Make optional AST slots nullable

Optional single-child slots return T? with a real null instead of a role
null-object, taking the C# grammar as the oracle for which slots are optional.
The generator emits the property type as T? and matches it with MatchOptional,
and consumers move from .IsNull to is null / ?.. This covers the optional
statement, member, try-catch, creation-initializer and pattern slots and the
optional NameSlot tokens. A few slots the grammar marks required but the
decompiler legitimately leaves empty (the implicit-element-access target, an
implicitly-typed lambda parameter's type) are flipped to nullable as well.
Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler.Generators/DecompilerSyntaxTreeGenerator.cs

@@ -143,7 +143,8 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 				string roleExpr = (string)nameSlotAttr.ConstructorArguments[0].Value!;
 				bool nullOnEmpty = nameSlotAttr.ConstructorArguments.Length > 1 && (bool)nameSlotAttr.ConstructorArguments[1].Value!;
 				string tokenName = property.Name + "Token";
-				slots.Add((roleExpr, false, tokenName, "Identifier", "Identifier", false, false, SlotKindName(roleExpr), false));
+				// An optional name (nullOnEmpty) makes the backing token a real nullable slot: an absent name is a null token.
+				slots.Add((roleExpr, false, tokenName, "Identifier", "Identifier", false, nullOnEmpty, SlotKindName(roleExpr), false));
 				nameSlots.Add((property.Name, tokenName, nullOnEmpty));
 			}
 		}
@@ -363,7 +364,7 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 				else
 				{
 					builder.AppendLine($"\t{type}? {field};");
-					builder.AppendLine($"\tpublic {(isOverride ? "override " : "")}{partialKw}{type} {name}");
+					builder.AppendLine($"\tpublic {(isOverride ? "override " : "")}{partialKw}{type}{(isNullable ? "?" : "")} {name}");
 					builder.AppendLine("\t{");
 					builder.AppendLine($"\t\tget => {field}{(isNullable ? "" : $" ?? {roleExpr}.NullObject")};");
 					builder.AppendLine($"\t\tset => SetChildNode(ref {field}, value, {roleExpr});");
@@ -379,11 +380,17 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 				{
 					builder.AppendLine($"\tpublic partial string {stringName}");
 					builder.AppendLine("\t{");
-					builder.AppendLine($"\t\tget => {tokenName}.Name;");
 					if (nullOnEmpty)
-						builder.AppendLine($"\t\tset => {tokenName} = string.IsNullOrEmpty(value) ? null! : global::ICSharpCode.Decompiler.CSharp.Syntax.Identifier.Create(value);");
+					{
+						// The token is a nullable slot, so guard the read and clear it (to null) on an empty name.
+						builder.AppendLine($"\t\tget => {tokenName}?.Name ?? string.Empty;");
+						builder.AppendLine($"\t\tset => {tokenName} = string.IsNullOrEmpty(value) ? null : global::ICSharpCode.Decompiler.CSharp.Syntax.Identifier.Create(value);");
+					}
 					else
+					{
+						builder.AppendLine($"\t\tget => {tokenName}.Name;");
 						builder.AppendLine($"\t\tset => {tokenName} = global::ICSharpCode.Decompiler.CSharp.Syntax.Identifier.Create(value);");
+					}
 					builder.AppendLine("\t}");
 					builder.AppendLine();
 				}

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -1663,7 +1663,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					typeDecl.Members.AddRange(entityMap[member]);
 				}
 
-				if (typeDecl.Members.OfType<IndexerDeclaration>().Any(idx => idx.PrivateImplementationType.IsNull))
+				if (typeDecl.Members.OfType<IndexerDeclaration>().Any(idx => idx.PrivateImplementationType is null))
 				{
 					// Remove the [DefaultMember] attribute if the class contains indexers
 					RemoveAttribute(typeDecl, KnownAttribute.DefaultMember);

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -731,7 +731,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			var assignment = HandleAccessorCall(expectedTargetDetails, method, unused,
 				argumentList.Arguments.ToList(), argumentList.ArgumentNames);
 
-			if (((AssignmentExpression)assignment).Left is IndexerExpression indexer && !indexer.Target.IsNull)
+			if (((AssignmentExpression)assignment).Left is IndexerExpression indexer && indexer.Target is not null)
 				indexer.Target.Remove();
 
 			if (value != null)
@@ -1093,7 +1093,7 @@ namespace ICSharpCode.Decompiler.CSharp
 						return false;
 					case ArrayCreateResolveResult { Type: ArrayType { ElementType: var type3 }, SizeArguments: [{ ConstantValue: int arrayLength }] }:
 						elementType = type3;
-						arguments = new(((ArrayCreateExpression)paramsArgument.Expression).Initializer.Elements.Select(e => new TranslatedExpression(e)));
+						arguments = new(((ArrayCreateExpression)paramsArgument.Expression).Initializer?.Elements.Select(e => new TranslatedExpression(e)) ?? []);
 						parameters = new List<IParameter>(arrayLength);
 						for (int i = 0; i < arrayLength; i++)
 						{
@@ -1423,7 +1423,8 @@ namespace ICSharpCode.Decompiler.CSharp
 					continue;
 				if (child is ReturnStatement ret)
 				{
-					ret.Expression = new TranslatedExpression(ret.Expression.Detach()).ConvertTo(returnType, expressionBuilder);
+					if (ret.Expression is not null)
+						ret.Expression = new TranslatedExpression(ret.Expression.Detach()).ConvertTo(returnType, expressionBuilder);
 					continue;
 				}
 				ModifyReturnStatementInsideLambda(returnType, child);

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -2469,7 +2469,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 
 			bool isLambda = false;
-			if (ame.Parameters.Any(p => p.Type.IsNull))
+			if (ame.Parameters.Any(p => p.Type is null))
 			{
 				// if there is an anonymous type involved, we are forced to use a lambda expression.
 				isLambda = true;
@@ -2532,7 +2532,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				function, delegateType, inferredReturnType,
 				hasParameterList: isLambda || ame.HasParameterList,
 				isAnonymousMethod: !isLambda,
-				isImplicitlyTyped: ame.Parameters.Any(p => p.Type.IsNull));
+				isImplicitlyTyped: ame.Parameters.Any(p => p.Type is null));
 
 			TranslatedExpression translatedLambda = replacement.WithILInstruction(function).WithRR(rr);
 			return new CastExpression(ConvertType(delegateType), translatedLambda)
@@ -2558,7 +2558,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			{
 				if (node is ReturnStatement ret)
 				{
-					if (!ret.Expression.IsNull)
+					if (ret.Expression is not null)
 					{
 						returnExpressions.Add(ret.Expression.GetResolveResult());
 					}

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpAmbience.cs

@@ -162,7 +162,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 					}
 					if ((ConversionFlags & ConversionFlags.ShowParameterDefaultValues) == 0)
 					{
-						param.DefaultExpression.Detach();
+						param.DefaultExpression?.Detach();
 					}
 					if (first)
 					{

