Enable nullable reference types across the C# AST consumers

Turn on #nullable enable across the AST consumer layer: the output visitor, the
IL-to-C# builders (statement, call and expression builders, CSharpDecompiler,
TypeSystemAstBuilder), the translation-result wrappers, the sequence-point and
required-namespace collectors, and the annotation helpers. Optional inputs,
fields and returns are typed nullable, detector out-parameters use
[NotNullWhen(true)], and structurally-guaranteed dereferences use the
null-forgiving operator. A few public parameters that already tolerate null are
widened to match their downstream callers. The annotations emit no IL, so the
Pretty suite stays byte-identical.
Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler/CSharp/Annotations.cs

@@ -26,6 +26,8 @@ using ICSharpCode.Decompiler.IL;
 using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	// Annotations:
@@ -134,7 +136,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// Retrieves the <see cref="ISymbol"/> associated with this AstNode, or null if no symbol
 		/// is associated with the node.
 		/// </summary>
-		public static ISymbol GetSymbol(this AstNode node)
+		public static ISymbol? GetSymbol(this AstNode node)
 		{
 			var rr = node.Annotation<ResolveResult>();
 			if (rr is MethodGroupResolveResult mgrr)
@@ -157,7 +159,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// Retrieves the <see cref="ILVariable"/> associated with this <see cref="IdentifierExpression"/>,
 		/// or <c>null</c> if no variable is associated with this identifier.
 		/// </summary>
-		public static ILVariable GetILVariable(this IdentifierExpression expr)
+		public static ILVariable? GetILVariable(this IdentifierExpression expr)
 		{
 			if (expr.Annotation<ResolveResult>() is ILVariableResolveResult rr)
 				return rr.Variable;
@@ -169,7 +171,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// Retrieves the <see cref="ILVariable"/> associated with this <see cref="VariableInitializer"/>,
 		/// or <c>null</c> if no variable is associated with this initializer.
 		/// </summary>
-		public static ILVariable GetILVariable(this VariableInitializer vi)
+		public static ILVariable? GetILVariable(this VariableInitializer vi)
 		{
 			if (vi.Annotation<ResolveResult>() is ILVariableResolveResult rr)
 				return rr.Variable;
@@ -181,7 +183,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// Retrieves the <see cref="ILVariable"/> associated with this <see cref="ForeachStatement"/>,
 		/// or <c>null</c> if no variable is associated with this foreach statement.
 		/// </summary>
-		public static ILVariable GetILVariable(this ForeachStatement loop)
+		public static ILVariable? GetILVariable(this ForeachStatement loop)
 		{
 			if (loop.Annotation<ResolveResult>() is ILVariableResolveResult rr)
 				return rr.Variable;

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -20,6 +20,7 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using System.Reflection.Metadata;
@@ -45,6 +46,8 @@ using ICSharpCode.Decompiler.Util;
 
 using SRM = System.Reflection.Metadata;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	/// <summary>
@@ -60,7 +63,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		readonly MetadataModule module;
 		readonly MetadataReader metadata;
 		readonly DecompilerSettings settings;
-		SyntaxTree syntaxTree;
+		SyntaxTree? syntaxTree;
 
 		List<IILTransform> ilTransforms = GetILTransforms();
 
@@ -210,12 +213,12 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// <summary>
 		/// Gets or sets the optional provider for debug info.
 		/// </summary>
-		public IDebugInfoProvider DebugInfoProvider { get; set; }
+		public IDebugInfoProvider? DebugInfoProvider { get; set; }
 
 		/// <summary>
 		/// Gets or sets the optional provider for XML documentation strings.
 		/// </summary>
-		public IDocumentationProvider DocumentationProvider { get; set; }
+		public IDocumentationProvider? DocumentationProvider { get; set; }
 
 		/// <summary>
 		/// IL transforms.
@@ -275,7 +278,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// <param name="module">The module containing the member.</param>
 		/// <param name="member">The metadata token/handle of the member. Can be a TypeDef, MethodDef or FieldDef.</param>
 		/// <param name="settings">The settings used to determine whether code should be hidden. E.g. if async methods are not transformed, async state machines are included in the decompiled code.</param>
-		public static bool MemberIsHidden(MetadataFile module, EntityHandle member, DecompilerSettings settings)
+		public static bool MemberIsHidden(MetadataFile? module, EntityHandle member, DecompilerSettings settings)
 		{
 			if (module == null || member.IsNil)
 				return false;
@@ -671,7 +674,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return typeSystemAstBuilder;
 		}
 
-		IDocumentationProvider CreateDefaultDocumentationProvider()
+		IDocumentationProvider? CreateDefaultDocumentationProvider()
 		{
 			try
 			{
@@ -783,8 +786,8 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		void DoDecompileTypes(IEnumerable<TypeDefinitionHandle> types, DecompileRun decompileRun, ITypeResolveContext decompilationContext, SyntaxTree syntaxTree)
 		{
-			string currentNamespace = null;
-			AstNode groupNode = null;
+			string? currentNamespace = null;
+			AstNode? groupNode = null;
 			foreach (var typeDefHandle in types)
 			{
 				var typeDef = module.GetDefinition(typeDefHandle);
@@ -806,7 +809,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				}
 				currentNamespace = typeDef.Namespace;
 				var typeDecl = DoDecompile(typeDef, decompileRun, decompilationContext.WithCurrentTypeDefinition(typeDef));
-				groupNode.AddChild(typeDecl, SlotKind.Member);
+				groupNode!.AddChild(typeDecl, SlotKind.Member);
 			}
 		}
 
@@ -1189,7 +1192,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (type == null)
 				throw new InvalidOperationException($"Could not find type definition {fullTypeName} in type system.");
 			if (type.ParentModule != typeSystem.MainModule)
-				throw new NotSupportedException($"Type {fullTypeName} was not found in the module being decompiled, but only in {type.ParentModule.Name}");
+				throw new NotSupportedException($"Type {fullTypeName} was not found in the module being decompiled, but only in {type.ParentModule!.Name}");
 			var decompilationContext = new SimpleTypeResolveContext(typeSystem.MainModule);
 			var namespaces = new HashSet<string>();
 			syntaxTree = new SyntaxTree();
@@ -1237,8 +1240,8 @@ namespace ICSharpCode.Decompiler.CSharp
 			var decompileRun = CreateDecompileRun(namespaces);
 
 			bool first = true;
-			ITypeDefinition parentTypeDef = null;
-			ExtensionInfo parentExtensionInfo = null;
+			ITypeDefinition? parentTypeDef = null;
+			ExtensionInfo? parentExtensionInfo = null;
 
 			foreach (var entity in definitions)
 			{
@@ -1341,7 +1344,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					extensionInfo = propDef.ResolveExtensionInfo();
 					Debug.Assert(extensionInfo != null);
 					var accessor = propDef.Getter ?? propDef.Setter;
-					memberInfo = extensionInfo.InfoOfExtensionMember((IMethod)accessor.MemberDefinition).GetValueOrDefault();
+					memberInfo = extensionInfo.InfoOfExtensionMember((IMethod)accessor!.MemberDefinition).GetValueOrDefault();
 					subst = new TypeParameterSubstitution(memberInfo.ExtensionGroupingTypeParameters, null);
 					propDef = (IProperty)propDef.Specialize(subst);
 					EntityDeclaration prop = DoDecompile(propDef, decompileRun, new SimpleTypeResolveContext(propDef), extensionInfo);
@@ -1363,7 +1366,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return syntaxTree;
 		}
 
-		ITypeDefinition FindCommonDeclaringTypeDefinition(ITypeDefinition a, ITypeDefinition b)
+		ITypeDefinition? FindCommonDeclaringTypeDefinition(ITypeDefinition? a, ITypeDefinition? b)
 		{
 			if (a == null || b == null)
 				return null;
@@ -1427,7 +1430,10 @@ namespace ICSharpCode.Decompiler.CSharp
 				if (m == null || m.DeclaringType.Kind != TypeKind.Interface)
 					continue;
 				var methodDecl = new MethodDeclaration();
-				methodDecl.ReturnType = memberDecl.ReturnType?.Clone();
+				// EntityDeclaration.ReturnType is typed non-null but its getter yields null when the Type
+				// slot is empty; leave the forwarder's (already empty) return-type slot untouched in that case.
+				if (memberDecl.ReturnType is { } memberReturnType)
+					methodDecl.ReturnType = memberReturnType.Clone();
 				methodDecl.PrivateImplementationType = astBuilder.ConvertType(m.DeclaringType);
 				methodDecl.Name = m.Name;
 				methodDecl.TypeParameters.AddRange(memberDecl.GetChildrenByRole<TypeParameterDeclaration>(SlotKind.TypeParameter)
@@ -1483,7 +1489,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (member is ExtensionDeclaration)
 				return;
 
-			var entity = (IEntity)member.GetSymbol();
+			var entity = (IEntity)member.GetSymbol()!;
 			var lookup = new MemberLookup(entity.DeclaringTypeDefinition, entity.ParentModule);
 
 			var baseTypes = entity.DeclaringType.GetNonInterfaceBaseTypes().Where(t => entity.DeclaringType != t).ToList();
@@ -1592,8 +1598,8 @@ namespace ICSharpCode.Decompiler.CSharp
 				{
 					if (entityDecl is ExtensionDeclaration ext && settings.ExtensionMembers)
 					{
-						var extensionInfo = typeDef.DeclaringTypeDefinition.ExtensionInfo ?? typeDef.DeclaringTypeDefinition.DeclaringTypeDefinition.ExtensionInfo;
-						extensionInfo.IsExtensionMarkerType(typeDef, out var group);
+						var extensionInfo = typeDef.DeclaringTypeDefinition!.ExtensionInfo ?? typeDef.DeclaringTypeDefinition.DeclaringTypeDefinition!.ExtensionInfo;
+						extensionInfo!.IsExtensionMarkerType(typeDef, out var group);
 						DoDecompileExtensionMembers(ext, group.Marker, extensionInfo);
 					}
 					// e.g. DelegateDeclaration
@@ -1604,13 +1610,13 @@ namespace ICSharpCode.Decompiler.CSharp
 					TypeKind.Struct => settings.RecordStructs && typeDef.IsRecord,
 					_ => false,
 				};
-				RecordDecompiler recordDecompiler = isRecord ? new RecordDecompiler(typeSystem, typeDef, settings, CancellationToken) : null;
+				RecordDecompiler? recordDecompiler = isRecord ? new RecordDecompiler(typeSystem, typeDef, settings, CancellationToken) : null;
 				if (recordDecompiler != null)
 					decompileRun.RecordDecompilers.Add(typeDef, recordDecompiler);
 
 				// With C# 9 records, the relative order of fields and properties matters:
 				IEnumerable<IMember> fieldsAndProperties = isRecord
-					? recordDecompiler.FieldsAndProperties
+					? recordDecompiler!.FieldsAndProperties
 					: typeDef.Fields.Concat<IMember>(typeDef.Properties);
 
