnit, TransformContext context)
 		{
-			compilationUnit.AcceptVisitor(this);
+			this.context = context;
+			try
+			{
+				compilationUnit.AcceptVisitor(this);
+			}
+			finally
+			{
+				this.context = null;
+			}
 		}
 
 		public static bool IsUnsafe(AstNode node)
@@ -52,6 +62,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 			if (result && node is EntityDeclaration && !(node is Accessor))
 			{
+				if (context != null)
+					context.Step("Add unsafe modifier", node);
 				((EntityDeclaration)node).Modifiers |= Modifiers.Unsafe;
 				return false;
 			}
@@ -95,6 +107,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					&& bop.GetResolveResult() is OperatorResolveResult orr
 					&& orr.Operands.FirstOrDefault()?.Type.Kind == TypeKind.Pointer)
 				{
+					if (context != null)
+						context.Step("Replace pointer addition with indexer", unaryOperatorExpression);
 					// transform "*(ptr + int)" to "ptr[int]"
 					IndexerExpression indexer = new IndexerExpression();
 					indexer.Target = bop.Left!.Detach();
@@ -102,6 +116,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					indexer.CopyAnnotationsFrom(unaryOperatorExpression);
 					indexer.CopyAnnotationsFrom(bop);
 					unaryOperatorExpression.ReplaceWith(indexer);
+					if (context != null)
+						context.EndStep(indexer);
 				}
 				return true;
 			}
@@ -121,6 +137,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			UnaryOperatorExpression? uoe = memberReferenceExpression.Target as UnaryOperatorExpression;
 			if (uoe != null && uoe.Operator == UnaryOperatorType.Dereference)
 			{
+				if (context != null)
+					context.Step("Replace pointer member access", memberReferenceExpression);
 				PointerReferenceExpression pre = new PointerReferenceExpression();
 				pre.Target = uoe.Expression.Detach();
 				pre.MemberName = memberReferenceExpression.MemberName;
@@ -129,6 +147,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				pre.RemoveAnnotations<ResolveResult>(); // only copy the ResolveResult from the MRE
 				pre.CopyAnnotationsFrom(memberReferenceExpression);
 				memberReferenceExpression.ReplaceWith(pre);
+				if (context != null)
+					context.EndStep(pre);
 			}
 			if (HasUnsafeResolveResult(memberReferenceExpression))
 				return true;

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUsingDeclarations.cs

@@ -26,6 +26,9 @@ using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
 using ICSharpCode.Decompiler.CSharp.Syntax;
+#if STEP
+using ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching;
+#endif
 using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.IL;
 using ICSharpCode.Decompiler.Semantics;
@@ -73,7 +76,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						resolvedNamespaces.Add(resolvedNamespace);
 					}
-					rootNode.InsertChildAfter(insertionPoint, new UsingDeclaration { Import = nsType }, Slots.Member);
+					context.Step("Add using declaration", rootNode);
+					var node = new UsingDeclaration { Import = nsType };
+					rootNode.InsertChildAfter(insertionPoint, node, Slots.Member);
+					context.EndStep(node);
 				}
 			}
 
@@ -189,6 +195,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			readonly bool ignoreUsingScope;
 			readonly DecompilerSettings settings;
+			readonly TransformContext context;
 
 			CSharpResolver resolver;
 			TypeSystemAstBuilder astBuilder;
@@ -197,6 +204,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 			public FullyQualifyAmbiguousTypeNamesVisitor(TransformContext context, UsingScope usingScope)
 			{
+				this.context = context;
 				this.ignoreUsingScope = !context.Settings.UsingDeclarations;
 				this.settings = context.Settings;
 				this.resolver = new CSharpResolver(new CSharpTypeResolveContext(context.TypeSystem.MainModule));
@@ -401,13 +409,31 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				}
 				if (simpleType.Parent is Syntax.Attribute)
 				{
-					simpleType.ReplaceWith(astBuilder.ConvertAttributeType(rr.Type));
+					ReplaceAndRecordStep("Qualify ambiguous attribute type", simpleType, astBuilder.ConvertAttributeType(rr.Type));
 				}
 				else
 				{
-					simpleType.ReplaceWith(astBuilder.ConvertType(rr.Type));
+					ReplaceAndRecordStep("Qualify ambiguous type", simpleType, astBuilder.ConvertType(rr.Type));
 				}
 			}
+
+			void ReplaceAndRecordStep(string stepDescription, SimpleType simpleType, AstType replacement)
+			{
+#if STEP
+				// Record a debug step only when the converted type actually differs from the
+				// original, so the step list stays meaningful. This structural comparison is
+				// debug-only (it allocates a pattern Match); release builds skip it entirely and
+				// just perform the replacement below.
+				bool changed = !simpleType.IsMatch(replacement);
+				if (changed)
+					context.Step(stepDescription, simpleType);
+#endif
+				simpleType.ReplaceWith(replacement);
+#if STEP
+				if (changed)
+					context.EndStep(replacement);
+#endif
+			}
 		}
 	}
 }

ICSharpCode.Decompiler/CSharp/Transforms/NormalizeBlockStatements.cs

@@ -26,6 +26,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			base.VisitSyntaxTree(syntaxTree);
 			if (context.Settings.FileScopedNamespaces && singleNamespaceDeclaration != null)
 			{
+				context.Step("Use file-scoped namespace", singleNamespaceDeclaration);
 				singleNamespaceDeclaration.IsFileScoped = true;
 			}
 		}
@@ -106,7 +107,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				if (statement is BlockStatement b && b.Statements.Count == 1 && IsAllowedAsEmbeddedStatement(b.Statements.First(), parent))
 				{
-					statement.ReplaceWith(b.Statements.First().Detach());
+					context.Step("Remove redundant block statement", statement);
+					var innerStatement = b.Statements.First().Detach();
+					statement.ReplaceWith(innerStatement);
+					context.EndStep(innerStatement);
 				}
 				else if (!IsAllowedAsEmbeddedStatement(statement, parent))
 				{
@@ -120,11 +124,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return parent is IfElseStatement && statement.Slot?.Kind == Slots.FalseStatement;
 		}
 
-		static void InsertBlock(Statement statement)
+		void InsertBlock(Statement statement)
 		{
 			if (!(statement is BlockStatement))
 			{
 				var b = new BlockStatement();
+				context.Step("Add block statement", statement);
 				statement.ReplaceWith(b);
 				if (statement is EmptyStatement && !statement.HasChildren)
 				{
@@ -134,6 +139,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				{
 					b.Add(statement);
 				}
+				context.EndStep(b);
 			}
 		}
 
@@ -221,6 +227,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return;
 			if ((getter.Modifiers & ~movableModifiers) != 0)
 				return;
+			context.Step("Use expression-bodied property", propertyDeclaration);
 			propertyDeclaration.Modifiers |= getter.Modifiers;
 			propertyDeclaration.ExpressionBody = m.Get<Expression>("expression").Single().Detach();
 			propertyDeclaration.CopyAnnotationsFrom(getter);
@@ -236,6 +243,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return;
 			if ((getter.Modifiers & ~movableModifiers) != 0)
 				return;
+			context.Step("Use expression-bodied indexer", indexerDeclaration);
 			indexerDeclaration.Modifiers |= getter.Modifiers;
 			indexerDeclaration.ExpressionBody = m.Get<Expression>("expression").Single().Detach();
 			indexerDeclaration.CopyAnnotationsFrom(getter);

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -191,6 +191,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return null;
 			if (next is ForStatement forStatement && ForStatementUsesVariable(forStatement, variable))
 			{
+				context.Step("Move declaration into for initializer", node);
 				node.Remove();
 				next.InsertChildAfter(null, node, Slots.ForInitializer);
 				return (ForStatement)next;
@@ -212,6 +213,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			// Whereas continue in for jumps to the increment block.
 			if (loop.DescendantNodes(DescendIntoStatement).OfType<Statement>().Any(s => s is ContinueStatement))
 				return null;
+			context.Step("Transform while loop to for", loop);
 			node.Remove();
 			BlockStatement newBody = new BlockStatement();
 			foreach (Statement stmt in m3.Get<Statement>("statement"))
@@ -223,6 +225,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			forStatement.Iterators.Add(iteratorStatement.Detach());
 			forStatement.EmbeddedStatement = newBody;
 			loop.ReplaceWith(forStatement);
+			context.EndStep(forStatement);
 			return forStatement;
 		}
 