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpOutputVisitor.cs

@@ -244,7 +244,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 					return false;
 				if (!(kind == SlotKind.Getter || kind == SlotKind.Setter))
 					return false;
-				bool isAutoProperty = accessor.Body.IsNull
+				bool isAutoProperty = accessor.Body is null
 					&& !accessor.Attributes.Any()
 					&& policy.AutoPropertyFormatting == PropertyFormatting.SingleLine;
 				return isAutoProperty;
@@ -531,7 +531,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 
 		protected virtual void WriteMethodBody(BlockStatement body, BraceStyle style, bool newLine = true)
 		{
-			if (body.IsNull)
+			if (body is null || body.IsNull)
 			{
 				Semicolon();
 			}
@@ -552,7 +552,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 
 		protected virtual void WritePrivateImplementationType(AstType privateImplementationType)
 		{
-			if (!privateImplementationType.IsNull)
+			if (privateImplementationType is not null && !privateImplementationType.IsNull)
 			{
 				privateImplementationType.AcceptVisitor(this);
 				WriteToken(Roles.Dot);
@@ -620,7 +620,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			{
 				specifier.AcceptVisitor(this);
 			}
-			arrayCreateExpression.Initializer.AcceptVisitor(this);
+			arrayCreateExpression.Initializer?.AcceptVisitor(this);
 			EndNode(arrayCreateExpression);
 		}
 
@@ -925,7 +925,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(recursivePatternExpression);
 
-			recursivePatternExpression.Type.AcceptVisitor(this);
+			recursivePatternExpression.Type?.AcceptVisitor(this);
 			Space();
 			if (recursivePatternExpression.IsPositional)
 			{
@@ -946,7 +946,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			{
 				WriteToken(Roles.RBrace);
 			}
-			if (!recursivePatternExpression.Designation.IsNull)
+			if (recursivePatternExpression.Designation is not null)
 			{
 				Space();
 				recursivePatternExpression.Designation.AcceptVisitor(this);
@@ -979,7 +979,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		public virtual void VisitIndexerExpression(IndexerExpression indexerExpression)
 		{
 			StartNode(indexerExpression);
-			indexerExpression.Target.AcceptVisitor(this);
+			indexerExpression.Target?.AcceptVisitor(this);
 			Space(policy.SpaceBeforeMethodCallParentheses);
 			WriteCommaSeparatedListInBrackets(indexerExpression.Arguments);
 			EndNode(indexerExpression);
@@ -1050,7 +1050,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				return true;
 			}
 			var p = lambdaExpression.Parameters.Single();
-			return !(p.Type.IsNull && p.ParameterModifier == ReferenceKind.None && !p.IsParams);
+			return !(p.Type is null && p.ParameterModifier == ReferenceKind.None && !p.IsParams);
 		}
 
 		public virtual void VisitMemberReferenceExpression(MemberReferenceExpression memberReferenceExpression)
@@ -1102,13 +1102,13 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			StartNode(objectCreateExpression);
 			WriteKeyword(ObjectCreateExpression.NewKeywordRole);
 			objectCreateExpression.Type.AcceptVisitor(this);
-			bool useParenthesis = objectCreateExpression.Arguments.Any() || objectCreateExpression.Initializer.IsNull;
+			bool useParenthesis = objectCreateExpression.Arguments.Any() || objectCreateExpression.Initializer is null;
 			if (useParenthesis)
 			{
 				Space(policy.SpaceBeforeMethodCallParentheses);
 				WriteCommaSeparatedListInParenthesis(objectCreateExpression.Arguments, policy.SpaceWithinMethodCallParentheses);
 			}
-			objectCreateExpression.Initializer.AcceptVisitor(this);
+			objectCreateExpression.Initializer?.AcceptVisitor(this);
 			EndNode(objectCreateExpression);
 		}
 
@@ -1212,9 +1212,9 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(stackAllocExpression);
 			WriteKeyword(StackAllocExpression.StackallocKeywordRole);
-			stackAllocExpression.Type.AcceptVisitor(this);
-			WriteCommaSeparatedListInBrackets(new[] { stackAllocExpression.CountExpression });
-			stackAllocExpression.Initializer.AcceptVisitor(this);
+			stackAllocExpression.Type?.AcceptVisitor(this);
+			WriteCommaSeparatedListInBrackets(stackAllocExpression.CountExpression is { } stackAllocCount ? new[] { stackAllocCount } : Array.Empty<Expression>());
+			stackAllocExpression.Initializer?.AcceptVisitor(this);
 			EndNode(stackAllocExpression);
 		}
 
@@ -1831,7 +1831,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			WriteToken(Roles.Semicolon);
 			Space(policy.SpaceAfterForSemicolon);
 
-			forStatement.Condition.AcceptVisitor(this);
+			forStatement.Condition?.AcceptVisitor(this);
 			Space(policy.SpaceBeforeForSemicolon);
 			WriteToken(Roles.Semicolon);
 			if (forStatement.Iterators.Any())
@@ -1886,7 +1886,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			Space(policy.SpacesWithinIfParentheses);
 			RPar();
 
-			if (ifElseStatement.FalseStatement.IsNull)
+			if (ifElseStatement.FalseStatement is null)
 			{
 				WriteEmbeddedStatement(ifElseStatement.TrueStatement);
 			}
@@ -1947,7 +1947,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(returnStatement);
 			WriteKeyword(ReturnStatement.ReturnKeywordRole);
-			if (!returnStatement.Expression.IsNull)
+			if (returnStatement.Expression is not null)
 			{
 				Space();
 				returnStatement.Expression.AcceptVisitor(this);
@@ -2024,7 +2024,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		public virtual void VisitCaseLabel(CaseLabel caseLabel)
 		{
 			StartNode(caseLabel);
-			if (caseLabel.Expression.IsNull)
+			if (caseLabel.Expression is null)
 			{
 				WriteKeyword(CaseLabel.DefaultKeywordRole);
 			}
@@ -2070,7 +2070,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(throwStatement);
 			WriteKeyword(ThrowStatement.ThrowKeywordRole);
-			if (!throwStatement.Expression.IsNull)
+			if (throwStatement.Expression is not null)
 			{
 				Space();
 				throwStatement.Expression.AcceptVisitor(this);
@@ -2092,7 +2092,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 					NewLine();
 				catchClause.AcceptVisitor(this);
 			}
-			if (!tryCatchStatement.FinallyBlock.IsNull)
+			if (tryCatchStatement.FinallyBlock is not null)
 			{
 				if (policy.FinallyNewLinePlacement == NewLinePlacement.SameLine)
 					Space();
@@ -2109,7 +2109,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(catchClause);
 			WriteKeyword(CatchClause.CatchKeywordRole);
-			if (!catchClause.Type.IsNull)
+			if (catchClause.Type is not null)
 			{
 				Space(policy.SpaceBeforeCatchParentheses);
 				LPar();
@@ -2123,7 +2123,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				Space(policy.SpacesWithinCatchParentheses);
 				RPar();
 			}
-			if (!catchClause.Condition.IsNull)
+			if (catchClause.Condition is not null)
 			{
 				Space();
 				WriteKeyword(CatchClause.WhenKeywordRole);
@@ -2313,7 +2313,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				WriteIdentifier(constructorDeclaration.NameToken);
 			Space(policy.SpaceBeforeConstructorDeclarationParentheses);
 			WriteCommaSeparatedListInParenthesis(constructorDeclaration.Parameters, policy.SpaceWithinMethodDeclarationParentheses);
-			if (!constructorDeclaration.Initializer.IsNull)
+			if (constructorDeclaration.Initializer is not null)
 			{
 				NewLine();
 				writer.Indent();
@@ -2370,7 +2370,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			WriteAttributes(enumMemberDeclaration.Attributes);
 			WriteModifiers(enumMemberDeclaration.Modifiers);
 			WriteIdentifier(enumMemberDeclaration.NameToken);
-			if (!enumMemberDeclaration.Initializer.IsNull)
+			if (enumMemberDeclaration.Initializer is not null)
 			{
 				Space(policy.SpaceAroundAssignment);
 				WriteToken(Roles.Assign);
@@ -2500,14 +2500,14 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			Space(policy.SpaceBeforeMethodDeclarationParentheses);
 			WriteCommaSeparatedListInBrackets(indexerDeclaration.Parameters, policy.SpaceWithinMethodDeclarationParentheses);
 