 				// For COM interop scenarios, the relative order of virtual functions/properties matters:
@@ -1629,7 +1635,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					foreach (var group in typeDef.ExtensionInfo?.ExtensionGroups ?? [])
 					{
 						var ext = (ExtensionDeclaration)typeSystemAstBuilder.ConvertExtension(group);
-						DoDecompileExtensionMembers(ext, group.Marker, typeDef.ExtensionInfo);
+						DoDecompileExtensionMembers(ext, group.Marker, typeDef.ExtensionInfo!);
 
 						typeDecl.Members.Add(ext);
 					}
@@ -1709,7 +1715,7 @@ namespace ICSharpCode.Decompiler.CSharp
 						case EnumValueDisplayMode.AllHex:
 							foreach (var enumMember in typeDecl.Members.OfType<EnumMemberDeclaration>())
 							{
-								var constantValue = (enumMember.GetSymbol() as IField).GetConstantValue();
+								var constantValue = (enumMember.GetSymbol() as IField)!.GetConstantValue();
 								if (constantValue == null || enumMember.Initializer is not PrimitiveExpression pe)
 								{
 									continue;
@@ -1744,7 +1750,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				Instrumentation.DecompilerEventSource.Log.DoDecompileTypeDefinition(typeDef.FullName, watch.ElapsedMilliseconds);
 			}
 
-			void DoDecompileMember(IEntity entity, RecordDecompiler recordDecompiler, PartialTypeInfo partialType, ExtensionInfo extensionInfo)
+			void DoDecompileMember(IEntity entity, RecordDecompiler? recordDecompiler, PartialTypeInfo? partialType, ExtensionInfo? extensionInfo)
 			{
 				if (partialType != null && partialType.IsDeclaredMember(entity.MetadataToken))
 				{
@@ -1822,7 +1828,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					else if (rr is TypeResolveResult trr
 						&& trr.Type.GetDefinition()?.DeclaringTypeDefinition == typeDef)
 					{
-						workList.Enqueue(trr.Type.GetDefinition());
+						workList.Enqueue(trr.Type.GetDefinition()!);
 					}
 				}
 			}
@@ -1879,7 +1885,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			{
 				if (MemberIsHidden(module, field.MetadataToken, settings))
 					continue;
-				object constantValue = field.GetConstantValue();
+				object? constantValue = field.GetConstantValue();
 				if (constantValue == null)
 					continue;
 				long currentValue = (long)CSharpPrimitiveCast.Cast(TypeCode.Int64, constantValue, false);
@@ -1928,7 +1934,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return firstValue == 0 ? EnumValueDisplayMode.None : EnumValueDisplayMode.FirstOnly;
 		}
 
-		EntityDeclaration DoDecompile(IMethod method, DecompileRun decompileRun, ITypeResolveContext decompilationContext, ExtensionInfo extensionInfo)
+		EntityDeclaration DoDecompile(IMethod method, DecompileRun decompileRun, ITypeResolveContext decompilationContext, ExtensionInfo? extensionInfo)
 		{
 			Debug.Assert(decompilationContext.CurrentMember == method);
 			var watch = System.Diagnostics.Stopwatch.StartNew();
@@ -1943,7 +1949,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				}
 				FixParameterNames(methodDecl);
 				var methodDefinition = metadata.GetMethodDefinition((MethodDefinitionHandle)method.MetadataToken);
-				if (!settings.LocalFunctions && LocalFunctionDecompiler.LocalFunctionNeedsAccessibilityChange(method.ParentModule.MetadataFile, (MethodDefinitionHandle)method.MetadataToken))
+				if (!settings.LocalFunctions && LocalFunctionDecompiler.LocalFunctionNeedsAccessibilityChange(method.ParentModule!.MetadataFile, (MethodDefinitionHandle)method.MetadataToken))
 				{
 					// if local functions are not active and we're dealing with a local function,
 					// reduce the visibility of the method to private,
@@ -1970,7 +1976,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					&& InheritanceHelper.GetBaseMember(method) == null && IsTypeHierarchyKnown(method.DeclaringType))
 				{
 					methodDecl.Modifiers &= ~Modifiers.Override;
-					if (!method.DeclaringTypeDefinition.IsSealed)
+					if (!method.DeclaringTypeDefinition!.IsSealed)
 					{
 						methodDecl.Modifiers |= Modifiers.Virtual;
 					}
@@ -2020,10 +2026,10 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		internal static bool IsWindowsFormsInitializeComponentMethod(IMethod method)
 		{
-			return method.ReturnType.Kind == TypeKind.Void && method.Name == "InitializeComponent" && method.DeclaringTypeDefinition.GetNonInterfaceBaseTypes().Any(t => t.FullName == "System.Windows.Forms.Control");
+			return method.ReturnType.Kind == TypeKind.Void && method.Name == "InitializeComponent" && method.DeclaringTypeDefinition!.GetNonInterfaceBaseTypes().Any(t => t.FullName == "System.Windows.Forms.Control");
 		}
 
-		void DecompileBody(IMethod method, EntityDeclaration entityDecl, DecompileRun decompileRun, ITypeResolveContext decompilationContext, ExtensionInfo extensionInfo)
+		void DecompileBody(IMethod method, EntityDeclaration entityDecl, DecompileRun decompileRun, ITypeResolveContext decompilationContext, ExtensionInfo? extensionInfo)
 		{
 			try
 			{
@@ -2037,7 +2043,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				{
 					if (!method.IsStatic)
 						parameterOffset = 1; // implementation method has an additional receiver parameter
-					method = extensionInfo.InfoOfExtensionMember((IMethod)method.MemberDefinition).Value.ImplementationMethod;
+					method = extensionInfo.InfoOfExtensionMember((IMethod)method.MemberDefinition)!.Value.ImplementationMethod;
 				}
 
 				var methodDef = metadata.GetMethodDefinition((MethodDefinitionHandle)method.MetadataToken);
@@ -2126,7 +2132,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		internal static void AddAnnotationsToDeclaration(IMethod method, EntityDeclaration entityDecl, ILFunction function, int parameterOffset = 0)
 		{
 			int i = parameterOffset;
-			var parameters = function.Variables.Where(v => v.Kind == VariableKind.Parameter).ToDictionary(v => v.Index);
+			var parameters = function.Variables.Where(v => v.Kind == VariableKind.Parameter).ToDictionary(v => v.Index!.Value);
 			foreach (var parameter in entityDecl.GetChildrenByRole<ParameterDeclaration>(SlotKind.Parameter))
 			{
 				if (parameters.TryGetValue(i, out var v))
@@ -2136,11 +2142,11 @@ namespace ICSharpCode.Decompiler.CSharp
 			entityDecl.AddAnnotation(function);
 		}
 
-		internal static void CleanUpMethodDeclaration(EntityDeclaration entityDecl, BlockStatement body, ILFunction function, bool decompileBody = true)
+		internal static void CleanUpMethodDeclaration(EntityDeclaration entityDecl, BlockStatement? body, ILFunction function, bool decompileBody = true)
 		{
 			if (function.IsIterator)
 			{
-				if (decompileBody && !body.Descendants.Any(d => d is YieldReturnStatement || d is YieldBreakStatement))
+				if (decompileBody && !body!.Descendants.Any(d => d is YieldReturnStatement || d is YieldBreakStatement))
 				{
 					body.Add(new YieldBreakStatement());
 				}
@@ -2243,7 +2249,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return found;
 		}
 
-		bool FindAttribute(EntityDeclaration entityDecl, KnownAttribute attributeType, out Syntax.Attribute attribute)
+		bool FindAttribute(EntityDeclaration entityDecl, KnownAttribute attributeType, [NotNullWhen(true)] out Syntax.Attribute? attribute)
 		{
 			attribute = null;
 			foreach (var section in entityDecl.Attributes)
@@ -2269,7 +2275,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				var symbolName = attr?.FixedArguments.FirstOrDefault().Value as string;
 				if (symbolName == null || !decompileRun.DefinedSymbols.Add(symbolName))
 					continue;
-				syntaxTree.AddLeadingTrivia(new PreProcessorDirective(PreProcessorDirectiveType.Define, symbolName));
+				syntaxTree!.AddLeadingTrivia(new PreProcessorDirective(PreProcessorDirectiveType.Define, symbolName));
 			}
 		}
 
@@ -2280,13 +2286,13 @@ namespace ICSharpCode.Decompiler.CSharp
 			try
 			{
 				var typeSystemAstBuilder = CreateAstBuilder(decompileRun.Settings);
-				if (decompilationContext.CurrentTypeDefinition.Kind == TypeKind.Enum && field.IsConst)
+				if (decompilationContext.CurrentTypeDefinition!.Kind == TypeKind.Enum && field.IsConst)
 				{
 					var enumDec = new EnumMemberDeclaration { Name = field.Name };
-					object constantValue = field.GetConstantValue();
+					object? constantValue = field.GetConstantValue();
 					if (constantValue != null)
 					{
-						enumDec.Initializer = typeSystemAstBuilder.ConvertConstantValue(decompilationContext.CurrentTypeDefinition.EnumUnderlyingType, constantValue);
+						enumDec.Initializer = typeSystemAstBuilder.ConvertConstantValue(decompilationContext.CurrentTypeDefinition.EnumUnderlyingType!, constantValue);
 					}
 					enumDec.Attributes.AddRange(field.GetAttributes().Select(a => new AttributeSection(typeSystemAstBuilder.ConvertAttribute(a))));
 					enumDec.AddAnnotation(new MemberResolveResult(null, field));
@@ -2343,11 +2349,11 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		internal static bool IsFixedField(IField field, out IType type, out int elementCount)
+		internal static bool IsFixedField(IField field, [NotNullWhen(true)] out IType? type, out int elementCount)
 		{
 			type = null;
 			elementCount = 0;
-			IAttribute attr = field.GetAttribute(KnownAttribute.FixedBuffer);
+			IAttribute? attr = field.GetAttribute(KnownAttribute.FixedBuffer);
 			if (attr != null && attr.FixedArguments.Length == 2)
 			{
 				if (attr.FixedArguments[0].Value is IType trr && attr.FixedArguments[1].Value is int length)
@@ -2360,7 +2366,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return false;
 		}
 
-		EntityDeclaration DoDecompile(IProperty property, DecompileRun decompileRun, ITypeResolveContext decompilationContext, ExtensionInfo extensionInfo)
+		EntityDeclaration DoDecompile(IProperty property, DecompileRun decompileRun, ITypeResolveContext decompilationContext, ExtensionInfo? extensionInfo)
 		{
 			Debug.Assert(decompilationContext.CurrentMember == property);
 			var watch = System.Diagnostics.Stopwatch.StartNew();
@@ -2374,7 +2380,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					propertyDecl.Name = property.Name.Substring(lastDot + 1);
 				}
 				FixParameterNames(propertyDecl);
-				Accessor getter, setter;
+				Accessor? getter, setter;
 				if (propertyDecl is PropertyDeclaration)
 				{
 					getter = ((PropertyDeclaration)propertyDecl).Getter;
@@ -2386,29 +2392,29 @@ namespace ICSharpCode.Decompiler.CSharp
 					setter = ((IndexerDeclaration)propertyDecl).Setter;
 				}
 