@@ -363,6 +366,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return null;
 			if (indexVariable.StoreCount != 2 || indexVariable.LoadCount != 3 || indexVariable.AddressCount != 0)
 				return null;
+			context.Step("Introduce foreach over array", forStatement);
 			var body = new BlockStatement();
 			foreach (var statement in m.Get<Statement>("statements"))
 				body.Statements.Add(statement.Detach());
@@ -378,6 +382,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			foreachStmt.VariableDesignation.AddAnnotation(new ILVariableResolveResult(itemVariable, itemVariable.Type));
 			// TODO : add ForeachAnnotation
 			forStatement.ReplaceWith(foreachStmt);
+			context.EndStep(foreachStmt);
 			return foreachStmt;
 		}
 
@@ -533,6 +538,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				|| !upperBounds.All(ub => ub.IsSingleDefinition && ub.LoadCount == 1)
 				|| !lowerBounds.All(lb => lb.StoreCount == 2 && lb.LoadCount == 3 && lb.AddressCount == 0))
 				return null;
+			context.Step("Introduce foreach over multidimensional array", expressionStatement);
 			var body = new BlockStatement();
 			foreach (var statement in statements)
 				body.Statements.Add(statement.Detach());
@@ -550,6 +556,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			foreachStmt.VariableDesignation.AddAnnotation(new ILVariableResolveResult(itemVariable, itemVariable.Type));
 			// TODO : add ForeachAnnotation
 			expressionStatement.ReplaceWith(foreachStmt);
+			context.EndStep(foreachStmt);
 			return foreachStmt;
 		}
 
@@ -643,6 +650,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return null;
 			if (field.IsCompilerGenerated() && field.DeclaringTypeDefinition == property.DeclaringTypeDefinition)
 			{
+				context.Step("Convert property to auto-property", propertyDeclaration);
 				// Clearing the accessor body turns it into an auto-property accessor.
 				var getter = propertyDeclaration.Getter;
 				var setter = propertyDeclaration.Setter;
@@ -710,6 +718,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (newIdentifier != null)
 				{
 					identifier.ReplaceWith(newIdentifier);
+					context.EndStep(newIdentifier);
 					return newIdentifier;
 				}
 			}
@@ -769,6 +778,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				{
 					if (!property.CanSet && !context.Settings.GetterOnlyAutomaticProperties)
 						return null;
+					context.Step("Replace backing field use with property", identifier);
 					parent.RemoveAnnotations<MemberResolveResult>();
 					parent.AddAnnotation(new MemberResolveResult(mrr.TargetResult, property));
 					return Identifier.Create(property.Name);
@@ -793,6 +803,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				var eventDef = module.ResolveEntity(eventHandle) as IEvent;
 				if (eventDef != null && currentMethod?.AccessorOwner != eventDef)
 				{
+					context.Step("Replace event backing field use with event", identifier);
 					parent.RemoveAnnotations<MemberResolveResult>();
 					parent.AddAnnotation(new MemberResolveResult(mrr.TargetResult, eventDef));
 					identifier.Name = eventDef.Name;
@@ -1026,6 +1037,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 			if (ev.AddAccessor is not { } addAccessor)
 				return null;
+			context.Step("Convert custom event to field-like event", ev);
 			RemoveCompilerGeneratedAttribute(addAccessor.Attributes, attributeTypesToRemoveFromAutoEvents);
 			EventDeclaration ed = new EventDeclaration();
 			ev.Attributes.MoveTo(ed.Attributes);
@@ -1054,6 +1066,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 
 			ev.ReplaceWith(ed);
+			context.EndStep(ed);
 			return ed;
 
 			bool IsEventBackingField(FieldDeclaration fd)
@@ -1094,6 +1107,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			Match m = destructorPattern.Match(methodDef);
 			if (m.Success)
 			{
+				context.Step("Convert Finalize method to destructor", methodDef);
 				DestructorDeclaration dd = new DestructorDeclaration();
 				methodDef.Attributes.MoveTo(dd.Attributes);
 				dd.CopyAnnotationsFrom(methodDef);
@@ -1103,6 +1117,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				// has an enclosing type at this point.
 				dd.Name = currentTypeDefinition!.Name;
 				methodDef.ReplaceWith(dd);
+				context.EndStep(dd);
 				return dd;
 			}
 			return null;
@@ -1113,6 +1128,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			Match m = destructorBodyPattern.Match(dtorDef.Body);
 			if (m.Success)
 			{
+				context.Step("Simplify destructor body", dtorDef);
 				dtorDef.Body = m.Get<BlockStatement>("body").Single().Detach();
 				return dtorDef;
 			}
@@ -1139,6 +1155,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			if (tryCatchFinallyPattern.IsMatch(tryFinally))
 			{
+				context.Step("Merge nested try-catch-finally", tryFinally);
 				TryCatchStatement tryCatch = (TryCatchStatement)tryFinally.TryBlock.Statements.Single();
 				tryFinally.TryBlock = tryCatch.TryBlock.Detach();
 				tryCatch.CatchClauses.MoveTo(tryFinally.CatchClauses);
@@ -1171,6 +1188,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			Match m = cascadingIfElsePattern.Match(node);
 			if (m.Success)
 			{
+				context.Step("Simplify cascading if-else", node);
 				IfElseStatement elseIf = m.Get<IfElseStatement>("nestedIfStatement").Single();
 				node.FalseStatement = elseIf.Detach();
 			}
@@ -1191,6 +1209,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					var bAndC = expr.Right as BinaryOperatorExpression;
 					if (bAndC != null && bAndC.Operator == expr.Operator)
 					{
+						context.Step("Reassociate conditional logic", expr);
 						// make bAndC the parent and expr the child.
 						// A conditional-and/or operator always has both operands present.
 						var b = bAndC.Left!.Detach();
@@ -1199,6 +1218,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						bAndC.Left = expr;
 						bAndC.Right = c;
 						expr.Right = b;
+						context.EndStep(bAndC);
 						return base.VisitBinaryOperatorExpression(bAndC);
 					}
 					break;
@@ -1210,8 +1230,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			if (expr.Operator == UnaryOperatorType.Not && expr.Expression is BinaryOperatorExpression { Operator: BinaryOperatorType.Equality } binary)
 			{
+				context.Step("Replace negated equality with inequality", expr);
 				binary.Operator = BinaryOperatorType.InEquality;
 				expr.ReplaceWith(binary.Detach());
+				context.EndStep(binary);
 				return VisitBinaryOperatorExpression(binary);
 			}
 			return base.VisitUnaryOperatorExpression(expr);
@@ -1240,6 +1262,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						Expression target = m.Get<Expression>("target").Single();
 						if (target.GetResolveResult().Type.IsReferenceType == false)
 						{
+							context.Step("Use pattern-based fixed statement", fixedStatement);
 							v.Initializer = target.Detach();
 						}
 					}
@@ -1262,6 +1285,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (!(usingStatement.ResourceAcquisition is VariableDeclarationStatement))
 				return usingStatement;
 
+			context.Step("Use enhanced using statement", usingStatement);
 			usingStatement.IsEnhanced = true;
 			return usingStatement;
 		}

ICSharpCode.Decompiler/CSharp/Transforms/PrettifyAssignments.cs

@@ -63,9 +63,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (CanConvertToCompoundAssignment(assignment.Left) && assignment.Left.IsMatch(binary.Left)
 					&& binary.Right != null && IsImplicitlyConvertible(binary.Right, expectedType))
 				{
-					assignment.Operator = GetAssignmentOperatorForBinaryOperator(binary.Operator);
+					var newOperator = GetAssignmentOperatorForBinaryOperator(binary.Operator);
-					if (assignment.Operator != AssignmentOperatorType.Assign)
+					if (newOperator != AssignmentOperatorType.Assign)
 					{
+						context.Step("Convert assignment to compound assignment", assignment);
+						assignment.Operator = newOperator;
 						// If we found a shorter operator, get rid of the BinaryOperatorExpression:
 						assignment.CopyAnnotationsFrom(binary);
 						assignment.Right = binary.Right;
@@ -88,7 +90,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 							type = (assignment.Operator == AssignmentOperatorType.Add) ? UnaryOperatorType.PostIncrement : UnaryOperatorType.PostDecrement;
 						else
 							type = (assignment.Operator == AssignmentOperatorType.Add) ? UnaryOperatorType.Increment : UnaryOperatorType.Decrement;
-						assignment.ReplaceWith(new UnaryOperatorExpression(type, assignment.Left.Detach()).CopyAnnotationsFrom(assignment));
+						context.Step(type is UnaryOperatorType.Increment or UnaryOperatorType.PostIncrement ? "Convert assignment to increment" : "Convert assignment to decrement", assignment);
+						var unaryOperator = new UnaryOperatorExpression(type, assignment.Left.Detach()).CopyAnnotationsFrom(assignment);
+						assignment.ReplaceWith(unaryOperator);
+						context.EndStep(unaryOperator);
 					}
 				}
 			}