-			if (indexerDeclaration.ExpressionBody.IsNull)
+			if (indexerDeclaration.ExpressionBody is null)
 			{
 				bool isSingleLine =
 					(policy.AutoPropertyFormatting == PropertyFormatting.SingleLine)
-					&& (indexerDeclaration.Getter.IsNull || indexerDeclaration.Getter.Body.IsNull)
-					&& (indexerDeclaration.Setter.IsNull || indexerDeclaration.Setter.Body.IsNull)
-					&& !indexerDeclaration.Getter.Attributes.Any()
-					&& !indexerDeclaration.Setter.Attributes.Any();
+					&& (indexerDeclaration.Getter is null || indexerDeclaration.Getter.Body is null)
+					&& (indexerDeclaration.Setter is null || indexerDeclaration.Setter.Body is null)
+					&& (indexerDeclaration.Getter is null || !indexerDeclaration.Getter.Attributes.Any())
+					&& (indexerDeclaration.Setter is null || !indexerDeclaration.Setter.Attributes.Any());
 				OpenBrace(isSingleLine ? BraceStyle.EndOfLine : policy.PropertyBraceStyle, newLine: !isSingleLine);
 				if (isSingleLine)
 					Space();
@@ -2634,8 +2634,8 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 					Space();
 					break;
 			}
-			parameterDeclaration.Type.AcceptVisitor(this);
-			if (!parameterDeclaration.Type.IsNull && !string.IsNullOrEmpty(parameterDeclaration.Name))
+			parameterDeclaration.Type?.AcceptVisitor(this);
+			if (parameterDeclaration.Type is not null && !string.IsNullOrEmpty(parameterDeclaration.Name))
 			{
 				Space();
 			}
@@ -2643,7 +2643,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			{
 				WriteIdentifier(parameterDeclaration.NameToken);
 			}
-			if (!parameterDeclaration.DefaultExpression.IsNull)
+			if (parameterDeclaration.DefaultExpression is not null)
 			{
 				Space(policy.SpaceAroundAssignment);
 				WriteToken(Roles.Assign);
@@ -2662,14 +2662,14 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			Space();
 			WritePrivateImplementationType(propertyDeclaration.PrivateImplementationType);
 			WriteIdentifier(propertyDeclaration.NameToken);
-			if (propertyDeclaration.ExpressionBody.IsNull)
+			if (propertyDeclaration.ExpressionBody is null)
 			{
 				bool isSingleLine =
 					(policy.AutoPropertyFormatting == PropertyFormatting.SingleLine)
-					&& (propertyDeclaration.Getter.IsNull || propertyDeclaration.Getter.Body.IsNull)
-					&& (propertyDeclaration.Setter.IsNull || propertyDeclaration.Setter.Body.IsNull)
-					&& !propertyDeclaration.Getter.Attributes.Any()
-					&& !propertyDeclaration.Setter.Attributes.Any();
+					&& (propertyDeclaration.Getter is null || propertyDeclaration.Getter.Body is null)
+					&& (propertyDeclaration.Setter is null || propertyDeclaration.Setter.Body is null)
+					&& (propertyDeclaration.Getter is null || !propertyDeclaration.Getter.Attributes.Any())
+					&& (propertyDeclaration.Setter is null || !propertyDeclaration.Setter.Attributes.Any());
 				OpenBrace(isSingleLine ? BraceStyle.EndOfLine : policy.PropertyBraceStyle, newLine: !isSingleLine);
 				if (isSingleLine)
 					Space();
@@ -2714,7 +2714,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(variableInitializer);
 			WriteIdentifier(variableInitializer.NameToken);
-			if (!variableInitializer.Initializer.IsNull)
+			if (variableInitializer.Initializer is not null)
 			{
 				Space(policy.SpaceAroundAssignment);
 				WriteToken(Roles.Assign);
@@ -2788,7 +2788,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(tupleTypeElement);
 			tupleTypeElement.Type.AcceptVisitor(this);
-			if (!tupleTypeElement.NameToken.IsNull)
+			if (tupleTypeElement.NameToken is not null)
 			{
 				Space();
 				tupleTypeElement.NameToken.AcceptVisitor(this);
@@ -3098,7 +3098,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		public virtual void VisitDocumentationReference(DocumentationReference documentationReference)
 		{
 			StartNode(documentationReference);
-			if (!documentationReference.DeclaringType.IsNull)
+			if (documentationReference.DeclaringType is not null)
 			{
 				documentationReference.DeclaringType.AcceptVisitor(this);
 				if (documentationReference.SymbolKind != SymbolKind.TypeDefinition)