-				bool getterHasBody = property.CanGet && property.Getter.HasBody;
-				bool setterHasBody = property.CanSet && property.Setter.HasBody;
+				bool getterHasBody = property.CanGet && property.Getter!.HasBody;
+				bool setterHasBody = property.CanSet && property.Setter!.HasBody;
 				if (getterHasBody)
 				{
-					DecompileBody(property.Getter, getter, decompileRun, decompilationContext, extensionInfo);
+					DecompileBody(property.Getter!, getter!, decompileRun, decompilationContext, extensionInfo);
 				}
 				if (setterHasBody)
 				{
-					DecompileBody(property.Setter, setter, decompileRun, decompilationContext, extensionInfo);
+					DecompileBody(property.Setter!, setter!, decompileRun, decompilationContext, extensionInfo);
 				}
 				if (!getterHasBody && !setterHasBody && !property.IsAbstract && property.DeclaringType.Kind != TypeKind.Interface)
 				{
 					propertyDecl.Modifiers |= Modifiers.Extern;
 				}
-				var accessorHandle = (MethodDefinitionHandle)(property.Getter ?? property.Setter).MetadataToken;
+				var accessorHandle = (MethodDefinitionHandle)(property.Getter ?? property.Setter)!.MetadataToken;
 				var accessor = metadata.GetMethodDefinition(accessorHandle);
 				if (!accessorHandle.GetMethodImplementations(metadata).Any() && accessor.HasFlag(System.Reflection.MethodAttributes.Virtual) == accessor.HasFlag(System.Reflection.MethodAttributes.NewSlot))
 				{
 					SetNewModifier(propertyDecl);
 				}
-				if (property.CanGet && IsCovariantReturnOverride(property.Getter))
+				if (property.CanGet && IsCovariantReturnOverride(property.Getter!))
 				{
-					RemoveAttribute(getter, KnownAttribute.PreserveBaseOverrides);
+					RemoveAttribute(getter!, KnownAttribute.PreserveBaseOverrides);
 					propertyDecl.Modifiers &= ~(Modifiers.New | Modifiers.Virtual);
 					propertyDecl.Modifiers |= Modifiers.Override;
 				}
@@ -2435,10 +2441,10 @@ namespace ICSharpCode.Decompiler.CSharp
 			var watch = System.Diagnostics.Stopwatch.StartNew();
 			try
 			{
-				bool adderHasBody = ev.CanAdd && ev.AddAccessor.HasBody;
-				bool removerHasBody = ev.CanRemove && ev.RemoveAccessor.HasBody;
+				bool adderHasBody = ev.CanAdd && ev.AddAccessor!.HasBody;
+				bool removerHasBody = ev.CanRemove && ev.RemoveAccessor!.HasBody;
 				var typeSystemAstBuilder = CreateAstBuilder(decompileRun.Settings);
-				typeSystemAstBuilder.UseCustomEvents = ev.DeclaringTypeDefinition.Kind != TypeKind.Interface
+				typeSystemAstBuilder.UseCustomEvents = ev.DeclaringTypeDefinition!.Kind != TypeKind.Interface
 					|| ev.IsExplicitInterfaceImplementation
 					|| adderHasBody
 					|| removerHasBody;
@@ -2450,17 +2456,17 @@ namespace ICSharpCode.Decompiler.CSharp
 				}
 				if (adderHasBody)
 				{
-					DecompileBody(ev.AddAccessor, ((CustomEventDeclaration)eventDecl).AddAccessor, decompileRun, decompilationContext, null);
+					DecompileBody(ev.AddAccessor!, ((CustomEventDeclaration)eventDecl).AddAccessor!, decompileRun, decompilationContext, null);
 				}
 				if (removerHasBody)
 				{
-					DecompileBody(ev.RemoveAccessor, ((CustomEventDeclaration)eventDecl).RemoveAccessor, decompileRun, decompilationContext, null);
+					DecompileBody(ev.RemoveAccessor!, ((CustomEventDeclaration)eventDecl).RemoveAccessor!, decompileRun, decompilationContext, null);
 				}
 				if (!adderHasBody && !removerHasBody && !ev.IsAbstract && ev.DeclaringType.Kind != TypeKind.Interface)
 				{
 					eventDecl.Modifiers |= Modifiers.Extern;
 				}
-				var accessor = metadata.GetMethodDefinition((MethodDefinitionHandle)(ev.AddAccessor ?? ev.RemoveAccessor).MetadataToken);
+				var accessor = metadata.GetMethodDefinition((MethodDefinitionHandle)(ev.AddAccessor ?? ev.RemoveAccessor)!.MetadataToken);
 				if (accessor.HasFlag(System.Reflection.MethodAttributes.Virtual) == accessor.HasFlag(System.Reflection.MethodAttributes.NewSlot))
 				{
 					SetNewModifier(eventDecl);

ICSharpCode.Decompiler/CSharp/CSharpLanguageVersion.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
 {
 	public enum LanguageVersion

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -20,6 +20,7 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Text;
 
@@ -32,6 +33,8 @@ using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	struct CallBuilder
@@ -47,10 +50,10 @@ namespace ICSharpCode.Decompiler.CSharp
 			public TranslatedExpression[] Arguments;
 			public IParameter[] ExpectedParameters;
 			public string[] ParameterNames;
-			public string[] ArgumentNames;
+			public string[]? ArgumentNames;
 			public int FirstOptionalArgumentIndex;
 			public BitSet IsPrimitiveValue;
-			public IReadOnlyList<int> ArgumentToParameterMap;
+			public IReadOnlyList<int>? ArgumentToParameterMap;
 
 			public bool AddNamesToPrimitiveValues;
 			public bool UseImplicitlyTypedOut;
@@ -64,9 +67,9 @@ namespace ICSharpCode.Decompiler.CSharp
 				return FirstOptionalArgumentIndex;
 			}
 
-			public string[] GetArgumentNames(int skipCount = 0)
+			public string[]? GetArgumentNames(int skipCount = 0)
 			{
-				string[] argumentNames = ArgumentNames;
+				string[]? argumentNames = ArgumentNames;
 				if (AddNamesToPrimitiveValues && IsPrimitiveValue.Any() && !IsExpandedForm
 						&& !ParameterNames.Any(string.IsNullOrEmpty))
 				{
@@ -154,7 +157,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			{
 				for (int i = 0; i < Arguments.Length; i++)
 				{
-					string inferredName;
+					string? inferredName;
 					switch (Arguments[i].Expression)
 					{
 						case IdentifierExpression identifier:
@@ -196,7 +199,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			this.typeSystem = typeSystem;
 		}
 
-		public TranslatedExpression Build(CallInstruction inst, IType typeHint = null)
+		public TranslatedExpression Build(CallInstruction inst, IType? typeHint = null)
 		{
 			if (inst is NewObj newobj && IL.Transforms.DelegateConstruction.MatchDelegateConstruction(newobj, out _, out _, out _))
 			{
@@ -255,7 +258,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return result;
 		}
 
-		static bool IsSpanBasedStringConcat(CallInstruction call, out List<(ILInstruction, KnownTypeCode)> operands)
+		static bool IsSpanBasedStringConcat(CallInstruction call, [NotNullWhen(true)] out List<(ILInstruction, KnownTypeCode)>? operands)
 		{
 			operands = null;
 
@@ -321,8 +324,8 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		public ExpressionWithResolveResult Build(OpCode callOpCode, IMethod method,
 			IReadOnlyList<ILInstruction> callArguments,
-			IReadOnlyList<int> argumentToParameterMap = null,
-			IType constrainedTo = null)
+			IReadOnlyList<int>? argumentToParameterMap = null,
+			IType? constrainedTo = null)
 		{
 			if (method.IsExplicitInterfaceImplementation && callOpCode == OpCode.Call)
 			{
@@ -340,7 +343,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			var expectedTargetDetails = new ExpectedTargetDetails {
 				CallOpCode = callOpCode
 			};
-			ILFunction localFunction = null;
+			ILFunction? localFunction = null;
 			if (method.IsLocalFunction)
 			{
 				localFunction = expressionBuilder.ResolveLocalFunction(method);
@@ -353,7 +356,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 			else if (localFunction != null)
 			{
-				var ide = new IdentifierExpression(localFunction.Name);
+				var ide = new IdentifierExpression(localFunction.Name!);
 				if (method.TypeArguments.Count > 0)
 				{
 					ide.TypeArguments.AddRange(method.TypeArguments.Select(expressionBuilder.ConvertType));
@@ -522,13 +525,15 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 
 			var transform = GetRequiredTransformationsForCall(expectedTargetDetails, method, ref target,
-				ref argumentList, CallTransformation.All, out IParameterizedMember foundMethod);
+				ref argumentList, CallTransformation.All, out IParameterizedMember? foundMethod);
+			// GetRequiredTransformationsForCall always assigns foundMethod (the resolved overload or 'method').
+			Debug.Assert(foundMethod != null);
 
 			// Note: after this, 'method' and 'foundMethod' may differ,
 			// but as far as allowed by IsAppropriateCallTarget().
 
 			// Need to update list of parameter names, because foundMethod is different and thus might use different names.
-			if (!method.Equals(foundMethod) && argumentList.ParameterNames.Length >= foundMethod.Parameters.Count)
+			if (!method.Equals(foundMethod) && argumentList.ParameterNames.Length >= foundMethod!.Parameters.Count)
 			{
 				for (int i = 0; i < foundMethod.Parameters.Count; i++)
 				{
@@ -582,7 +587,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		private ExpressionWithResolveResult HandleStringInterpolation(IMethod method, ArgumentList argumentList)
 		{
-			if (!TryGetStringInterpolationTokens(argumentList, out string format, out var tokens))
+			if (!TryGetStringInterpolationTokens(argumentList, out string? format, out var tokens))
 				return default;
 
 			var arguments = argumentList.Arguments;
@@ -598,7 +603,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					return;
 				var arrayCreation = (ArrayCreateExpression)argumentList.Arguments[1].Expression;
 				var arrayCreationRR = (ArrayCreateResolveResult)argumentList.Arguments[1].ResolveResult;
-				var element = arrayCreation.Initializer.Elements.First().Detach();
+				var element = arrayCreation.Initializer!.Elements.First().Detach();
 				argument = new TranslatedExpression(element, arrayCreationRR.InitializerElements.First());
 			}
 
@@ -714,7 +719,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		public ExpressionWithResolveResult BuildDictionaryInitializerExpression(OpCode callOpCode, IMethod method,
-			InitializedObjectResolveResult target, IReadOnlyList<ILInstruction> indices, ILInstruction value = null)
+			InitializedObjectResolveResult target, IReadOnlyList<ILInstruction> indices, ILInstruction? value = null)
 		{
 			if (method is null)
 				throw new ArgumentNullException(nameof(method));
@@ -755,7 +760,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			);
 		}
 