ICSharpCode.Decompiler/CSharp/Transforms/ReplaceMethodCallsWithOperators.cs

@@ -75,6 +75,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 				bool isInExpressionTree = invocationExpression.Ancestors.OfType<LambdaExpression>().Any(
 					lambda => lambda.Annotation<IL.ILFunction>()?.Kind == IL.ILFunctionKind.ExpressionTree);
+				context.Step("Replace String.Concat with +", invocationExpression);
 				Expression arg0 = arguments[0].Detach();
 				Expression arg1 = arguments[1].Detach();
 				if (!isInExpressionTree)
@@ -97,6 +98,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				}
 				expr.CopyAnnotationsFrom(invocationExpression);
 				invocationExpression.ReplaceWith(expr);
+				context.EndStep(expr);
 				return;
 			}
 
@@ -107,9 +109,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						if (typeHandleOnTypeOfPattern.IsMatch(arguments[0]))
 						{
+							context.Step("Replace GetTypeFromHandle with typeof", invocationExpression);
 							Expression target = ((MemberReferenceExpression)arguments[0]).Target;
 							target.CopyInstructionsFrom(invocationExpression);
 							invocationExpression.ReplaceWith(target);
+							context.EndStep(target);
 							return;
 						}
 					}
@@ -147,15 +151,20 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						method.TypeArguments.Count == 1 &&
 						IsInstantiableTypeParameter(method.TypeArguments[0]))
 					{
-						invocationExpression.ReplaceWith(new ObjectCreateExpression(context.TypeSystemAstBuilder.ConvertType(method.TypeArguments.First())));
+						context.Step("Replace Activator.CreateInstance with new", invocationExpression);
+						var objectCreate = new ObjectCreateExpression(context.TypeSystemAstBuilder.ConvertType(method.TypeArguments.First()));
+						invocationExpression.ReplaceWith(objectCreate);
+						context.EndStep(objectCreate);
 					}
 					break;
 				case "System.Runtime.CompilerServices.RuntimeHelpers.GetSubArray":
 					if (arguments.Length == 2 && context.Settings.Ranges)
 					{
+						context.Step("Replace RuntimeHelpers.GetSubArray with range indexer", invocationExpression);
 						var slicing = new IndexerExpression(arguments[0].Detach(), arguments[1].Detach());
 						slicing.CopyAnnotationsFrom(invocationExpression);
 						invocationExpression.ReplaceWith(slicing);
+						context.EndStep(slicing);
 					}
 					break;
 			}
@@ -164,6 +173,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			BinaryOperatorType? bop = GetBinaryOperatorTypeFromMetadataName(method.Name, out isChecked, context.Settings);
 			if (bop != null && arguments.Length == 2)
 			{
+				context.Step("Replace operator method with binary operator", invocationExpression);
 				invocationExpression.Arguments.Clear(); // detach arguments from invocationExpression
 				if (isChecked)
 				{
@@ -173,13 +183,13 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				{
 					invocationExpression.AddAnnotation(AddCheckedBlocks.UncheckedAnnotation);
 				}
-				invocationExpression.ReplaceWith(
+				var binaryOperator = new BinaryOperatorExpression(
-					new BinaryOperatorExpression(
+					arguments[0].UnwrapInDirectionExpression(),
-						arguments[0].UnwrapInDirectionExpression(),
+					bop.Value,
-						bop.Value,
+					arguments[1].UnwrapInDirectionExpression()
-						arguments[1].UnwrapInDirectionExpression()
+				).CopyAnnotationsFrom(invocationExpression);
-					).CopyAnnotationsFrom(invocationExpression)
+				invocationExpression.ReplaceWith(binaryOperator);
-				);
+				context.EndStep(binaryOperator);
 				return;
 			}
 			UnaryOperatorType? uop = GetUnaryOperatorTypeFromMetadataName(method.Name, out isChecked, context.Settings);
@@ -199,28 +209,31 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					// because it doesn't assign the incremented value to a.
 					if (method.DeclaringType.IsKnownType(KnownTypeCode.Decimal))
 					{
+						context.Step("Replace decimal increment method with arithmetic", invocationExpression);
 						// Legacy csc optimizes "d + 1m" to "op_Increment(d)",
 						// so reverse that optimization here:
-						invocationExpression.ReplaceWith(
+						var arithmetic = new BinaryOperatorExpression(
-							new BinaryOperatorExpression(
+							arguments[0].UnwrapInDirectionExpression().Detach(),
-								arguments[0].UnwrapInDirectionExpression().Detach(),
+							(uop == UnaryOperatorType.Increment ? BinaryOperatorType.Add : BinaryOperatorType.Subtract),
-								(uop == UnaryOperatorType.Increment ? BinaryOperatorType.Add : BinaryOperatorType.Subtract),
+							new PrimitiveExpression(1m)
-								new PrimitiveExpression(1m)
+						).CopyAnnotationsFrom(invocationExpression);
-							).CopyAnnotationsFrom(invocationExpression)
+						invocationExpression.ReplaceWith(arithmetic);
-						);
+						context.EndStep(arithmetic);
 					}
 				}
 				else
 				{
+					context.Step("Replace operator method with unary operator", invocationExpression);
 					arguments[0].Remove(); // detach argument
-					invocationExpression.ReplaceWith(
+					var unaryOperator = new UnaryOperatorExpression(uop.Value, arguments[0].UnwrapInDirectionExpression()).CopyAnnotationsFrom(invocationExpression);
-						new UnaryOperatorExpression(uop.Value, arguments[0].UnwrapInDirectionExpression()).CopyAnnotationsFrom(invocationExpression)
+					invocationExpression.ReplaceWith(unaryOperator);
-					);
+					context.EndStep(unaryOperator);
 				}
 				return;
 			}
 			if (method.Name is "op_Explicit" or "op_CheckedExplicit" && arguments.Length == 1)
 			{
+				context.Step("Replace conversion operator method with cast", invocationExpression);
 				arguments[0].Remove(); // detach argument
 				if (method.Name == "op_CheckedExplicit")
 				{
@@ -230,15 +243,18 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				{
 					invocationExpression.AddAnnotation(AddCheckedBlocks.UncheckedAnnotation);
 				}
-				invocationExpression.ReplaceWith(
+				var cast = new CastExpression(context.TypeSystemAstBuilder.ConvertType(method.ReturnType), arguments[0].UnwrapInDirectionExpression())
-					new CastExpression(context.TypeSystemAstBuilder.ConvertType(method.ReturnType), arguments[0].UnwrapInDirectionExpression())
+					.CopyAnnotationsFrom(invocationExpression);
-						.CopyAnnotationsFrom(invocationExpression)
+				invocationExpression.ReplaceWith(cast);
-				);
+				context.EndStep(cast);
 				return;
 			}
 			if (method.Name == "op_True" && arguments.Length == 1 && invocationExpression.Slot?.Kind == Slots.Condition)
 			{
-				invocationExpression.ReplaceWith(arguments[0].UnwrapInDirectionExpression());
+				context.Step("Remove op_True from condition", invocationExpression);
+				var condition = arguments[0].UnwrapInDirectionExpression();
+				invocationExpression.ReplaceWith(condition);
+				context.EndStep(condition);
 				return;
 			}
 

ICSharpCode.Decompiler/CSharp/Transforms/TransformContext.cs

@@ -19,9 +19,11 @@
 #nullable enable
 
 using System.Collections.Immutable;
+using System.Diagnostics;
 using System.Threading;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
+using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.CSharp.Transforms
@@ -36,6 +38,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		public readonly TypeSystemAstBuilder TypeSystemAstBuilder;
 		public readonly DecompilerSettings Settings;
 		internal readonly DecompileRun DecompileRun;
+		public Stepper Stepper { get; set; }
 
 		readonly ITypeResolveContext decompilationContext;
 