ICSharpCode.Decompiler/CSharp/OutputVisitor/InsertParenthesesVisitor.cs

@@ -199,17 +199,21 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 
 		public override void VisitIndexerExpression(IndexerExpression indexerExpression)
 		{
-			ParenthesizeIfRequired(indexerExpression.Target, PrecedenceLevel.Primary);
-			switch (indexerExpression.Target)
+			// An implicit-element-access indexer (e.g. the '[i]' in a dictionary initializer) has no target.
+			if (indexerExpression.Target is not null)
 			{
-				case ArrayCreateExpression ace when InsertParenthesesForReadability || ace.Initializer.IsNull:
-					// require parentheses for "(new int[1])[0]"
-					Parenthesize(indexerExpression.Target);
-					break;
-				case StackAllocExpression sae when InsertParenthesesForReadability || sae.Initializer.IsNull:
-					// require parentheses for "(stackalloc int[1])[0]"
-					Parenthesize(indexerExpression.Target);
-					break;
+				ParenthesizeIfRequired(indexerExpression.Target, PrecedenceLevel.Primary);
+				switch (indexerExpression.Target)
+				{
+					case ArrayCreateExpression ace when InsertParenthesesForReadability || ace.Initializer is null:
+						// require parentheses for "(new int[1])[0]"
+						Parenthesize(indexerExpression.Target);
+						break;
+					case StackAllocExpression sae when InsertParenthesesForReadability || sae.Initializer is null:
+						// require parentheses for "(stackalloc int[1])[0]"
+						Parenthesize(indexerExpression.Target);
+						break;
+				}
 			}
 			base.VisitIndexerExpression(indexerExpression);
 		}
@@ -487,7 +491,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 
 		public override void VisitVariableInitializer(VariableInitializer variableInitializer)
 		{
-			if (!variableInitializer.Initializer.IsNull)
+			if (variableInitializer.Initializer is not null)
 				HandleAssignmentRHS(variableInitializer.Initializer);
 			base.VisitVariableInitializer(variableInitializer);
 		}

ICSharpCode.Decompiler/CSharp/SequencePointBuilder.cs

@@ -96,7 +96,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		void VisitAsSequencePoint(AstNode node)
 		{
-			if (node.IsNull)
+			if (node is null || node.IsNull)
 				return;
 			StartSequencePoint(node);
 			node.AcceptVisitor(this);
@@ -165,7 +165,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		public override void VisitPropertyDeclaration(PropertyDeclaration propertyDeclaration)
 		{
-			if (!propertyDeclaration.ExpressionBody.IsNull)
+			if (propertyDeclaration.ExpressionBody is not null)
 			{
 				VisitAsSequencePoint(propertyDeclaration.ExpressionBody);
 			}
@@ -177,7 +177,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		public override void VisitIndexerDeclaration(IndexerDeclaration indexerDeclaration)
 		{
-			if (!indexerDeclaration.ExpressionBody.IsNull)
+			if (indexerDeclaration.ExpressionBody is not null)
 			{
 				VisitAsSequencePoint(indexerDeclaration.ExpressionBody);
 			}
@@ -385,7 +385,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		public override void VisitCatchClause(CatchClause catchClause)
 		{
-			if (catchClause.Condition.IsNull)
+			if (catchClause.Condition is null)
 			{
 				var tryCatchHandler = catchClause.Annotation<TryCatchHandler>();
 				if (tryCatchHandler != null && !tryCatchHandler.ExceptionSpecifierILRange.IsEmpty)
@@ -432,9 +432,9 @@ namespace ICSharpCode.Decompiler.CSharp
 		// the 'catch' keyword when there is no exception specifier.
 		static TextLocation CatchHeaderEnd(CatchClause catchClause)
 		{
-			if (catchClause.Type.IsNull)
+			if (catchClause.Type is null)
 				return Offset(catchClause.StartLocation, "catch".Length);
-			AstNode beforeRParen = catchClause.VariableNameToken.IsNull ? catchClause.Type : catchClause.VariableNameToken;
+			AstNode beforeRParen = catchClause.VariableNameToken is null ? catchClause.Type : catchClause.VariableNameToken;
 			return Offset(beforeRParen.EndLocation, 1);
 		}
 

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -268,7 +268,7 @@ namespace ICSharpCode.Decompiler.CSharp
 							ConvertSwitchSectionBody(astSection, section.Body);
 						break;
 					case Leave leave:
-						if (astSection.CaseLabels.Count == 1 && astSection.CaseLabels.First().Expression.IsNull && leave.TargetContainer == switchContainer)
+						if (astSection.CaseLabels.Count == 1 && astSection.CaseLabels.First().Expression is null && leave.TargetContainer == switchContainer)
 						{
 							stmt.SwitchSections.Remove(astSection);
 							break;
@@ -442,7 +442,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			var tryBlockConverted = Convert(tryBlock);
 			var tryCatch = tryBlockConverted as TryCatchStatement;
-			if (tryCatch != null && tryCatch.FinallyBlock.IsNull)
+			if (tryCatch != null && tryCatch.FinallyBlock is null)
 				return tryCatch; // extend existing try-catch
 			tryCatch = new TryCatchStatement();
 			tryCatch.TryBlock = tryBlockConverted as BlockStatement ?? new BlockStatement { tryBlockConverted };

ICSharpCode.Decompiler/CSharp/Syntax/DocumentationReference.cs

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -79,7 +81,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// Gets/Sets the declaring type.
 		/// </summary>
 		[Slot("DeclaringTypeRole")]
-		public partial AstType DeclaringType { get; set; }
+		public partial AstType? DeclaringType { get; set; }
 
 		/// <summary>
 		/// Gets/sets the member name.
@@ -106,9 +108,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		[Slot("Roles.Parameter")]
 		public partial AstNodeCollection<ParameterDeclaration> Parameters { get; }
 
-		protected internal override bool DoMatch(AstNode other, PatternMatching.Match match)
+		protected internal override bool DoMatch(AstNode? other, PatternMatching.Match match)
 		{
-			DocumentationReference o = other as DocumentationReference;
+			DocumentationReference? o = other as DocumentationReference;
 			if (!(o != null && this.SymbolKind == o.SymbolKind && this.HasParameterList == o.HasParameterList))
 				return false;
 			if (this.SymbolKind == SymbolKind.Operator)

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

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -42,6 +44,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial AstNodeCollection<ArraySpecifier> AdditionalArraySpecifiers { get; }
 
 		[Slot("InitializerRole")]
-		public partial ArrayInitializerExpression Initializer { get; set; }
+		public partial ArrayInitializerExpression? Initializer { get; set; }
 	}
 }

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

@@ -26,6 +26,8 @@
 
 using System.Collections.Generic;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -35,7 +37,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	public partial class IndexerExpression : Expression
 	{
 		[Slot("Roles.TargetExpression")]
-		public partial Expression Target { get; set; }
+		public partial Expression? Target { get; set; }
 
 		[Slot("Roles.Argument")]
 		public partial AstNodeCollection<Expression> Arguments { get; }
@@ -44,9 +46,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		{
 		}
 
-		public IndexerExpression(Expression target, IEnumerable<Expression> arguments)
+		public IndexerExpression(Expression? target, IEnumerable<Expression>? arguments)
 		{
-			AddChild(target, Roles.TargetExpression);
+			Target = target;
 			if (arguments != null)
 			{
 				foreach (var arg in arguments)
@@ -56,7 +58,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		public IndexerExpression(Expression target, params Expression[] arguments) : this(target, (IEnumerable<Expression>)arguments)
+		public IndexerExpression(Expression? target, params Expression[] arguments) : this(target, (IEnumerable<Expression>)arguments)
 		{
 		}
 	}