-		private bool TryGetStringInterpolationTokens(ArgumentList argumentList, out string format, out List<(TokenKind Kind, int Index, int Alignment, string Format)> tokens)
+		private bool TryGetStringInterpolationTokens(ArgumentList argumentList, [NotNullWhen(true)] out string? format, [NotNullWhen(true)] out List<(TokenKind Kind, int Index, int Alignment, string? Format)>? tokens)
 		{
 			tokens = null;
 			format = null;
@@ -766,9 +771,9 @@ namespace ICSharpCode.Decompiler.CSharp
 				return false;
 			if (!arguments.Skip(1).All(a => !a.Expression.DescendantsAndSelf.OfType<PrimitiveExpression>().Any(p => p.Value is string)))
 				return false;
-			tokens = new List<(TokenKind Kind, int Index, int Alignment, string Format)>();
+			tokens = new List<(TokenKind Kind, int Index, int Alignment, string? Format)>();
 			int i = 0;
-			format = (string)crr.ConstantValue;
+			format = (string)crr.ConstantValue!;
 			foreach (var (kind, data) in TokenizeFormatString(format))
 			{
 				int index;
@@ -787,7 +792,7 @@ namespace ICSharpCode.Decompiler.CSharp
 						tokens.Add((kind, index, 0, null));
 						break;
 					case TokenKind.ArgumentWithFormat:
-						arg = data.Split(new[] { ':' }, 2);
+						arg = data!.Split(new[] { ':' }, 2);
 						if (arg.Length != 2 || arg[1].Length == 0)
 							return false;
 						if (!int.TryParse(arg[0], out index) || index != i)
@@ -796,7 +801,7 @@ namespace ICSharpCode.Decompiler.CSharp
 						tokens.Add((kind, index, 0, arg[1]));
 						break;
 					case TokenKind.ArgumentWithAlignment:
-						arg = data.Split(new[] { ',' }, 2);
+						arg = data!.Split(new[] { ',' }, 2);
 						if (arg.Length != 2 || arg[1].Length == 0)
 							return false;
 						if (!int.TryParse(arg[0], out index) || index != i)
@@ -807,7 +812,7 @@ namespace ICSharpCode.Decompiler.CSharp
 						tokens.Add((kind, index, alignment, null));
 						break;
 					case TokenKind.ArgumentWithAlignmentAndFormat:
-						arg = data.Split(new[] { ',', ':' }, 3);
+						arg = data!.Split(new[] { ',', ':' }, 3);
 						if (arg.Length != 3 || arg[1].Length == 0 || arg[2].Length == 0)
 							return false;
 						if (!int.TryParse(arg[0], out index) || index != i)
@@ -834,7 +839,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			ArgumentWithAlignmentAndFormat,
 		}
 
-		private IEnumerable<(TokenKind, string)> TokenizeFormatString(string value)
+		private IEnumerable<(TokenKind, string?)> TokenizeFormatString(string value)
 		{
 			int pos = -1;
 
@@ -926,14 +931,14 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		private ArgumentList BuildArgumentList(ExpectedTargetDetails expectedTargetDetails, ResolveResult target, IMethod method,
-			int firstParamIndex, IReadOnlyList<ILInstruction> callArguments, IReadOnlyList<int> argumentToParameterMap)
+		private ArgumentList BuildArgumentList(ExpectedTargetDetails expectedTargetDetails, ResolveResult? target, IMethod method,
+			int firstParamIndex, IReadOnlyList<ILInstruction> callArguments, IReadOnlyList<int>? argumentToParameterMap)
 		{
 			ArgumentList list = new ArgumentList();
 
 			// Translate arguments to the expected parameter types
 			var arguments = new List<TranslatedExpression>(method.Parameters.Count);
-			string[] argumentNames = null;
+			string[]? argumentNames = null;
 			Debug.Assert(callArguments.Count == firstParamIndex + method.Parameters.Count);
 			var expectedParameters = new List<IParameter>(method.Parameters.Count); // parameters, but in argument order
 			bool isExpandedForm = false;
@@ -1038,12 +1043,12 @@ namespace ICSharpCode.Decompiler.CSharp
 			return p.Type.IsKnownType(KnownTypeCode.Boolean);
 		}
 
-		private bool TransformParamsArgument(ExpectedTargetDetails expectedTargetDetails, ResolveResult targetResolveResult,
+		private bool TransformParamsArgument(ExpectedTargetDetails expectedTargetDetails, ResolveResult? targetResolveResult,
 			IMethod method, IParameter parameter, TranslatedExpression paramsArgument, ref List<IParameter> expectedParameters,
 			ref List<TranslatedExpression> arguments)
 		{
 			var expressionBuilder = this.expressionBuilder;
-			if (ExtractArguments(out IType elementType, out var expandedParameters, out var expandedArguments))
+			if (ExtractArguments(out var elementType, out var expandedParameters, out var expandedArguments))
 			{
 				expandedParameters.InsertRange(0, expectedParameters);
 				expandedArguments.InsertRange(0, arguments);
@@ -1059,7 +1064,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 			return false;
 
-			bool ExtractArguments(out IType elementType, out List<IParameter> parameters, out List<TranslatedExpression> arguments)
+			bool ExtractArguments([NotNullWhen(true)] out IType? elementType, [NotNullWhen(true)] out List<IParameter>? parameters, [NotNullWhen(true)] out List<TranslatedExpression>? arguments)
 			{
 				elementType = null;
 				parameters = null;
@@ -1138,13 +1143,13 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		private CallTransformation GetRequiredTransformationsForCall(ExpectedTargetDetails expectedTargetDetails, IMethod method,
-			ref TranslatedExpression target, ref ArgumentList argumentList, CallTransformation allowedTransforms, out IParameterizedMember foundMethod)
+			ref TranslatedExpression target, ref ArgumentList argumentList, CallTransformation allowedTransforms, out IParameterizedMember? foundMethod)
 		{
 			CallTransformation transform = CallTransformation.None;
 
 			// initialize requireTarget flag
 			bool requireTarget;
-			ResolveResult targetResolveResult;
+			ResolveResult? targetResolveResult;
 			if ((allowedTransforms & CallTransformation.RequireTarget) != 0)
 			{
 				if (settings.AlwaysQualifyMemberReferences || expressionBuilder.HidesVariableWithName(method.Name))
@@ -1475,9 +1480,9 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		OverloadResolutionErrors IsUnambiguousCall(ExpectedTargetDetails expectedTargetDetails, IMethod method,
-			ResolveResult target, IType[] typeArguments, ResolveResult[] arguments,
-			string[] argumentNames, int firstOptionalArgumentIndex,
-			out IParameterizedMember foundMember, out bool bestCandidateIsExpandedForm)
+			ResolveResult? target, IType[] typeArguments, ResolveResult[] arguments,
+			string[]? argumentNames, int firstOptionalArgumentIndex,
+			out IParameterizedMember? foundMember, out bool bestCandidateIsExpandedForm)
 		{
 			foundMember = null;
 			bestCandidateIsExpandedForm = false;
@@ -1585,8 +1590,8 @@ namespace ICSharpCode.Decompiler.CSharp
 			return OverloadResolutionErrors.None;
 		}
 
-		bool IsUnambiguousAccess(ExpectedTargetDetails expectedTargetDetails, ResolveResult target, IMethod method,
-			IList<TranslatedExpression> arguments, string[] argumentNames, out IMember foundMember)
+		bool IsUnambiguousAccess(ExpectedTargetDetails expectedTargetDetails, ResolveResult? target, IMethod method,
+			IList<TranslatedExpression> arguments, string[]? argumentNames, [NotNullWhen(true)] out IMember? foundMember)
 		{
 			Log.WriteLine("IsUnambiguousAccess: Performing overload resolution for " + method);
 			Log.WriteCollection("  Arguments: ", arguments.Select(a => a.ResolveResult));
@@ -1594,7 +1599,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			foundMember = null;
 			if (target == null)
 			{
-				var result = resolver.ResolveSimpleName(method.AccessorOwner.Name,
+				var result = resolver.ResolveSimpleName(method.AccessorOwner!.Name,
 					EmptyList<IType>.Instance,
 					isInvocationTarget: false) as MemberResolveResult;
 				if (result == null || result.IsError)
@@ -1604,7 +1609,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			else
 			{
 				var lookup = new MemberLookup(resolver.CurrentTypeDefinition, resolver.CurrentTypeDefinition.ParentModule);
-				if (method.AccessorOwner.SymbolKind == SymbolKind.Indexer)
+				if (method.AccessorOwner!.SymbolKind == SymbolKind.Indexer)
 				{
 					var or = new OverloadResolution(resolver.Compilation,
 						arguments.SelectArray(a => a.ResolveResult),
@@ -1621,7 +1626,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				else
 				{
 					var result = lookup.Lookup(target,
-						method.AccessorOwner.Name,
+						method.AccessorOwner!.Name,
 						EmptyList<IType>.Instance,
 						isInvocation: false) as MemberResolveResult;
 					if (result == null || result.IsError)
@@ -1633,10 +1638,10 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		ExpressionWithResolveResult HandleAccessorCall(ExpectedTargetDetails expectedTargetDetails, IMethod method,
-			TranslatedExpression target, List<TranslatedExpression> arguments, string[] argumentNames)
+			TranslatedExpression target, List<TranslatedExpression> arguments, string[]? argumentNames)
 		{
 			bool requireTarget;
-			if (settings.AlwaysQualifyMemberReferences || method.AccessorOwner.SymbolKind == SymbolKind.Indexer || expressionBuilder.HidesVariableWithName(method.AccessorOwner.Name))
+			if (settings.AlwaysQualifyMemberReferences || method.AccessorOwner!.SymbolKind == SymbolKind.Indexer || expressionBuilder.HidesVariableWithName(method.AccessorOwner.Name))
 				requireTarget = true;
 			else if (method.IsStatic)
 				requireTarget = !expressionBuilder.IsCurrentOrContainingType(method.DeclaringTypeDefinition);
@@ -1654,7 +1659,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				arguments.Remove(value);
 			}
 
-			IMember foundMember;
+			IMember? foundMember;
 			while (!IsUnambiguousAccess(expectedTargetDetails, targetResolveResult, method, arguments, argumentNames, out foundMember))
 			{
 				if (!argumentsCasted)
@@ -1670,12 +1675,12 @@ namespace ICSharpCode.Decompiler.CSharp
 				else if (!targetCasted)
 				{
 					targetCasted = true;
-					target = target.ConvertTo(method.AccessorOwner.DeclaringType, expressionBuilder);
+					target = target.ConvertTo(method.AccessorOwner!.DeclaringType, expressionBuilder);
 					targetResolveResult = target.ResolveResult;
 				}
 				else
 				{
-					foundMember = method.AccessorOwner;
+					foundMember = method.AccessorOwner!;
 					break;
 				}
 			}
@@ -1693,12 +1698,12 @@ namespace ICSharpCode.Decompiler.CSharp
 				}
 				else if (requireTarget)
 				{
-					expr = new MemberReferenceExpression(target.Expression, method.AccessorOwner.Name)
+					expr = new MemberReferenceExpression(target.Expression, method.AccessorOwner!.Name)
 						.WithoutILInstruction().WithRR(rr);
 				}
 				else
 				{
-					expr = new IdentifierExpression(method.AccessorOwner.Name)
+					expr = new IdentifierExpression(method.AccessorOwner!.Name)
 						.WithoutILInstruction().WithRR(rr);
 				}
 
@@ -1714,7 +1719,7 @@ namespace ICSharpCode.Decompiler.CSharp
 						op = AssignmentOperatorType.Subtract;
 					}
 				}
-				return new AssignmentExpression(expr, op, value.Expression).WithRR(new TypeResolveResult(method.AccessorOwner.ReturnType));
+				return new AssignmentExpression(expr, op, value.Expression!).WithRR(new TypeResolveResult(method.AccessorOwner!.ReturnType));
 			}
 			else
 			{
@@ -1725,12 +1730,12 @@ namespace ICSharpCode.Decompiler.CSharp
 				}
 				else if (requireTarget)
 				{
-					return new MemberReferenceExpression(target.Expression, method.AccessorOwner.Name)
+					return new MemberReferenceExpression(target.Expression, method.AccessorOwner!.Name)
 						.WithoutILInstruction().WithRR(rr);
 				}
 				else
 				{
-					return new IdentifierExpression(method.AccessorOwner.Name)
+					return new IdentifierExpression(method.AccessorOwner!.Name)
 						.WithoutILInstruction().WithRR(rr);
 				}
 			}
@@ -1756,7 +1761,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return false;
 		}
 