@@ -67,6 +70,74 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			this.TypeSystemAstBuilder = typeSystemAstBuilder;
 			this.CancellationToken = decompileRun.CancellationToken;
 			this.Settings = decompileRun.Settings;
+			this.Stepper = new Stepper();
+		}
+
+		/// <summary>
+		/// Call this method immediately before performing a transform step.
+		/// Unlike <c>context.Stepper.Step()</c>, calls to this method are only compiled in debug builds.
+		/// </summary>
+		[Conditional("STEP")]
+		[DebuggerStepThrough]
+		internal void Step(string description, AstNode? near = null)
+		{
+			TrackModifiedNode(Stepper.Step(description, modifiedNode: near), near);
+		}
+
+		[Conditional("STEP")]
+		[DebuggerStepThrough]
+		internal void StepStartGroup(string description, AstNode? near = null)
+		{
+			TrackModifiedNode(Stepper.StartGroup(description, modifiedNode: near), near);
+		}
+
+		[Conditional("STEP")]
+		internal void StepEndGroup(bool keepIfEmpty = false)
+		{
+			Stepper.EndGroup(keepIfEmpty);
+		}
+
+		/// <summary>
+		/// Points the most recently recorded step at the node its mutation produced.
+		/// Call this after a <see cref="Step"/> whose modified node only comes into existence
+		/// during the mutation (e.g. the result of a ReplaceWith or a freshly inserted node).
+		/// </summary>
+		[Conditional("STEP")]
+		internal void EndStep(AstNode? modifiedNode)
+		{
+			if (Stepper.LastStep != null)
+			{
+				Stepper.LastStep.ModifiedNode = modifiedNode;
+				TrackModifiedNode(Stepper.LastStep, modifiedNode, insertFirst: true);
+			}
+		}
+
+		static void TrackModifiedNode(Stepper.Node step, AstNode? modifiedNode, bool insertFirst = false)
+		{
+			if (modifiedNode == null)
+				return;
+			AddCandidate(step, modifiedNode, insertFirst);
+			var marker = new DebugStepMarker();
+			modifiedNode.AddAnnotation(marker);
+			AddCandidate(step, marker, insertFirst: false);
+			for (var parent = modifiedNode.Parent; parent != null; parent = parent.Parent)
+			{
+				AddCandidate(step, parent, insertFirst: false);
+			}
+		}
+
+		static void AddCandidate(Stepper.Node step, object candidate, bool insertFirst)
+		{
+			if (step.ModifiedNodeCandidates.Contains(candidate))
+				return;
+			if (insertFirst)
+				step.ModifiedNodeCandidates.Insert(0, candidate);
+			else
+				step.ModifiedNodeCandidates.Add(candidate);
+		}
+
+		sealed class DebugStepMarker
+		{
 		}
 	}
 }

ICSharpCode.Decompiler/CSharp/Transforms/TransformFieldAndConstructorInitializers.cs

@@ -481,6 +481,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 				var ci = new ConstructorInitializer { ConstructorInitializerType = type };
 
+				context.Step("Move constructor call to initializer", stmt);
 				// Move arguments from invocation to initializer:
 				invocation.GetChildren(Slots.Argument).MoveTo(ci.Arguments);
 				// Add the initializer: (unless it is the default 'base()')
@@ -489,6 +490,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 				// Remove the statement
 				stmt.Remove();
+				context.EndStep(constructorDeclaration.Initializer);
 
 				return true;
 			}
@@ -507,7 +509,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						// e.g. when a single static constructor is decompiled in isolation -- so the
 						// assignment must remain in the constructor body.
 						if (kind is InitializerKind.Primary)
+						{
+							context.Step("Remove redundant primary constructor assignment", stmt);
 							stmt.Remove();
+						}
 						continue;
 					}
 
@@ -518,7 +523,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 							v = fd.Variables.Single();
 							if (v.Initializer is null)
 							{
-								v.Initializer = initializer.Detach();
+								context.Step("Move assignment to field initializer", stmt);
+								var movedInitializer = initializer.Detach();
+								v.Initializer = movedInitializer;
+								context.EndStep(movedInitializer);
 							}
 							else if (kind == InitializerKind.Static)
 							{
@@ -548,7 +556,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 							Debug.Assert(pd.IsAutomaticProperty);
 							if (pd.Initializer is null)
 							{
-								pd.Initializer = initializer.Detach();
+								context.Step("Move assignment to property initializer", stmt);
+								var movedInitializer = initializer.Detach();
+								pd.Initializer = movedInitializer;
+								context.EndStep(movedInitializer);
 							}
 							else
 							{
@@ -560,7 +571,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 							v = ev.Variables.Single();
 							if (v.Initializer is null)
 							{
-								v.Initializer = initializer.Detach();
+								context.Step("Move assignment to event initializer", stmt);
+								var movedInitializer = initializer.Detach();
+								v.Initializer = movedInitializer;
+								context.EndStep(movedInitializer);
 							}
 							else
 							{
@@ -594,6 +608,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 					if (sequence.IsUnsafe && IntroduceUnsafeModifier.IsUnsafe(initializer))
 					{
+						context.Step("Add unsafe modifier to initialized member", declaringSyntaxNode);
 						declaringSyntaxNode.Modifiers |= Modifiers.Unsafe;
 					}
 				}
@@ -626,6 +641,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						var insertionPoint = (AstNode?)this.TypeDeclaration.TypeParameters.LastOrDefault() ?? this.TypeDeclaration.NameToken;
 						foreach (var param in PrimaryConstructorDecl.Parameters)
 						{
+							context.Step("Move primary constructor parameter to type", param);
 							param.Remove();
 							this.TypeDeclaration.InsertChildAfter(insertionPoint, param, Slots.Parameter);
 							insertionPoint = param;
@@ -680,6 +696,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 					if (PrimaryConstructorDecl.HasModifier(Modifiers.Unsafe))
 					{
+						context.Step("Move unsafe modifier from primary constructor to type", this.TypeDeclaration);
 						this.TypeDeclaration.Modifiers |= Modifiers.Unsafe;
 					}
 
@@ -689,11 +706,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 						var baseType = TypeDeclaration.BaseTypes.First();
 						var newBaseType = new InvocationAstType();
+						context.Step("Move primary constructor initializer to base type", baseType);
 						baseType.ReplaceWith(newBaseType);
 						newBaseType.BaseType = baseType;
 						initializer.Arguments.MoveTo(newBaseType.Arguments);
+						context.EndStep(newBaseType);
 					}
 
+					context.Step("Remove primary constructor body", PrimaryConstructorDecl);
 					PrimaryConstructorDecl.Remove();
 				}
 
@@ -704,6 +724,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 					if (IsBeforeFieldInit && StaticConstructorDecl.Body is { Statements.Count: 0 })
 					{
+						context.Step("Remove empty static constructor", StaticConstructorDecl);
 						StaticConstructorDecl.Remove();
 					}
 				}
@@ -733,7 +754,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					bool retainBecauseOfDocumentation = context.Settings.ShowXmlDocumentation
 						&& context.DecompileRun.DocumentationProvider?.GetDocumentation(ctorMethod) != null;
 					if (!retainBecauseOfDocumentation)
+					{
+						context.Step("Remove implicit constructor", ctor);
 						ctor.Remove();
+					}
 				}
 			}
 

ICSharpCode.Decompiler/IL/Transforms/Stepper.cs

@@ -54,6 +54,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		}
 
 		public IList<Node> Steps => steps;
+		public Node? LimitReachedStep { get; private set; }
+		public Node? LastStep { get; private set; }
 
 		public int StepLimit { get; set; } = int.MaxValue;
 		public bool IsDebug { get; set; }
@@ -62,6 +64,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		{
 			public string Description { get; }
 			public ILInstruction? Position { get; set; }
+			public object? ModifiedNode { get; set; }
+			public IList<object> ModifiedNodeCandidates { get; } = new List<object>();
 			/// <summary>
 			/// BeginStep is inclusive.
 			/// </summary>
@@ -96,39 +100,62 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// May throw <see cref="StepLimitReachedException"/> in debug mode.
 		/// </summary>
 		[DebuggerStepThrough]
-		public void Step(string description, ILInstruction? near = null)
+		public Node Step(string description, ILInstruction? near = null, object? modifiedNode = null)
 		{
-			StepInternal(description, near);
+			return StepInternal(description, near, modifiedNode ?? near);
 		}
 
 		[DebuggerStepThrough]
-		private Node StepInternal(string description, ILInstruction? near)
+		private Node StepInternal(string description, ILInstruction? near, object? modifiedNode)
 		{
+			var stepNode = new Node($"{step}: {description}") {
+				Position = near,
+				ModifiedNode = modifiedNode,
+				BeginStep = step,
+				EndStep = step + 1
+			};
 			if (step == StepLimit)
 			{
+				LimitReachedStep = stepNode;
 				if (IsDebug)
 					Debugger.Break();
 				else
 					throw new StepLimitReachedException();
 			}
-			var stepNode = new Node($"{step}: {description}") {
-				Position = near,
-				BeginStep = step,
-				EndStep = step + 1
-			};
 			var p = groups.PeekOrDefault();
 			if (p != null)
 				p.Children.Add(stepNode);
 			else
 				steps.Add(stepNode);
+			LastStep = stepNode;
 			step++;
 			return stepNode;
 		}
 