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -44,13 +46,13 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial AstNodeCollection<Expression> Arguments { get; }
 
 		[Slot("InitializerRole")]
-		public partial ArrayInitializerExpression Initializer { get; set; }
+		public partial ArrayInitializerExpression? Initializer { get; set; }
 
 		public ObjectCreateExpression()
 		{
 		}
 
-		public ObjectCreateExpression(AstType type, IEnumerable<Expression> arguments = null)
+		public ObjectCreateExpression(AstType type, IEnumerable<Expression>? arguments = null)
 		{
 			AddChild(type, Roles.Type);
 			if (arguments != null)

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

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -32,13 +34,13 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public static readonly Role<Expression> SubPatternRole = new Role<Expression>("SubPattern", Syntax.Expression.Null);
 
 		[Slot("Roles.Type")]
-		public partial AstType Type { get; set; }
+		public partial AstType? Type { get; set; }
 
 		[Slot("SubPatternRole")]
 		public partial AstNodeCollection<Expression> SubPatterns { get; }
 
 		[Slot("Roles.VariableDesignationRole")]
-		public partial VariableDesignation Designation { get; set; }
+		public partial VariableDesignation? Designation { get; set; }
 
 		public bool IsPositional { get; set; }
 	}

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

@@ -24,6 +24,10 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
+using System.Diagnostics.CodeAnalysis;
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -41,12 +45,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public readonly static Role<ArrayInitializerExpression> InitializerRole = new Role<ArrayInitializerExpression>("Initializer", ArrayInitializerExpression.Null);
 
 		[Slot("Roles.Type")]
-		public partial AstType Type { get; set; }
+		public partial AstType? Type { get; set; }
 
 		[Slot("Roles.Expression")]
-		public partial Expression CountExpression { get; set; }
+		public partial Expression? CountExpression { get; set; }
 
 		[Slot("InitializerRole")]
-		public partial ArrayInitializerExpression Initializer { get; set; }
+		public partial ArrayInitializerExpression? Initializer { get; set; }
 	}
 }

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -45,7 +47,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial AstNodeCollection<Statement> Initializers { get; }
 
 		[Slot("Roles.Condition")]
-		public partial Expression Condition { get; set; }
+		public partial Expression? Condition { get; set; }
 
 		[Slot("IteratorRole")]
 		public partial AstNodeCollection<Statement> Iterators { get; }

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -45,13 +47,13 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial Statement TrueStatement { get; set; }
 
 		[Slot("FalseRole")]
-		public partial Statement FalseStatement { get; set; }
+		public partial Statement? FalseStatement { get; set; }
 
 		public IfElseStatement()
 		{
 		}
 
-		public IfElseStatement(Expression condition, Statement trueStatement, Statement falseStatement = null)
+		public IfElseStatement(Expression condition, Statement trueStatement, Statement? falseStatement = null)
 		{
 			this.Condition = condition;
 			this.TrueStatement = trueStatement;

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -35,7 +37,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public static readonly TokenRole ReturnKeywordRole = new TokenRole("return");
 
 		[Slot("Roles.Expression")]
-		public partial Expression Expression { get; set; }
+		public partial Expression? Expression { get; set; }
 
 		public ReturnStatement()
 		{

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -70,7 +72,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// Gets or sets the expression. The expression can be null - if the expression is null, it's the default switch section.
 		/// </summary>
 		[Slot("Roles.Expression")]
-		public partial Expression Expression { get; set; }
+		public partial Expression? Expression { get; set; }
 
 		public CaseLabel()
 		{

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -35,7 +37,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public static readonly TokenRole ThrowKeywordRole = new TokenRole("throw");
 
 		[Slot("Roles.Expression")]
-		public partial Expression Expression { get; set; }
+		public partial Expression? Expression { get; set; }
 
 		public ThrowStatement()
 		{

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -45,7 +47,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial AstNodeCollection<CatchClause> CatchClauses { get; }
 
 		[Slot("FinallyBlockRole")]
-		public partial BlockStatement FinallyBlock { get; set; }
+		public partial BlockStatement? FinallyBlock { get; set; }
 	}
 
 	/// <summary>
@@ -61,13 +63,13 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public static readonly TokenRole CondRPar = new TokenRole(")");
 
 		[Slot("Roles.Type")]
-		public partial AstType Type { get; set; }
+		public partial AstType? Type { get; set; }
 
 		[NameSlot("Roles.Identifier", nullOnEmpty: true)]
 		public partial string VariableName { get; set; }
 
 		[Slot("ConditionRole")]
-		public partial Expression Condition { get; set; }
+		public partial Expression? Condition { get; set; }
 
 		[Slot("Roles.Body")]
 		public partial BlockStatement Body { get; set; }

ICSharpCode.Decompiler/CSharp/Syntax/TupleAstType.cs

@@ -39,7 +39,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		[Slot("Roles.Type")]
 		public partial AstType Type { get; set; }
 
-		[NameSlot("Roles.Identifier")]
+		[NameSlot("Roles.Identifier", nullOnEmpty: true)]
 		public partial string Name { get; set; }
 
 	}

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

@@ -71,7 +71,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public override partial AstNodeCollection<AttributeSection> Attributes { get; }
 
 		[Slot("Roles.Body")]
-		public partial BlockStatement Body { get; set; }
+		public partial BlockStatement? Body { get; set; }
 
 		public static TokenRole? GetAccessorKeywordRole(AccessorKind kind)
 		{

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -52,10 +54,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial AstNodeCollection<ParameterDeclaration> Parameters { get; }
 
 		[Slot("InitializerRole")]
-		public partial ConstructorInitializer Initializer { get; set; }
+		public partial ConstructorInitializer? Initializer { get; set; }
 
 		[Slot("Roles.Body")]
-		public partial BlockStatement Body { get; set; }
+		public partial BlockStatement? Body { get; set; }
 	}
 
 	public enum ConstructorInitializerType

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -49,6 +51,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public override partial Identifier NameToken { get; set; }
 
 		[Slot("Roles.Body")]
-		public partial BlockStatement Body { get; set; }
+		public partial BlockStatement? Body { get; set; }
 	}
 }

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -47,7 +49,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public override partial Identifier NameToken { get; set; }
 
 		[Slot("InitializerRole")]
-		public partial Expression Initializer { get; set; }
+		public partial Expression? Initializer { get; set; }
 	}
 }
 

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.ComponentModel;
 
@@ -95,15 +97,15 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// Null node if this member is not an explicit interface implementation.
 		/// </summary>
 		[Slot("PrivateImplementationTypeRole")]
-		public partial AstType PrivateImplementationType { get; set; }
+		public partial AstType? PrivateImplementationType { get; set; }
 