-		ExpressionWithResolveResult HandleConstructorCall(ExpectedTargetDetails expectedTargetDetails, ResolveResult target, IMethod method, ArgumentList argumentList)
+		ExpressionWithResolveResult HandleConstructorCall(ExpectedTargetDetails expectedTargetDetails, ResolveResult? target, IMethod method, ArgumentList argumentList)
 		{
 			if (settings.AnonymousTypes && method.DeclaringType.IsAnonymousType())
 			{
@@ -1802,7 +1807,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					CastArguments(argumentList.Arguments, argumentList.ExpectedParameters);
 					break; // make sure that we don't not end up in an infinite loop
 				}
-				IType returnTypeOverride = null;
+				IType? returnTypeOverride = null;
 				if (typeSystem.MainModule.TypeSystemOptions.HasFlag(TypeSystemOptions.NativeIntegersWithoutAttribute))
 				{
 					// For DeclaringType, we don't use nint/nuint (so that DeclaringType.GetConstructors etc. works),
@@ -1908,7 +1913,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return expr.Expression.WithRR(new MemberResolveResult(null, method));
 		}
 
-		ExpressionWithResolveResult BuildDelegateReference(IMethod method, IMethod invokeMethod, ExpectedTargetDetails expectedTargetDetails, ILInstruction thisArg)
+		ExpressionWithResolveResult BuildDelegateReference(IMethod method, IMethod? invokeMethod, ExpectedTargetDetails expectedTargetDetails, ILInstruction? thisArg)
 		{
 			ExpressionBuilder expressionBuilder = this.expressionBuilder;
 			ExpressionWithResolveResult targetExpression;
@@ -1935,13 +1940,13 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		}
 
-		(TranslatedExpression target, bool addTypeArguments, string methodName, ResolveResult result) DisambiguateDelegateReference(IMethod method, IMethod invokeMethod, ExpectedTargetDetails expectedTargetDetails, ILInstruction thisArg)
+		(TranslatedExpression target, bool addTypeArguments, string methodName, ResolveResult result) DisambiguateDelegateReference(IMethod method, IMethod? invokeMethod, ExpectedTargetDetails expectedTargetDetails, ILInstruction? thisArg)
 		{
 			if (method.IsLocalFunction)
 			{
-				ILFunction localFunction = expressionBuilder.ResolveLocalFunction(method);
+				ILFunction? localFunction = expressionBuilder.ResolveLocalFunction(method);
 				Debug.Assert(localFunction != null);
-				return (default, addTypeArguments: true, localFunction.Name, ToMethodGroup(method, localFunction));
+				return (default, addTypeArguments: true, localFunction.Name!, ToMethodGroup(method, localFunction));
 			}
 			if (method.IsExtensionMethod && method.Parameters.Count - 1 == invokeMethod?.Parameters.Count)
 			{
@@ -1951,21 +1956,21 @@ namespace ICSharpCode.Decompiler.CSharp
 					targetType = ((ByReferenceType)targetType).ElementType;
 					thisArg = thisArgBox.Argument;
 				}
-				TranslatedExpression target = expressionBuilder.Translate(thisArg, targetType);
+				TranslatedExpression target = expressionBuilder.Translate(thisArg!, targetType);
 				var currentTarget = target;
 				bool targetCasted = false;
 				bool addTypeArguments = false;
 				// Initial inputs for IsUnambiguousMethodReference:
 				ResolveResult targetResolveResult = target.ResolveResult;
 				IReadOnlyList<IType> typeArguments = EmptyList<IType>.Instance;
-				if (thisArg.MatchLdNull())
+				if (thisArg!.MatchLdNull())
 				{
 					targetCasted = true;
 					currentTarget = currentTarget.ConvertTo(targetType, expressionBuilder);
 					targetResolveResult = currentTarget.ResolveResult;
 				}
 				// Find somewhat minimal solution:
-				ResolveResult result;
+				ResolveResult? result;
 				while (!IsUnambiguousMethodReference(expectedTargetDetails, method, targetResolveResult, typeArguments, true, out result))
 				{
 					if (!targetCasted)
@@ -1985,7 +1990,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 					break;
 				}
-				return (currentTarget, addTypeArguments, method.Name, result);
+				return (currentTarget, addTypeArguments, method.Name, result!);
 			}
 			else
 			{
@@ -2019,10 +2024,10 @@ namespace ICSharpCode.Decompiler.CSharp
 				bool targetCasted = false;
 				bool addTypeArguments = false;
 				// Initial inputs for IsUnambiguousMethodReference:
-				ResolveResult targetResolveResult = targetAdded ? target.ResolveResult : null;
+				ResolveResult? targetResolveResult = targetAdded ? target.ResolveResult : null;
 				IReadOnlyList<IType> typeArguments = EmptyList<IType>.Instance;
 				// Find somewhat minimal solution:
-				ResolveResult result;
+				ResolveResult? result;
 				while (!IsUnambiguousMethodReference(expectedTargetDetails, method, targetResolveResult, typeArguments, false, out result))
 				{
 					if (!addTypeArguments)
@@ -2054,7 +2059,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				{
 					result = mgrr.WithChosenMethod(method);
 				}
-				return (currentTarget, addTypeArguments, method.Name, result);
+				return (currentTarget, addTypeArguments, method.Name, result!);
 			}
 		}
 
@@ -2071,7 +2076,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return oce;
 		}
 
-		bool IsUnambiguousMethodReference(ExpectedTargetDetails expectedTargetDetails, IMethod method, ResolveResult target, IReadOnlyList<IType> typeArguments, bool isExtensionMethodReference, out ResolveResult result)
+		bool IsUnambiguousMethodReference(ExpectedTargetDetails expectedTargetDetails, IMethod method, ResolveResult? target, IReadOnlyList<IType> typeArguments, bool isExtensionMethodReference, [NotNullWhen(true)] out ResolveResult? result)
 		{
 			Log.WriteLine("IsUnambiguousMethodReference: Performing overload resolution for " + method);
 

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -38,6 +38,8 @@ using ICSharpCode.Decompiler.Util;
 using ExpressionType = System.Linq.Expressions.ExpressionType;
 using PrimitiveType = ICSharpCode.Decompiler.CSharp.Syntax.PrimitiveType;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	/// <summary>
@@ -128,7 +130,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				}
 				else if (rr.Type.IsCSharpSmallIntegerType())
 				{
-					expr = new CastExpression(new PrimitiveType(KnownTypeReference.GetCSharpNameByTypeCode(rr.Type.GetDefinition().KnownTypeCode)), expr);
+					expr = new CastExpression(new PrimitiveType(KnownTypeReference.GetCSharpNameByTypeCode(rr.Type.GetDefinition()!.KnownTypeCode)), expr);
 					// Note: no unchecked annotation necessary, because the constant was folded to be in-range
 				}
 				else if (rr.Type.IsCSharpNativeIntegerType())
@@ -160,7 +162,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		public TranslatedExpression Translate(ILInstruction inst, IType typeHint = null)
+		public TranslatedExpression Translate(ILInstruction inst, IType? typeHint = null)
 		{
 			Debug.Assert(inst != null);
 			cancellationToken.ThrowIfCancellationRequested();
@@ -224,7 +226,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (variable.Kind == VariableKind.Parameter && variable.Index < 0)
 				expr = new ThisReferenceExpression();
 			else
-				expr = new IdentifierExpression(variable.Name);
+				expr = new IdentifierExpression(variable.Name!);
 			if (variable.Type.Kind == TypeKind.ByReference)
 			{
 				// When loading a by-ref parameter, use 'ref paramName'.
@@ -272,13 +274,13 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		internal ILFunction ResolveLocalFunction(IMethod method)
+		internal ILFunction? ResolveLocalFunction(IMethod method)
 		{
 			Debug.Assert(method.IsLocalFunction);
-			method = (IMethod)((IMethod)method.MemberDefinition).ReducedFrom.MemberDefinition;
+			method = (IMethod)((IMethod)method.MemberDefinition!).ReducedFrom!.MemberDefinition;
 			foreach (var parent in currentFunction.Ancestors.OfType<ILFunction>())
 			{
-				var definition = parent.LocalFunctions.FirstOrDefault(f => f.Method.MemberDefinition.Equals(method));
+				var definition = parent.LocalFunctions.FirstOrDefault(f => f.Method!.MemberDefinition.Equals(method));
 				if (definition != null)
 				{
 					return definition;
@@ -296,7 +298,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return !(target.Expression is ThisReferenceExpression || target.Expression is BaseReferenceExpression);
 		}
 
-		ExpressionWithResolveResult ConvertField(IField field, ILInstruction targetInstruction = null)
+		ExpressionWithResolveResult ConvertField(IField field, ILInstruction? targetInstruction = null)
 		{
 			var target = TranslateTarget(targetInstruction,
 				nonVirtualInvocation: true,
@@ -321,7 +323,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			bool targetCasted = false;
 			var targetResolveResult = requireTarget ? target.ResolveResult : null;
 
-			bool IsAmbiguousAccess(out MemberResolveResult result)
+			bool IsAmbiguousAccess(out MemberResolveResult? result)
 			{
 				if (targetResolveResult == null)
 				{
@@ -335,7 +337,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				return result == null || result.IsError || !result.Member.Equals(field, NormalizeTypeVisitor.TypeErasure);
 			}
 
-			MemberResolveResult mrr;
+			MemberResolveResult? mrr;
 			while (IsAmbiguousAccess(out mrr))
 			{
 				if (!requireTarget)
@@ -495,11 +497,12 @@ namespace ICSharpCode.Decompiler.CSharp
 		StackAllocExpression TranslateLocAlloc(LocAlloc inst, IType typeHint, out IType elementType)
 		{
 			TranslatedExpression countExpression;
-			PointerType pointerType;
+			PointerType? pointerType;
 			if (inst.Argument.MatchBinaryNumericInstruction(BinaryNumericOperator.Mul, out var left, out var right)
-				&& right.UnwrapConv(ConversionKind.SignExtend).UnwrapConv(ConversionKind.ZeroExtend).MatchSizeOf(out elementType))
+				&& right.UnwrapConv(ConversionKind.SignExtend).UnwrapConv(ConversionKind.ZeroExtend).MatchSizeOf(out var sizeOfElementType))
 			{
 				// Determine the element type from the sizeof
+				elementType = sizeOfElementType;
 				countExpression = Translate(left.UnwrapConv(ConversionKind.ZeroExtend));
 				pointerType = new PointerType(elementType);
 			}
@@ -634,7 +637,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			Expression expr;
 			IType constantType;
-			object constantValue;
+			object? constantValue;
 			if (type.IsReferenceType == true)
 			{
 				expr = new NullReferenceExpression();
@@ -966,7 +969,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					.WithILInstruction(inst);
 			}
 