 		[DebuggerStepThrough]
-		public void StartGroup(string description, ILInstruction? near = null)
+		public Node StartGroup(string description, ILInstruction? near = null, object? modifiedNode = null)
+		{
+			var stepNode = StepInternal(description, near, modifiedNode ?? near);
+			groups.Push(stepNode);
+			return stepNode;
+		}
+
+		public Node? GetStepByBeginStep(int beginStep)
 		{
-			groups.Push(StepInternal(description, near));
+			return FindStep(steps, beginStep);
+
+			static Node? FindStep(IEnumerable<Node> nodes, int beginStep)
+			{
+				foreach (var node in nodes)
+				{
+					if (node.BeginStep == beginStep)
+						return node;
+					var child = FindStep(node.Children, beginStep);
+					if (child != null)
+						return child;
+				}
+				return null;
+			}
 		}
 
 		public void EndGroup(bool keepIfEmpty = false)

ILSpy.Tests/Views/DebugStepsTests.cs

@@ -18,6 +18,7 @@
 
 #if DEBUG
 
+using System;
 using System.Linq;
 using System.Threading.Tasks;
 
@@ -27,10 +28,14 @@ using Avalonia.VisualTree;
 
 using AwesomeAssertions;
 
+using ICSharpCode.Decompiler.CSharp;
+using ICSharpCode.Decompiler.CSharp.Syntax;
+
 using ICSharpCode.ILSpy;
 using ICSharpCode.ILSpy.AppEnv;
 using ICSharpCode.ILSpy.Docking;
 using ICSharpCode.ILSpy.Languages;
+using ICSharpCode.ILSpy.TextView;
 using ICSharpCode.ILSpy.TreeNodes;
 using ICSharpCode.ILSpy.ViewModels;
 using ICSharpCode.ILSpy.Views;
@@ -119,6 +124,75 @@ public class DebugStepsTests
 			"after switching to ILAst and decompiling, the VM's Steps must list the stepper's recorded transforms");
 	}
 
+	[AvaloniaTest]
+	public async Task CSharp_DebugSteps_Are_Grouped_By_Ast_Transform()
+	{
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+
+		var languageService = AppComposition.Current.GetExport<LanguageService>();
+		var csharp = languageService.Languages.OfType<CSharpLanguage>().First();
+		languageService.CurrentLanguage = csharp;
+
+		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
+			"System.Linq", "System.Linq", "System.Linq.Enumerable");
+		typeNode.IsExpanded = true;
+		var method = typeNode.Children.OfType<MethodTreeNode>()
+			.First(m => m.MethodDefinition.Name == "Range");
+		vm.AssemblyTreeModel.SelectNode(method);
+		await vm.DockWorkspace.WaitForDecompiledTextAsync();
+
+		var debugStepsVm = AppComposition.Current.GetExport<DebugStepsPaneModel>();
+		await Waiters.WaitForAsync(
+			() => debugStepsVm.Steps?.Count > 0,
+			description: "DebugStepsPaneModel.Steps to be populated after the C# decompile");
+
+		var astTransformNames = CSharpDecompiler.GetAstTransforms()
+			.Select(transform => transform.GetType().Name)
+			.ToArray();
+
+		debugStepsVm.Steps!
+			.Select(step => StripStepNumber(step.Description))
+			.Should().Equal(astTransformNames,
+				"C# debug steps must use AST transforms as top-level groups");
+
+		var transformGroupWithChanges = debugStepsVm.Steps!
+			.FirstOrDefault(step => step.Children.Count > 0);
+		transformGroupWithChanges.Should().NotBeNull(
+			"individual C# AST mutation steps must be nested under their transform group");
+		transformGroupWithChanges!.Children
+			.Select(step => StripStepNumber(step.Description))
+			.Should().Contain(
+				description => !astTransformNames.Contains(description),
+				"nested C# debug steps must describe individual AST mutation points");
+
+		var collectedSteps = debugStepsVm.Steps;
+		var replayStep = transformGroupWithChanges.Children.First();
+		var tab = vm.DockWorkspace.ActiveDecompilerTab!;
+
+		tab.RestartDecompileWithStepLimit(replayStep.BeginStep, isDebug: false, replayStep.BeginStep);
+		tab = await vm.DockWorkspace.WaitForDecompiledTextAsync();
+		tab.Text.Should().NotBeNullOrWhiteSpace("C# replay before a selected AST mutation step must still emit code");
+		tab.DebugStepHighlight.Should().NotBeNull("C# replay before a selected AST mutation step must locate the changed node");
+		debugStepsVm.Steps.Should().BeSameAs(collectedSteps,
+			"a step-limited C# replay must not replace the full step tree shown by the pane");
+
+		tab.RestartDecompileWithStepLimit(replayStep.EndStep, isDebug: false, replayStep.BeginStep);
+		tab = await vm.DockWorkspace.WaitForDecompiledTextAsync();
+		tab.Text.Should().NotBeNullOrWhiteSpace("C# replay after a selected AST mutation step must still emit code");
+		tab.DebugStepHighlight.Should().NotBeNull("C# replay after a selected AST mutation step must locate the changed node");
+		debugStepsVm.Steps.Should().BeSameAs(collectedSteps,
+			"a step-limited C# replay must preserve the current full-run step tree and selection context");
+
+		static string StripStepNumber(string description)
+		{
+			var separatorIndex = description.IndexOf(": ", StringComparison.Ordinal);
+			return separatorIndex >= 0 ? description[(separatorIndex + 2)..] : description;
+		}
+	}
+
 	[AvaloniaTest]
 	public Task ILAst_And_TypedIL_Languages_Are_Registered_In_Debug_Builds()
 	{
@@ -133,6 +207,25 @@ public class DebugStepsTests
 		languageService.Languages.Should().Contain(l => l.Name == "Typed IL");
 		return Task.CompletedTask;
 	}
+
+	[AvaloniaTest]
+	public Task NodeLookup_Resolves_Copied_Ast_Annotations()
+	{
+		var marker = new object();
+		var original = new IdentifierExpression("old");
+		original.AddAnnotation(marker);
+		var replacement = new IdentifierExpression("new").CopyAnnotationsFrom(original);
+		var lookup = new NodeLookup();
+
+		lookup.AddNode(replacement, 12, 3);
+
+		lookup.TryGetRange(marker, out var range).Should().BeTrue(
+			"C# debug-step markers copied by AST replacements must still resolve to emitted text");
+		range.Start.Should().Be(12);
+		range.Length.Should().Be(3);
+		return Task.CompletedTask;
+	}
+
 	[AvaloniaTest]
 	public Task Pane_Reports_Not_Available_For_Languages_Without_Debug_Steps()
 	{

ILSpy/DecompilationOptions.cs

@@ -60,6 +60,13 @@ namespace ICSharpCode.ILSpy
 		/// </summary>
 		public int StepLimit { get; set; } = int.MaxValue;
 
+		/// <summary>
+		/// Step whose changed node should be highlighted after a debug-stepper re-decompile.
+		/// This can differ from <see cref="StepLimit"/> when showing the state after a step,
+		/// because the next recorded step is where the pipeline stops.
+		/// </summary>
+		public int? HighlightStep { get; set; }
+
 		/// <summary>
 		/// When true, transforms emit verbose debug information about their behaviour. Only
 		/// meaningful in combination with <see cref="StepLimit"/> — the Debug Steps pane sets

ILSpy/Languages/CSharpHighlightingTokenWriter.cs

@@ -124,6 +124,12 @@ namespace ICSharpCode.ILSpy.Languages
 			//this.externAliasKeywordColor = ...;
 		}
 
+		public CSharpHighlightingTokenWriter(TokenWriter decoratedWriter, AvaloniaEditTextOutput textOutput, ILocatable? locatable = null)
+			: this(decoratedWriter, (ISmartTextOutput?)textOutput, locatable)
+		{
+			this.nodeTrackingOutput = textOutput;
+		}
+
 		public override void WriteKeyword(string keyword)
 		{
 			HighlightingColor? color = null;
@@ -496,6 +502,7 @@ namespace ICSharpCode.ILSpy.Languages
 
 		public override void StartNode(AstNode node)
 		{
+			nodeTrackingOutput?.MarkNodeStart(node);
 			nodeStack.Push(node);
 			base.StartNode(node);
 		}
@@ -503,6 +510,7 @@ namespace ICSharpCode.ILSpy.Languages
 		public override void EndNode(AstNode node)
 		{
 			base.EndNode(node);
+			nodeTrackingOutput?.MarkNodeEnd(node);
 			nodeStack.Pop();
 		}
 
@@ -511,6 +519,7 @@ namespace ICSharpCode.ILSpy.Languages
 		int currentColorBegin = -1;
 		readonly ILocatable? locatable;
 		readonly ISmartTextOutput? textOutput;
+		readonly AvaloniaEditTextOutput? nodeTrackingOutput;
 
 		// Wraps a base WriteX call so its output lands inside a highlighting span for the given colour
 		// (or no span when null) -- replacing the begin/end guard each WriteX override used to repeat.