 		[Slot("Roles.Identifier")]
 		public override partial Identifier NameToken { get; set; }
 
 		[Slot("AddAccessorRole")]
-		public partial Accessor AddAccessor { get; set; }
+		public partial Accessor? AddAccessor { get; set; }
 
 		[Slot("RemoveAccessorRole")]
-		public partial Accessor RemoveAccessor { get; set; }
+		public partial Accessor? RemoveAccessor { get; set; }
 	}
 }

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.ComponentModel;
 
@@ -62,7 +64,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public override partial AstType ReturnType { get; set; }
 
 		[Slot("PrivateImplementationTypeRole")]
-		public partial AstType PrivateImplementationType { get; set; }
+		public partial AstType? PrivateImplementationType { get; set; }
 
 		public override string Name {
 			get { return "Item"; }
@@ -79,12 +81,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial AstNodeCollection<ParameterDeclaration> Parameters { get; }
 
 		[Slot("GetterRole")]
-		public partial Accessor Getter { get; set; }
+		public partial Accessor? Getter { get; set; }
 
 		[Slot("SetterRole")]
-		public partial Accessor Setter { get; set; }
+		public partial Accessor? Setter { get; set; }
 
 		[Slot("ExpressionBodyRole")]
-		public partial Expression ExpressionBody { get; set; }
+		public partial Expression? ExpressionBody { get; set; }
 	}
 }

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -51,7 +53,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// Null node if this member is not an explicit interface implementation.
 		/// </summary>
 		[Slot("PrivateImplementationTypeRole")]
-		public partial AstType PrivateImplementationType { get; set; }
+		public partial AstType? PrivateImplementationType { get; set; }
 
 		[Slot("Roles.Identifier")]
 		public override partial Identifier NameToken { get; set; }
@@ -66,7 +68,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial AstNodeCollection<Constraint> Constraints { get; }
 
 		[Slot("Roles.Body")]
-		public partial BlockStatement Body { get; set; }
+		public partial BlockStatement? Body { get; set; }
 
 		public bool IsExtensionMethod {
 			get {

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.ComponentModel;
 
@@ -174,7 +176,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public override partial AstType ReturnType { get; set; }
 
 		[Slot("PrivateImplementationTypeRole")]
-		public partial AstType PrivateImplementationType { get; set; }
+		public partial AstType? PrivateImplementationType { get; set; }
 
 		OperatorType operatorType;
 
@@ -189,7 +191,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial AstNodeCollection<ParameterDeclaration> Parameters { get; }
 
 		[Slot("Roles.Body")]
-		public partial BlockStatement Body { get; set; }
+		public partial BlockStatement? Body { get; set; }
 
 		/// <summary>
 		/// Gets the operator type from the method name, or null, if the method does not represent one of the known operator types.
@@ -281,7 +283,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// <summary>
 		/// Gets the method name for the operator type. ("op_Addition", "op_Implicit", etc.)
 		/// </summary>
-		public static string GetName(OperatorType? type)
+		public static string? GetName(OperatorType? type)
 		{
 			if (type == null)
 				return null;
@@ -316,7 +318,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 
 		public override string Name {
-			get { return GetName(this.OperatorType); }
+			get { return GetName(this.OperatorType)!; }
 			set { throw new NotSupportedException(); }
 		}
 

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

@@ -84,13 +84,13 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 
 		[Slot("Roles.Type")]
-		public partial AstType Type { get; set; }
+		public partial AstType? Type { get; set; }
 
 		[NameSlot("Roles.Identifier")]
 		public partial string Name { get; set; }
 
 		[Slot("Roles.Expression")]
-		public partial Expression DefaultExpression { get; set; }
+		public partial Expression? DefaultExpression { get; set; }
 
 		public ParameterDeclaration()
 		{

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
@@ -64,28 +66,28 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public override partial Identifier NameToken { get; set; }
 
 		[Slot("PrivateImplementationTypeRole")]
-		public partial AstType PrivateImplementationType { get; set; }
+		public partial AstType? PrivateImplementationType { get; set; }
 
 		[Slot("GetterRole")]
-		public partial Accessor Getter { get; set; }
+		public partial Accessor? Getter { get; set; }
 
 		[Slot("SetterRole")]
-		public partial Accessor Setter { get; set; }
+		public partial Accessor? Setter { get; set; }
 
 		[Slot("Roles.Expression")]
 		public partial Expression Initializer { get; set; }
 
 		[Slot("ExpressionBodyRole")]
-		public partial Expression ExpressionBody { get; set; }
+		public partial Expression? ExpressionBody { get; set; }
 
 		public bool IsAutomaticProperty {
 			get {
-				if (!Getter.IsNull && !Getter.Body.IsNull)
+				if (Getter is { Body: not null })
 				{
 					return false;
 				}
 
-				if (!Setter.IsNull && !Setter.Body.IsNull)
+				if (Setter is { Body: not null })
 				{
 					return false;
 				}

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

@@ -24,6 +24,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -37,7 +39,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		{
 		}
 
-		public VariableInitializer(string name, Expression initializer = null)
+		public VariableInitializer(string name, Expression? initializer = null)
 		{
 			this.Name = name;
 			this.Initializer = initializer;
@@ -47,6 +49,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public partial string Name { get; set; }
 
 		[Slot("Roles.Expression")]
-		public partial Expression Initializer { get; set; }
+		public partial Expression? Initializer { get; set; }
 	}
 }

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -2157,7 +2157,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		static void MergeReadOnlyModifiers(EntityDeclaration decl, Accessor accessor1, Accessor accessor2)
 		{
-			if (accessor1.HasModifier(Modifiers.Readonly) && accessor2.IsNull)
+			if (accessor1 is null)
+				return;
+			if (accessor1.HasModifier(Modifiers.Readonly) && accessor2 is null)
 			{
 				accessor1.Modifiers &= ~Modifiers.Readonly;
 				decl.Modifiers |= Modifiers.Readonly;

ICSharpCode.Decompiler/CSharp/Transforms/NormalizeBlockStatements.cs

@@ -91,9 +91,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			DoTransform(usingStatement.EmbeddedStatement, usingStatement);
 		}
 