-			OperatorResolveResult rr = resolver.ResolveBinaryOperator(inst.Kind.ToBinaryOperatorType(), left.ResolveResult, right.ResolveResult) as OperatorResolveResult;
+			OperatorResolveResult? rr = resolver.ResolveBinaryOperator(inst.Kind.ToBinaryOperatorType(), left.ResolveResult, right.ResolveResult) as OperatorResolveResult;
 			if (rr == null || rr.IsError || rr.UserDefinedOperatorMethod != null
 				|| NullableType.GetUnderlyingType(rr.Operands[0].Type).GetStackType() != inst.InputType
 				|| !rr.Type.IsKnownType(KnownTypeCode.Boolean))
@@ -1375,12 +1378,12 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 					right = Translate(offsetInst);
 					right = ConvertArrayIndex(right, inst.RightInputType, allowIntPtr: true);
-					return CallUnsafeIntrinsic(name, new[] { left.Expression, right.Expression }, brt, inst);
+					return CallUnsafeIntrinsic(name, new[] { left.Expression, right.Expression }, brt!, inst);
 				}
 				else
 				{
 					right = ConvertArrayIndex(right, inst.RightInputType, allowIntPtr: true);
-					return CallUnsafeIntrinsic(name + "ByteOffset", new[] { left.Expression, right.Expression }, brt, inst);
+					return CallUnsafeIntrinsic(name + "ByteOffset", new[] { left.Expression, right.Expression }, brt!, inst);
 				}
 			}
 
@@ -1428,7 +1431,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		internal TranslatedExpression CallUnsafeIntrinsic(string name, Expression[] arguments, IType returnType, ILInstruction inst = null, IEnumerable<IType> typeArguments = null)
+		internal TranslatedExpression CallUnsafeIntrinsic(string name, Expression[] arguments, IType returnType, ILInstruction? inst = null, IEnumerable<IType>? typeArguments = null)
 		{
 			var target = new MemberReferenceExpression {
 				Target = new TypeReferenceExpression(astBuilder.ConvertType(compilation.FindType(KnownTypeCode.Unsafe))),
@@ -1499,7 +1502,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (sub.CheckForOverflow)
 				return null;
 			// First, attempt to parse the 'sizeof' on the RHS
-			IType elementType;
+			IType? elementType;
 			if (inst.Right.MatchLdcI(out long elementSize))
 			{
 				elementType = null;
@@ -2403,7 +2406,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return expr;
 		}
 
-		internal bool IsCurrentOrContainingType(ITypeDefinition type)
+		internal bool IsCurrentOrContainingType(ITypeDefinition? type)
 		{
 			var currentTypeDefinition = decompilationContext.CurrentTypeDefinition;
 			while (currentTypeDefinition != null)
@@ -2415,7 +2418,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return false;
 		}
 
-		internal bool IsBaseTypeOfCurrentType(ITypeDefinition type)
+		internal bool IsBaseTypeOfCurrentType(ITypeDefinition? type)
 		{
 			return decompilationContext.CurrentTypeDefinition.GetAllBaseTypeDefinitions().Any(t => t == type);
 		}
@@ -2506,7 +2509,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				if (body.Statements.Count == 1 && body.Statements.Single() is ReturnStatement returnStmt)
 				{
 					lambda.Body = returnStmt.Expression.Detach();
-					inferredReturnType = lambda.Body.GetResolveResult().Type;
+					inferredReturnType = lambda.Body!.GetResolveResult().Type;
 				}
 				else
 				{
@@ -2539,7 +2542,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		protected internal override TranslatedExpression VisitILFunction(ILFunction function, TranslationContext context)
 		{
-			return TranslateFunction(function.DelegateType, function)
+			return TranslateFunction(function.DelegateType!, function)
 				.WithILInstruction(function);
 		}
 
@@ -2580,7 +2583,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (resultType.Kind == TypeKind.Void)
 				return compilation.FindType(KnownTypeCode.Task);
 
-			ITypeDefinition def = compilation.FindType(KnownTypeCode.TaskOfT).GetDefinition();
+			ITypeDefinition? def = compilation.FindType(KnownTypeCode.TaskOfT).GetDefinition();
 			if (def != null)
 				return new ParameterizedType(def, new[] { resultType });
 			else
@@ -2589,7 +2592,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		IEnumerable<ParameterDeclaration> MakeParameters(IReadOnlyList<IParameter> parameters, ILFunction function)
 		{
-			var variables = function.Variables.Where(v => v.Kind == VariableKind.Parameter).ToDictionary(v => v.Index);
+			var variables = function.Variables.Where(v => v.Kind == VariableKind.Parameter).ToDictionary(v => v.Index!.Value);
 			int i = 0;
 			foreach (var parameter in parameters)
 			{
@@ -2597,7 +2600,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				if (variables.TryGetValue(i, out var v))
 				{
 					pd.AddAnnotation(new ILVariableResolveResult(v, parameters[i].Type));
-					pd.Name = v.Name;
+					pd.Name = v.Name!;
 				}
 				if (string.IsNullOrEmpty(pd.Name) && !pd.Type.IsArgList())
 				{
@@ -2661,8 +2664,8 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		internal TranslatedExpression TranslateTarget(ILInstruction target, bool nonVirtualInvocation,
-			bool memberStatic, IType memberDeclaringType, IType constrainedTo = null)
+		internal TranslatedExpression TranslateTarget(ILInstruction? target, bool nonVirtualInvocation,
+			bool memberStatic, IType memberDeclaringType, IType? constrainedTo = null)
 		{
 			// If references are missing member.IsStatic might not be set correctly.
 			// Additionally check target for null, in order to avoid a crash.
@@ -3036,7 +3039,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				memberName = "LongLength";
 				code = KnownTypeCode.Int64;
 			}
-			IProperty member = arrayType.GetProperties(p => p.Name == memberName).FirstOrDefault();
+			IProperty? member = arrayType.GetProperties(p => p.Name == memberName).FirstOrDefault();
 			ResolveResult rr = member == null
 				? new ResolveResult(compilation.FindType(code))
 				: new MemberResolveResult(arrayExpr.ResolveResult, member);
@@ -3365,9 +3368,9 @@ namespace ICSharpCode.Decompiler.CSharp
 			{
 				var call = (Call)block.Instructions[i];
 
-				Interpolation BuildInterpolation(int alignment = 0, string suffix = null)
+				Interpolation BuildInterpolation(int alignment = 0, string? suffix = null)
 				{
-					return new Interpolation(Translate(call.Arguments[1]).ConvertTo(call.GetParameter(1).Type, this, allowImplicitConversion: true), alignment, suffix);
+					return new Interpolation(Translate(call.Arguments[1]).ConvertTo(call.GetParameter(1)!.Type, this, allowImplicitConversion: true), alignment, suffix);
 				}
 
 				switch (call.Method.Name)
@@ -3465,7 +3468,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			var elementsStack = new Stack<List<TranslatedExpression>>();
 			var elements = new List<TranslatedExpression>(block.Instructions.Count);
 			elementsStack.Push(elements);
-			List<IL.Transforms.AccessPathElement> currentPath = null;
+			List<IL.Transforms.AccessPathElement>? currentPath = null;
 			var indexVariables = new Dictionary<ILVariable, ILInstruction>();
 			foreach (var inst in block.Instructions.Skip(1))
 			{
@@ -3512,7 +3515,7 @@ namespace ICSharpCode.Decompiler.CSharp
 						Debug.Assert(lastElement.Member is IMethod);
 						elementsStack.Peek().Add(
 							new CallBuilder(this, typeSystem, settings)
-								.BuildCollectionInitializerExpression(lastElement.OpCode, (IMethod)lastElement.Member, initObjRR, info.Values)
+								.BuildCollectionInitializerExpression(lastElement.OpCode, (IMethod)lastElement.Member, initObjRR, info.Values!)
 								.WithILInstruction(inst)
 						);
 						break;
@@ -3525,13 +3528,13 @@ namespace ICSharpCode.Decompiler.CSharp
 							Debug.Assert(property.Setter != null, $"Indexer property {property} has no setter");
 							elementsStack.Peek().Add(
 								new CallBuilder(this, typeSystem, settings)
-									.BuildDictionaryInitializerExpression(lastElement.OpCode, property.Setter, initObjRR, GetIndices(lastElement.Indices, indexVariables).ToList(), info.Values.Single())
+									.BuildDictionaryInitializerExpression(lastElement.OpCode, property.Setter, initObjRR, GetIndices(lastElement.Indices, indexVariables).ToList(), info.Values!.Single())
 									.WithILInstruction(inst)
 							);
 						}
 						else
 						{
-							var value = Translate(info.Values.Single(), typeHint: memberRR.Type)
+							var value = Translate(info.Values!.Single(), typeHint: memberRR.Type)
 								.ConvertTo(memberRR.Type, this, allowImplicitConversion: true);
 							var assignment = new NamedExpression(lastElement.Member.Name, value)
 								.WithILInstruction(inst).WithRR(memberRR);
@@ -3542,7 +3545,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 			while (elementsStack.Count > 1)
 			{
-				var methodElement = currentPath[elementsStack.Count - 1];
+				var methodElement = currentPath![elementsStack.Count - 1];
 				var pathElement = currentPath[elementsStack.Count - 2];
 				var values = elementsStack.Pop();
 				elementsStack.Peek().Add(
@@ -3616,7 +3619,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			var stloc = block.Instructions.FirstOrDefault() as StLoc;
 			var final = block.FinalInstruction as LdLoc;
-			if (stloc == null || final == null || !stloc.Value.MatchNewArr(out IType type))
+			if (stloc == null || final == null || !stloc.Value.MatchNewArr(out IType? type))
 				throw new ArgumentException("given Block is invalid!");
 			if (stloc.Variable != final.Variable || stloc.Variable.Kind != VariableKind.InitializerTarget)
 				throw new ArgumentException("given Block is invalid!");
@@ -3635,11 +3638,11 @@ namespace ICSharpCode.Decompiler.CSharp
 
 			for (int i = 1; i < block.Instructions.Count; i++)
 			{
-				if (!block.Instructions[i].MatchStObj(out ILInstruction target, out ILInstruction value, out IType t) || !type.Equals(t))
+				if (!block.Instructions[i].MatchStObj(out ILInstruction? target, out ILInstruction? value, out IType? t) || !type.Equals(t))
 					throw new ArgumentException("given Block is invalid!");
-				if (!target.MatchLdElema(out t, out ILInstruction array) || !type.Equals(t))
+				if (!target.MatchLdElema(out t, out ILInstruction? array) || !type.Equals(t))
 					throw new ArgumentException("given Block is invalid!");
-				if (!array.MatchLdLoc(out ILVariable v) || v != final.Variable)
+				if (!array.MatchLdLoc(out ILVariable? v) || v != final.Variable)
 					throw new ArgumentException("given Block is invalid!");
 				while (container.Count < dimensions)
 				{
@@ -3670,7 +3673,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				}
 			}
 			ArraySpecifier[] additionalSpecifiers;
-			AstType typeExpression;
+			AstType? typeExpression;
 			if (settings.AnonymousTypes && type.ContainsAnonymousType())
 			{
 				typeExpression = null;
@@ -3980,20 +3983,20 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		internal (TranslatedExpression, IType, StringToInt) TranslateSwitchValue(SwitchInstruction inst, bool isExpressionContext)
+		internal (TranslatedExpression, IType, StringToInt?) TranslateSwitchValue(SwitchInstruction inst, bool isExpressionContext)
 		{
 			TranslatedExpression value;
 			IType governingType;
 			// prepare expression and expected type
 			// first try to guess a governing type
-			if (inst.Value is StringToInt strToInt)
+			StringToInt? strToInt = inst.Value as StringToInt;
+			if (strToInt != null)
 			{
 				value = Translate(strToInt.Argument);
 				governingType = strToInt.ExpectedType ?? compilation.FindType(KnownTypeCode.String);
 			}
 			else
 			{
-				strToInt = null;
 				value = Translate(inst.Value);
 				governingType = inst.Type ?? value.Type;
 