ILSpy/Languages/CSharpLanguage.DebugSteps.cs

@@ -32,18 +32,16 @@ using ICSharpCode.ILSpy.ViewModels;
 namespace ICSharpCode.ILSpy.Languages
 {
 	/// <summary>
-	/// Debug Steps support for the C# language: coarse, one step per AST transform, shown in the
+	/// Debug Steps support for the C# language, shown in the Debug Steps pane like the ILAst
-	/// Debug Steps pane like the ILAst language already does for IL transforms. The step list is
+	/// language already does for IL transforms. A full decompile records AST transform groups with
-	/// static -- the C# AST pipeline (<see cref="CSharpDecompiler.GetAstTransforms"/>) is the same
+	/// individual mutation steps inside; a selected step's index is replayed by re-decompiling with <see
-	/// for every member -- so a selected step's index maps straight onto <see
+	/// cref="DecompilationOptions.StepLimit"/>.
-	/// cref="DecompilationOptions.StepLimit"/>, which CreateDecompiler turns into the number of AST
-	/// transforms to keep before re-rendering.
 	/// </summary>
 	partial class CSharpLanguage : IDebugStepProvider
 	{
-		Stepper? stepper;
+		Stepper stepper = new Stepper();
 
-		public Stepper Stepper => stepper ??= BuildStepper();
+		public Stepper Stepper => stepper;
 
 		public event EventHandler? StepperUpdated;
 
@@ -51,18 +49,7 @@ namespace ICSharpCode.ILSpy.Languages
 		// tree for C#, unlike ILAst's writing-options checkboxes.
 		public object? StepOptions => null;
 
-		// One node per AST transform, in pipeline order. Stepper.Step assigns BeginStep=i /
+		partial void OnCSharpDecompiled(CSharpDecompiler decompiler, ITextOutput output, DecompilationOptions options)
-		// EndStep=i+1 automatically, so "show state before step i" re-decompiles with StepLimit=i
-		// (run i transforms) and "after" with StepLimit=i+1 -- exactly the cap CreateDecompiler wants.
-		static Stepper BuildStepper()
-		{
-			var s = new Stepper();
-			foreach (var transform in CSharpDecompiler.GetAstTransforms())
-				s.Step(transform.GetType().Name);
-			return s;
-		}
-
-		partial void OnCSharpDecompiled(ITextOutput output, DecompilationOptions options)
 		{
 			// The button always shows so the pane is one click away; mirrors the ILAst language.
 			// DockWorkspace is resolved lazily (an ImportingConstructor import would form a MEF
@@ -74,7 +61,7 @@ namespace ICSharpCode.ILSpy.Languages
 			// pane itself) must leave the tree and the user's selection intact.
 			if (options.StepLimit == int.MaxValue)
 			{
-				_ = Stepper;
+				stepper = decompiler.Stepper;
 				StepperUpdated?.Invoke(this, EventArgs.Empty);
 			}
 		}

ILSpy/Languages/CSharpLanguage.cs

@@ -33,6 +33,7 @@ using ICSharpCode.Decompiler.CSharp.ProjectDecompiler;
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.CSharp.Transforms;
 using ICSharpCode.Decompiler.IL;
+using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.Metadata;
 using ICSharpCode.Decompiler.Output;
 using ICSharpCode.Decompiler.Solution;
@@ -313,11 +314,8 @@ namespace ICSharpCode.ILSpy.Languages
 				CancellationToken = options.CancellationToken,
 				DebugInfoProvider = module.GetDebugInfoOrNull(),
 			};
-			// The Debug Steps pane stops the AST pipeline at a chosen step by re-decompiling with
+			decompiler.Stepper.StepLimit = options.StepLimit;
-			// options.StepLimit = number of AST transforms to keep; pop the rest from the end.
+			decompiler.Stepper.IsDebug = options.IsDebug;
-			// StepLimit is int.MaxValue for a normal decompile, so nothing is removed.
-			while (decompiler.AstTransforms.Count > options.StepLimit)
-				decompiler.AstTransforms.RemoveAt(decompiler.AstTransforms.Count - 1);
 			if (options.EscapeInvalidIdentifiers)
 				decompiler.AstTransforms.Add(new EscapeInvalidIdentifiers());
 			return decompiler;
@@ -345,25 +343,25 @@ namespace ICSharpCode.ILSpy.Languages
 			{
 				var members = CollectFieldsAndCtors(methodDefinition.DeclaringTypeDefinition!, methodDefinition.IsStatic);
 				decompiler.AstTransforms.Add(new SelectCtorTransform(methodDefinition));
-				WriteCode(output, options.DecompilerSettings, decompiler, decompiler.Decompile(members));
+				WriteCode(output, options, decompiler.Decompile(members), decompiler);
 			}
 			else
 			{
-				WriteCode(output, options.DecompilerSettings, decompiler, decompiler.Decompile(method.MetadataToken));
+				WriteCode(output, options, decompiler.Decompile(method.MetadataToken), decompiler);
 			}
-			OnCSharpDecompiled(output, options);
+			OnCSharpDecompiled(decompiler, output, options);
 		}
 
 		// Implemented only under DEBUG (CSharpLanguage.DebugSteps.cs) to feed the Debug Steps pane;
 		// a no-op partial in Release.
-		partial void OnCSharpDecompiled(ITextOutput output, DecompilationOptions options);
+		partial void OnCSharpDecompiled(CSharpDecompiler decompiler, ITextOutput output, DecompilationOptions options);
 
 		public override void DecompileProperty(IProperty property, ITextOutput output, DecompilationOptions options)
 		{
 			CSharpDecompiler decompiler = BeginDecompile(property, output, options);
 			WriteCommentLine(output, TypeToString(property.DeclaringType));
-			WriteCode(output, options.DecompilerSettings, decompiler, decompiler.Decompile(property.MetadataToken));
+			WriteCode(output, options, decompiler.Decompile(property.MetadataToken), decompiler);
-			OnCSharpDecompiled(output, options);
+			OnCSharpDecompiled(decompiler, output, options);
 		}
 
 		public override void DecompileField(IField field, ITextOutput output, DecompilationOptions options)
@@ -372,16 +370,16 @@ namespace ICSharpCode.ILSpy.Languages
 			WriteCommentLine(output, TypeToString(field.DeclaringType));
 			if (field.IsConst)
 			{
-				WriteCode(output, options.DecompilerSettings, decompiler, decompiler.Decompile(field.MetadataToken));
+				WriteCode(output, options, decompiler.Decompile(field.MetadataToken), decompiler);
 			}
 			else
 			{
 				var members = CollectFieldsAndCtors(field.DeclaringTypeDefinition!, field.IsStatic);
 				var resolvedField = decompiler.TypeSystem.MainModule.GetDefinition((FieldDefinitionHandle)field.MetadataToken);
 				decompiler.AstTransforms.Add(new SelectFieldTransform(resolvedField));
-				WriteCode(output, options.DecompilerSettings, decompiler, decompiler.Decompile(members));
+				WriteCode(output, options, decompiler.Decompile(members), decompiler);
 			}
-			OnCSharpDecompiled(output, options);
+			OnCSharpDecompiled(decompiler, output, options);
 		}
 