-		void DoTransform(Statement statement, Statement parent)
+		void DoTransform(Statement? statement, Statement parent)
 		{
-			if (statement.IsNull)
+			if (statement is null || statement.IsNull)
 				return;
 			if (context.Settings.AlwaysUseBraces)
 			{
@@ -219,12 +219,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			var m = CalculatedGetterOnlyPropertyPattern.Match(propertyDeclaration);
 			if (!m.Success)
 				return;
-			if ((propertyDeclaration.Getter.Modifiers & ~movableModifiers) != 0)
+			if (propertyDeclaration.Getter is not { } getter)
+				return;
+			if ((getter.Modifiers & ~movableModifiers) != 0)
 				return;
-			propertyDeclaration.Modifiers |= propertyDeclaration.Getter.Modifiers;
+			propertyDeclaration.Modifiers |= getter.Modifiers;
 			propertyDeclaration.ExpressionBody = m.Get<Expression>("expression").Single().Detach();
-			propertyDeclaration.CopyAnnotationsFrom(propertyDeclaration.Getter);
-			propertyDeclaration.Getter.Remove();
+			propertyDeclaration.CopyAnnotationsFrom(getter);
+			getter.Remove();
 		}
 
 		void SimplifyIndexerDeclaration(IndexerDeclaration indexerDeclaration)
@@ -232,12 +234,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			var m = CalculatedGetterOnlyIndexerPattern.Match(indexerDeclaration);
 			if (!m.Success)
 				return;
-			if ((indexerDeclaration.Getter.Modifiers & ~movableModifiers) != 0)
+			if (indexerDeclaration.Getter is not { } getter)
+				return;
+			if ((getter.Modifiers & ~movableModifiers) != 0)
 				return;
-			indexerDeclaration.Modifiers |= indexerDeclaration.Getter.Modifiers;
+			indexerDeclaration.Modifiers |= getter.Modifiers;
 			indexerDeclaration.ExpressionBody = m.Get<Expression>("expression").Single().Detach();
-			indexerDeclaration.CopyAnnotationsFrom(indexerDeclaration.Getter);
-			indexerDeclaration.Getter.Remove();
+			indexerDeclaration.CopyAnnotationsFrom(getter);
+			getter.Remove();
 		}
 	}
 }

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -109,7 +109,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		public override AstNode VisitPropertyDeclaration(PropertyDeclaration propertyDeclaration)
 		{
 			if (context.Settings.AutomaticProperties
-				&& (!propertyDeclaration.Setter.IsNull || context.Settings.GetterOnlyAutomaticProperties))
+				&& (propertyDeclaration.Setter is not null || context.Settings.GetterOnlyAutomaticProperties))
 			{
 				AstNode? result = TransformAutomaticProperty(propertyDeclaration);
 				if (result != null)
@@ -237,7 +237,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		bool ForStatementUsesVariable(ForStatement statement, IL.ILVariable variable)
 		{
-			if (statement.Condition.DescendantsAndSelf.OfType<IdentifierExpression>().Any(ie => ie.GetILVariable() == variable))
+			if (statement.Condition?.DescendantsAndSelf.OfType<IdentifierExpression>().Any(ie => ie.GetILVariable() == variable) == true)
 				return true;
 			if (statement.Iterators.Any(i => i.DescendantsAndSelf.OfType<IdentifierExpression>().Any(ie => ie.GetILVariable() == variable)))
 				return true;
@@ -638,18 +638,26 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 			if (field == null || !NameCouldBeBackingFieldOfAutomaticProperty(field.Name, out _))
 				return null;
-			if (propertyDeclaration.Setter.HasModifier(Modifiers.Readonly) || (propertyDeclaration.HasModifier(Modifiers.Readonly) && !propertyDeclaration.Setter.IsNull))
+			if (propertyDeclaration.Setter?.HasModifier(Modifiers.Readonly) == true || (propertyDeclaration.HasModifier(Modifiers.Readonly) && propertyDeclaration.Setter is not null))
 				return null;
 			if (field.IsCompilerGenerated() && field.DeclaringTypeDefinition == property.DeclaringTypeDefinition)
 			{
-				RemoveCompilerGeneratedAttribute(propertyDeclaration.Getter.Attributes);
-				RemoveCompilerGeneratedAttribute(propertyDeclaration.Setter.Attributes);
-				// Clearing the accessor body turns it into an auto-property accessor; the slot setter
-				// tolerates null until optional slots become nullable in the AST declarations.
-				propertyDeclaration.Getter.Body = null!;
-				propertyDeclaration.Setter.Body = null!;
+				// Clearing the accessor body turns it into an auto-property accessor.
+				var getter = propertyDeclaration.Getter;
+				var setter = propertyDeclaration.Setter;
+				if (getter is not null)
+				{
+					RemoveCompilerGeneratedAttribute(getter.Attributes);
+					getter.Body = null;
+				}
+				if (setter is not null)
+				{
+					RemoveCompilerGeneratedAttribute(setter.Attributes);
+					setter.Body = null;
+				}
 				propertyDeclaration.Modifiers &= ~Modifiers.Readonly;
-				propertyDeclaration.Getter.Modifiers &= ~Modifiers.Readonly;
+				if (getter is not null)
+					getter.Modifiers &= ~Modifiers.Readonly;
 
 				var fieldDecl = propertyDeclaration.Parent?.Children.OfType<FieldDeclaration>()
 					.FirstOrDefault(fd => field.Equals(fd.GetSymbol()));
@@ -1005,7 +1013,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		EventDeclaration? TransformAutomaticEvents(CustomEventDeclaration ev)
 		{
-			if (!ev.PrivateImplementationType.IsNull)
+			if (ev.PrivateImplementationType is not null)
 				return null;
 			const Modifiers withoutBody = Modifiers.Abstract | Modifiers.Extern;
 			if (ev.GetSymbol() is not IEvent symbol)
@@ -1015,10 +1023,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (!CheckAutomaticEventV4AggressivelyInlined(ev) && !CheckAutomaticEventV4(ev) && !CheckAutomaticEventV2(ev) && !CheckAutomaticEventV4MCS(ev))
 					return null;
 			}
-			RemoveCompilerGeneratedAttribute(ev.AddAccessor.Attributes, attributeTypesToRemoveFromAutoEvents);
+			if (ev.AddAccessor is not { } addAccessor)
+				return null;
+			RemoveCompilerGeneratedAttribute(addAccessor.Attributes, attributeTypesToRemoveFromAutoEvents);
 			EventDeclaration ed = new EventDeclaration();
 			ev.Attributes.MoveTo(ed.Attributes);
-			foreach (var attr in ev.AddAccessor.Attributes)
+			foreach (var attr in addAccessor.Attributes)
 			{
 				attr.AttributeTarget = "method";
 				ed.Attributes.Add(attr.Detach());

ICSharpCode.Decompiler/CSharp/Transforms/PrettifyAssignments.cs

@@ -148,7 +148,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return IsWithoutSideEffects(left);
 		}
 
-		static bool IsWithoutSideEffects(Expression left)
+		static bool IsWithoutSideEffects(Expression? left)
 		{
 			return left is ThisReferenceExpression || left is IdentifierExpression || left is TypeReferenceExpression || left is BaseReferenceExpression;
 		}