@@ -4191,7 +4194,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		protected internal override TranslatedExpression VisitAwait(Await inst, TranslationContext context)
 		{
-			IType expectedType = null;
+			IType? expectedType = null;
 			if (inst.GetAwaiterMethod != null)
 			{
 				if (inst.GetAwaiterMethod.IsStatic)
@@ -4269,7 +4272,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		protected internal override TranslatedExpression VisitDynamicGetMemberInstruction(DynamicGetMemberInstruction inst, TranslationContext context)
 		{
 			var target = TranslateDynamicTarget(inst.Target, inst.TargetArgumentInfo);
-			return new MemberReferenceExpression(target, inst.Name)
+			return new MemberReferenceExpression(target, inst.Name!)
 				.WithILInstruction(inst)
 				.WithRR(new DynamicMemberResolveResult(target.ResolveResult, inst.Name));
 		}
@@ -4415,7 +4418,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			var target = TranslateDynamicTarget(inst.Target, inst.TargetArgumentInfo);
 			var value = TranslateDynamicArgument(inst.Value, inst.ValueArgumentInfo);
-			var member = new MemberReferenceExpression(target, inst.Name)
+			var member = new MemberReferenceExpression(target, inst.Name!)
 				.WithoutILInstruction()
 				.WithRR(new DynamicMemberResolveResult(target.ResolveResult, inst.Name));
 			return Assignment(member, value).WithILInstruction(inst);
@@ -4581,7 +4584,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 			var value = TranslateDynamicArgument(inst.Value, inst.ValueArgumentInfo);
 
-			var ae = new AssignmentExpression(target, AssignmentExpression.GetAssignmentOperatorTypeFromExpressionType(inst.Operation).Value, value);
+			var ae = new AssignmentExpression(target, AssignmentExpression.GetAssignmentOperatorTypeFromExpressionType(inst.Operation)!.Value, value);
 			if (inst.BinderFlags.HasFlag(CSharpBinderFlags.CheckedContext))
 				ae.AddAnnotation(AddCheckedBlocks.CheckedAnnotation);
 			else
@@ -4740,7 +4743,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					{
 						if (subPattern.HasDesignator)
 						{
-							if (!conversionMapping.TryGetValue(subPattern.Variable, out ILVariable value))
+							if (!conversionMapping.TryGetValue(subPattern.Variable, out ILVariable? value))
 							{
 								value = subPattern.Variable;
 							}
@@ -4845,7 +4848,7 @@ namespace ICSharpCode.Decompiler.CSharp
 								Debug.Fail("Invalid sub pattern");
 								continue;
 							}
-							IMember member;
+							IMember? member;
 							if (testedOperand is CallInstruction call)
 							{
 								member = call.Method.AccessorOwner;
@@ -4860,7 +4863,7 @@ namespace ICSharpCode.Decompiler.CSharp
 								continue;
 							}
 							recursivePatternExpression.SubPatterns.Add(
-								new NamedArgumentExpression { Name = member.Name, Expression = TranslatePattern(subPattern, member.ReturnType) }
+								new NamedArgumentExpression { Name = member!.Name, Expression = TranslatePattern(subPattern, member.ReturnType) }
 									.WithRR(new MemberResolveResult(null, member))
 							);
 						}

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpFormattingOptions.cs

@@ -26,6 +26,8 @@
 
 using System.ComponentModel;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 {
 	public enum BraceStyle
@@ -74,7 +76,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 	[TypeConverter(typeof(ExpandableObjectConverter))]
 	public class CSharpFormattingOptions
 	{
-		public string Name {
+		public string? Name {
 			get;
 			set;
 		}

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpOutputVisitor.cs

@@ -29,6 +29,8 @@ using ICSharpCode.Decompiler.Util;
 
 using Attribute = ICSharpCode.Decompiler.CSharp.Syntax.Attribute;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 {
 	/// <summary>
@@ -390,7 +392,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		/// Determines whether the specified identifier is a keyword in the given context.
 		/// If <paramref name="context"/> is <see langword="null" /> all keywords are treated as unconditional.
 		/// </summary>
-		public static bool IsKeyword(string identifier, AstNode context = null)
+		public static bool IsKeyword(string identifier, AstNode? context = null)
 		{
 			// only 2-10 char lower-case identifiers can be keywords
 			if (identifier.Length > maxKeywordLength || identifier.Length < 2 || identifier[0] < 'a')
@@ -497,7 +499,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				NewLine();
 				return;
 			}
-			BlockStatement block = embeddedStatement as BlockStatement;
+			BlockStatement? block = embeddedStatement as BlockStatement;
 			if (block != null)
 			{
 				WriteBlock(block, policy.StatementBraceStyle);
@@ -519,7 +521,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			}
 		}
 
-		protected virtual void WriteMethodBody(BlockStatement body, BraceStyle style, bool newLine = true)
+		protected virtual void WriteMethodBody(BlockStatement? body, BraceStyle style, bool newLine = true)
 		{
 			if (body is null)
 			{
@@ -540,7 +542,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			}
 		}
 
-		protected virtual void WritePrivateImplementationType(AstType privateImplementationType)
+		protected virtual void WritePrivateImplementationType(AstType? privateImplementationType)
 		{
 			if (privateImplementationType is not null)
 			{
@@ -640,11 +642,11 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			// "int a; new List<int> { a = 1 };" is an object initalizers and invalid, but
 			// "int a; new List<int> { { a = 1 } };" is a valid collection initializer.
-			AssignmentExpression ae = expr as AssignmentExpression;
+			AssignmentExpression? ae = expr as AssignmentExpression;
 			return ae != null && ae.Operator == AssignmentOperatorType.Assign;
 		}
 
-		protected bool IsObjectOrCollectionInitializer(AstNode node)
+		protected bool IsObjectOrCollectionInitializer(AstNode? node)
 		{
 			if (!(node is ArrayInitializerExpression))
 			{
@@ -1273,7 +1275,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			unaryOperatorExpression.Expression.AcceptVisitor(this);
 			if (IsPostfixOperator(opType))
 			{
-				WriteToken(opSymbol);
+				WriteToken(opSymbol!);
 			}
 			EndNode(unaryOperatorExpression);
 		}
@@ -1407,7 +1409,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			{
 				Space();
 				WriteKeyword(QueryJoinClause.IntoKeyword);
-				WriteIdentifier(queryJoinClause.IntoIdentifierToken);
+				WriteIdentifier(queryJoinClause.IntoIdentifierToken!);
 			}
 			EndNode(queryJoinClause);
 		}
@@ -1861,7 +1863,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(gotoStatement);
 			WriteKeyword(GotoStatement.GotoKeyword);
-			WriteIdentifier(gotoStatement.LabelToken);
+			WriteIdentifier(gotoStatement.LabelToken!);
 			Semicolon();
 			EndNode(gotoStatement);
 		}
@@ -1904,7 +1906,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			WriteIdentifier(labelStatement.GetChildByRole<Identifier>(SlotKind.Identifier));
 			WriteToken(Roles.Colon);
 			bool foundLabelledStatement = false;
-			for (AstNode tmp = labelStatement.NextSibling; tmp != null; tmp = tmp.NextSibling)
+			for (AstNode? tmp = labelStatement.NextSibling; tmp != null; tmp = tmp.NextSibling)
 			{
 				if (tmp.Slot?.Kind == labelStatement.Slot?.Kind)
 				{
@@ -2109,7 +2111,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				if (!string.IsNullOrEmpty(catchClause.VariableName))
 				{
 					Space();
-					WriteIdentifier(catchClause.VariableNameToken);
+					WriteIdentifier(catchClause.VariableNameToken!);
 				}
 				Space(policy.SpacesWithinCatchParentheses);
 				RPar();
@@ -2297,7 +2299,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			StartNode(constructorDeclaration);
 			WriteAttributes(constructorDeclaration.Attributes);
 			WriteModifiers(constructorDeclaration.Modifiers);
-			TypeDeclaration type = constructorDeclaration.Parent as TypeDeclaration;
+			TypeDeclaration? type = constructorDeclaration.Parent as TypeDeclaration;
 			if (type != null && type.Name != constructorDeclaration.Name)
 				WriteIdentifier((Identifier)type.NameToken.Clone());
 			else
@@ -2343,7 +2345,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				Space();
 			}
 			WriteToken(DestructorDeclaration.TildeToken);
-			TypeDeclaration type = destructorDeclaration.Parent as TypeDeclaration;
+			TypeDeclaration? type = destructorDeclaration.Parent as TypeDeclaration;
 			if (type != null && type.Name != destructorDeclaration.Name)
 				WriteIdentifier((Identifier)type.NameToken.Clone());
 			else
@@ -2632,7 +2634,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			}
 			if (!string.IsNullOrEmpty(parameterDeclaration.Name))
 			{
-				WriteIdentifier(parameterDeclaration.NameToken);
+				WriteIdentifier(parameterDeclaration.NameToken!);
 			}
 			if (parameterDeclaration.DefaultExpression is not null)
 			{

ICSharpCode.Decompiler/CSharp/OutputVisitor/FormattingOptionsFactory.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.OutputVisitor
 {
 	/// <summary>

ICSharpCode.Decompiler/CSharp/OutputVisitor/GenericGrammarAmbiguityVisitor.cs

@@ -22,6 +22,8 @@ using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 {
 	/// <summary>
@@ -53,7 +55,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			var v = new GenericGrammarAmbiguityVisitor();
 			v.genericNestingLevel = 1;
 
-			for (AstNode node = binaryOperatorExpression.Right; node != null; node = node.GetNextNode())
+			for (AstNode? node = binaryOperatorExpression.Right; node != null; node = node.GetNextNode())
 			{
 				if (node.AcceptVisitor(v))
 					return v.ambiguityFound;
@@ -78,7 +80,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 
 		public override bool VisitBinaryOperatorExpression(BinaryOperatorExpression binaryOperatorExpression)
 		{
-			if (binaryOperatorExpression.Left.AcceptVisitor(this))
+			if (binaryOperatorExpression.Left!.AcceptVisitor(this))
 				return true;
 			Debug.Assert(genericNestingLevel > 0);
 			switch (binaryOperatorExpression.Operator)
@@ -105,7 +107,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				ambiguityFound = binaryOperatorExpression.Right is ParenthesizedExpression;
 				return true; // stop visiting
 			}
-			return binaryOperatorExpression.Right.AcceptVisitor(this);
+			return binaryOperatorExpression.Right!.AcceptVisitor(this);
 		}
 
 		public override bool VisitIdentifierExpression(IdentifierExpression identifierExpression)