 		/// <summary>
@@ -408,24 +406,24 @@ namespace ICSharpCode.ILSpy.Languages
 			CSharpDecompiler decompiler = BeginDecompile(extension, output, options);
 			WriteCommentLine(output, TypeToString(commentType,
 				ConversionFlags.UseFullyQualifiedTypeNames | ConversionFlags.UseFullyQualifiedEntityNames | ConversionFlags.SupportExtensionDeclarations));
-			WriteCode(output, options.DecompilerSettings, decompiler, decompiler.DecompileExtension(extension.MetadataToken));
+			WriteCode(output, options, decompiler.DecompileExtension(extension.MetadataToken), decompiler);
-			OnCSharpDecompiled(output, options);
+			OnCSharpDecompiled(decompiler, output, options);
 		}
 
 		public override void DecompileEvent(IEvent ev, ITextOutput output, DecompilationOptions options)
 		{
 			CSharpDecompiler decompiler = BeginDecompile(ev, output, options);
 			WriteCommentLine(output, TypeToString(ev.DeclaringType));
-			WriteCode(output, options.DecompilerSettings, decompiler, decompiler.Decompile(ev.MetadataToken));
+			WriteCode(output, options, decompiler.Decompile(ev.MetadataToken), decompiler);
-			OnCSharpDecompiled(output, options);
+			OnCSharpDecompiled(decompiler, output, options);
 		}
 
 		public override void DecompileType(ITypeDefinition type, ITextOutput output, DecompilationOptions options)
 		{
 			CSharpDecompiler decompiler = BeginDecompile(type, output, options);
 			WriteCommentLine(output, TypeToString(type, ConversionFlags.UseFullyQualifiedTypeNames | ConversionFlags.UseFullyQualifiedEntityNames));
-			WriteCode(output, options.DecompilerSettings, decompiler, decompiler.Decompile(type.MetadataToken));
+			WriteCode(output, options, decompiler.Decompile(type.MetadataToken), decompiler);
-			OnCSharpDecompiled(output, options);
+			OnCSharpDecompiled(decompiler, output, options);
 		}
 
 		public override ProjectId? DecompileAssembly(LoadedAssembly assembly, ITextOutput output, DecompilationOptions options)
@@ -511,7 +509,7 @@ namespace ICSharpCode.ILSpy.Languages
 			SyntaxTree st = options.FullDecompilation
 				? decompiler.DecompileWholeModuleAsSingleFile()
 				: decompiler.DecompileModuleAndAssemblyAttributes();
-			WriteCode(output, options.DecompilerSettings, decompiler, st);
+			WriteCode(output, options, st, decompiler);
 			return null;
 		}
 
@@ -709,13 +707,15 @@ namespace ICSharpCode.ILSpy.Languages
 			}
 		}
 
-		static void WriteCode(ITextOutput output, DecompilerSettings settings, CSharpDecompiler decompiler, SyntaxTree syntaxTree)
+		static void WriteCode(ITextOutput output, DecompilationOptions options, SyntaxTree syntaxTree, CSharpDecompiler decompiler)
 		{
+			var settings = options.DecompilerSettings;
 			syntaxTree.AcceptVisitor(new InsertParenthesesVisitor { InsertParenthesesForReadability = true });
 			output.IndentationString = settings.CSharpFormattingOptions.IndentationString;
-
 			TokenWriter tokenWriter = new TextTokenWriter(output, settings, decompiler.TypeSystem);
-			if (output is TextView.ISmartTextOutput smartOutput)
+			if (output is TextView.AvaloniaEditTextOutput avaloniaOutput)
+				tokenWriter = new CSharpHighlightingTokenWriter(tokenWriter, avaloniaOutput);
+			else if (output is TextView.ISmartTextOutput smartOutput)
 				tokenWriter = new CSharpHighlightingTokenWriter(tokenWriter, smartOutput);
 
 			// For the on-screen C# view, harvest the IL-offset/line map for body bookmarks during this
@@ -728,6 +728,47 @@ namespace ICSharpCode.ILSpy.Languages
 
 			syntaxTree.AcceptVisitor(new CSharpOutputVisitor(tokenWriter, settings.CSharpFormattingOptions));
 			bookmarkCollector?.Publish();
+			if (output is TextView.AvaloniaEditTextOutput nodeOutput
+				&& TryGetDebugStepHighlightRange(decompiler, options, nodeOutput.NodeLookup, out var range))
+			{
+				nodeOutput.DebugStepHighlight = range;
+			}
+		}
+
+		static bool TryGetDebugStepHighlightRange(CSharpDecompiler decompiler, DecompilationOptions options, TextView.NodeLookup nodeLookup, out TextView.TextRange range)
+		{
+			range = default;
+			if (options.StepLimit == int.MaxValue)
+				return false;
+			if (options.HighlightStep is { } highlightStep)
+			{
+				if (TryGetRange(decompiler.Stepper.GetStepByBeginStep(highlightStep), nodeLookup, out range))
+					return true;
+				if (decompiler.Stepper.LimitReachedStep is { BeginStep: var limitStep } reachedStep
+					&& limitStep == highlightStep)
+				{
+					return TryGetRange(reachedStep, nodeLookup, out range);
+				}
+				return false;
+			}
+			if (TryGetRange(decompiler.Stepper.LimitReachedStep, nodeLookup, out range))
+				return true;
+			if (options.StepLimit > 0)
+				return TryGetRange(decompiler.Stepper.GetStepByBeginStep(options.StepLimit - 1), nodeLookup, out range);
+			return false;
+
+			static bool TryGetRange(Stepper.Node? step, TextView.NodeLookup nodeLookup, out TextView.TextRange range)
+			{
+				range = default;
+				if (step == null)
+					return false;
+				foreach (var candidate in step.ModifiedNodeCandidates)
+				{
+					if (nodeLookup.TryGetRange(candidate, out range))
+						return true;
+				}
+				return step.ModifiedNode != null && nodeLookup.TryGetRange(step.ModifiedNode, out range);
+			}
 		}
 
 		void AddWarningMessage(MetadataFile module, ITextOutput output, string line1, string? line2 = null,

ILSpy/TextView/AvaloniaEditTextOutput.cs

@@ -54,6 +54,7 @@ namespace ICSharpCode.ILSpy.TextView
 		public int LengthLimit { get; set; } = int.MaxValue;
 
 		readonly Stack<(int Offset, HighlightingColor Color)> openSpans = new();
+		readonly Stack<(object Node, int Offset)> openNodes = new();
 		readonly Stack<(NewFolding Folding, int StartLine)> openFoldings = new();
 		readonly List<NewFolding> foldings = new();
 		int indent;
@@ -87,6 +88,10 @@ namespace ICSharpCode.ILSpy.TextView
 		/// <summary>Maps reference targets to their definition offsets in the rendered text.</summary>
 		public DefinitionLookup DefinitionLookup { get; } = new();
 
+		internal NodeLookup NodeLookup { get; } = new();
+
+		internal TextRange? DebugStepHighlight { get; set; }
+
 		readonly List<Bookmarks.MethodDebugInfo> methodDebugInfos = new();
 
 		/// <summary>
@@ -300,5 +305,20 @@ namespace ICSharpCode.ILSpy.TextView
 				highlightingSpans.Add((start, length, color));
 			}
 		}
+
+		internal void MarkNodeStart(object node)
+		{
+			openNodes.Push((node, builder.Length));
+		}
+
+		internal void MarkNodeEnd(object node)
+		{
+			if (openNodes.Count == 0)
+				return;
+			var (currentNode, start) = openNodes.Pop();
+			if (!ReferenceEquals(currentNode, node))
+				return;
+			NodeLookup.AddNode(node, start, builder.Length - start);
+		}
 	}
 }

ILSpy/TextView/DecompilerTabPageModel.cs

@@ -183,6 +183,9 @@ namespace ICSharpCode.ILSpy.TextView
 		[ObservableProperty]
 		private DefinitionLookup? definitionLookup;
 
+		[ObservableProperty]
+		private TextRange? debugStepHighlight;
+
 		/// <summary>
 		/// IL-offset &lt;-&gt; line maps for the methods in this document (C# only). Lets bookmarks
 		/// anchor in-method lines by IL offset and the gutter place their icons. Null for non-C#
@@ -379,6 +382,7 @@ namespace ICSharpCode.ILSpy.TextView
 		// every run; set non-default by RestartDecompileWithStepLimit before kicking off a
 		// debug-stepper decompile. Set on the UI thread only — no inter-thread access.
 		int pendingStepLimit = int.MaxValue;
+		int? pendingHighlightStep;
 		bool pendingIsDebug;
 