ICSharpCode.Decompiler/CSharp/Transforms/ReplaceMethodCallsWithOperators.cs

@@ -63,9 +63,9 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			// Reduce "String.Concat(a, b)" to "a + b"
 			if (IsStringConcat(method) && context.Settings.StringConcat)
 			{
-				if (arguments is [ArrayCreateExpression ace] && method.Parameters is [{ Type: ArrayType }])
+				if (arguments is [ArrayCreateExpression { Initializer: { } aceInitializer }] && method.Parameters is [{ Type: ArrayType }])
 				{
-					arguments = ace.Initializer.Elements.ToArray();
+					arguments = aceInitializer.Elements.ToArray();
 				}
 
 				if (!CheckArgumentsForStringConcat(arguments))

ICSharpCode.Decompiler/CSharp/Transforms/TransformFieldAndConstructorInitializers.cs

@@ -131,7 +131,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				bool skippedStmts = false;
 
 				Statement? stmt;
-				for (stmt = ctor.Body.Statements.FirstOrDefault(); stmt != null; stmt = stmt.GetNextStatement())
+				for (stmt = ctor.Body?.Statements.FirstOrDefault(); stmt != null; stmt = stmt.GetNextStatement())
 				{
 					var m = memberInitializerPattern.Match(stmt);
 					if (!m.Success)
@@ -200,6 +200,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 			public bool IsMatch(ConstructorDeclaration ctor)
 			{
+				if (ctor.Body is null)
+					return false;
 				var stmts = ctor.Body.Statements;
 				var otherStmt = stmts.FirstOrDefault();
 				foreach (var (stmt, member, initializer, _) in Statements)
@@ -326,7 +328,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					else
 					{
 						// find this-ctor call
-						var stmt = ctor.Body.Statements.FirstOrDefault();
+						var stmt = ctor.Body?.Statements.FirstOrDefault();
 						var m = ctorMethod.DeclaringType.Kind == TypeKind.Struct
 							? ThisCallStructPattern.Match(stmt)
 							: ThisCallClassPattern.Match(stmt);
@@ -449,6 +451,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 			public bool MoveConstructorInitializer(ConstructorDeclaration constructorDeclaration, IMethod ctorMethod)
 			{
+				if (constructorDeclaration.Body is null)
+					return false;
 				Statement stmt = constructorDeclaration.Body.Statements.FirstOrDefault()!;
 				var isValueType = ctorMethod.DeclaringType.Kind == TypeKind.Struct;
 
@@ -512,7 +516,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						case FieldDeclaration fd:
 							v = fd.Variables.Single();
-							if (v.Initializer.IsNull)
+							if (v.Initializer is null)
 							{
 								v.Initializer = initializer.Detach();
 							}
@@ -554,7 +558,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 							break;
 						case EventDeclaration ev:
 							v = ev.Variables.Single();
-							if (v.Initializer.IsNull)
+							if (v.Initializer is null)
 							{
 								v.Initializer = initializer.Detach();
 							}
@@ -603,7 +607,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					Debug.Assert(PrimaryConstructorDecl != null);
 
 					this.TypeDeclaration.HasPrimaryConstructor = PrimaryConstructor.Parameters.Any()
-						|| !PrimaryConstructorDecl.Initializer.IsNull
+						|| PrimaryConstructorDecl.Initializer is not null
 						|| TypeDefinition.Kind == TypeKind.Struct;
 
 					// HACK: because our current AST model doesn't allow specifying an explicit order of roles,
@@ -671,15 +675,15 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						this.TypeDeclaration.Modifiers |= Modifiers.Unsafe;
 					}
 
-					if (!PrimaryConstructorDecl.Initializer.IsNull && TypeDeclaration is { BaseTypes.Count: > 0 })
+					if (PrimaryConstructorDecl.Initializer is { } initializer && TypeDeclaration is { BaseTypes.Count: > 0 })
 					{
-						Debug.Assert(PrimaryConstructorDecl.Initializer is { ConstructorInitializerType: ConstructorInitializerType.Base });
+						Debug.Assert(initializer.ConstructorInitializerType == ConstructorInitializerType.Base);
 
 						var baseType = TypeDeclaration.BaseTypes.First();
 						var newBaseType = new InvocationAstType();
 						baseType.ReplaceWith(newBaseType);
 						newBaseType.BaseType = baseType;
-						PrimaryConstructorDecl.Initializer.Arguments.MoveTo(newBaseType.Arguments);
+						initializer.Arguments.MoveTo(newBaseType.Arguments);
 					}
 
 					PrimaryConstructorDecl.Remove();
@@ -690,7 +694,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				{
 					Debug.Assert(StaticConstructorDecl != null);
 
-					if (IsBeforeFieldInit && StaticConstructorDecl.Body.Statements.Count == 0)
+					if (IsBeforeFieldInit && StaticConstructorDecl.Body is { Statements.Count: 0 })
 					{
 						StaticConstructorDecl.Remove();
 					}

ICSharpCode.Decompiler/DebugInfo/DebugInfoGenerator.cs

@@ -135,7 +135,7 @@ namespace ICSharpCode.Decompiler.DebugInfo
 
 		public override void VisitPropertyDeclaration(PropertyDeclaration propertyDeclaration)
 		{
-			if (!propertyDeclaration.ExpressionBody.IsNull)
+			if (propertyDeclaration.ExpressionBody is not null)
 			{
 				HandleMethod(propertyDeclaration.ExpressionBody, propertyDeclaration.Annotation<ILFunction>());
 			}
@@ -147,7 +147,7 @@ namespace ICSharpCode.Decompiler.DebugInfo
 
 		public override void VisitIndexerDeclaration(IndexerDeclaration indexerDeclaration)
 		{
-			if (!indexerDeclaration.ExpressionBody.IsNull)
+			if (indexerDeclaration.ExpressionBody is not null)
 			{
 				HandleMethod(indexerDeclaration.ExpressionBody, indexerDeclaration.Annotation<ILFunction>());
 			}

ILSpy/Languages/CSharpLanguage.cs

@@ -613,14 +613,14 @@ namespace ICSharpCode.ILSpy.Languages
 							}
 							break;
 						case FieldDeclaration fd:
-							if (fd.Variables.All(v => v.Initializer.IsNull))
+							if (fd.Variables.All(v => v.Initializer is null))
 							{
 								fd.Remove();
 								removedSymbols.Add(fd.GetSymbol());
 							}
 							break;
 						case EventDeclaration ed:
-							if (ed.Variables.All(v => v.Initializer.IsNull))
+							if (ed.Variables.All(v => v.Initializer is null))
 							{
 								ed.Remove();
 								removedSymbols.Add(ed.GetSymbol());
@@ -640,7 +640,7 @@ namespace ICSharpCode.ILSpy.Languages
 							break;
 					}
 				}
-				if (ctorDecl?.Initializer.ConstructorInitializerType == ConstructorInitializerType.This)
+				if (ctorDecl?.Initializer?.ConstructorInitializerType == ConstructorInitializerType.This)
 				{
 					foreach (var node in rootNode.Children)
 					{