 		/// <summary>
@@ -397,9 +401,10 @@ namespace ICSharpCode.ILSpy.TextView
 		/// is <see cref="int.MaxValue"/>). <paramref name="isDebug"/> toggles the transforms'
 		/// verbose-debug emission. No-op when there's nothing currently being decompiled.
 		/// </summary>
-		public void RestartDecompileWithStepLimit(int stepLimit, bool isDebug)
+		public void RestartDecompileWithStepLimit(int stepLimit, bool isDebug, int? highlightStep = null)
 		{
 			pendingStepLimit = stepLimit;
+			pendingHighlightStep = highlightStep;
 			pendingIsDebug = isDebug;
 			StartDecompile();
 		}
@@ -511,6 +516,7 @@ namespace ICSharpCode.ILSpy.TextView
 				References = null;
 				DefinitionLookup = null;
 				DebugInfo = null;
+				DebugStepHighlight = null;
 				UIElements = null;
 				Text = string.Empty;
 				IsDecompiling = false;
@@ -537,8 +543,10 @@ namespace ICSharpCode.ILSpy.TextView
 				// stable value, and reset the fields to defaults so the NEXT decompile runs
 				// at full fidelity unless RestartDecompileWithStepLimit sets them again.
 				var stepLimit = pendingStepLimit;
+				var highlightStep = pendingHighlightStep;
 				var isDebug = pendingIsDebug;
 				pendingStepLimit = int.MaxValue;
+				pendingHighlightStep = null;
 				pendingIsDebug = false;
 				var outputLengthLimit = pendingOutputLengthLimit;
 				pendingOutputLengthLimit = DefaultOutputLengthLimit;
@@ -553,6 +561,7 @@ namespace ICSharpCode.ILSpy.TextView
 							decompilerSettings ?? new ICSharpCode.Decompiler.DecompilerSettings()) {
 							CancellationToken = cts.Token,
 							StepLimit = stepLimit,
+							HighlightStep = highlightStep,
 							IsDebug = isDebug,
 						};
 						try
@@ -745,6 +754,7 @@ namespace ICSharpCode.ILSpy.TextView
 				: output.MethodDebugInfos.Count > 0
 					? new Bookmarks.DecompiledDebugInfo(output.MethodDebugInfos)
 					: Bookmarks.DecompiledDebugInfo.Empty;
+			DebugStepHighlight = output.DebugStepHighlight;
 			UIElements = output.UIElements;
 			Text = text;
 		}

ILSpy/TextView/DecompilerTextView.axaml.cs

@@ -64,6 +64,7 @@ namespace ICSharpCode.ILSpy.TextView
 		// softer green for the actual definition.
 		static readonly Color LocalMatchBackground = Colors.GreenYellow;
 		static readonly Color LocalDefinitionBackground = Color.FromArgb(0x80, 0xA0, 0xFF, 0xA0);
+		static readonly Color DebugStepBackground = Color.FromArgb(0x80, 0xFF, 0xD7, 0x66);
 
 		// Stay-open corridor for the rich popup: as long as the pointer is closer than
 		// `distanceToPopupLimit` to the popup edges, the popup stays. The limit shrinks toward
@@ -78,6 +79,7 @@ namespace ICSharpCode.ILSpy.TextView
 		TextMarkerService textMarkerService = null!;
 		BracketHighlightRenderer bracketHighlightRenderer = null!;
 		readonly List<TextMarker> localReferenceMarks = new();
+		readonly List<TextMarker> debugStepMarks = new();
 		readonly List<AvaloniaEdit.Rendering.VisualLineElementGenerator> activeCustomGenerators = new();
 		RichTextColorizer? activeColorizer;
 		FoldingManager? activeFoldingManager;
@@ -1043,6 +1045,7 @@ namespace ICSharpCode.ILSpy.TextView
 			// force-close even if the popup currently wants to stay (mouseClick: true).
 			TryCloseExistingPopup(mouseClick: true);
 			ClearLocalReferenceMarks();
+			ClearDebugStepMarks();
 			Editor.SyntaxHighlighting = HighlightingService.GetByExtension(model.SyntaxExtension);
 			Editor.Document.Text = model.Text;
 
@@ -1072,6 +1075,8 @@ namespace ICSharpCode.ILSpy.TextView
 			if (restoreViewState)
 				RestoreOrResetViewState(pendingState);
 
+			ApplyDebugStepHighlight(model.DebugStepHighlight);
+
 			// Position at a navigated-to bookmark once its document (and debug map) has landed. Only on
 			// the final content (the Text change, where restoreViewState is set), AFTER the view-state
 			// reset above, so the bookmark scroll is the last word on position and the highlight plays
@@ -1246,6 +1251,31 @@ namespace ICSharpCode.ILSpy.TextView
 			localReferenceMarks.Clear();
 		}
 
+		void ApplyDebugStepHighlight(TextRange? range)
+		{
+			ClearDebugStepMarks();
+			if (range is not { } r || r.Length <= 0)
+				return;
+			var start = Math.Clamp(r.Start, 0, Editor.Document.TextLength);
+			var end = Math.Clamp(r.Start + r.Length, start, Editor.Document.TextLength);
+			if (end <= start)
+				return;
+			var mark = textMarkerService.Create(start, end - start);
+			mark.BackgroundColor = DebugStepBackground;
+			debugStepMarks.Add(mark);
+			Editor.TextArea.Caret.Offset = start;
+			Editor.TextArea.Caret.BringCaretToView();
+			CaretHighlightAdorner.DisplayCaretHighlightAnimation(Editor.TextArea);
+		}
+
+
+		void ClearDebugStepMarks()
+		{
+			foreach (var mark in debugStepMarks)
+				textMarkerService.Remove(mark);
+			debugStepMarks.Clear();
+		}
+
 		// Live cursor + full ScrollOffset, queried at hover-event time. Returns null if the
 		// pointer is past the end of the line so we don't snap back to the last visual
 		// column when the user is hovering empty trailing space.

ILSpy/TextView/NodeLookup.cs

@@ -0,0 +1,50 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System.Collections.Generic;
+
+using ICSharpCode.Decompiler.CSharp.Syntax;
+
+namespace ICSharpCode.ILSpy.TextView
+{
+	/// <summary>
+	/// Maps syntax tree nodes to character ranges in the rendered text.
+	/// </summary>
+	internal sealed class NodeLookup
+	{
+		readonly Dictionary<object, TextRange> nodes = new(ReferenceEqualityComparer.Instance);
+
+		public bool TryGetRange(object node, out TextRange range)
+			=> nodes.TryGetValue(node, out range);
+
+		public void AddNode(object node, int start, int length)
+		{
+			if (length > 0)
+			{
+				nodes[node] = new TextRange(start, length);
+				if (node is IAnnotatable annotatable)
+				{
+					foreach (var annotation in annotatable.Annotations)
+					{
+						nodes[annotation] = new TextRange(start, length);
+					}
+				}
+			}
+		}
+	}
+}

ILSpy/TextView/TextRange.cs

@@ -0,0 +1,22 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+namespace ICSharpCode.ILSpy.TextView
+{
+	public readonly record struct TextRange(int Start, int Length);
+}

ILSpy/ViewModels/DebugStepsPaneModel.cs

@@ -109,8 +109,8 @@ namespace ICSharpCode.ILSpy.ViewModels
 		{
 			Id = PaneContentId;
 			Title = "Debug Steps";
-			ShowStateBeforeCommand = new RelayCommand(() => RequestRedecompile(SelectedStep?.BeginStep ?? int.MaxValue, isDebug: false));
+			ShowStateBeforeCommand = new RelayCommand(() => RequestRedecompile(SelectedStep?.BeginStep ?? int.MaxValue, isDebug: false, SelectedStep?.BeginStep));
-			ShowStateAfterCommand = new RelayCommand(() => RequestRedecompile(SelectedStep?.EndStep ?? int.MaxValue, isDebug: false));
+			ShowStateAfterCommand = new RelayCommand(() => RequestRedecompile(SelectedStep?.EndStep ?? int.MaxValue, isDebug: false, SelectedStep?.BeginStep));
 			DebugStepCommand = new RelayCommand(() => {
 				// "Debug this step" relies on Stepper.Step calling Debugger.Break() when
 				// step == StepLimit — which is a silent no-op without a debugger attached.
@@ -123,7 +123,7 @@ namespace ICSharpCode.ILSpy.ViewModels
 					if (!System.Diagnostics.Debugger.Launch())
 						AppEnv.AppLog.Mark("DebugStep: Debugger.Launch returned false; the upcoming Stepper.Step break is a no-op without a debugger attached.");
 				}
-				RequestRedecompile(SelectedStep?.BeginStep ?? int.MaxValue, isDebug: true);
+				RequestRedecompile(SelectedStep?.BeginStep ?? int.MaxValue, isDebug: true, SelectedStep?.BeginStep);
 			});
 		}
 
@@ -222,12 +222,12 @@ namespace ICSharpCode.ILSpy.ViewModels
 			RequestRedecompile(lastSelectedStep, isDebug: false);
 		}
 
-		void RequestRedecompile(int stepLimit, bool isDebug)
+		void RequestRedecompile(int stepLimit, bool isDebug, int? highlightStep = null)
 		{
 			lastSelectedStep = stepLimit;
 			// Composition unavailable in design-time previews; the gesture is a no-op there.
 			var dock = AppComposition.TryGetExport<DockWorkspace>();
-			dock?.ActiveDecompilerTab?.RestartDecompileWithStepLimit(stepLimit, isDebug);
+			dock?.ActiveDecompilerTab?.RestartDecompileWithStepLimit(stepLimit, isDebug, highlightStep);
 		}
 	}
 }