ICSharpCode.Decompiler/CSharp/OutputVisitor/ITokenWriter.cs

@@ -21,6 +21,8 @@ using System.IO;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 {
 	public abstract class TokenWriter
@@ -46,7 +48,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		/// <summary>
 		/// Writes a primitive/literal value
 		/// </summary>
-		public abstract void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None);
+		public abstract void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None);
 
 		public abstract void WritePrimitiveType(string type);
 
@@ -129,7 +131,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			decoratedWriter.WriteToken(token);
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			decoratedWriter.WritePrimitiveValue(value, format);
 		}

ICSharpCode.Decompiler/CSharp/OutputVisitor/InsertMissingTokensDecorator.cs

@@ -21,6 +21,8 @@ using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 {
 	class InsertMissingTokensDecorator : DecoratingTokenWriter
@@ -128,13 +130,13 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			TextLocation start = locationProvider.Location;
 			if (keyword == "this")
 			{
-				ThisReferenceExpression node = nodes.Peek().LastOrDefault() as ThisReferenceExpression;
+				ThisReferenceExpression? node = nodes.Peek().LastOrDefault() as ThisReferenceExpression;
 				if (node != null)
 					node.StorePrintStart(start);
 			}
 			else if (keyword == "base")
 			{
-				BaseReferenceExpression node = nodes.Peek().LastOrDefault() as BaseReferenceExpression;
+				BaseReferenceExpression? node = nodes.Peek().LastOrDefault() as BaseReferenceExpression;
 				if (node != null)
 					node.StorePrintStart(start);
 			}
@@ -145,17 +147,16 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		public override void WriteIdentifier(Identifier identifier)
 		{
 			AssignPendingStartLocations();
-			if (identifier is not null)
-				identifier.SetStartLocation(locationProvider.Location);
+			identifier.SetStartLocation(locationProvider.Location);
 			currentList.Add(identifier);
 			base.WriteIdentifier(identifier);
 			lastTokenEnd = locationProvider.Location;
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			AssignPendingStartLocations();
-			Expression node = nodes.Peek().LastOrDefault() as Expression;
+			Expression? node = nodes.Peek().LastOrDefault() as Expression;
 			var startLocation = locationProvider.Location;
 			base.WritePrimitiveValue(value, format);
 			if (node is PrimitiveExpression)
@@ -172,7 +173,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		public override void WritePrimitiveType(string type)
 		{
 			AssignPendingStartLocations();
-			PrimitiveType node = nodes.Peek().LastOrDefault() as PrimitiveType;
+			PrimitiveType? node = nodes.Peek().LastOrDefault() as PrimitiveType;
 			if (node != null)
 				node.StorePrintStart(locationProvider.Location);
 			base.WritePrimitiveType(type);

ICSharpCode.Decompiler/CSharp/OutputVisitor/InsertParenthesesVisitor.cs

@@ -20,6 +20,8 @@ using System;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 {
 	/// <summary>
@@ -165,8 +167,10 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		/// <summary>
 		/// Parenthesizes the expression if it does not have the minimum required precedence.
 		/// </summary>
-		static void ParenthesizeIfRequired(Expression expr, PrecedenceLevel minimumPrecedence)
+		static void ParenthesizeIfRequired(Expression? expr, PrecedenceLevel minimumPrecedence)
 		{
+			if (expr == null)
+				return;
 			if (GetPrecedence(expr) < minimumPrecedence)
 			{
 				Parenthesize(expr);
@@ -222,7 +226,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		public override void VisitUnaryOperatorExpression(UnaryOperatorExpression unaryOperatorExpression)
 		{
 			ParenthesizeIfRequired(unaryOperatorExpression.Expression, GetPrecedence(unaryOperatorExpression));
-			UnaryOperatorExpression child = unaryOperatorExpression.Expression as UnaryOperatorExpression;
+			UnaryOperatorExpression? child = unaryOperatorExpression.Expression as UnaryOperatorExpression;
 			if (child != null && InsertParenthesesForReadability)
 				Parenthesize(child);
 			base.VisitUnaryOperatorExpression(unaryOperatorExpression);
@@ -237,7 +241,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			}
 			// There's a nasty issue in the C# grammar: cast expressions including certain operators are ambiguous in some cases
 			// "(int)-1" is fine, but "(A)-b" is not a cast.
-			UnaryOperatorExpression uoe = castExpression.Expression as UnaryOperatorExpression;
+			UnaryOperatorExpression? uoe = castExpression.Expression as UnaryOperatorExpression;
 			if (uoe != null && !(uoe.Operator == UnaryOperatorType.BitNot || uoe.Operator == UnaryOperatorType.Not))
 			{
 				if (TypeCanBeMisinterpretedAsExpression(castExpression.Type))
@@ -246,7 +250,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				}
 			}
 			// The above issue can also happen with PrimitiveExpressions representing negative values:
-			PrimitiveExpression pe = castExpression.Expression as PrimitiveExpression;
+			PrimitiveExpression? pe = castExpression.Expression as PrimitiveExpression;
 			if (pe != null && pe.Value != null && TypeCanBeMisinterpretedAsExpression(castExpression.Type))
 			{
 				TypeCode typeCode = Type.GetTypeCode(pe.Value.GetType());
@@ -290,7 +294,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			// SimpleTypes can always be misinterpreted as IdentifierExpressions
 			// MemberTypes can be misinterpreted as MemberReferenceExpressions if they don't use double colon
 			// PrimitiveTypes or ComposedTypes can never be misinterpreted as expressions.
-			MemberType mt = type as MemberType;
+			MemberType? mt = type as MemberType;
 			if (mt != null)
 				return !mt.IsDoubleColon;
 			else
@@ -356,9 +360,9 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				|| op == BinaryOperatorType.ExclusiveOr;
 		}
 
-		BinaryOperatorType? GetBinaryOperatorType(Expression expr)
+		BinaryOperatorType? GetBinaryOperatorType(Expression? expr)
 		{
-			BinaryOperatorExpression boe = expr as BinaryOperatorExpression;
+			BinaryOperatorExpression? boe = expr as BinaryOperatorExpression;
 			if (boe != null)
 				return boe.Operator;
 			else

ICSharpCode.Decompiler/CSharp/OutputVisitor/InsertRequiredSpacesDecorator.cs

@@ -20,6 +20,8 @@ using System;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 {
 	class InsertRequiredSpacesDecorator : DecoratingTokenWriter
@@ -148,7 +150,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			lastWritten = LastWritten.Whitespace;
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			if (lastWritten == LastWritten.KeywordOrIdentifier)
 			{

ICSharpCode.Decompiler/CSharp/OutputVisitor/TextWriterTokenWriter.cs

@@ -23,6 +23,8 @@ using System.Text;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 {
 	/// <summary>
@@ -230,10 +232,10 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			TextWriter writer = new StringWriter();
 			TextWriterTokenWriter tokenWriter = new TextWriterTokenWriter(writer);
 			tokenWriter.WritePrimitiveValue(value);
-			return writer.ToString();
+			return writer.ToString()!;
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			if (value == null)
 			{
@@ -263,7 +265,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 
 			if (value is string)
 			{
-				string tmp = ConvertString(value.ToString());
+				string tmp = ConvertString(value.ToString()!);
 				column += tmp.Length + 2;
 				Length += tmp.Length + 2;
 				textWriter.Write('"');
@@ -409,7 +411,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			else
 			{
 				textWriter.Write(value.ToString());
-				int length = value.ToString().Length;
+				int length = value.ToString()!.Length;
 				column += length;
 				Length += length;
 			}
@@ -437,7 +439,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		/// Gets the escape sequence for the specified character.
 		/// </summary>
 		/// <remarks>This method does not convert ' or ".</remarks>
-		static string ConvertChar(char ch)
+		static string? ConvertChar(char ch)
 		{
 			switch (ch)
 			{
@@ -499,7 +501,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			StringBuilder sb = new StringBuilder();
 			foreach (char ch in str)
 			{
-				string s = ch == '"' ? "\\\"" : ConvertChar(ch);
+				string? s = ch == '"' ? "\\\"" : ConvertChar(ch);
 				if (s != null)
 					sb.Append(s);
 				else

ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs

@@ -11,6 +11,8 @@ using ICSharpCode.Decompiler.TypeSystem.Implementation;
 
 using static ICSharpCode.Decompiler.Metadata.ILOpCodeExtensions;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	class RequiredNamespaceCollector
@@ -37,7 +39,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			var collector = new RequiredNamespaceCollector(namespaces);
 			foreach (var type in module.TypeDefinitions)
 			{
-				collector.CollectNamespaces(type, module, (CodeMappingInfo)null);
+				collector.CollectNamespaces(type, module, (CodeMappingInfo?)null);
 			}
 			collector.HandleAttributes(module.GetAssemblyAttributes());
 			collector.HandleAttributes(module.GetModuleAttributes());
@@ -50,13 +52,13 @@ namespace ICSharpCode.Decompiler.CSharp
 			collector.HandleAttributes(module.GetModuleAttributes());
 		}
 
-		public static void CollectNamespaces(IEntity entity, MetadataModule module, HashSet<string> namespaces)
+		public static void CollectNamespaces(IEntity? entity, MetadataModule module, HashSet<string> namespaces)
 		{
 			var collector = new RequiredNamespaceCollector(namespaces);
 			collector.CollectNamespaces(entity, module);
 		}
 
-		void CollectNamespaces(IEntity entity, MetadataModule module, CodeMappingInfo mappingInfo = null)
+		void CollectNamespaces(IEntity? entity, MetadataModule module, CodeMappingInfo? mappingInfo = null)
 		{
 			if (entity == null || entity.MetadataToken.IsNil || module.MetadataFile is not MetadataFile corFile)
 				return;
@@ -230,7 +232,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		void HandleAttributeValue(IType type, object value)
+		void HandleAttributeValue(IType type, object? value)
 		{
 			CollectNamespacesForTypeReference(type);
 			if (value is IType typeofType)
@@ -343,7 +345,7 @@ namespace ICSharpCode.Decompiler.CSharp
 							case HandleKind.MethodDefinition:
 							case HandleKind.MethodSpecification:
 							case HandleKind.MemberReference:
-								IMember member;
+								IMember? member;
 								try
 								{
 									member = module.ResolveEntity(handle, genericContext) as IMember;
@@ -394,7 +396,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		void CollectNamespacesForMemberReference(IMember member)
+		void CollectNamespacesForMemberReference(IMember? member)
 		{
 			switch (member)
 			{

ICSharpCode.Decompiler/CSharp/SequencePointBuilder.cs

@@ -26,6 +26,8 @@ using ICSharpCode.Decompiler.DebugInfo;
 using ICSharpCode.Decompiler.IL;
 using ICSharpCode.Decompiler.Util;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	/// <summary>
@@ -75,7 +77,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			/// <summary>
 			/// The function containing this sequence point.
 			/// </summary>
-			internal ILFunction Function;
+			internal ILFunction? Function;
 
 			public StatePerSequencePoint(AstNode primaryNode)
 			{
@@ -94,7 +96,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		// Collects information for the current sequence point.
 		StatePerSequencePoint current;
 
-		void VisitAsSequencePoint(AstNode node)
+		void VisitAsSequencePoint(AstNode? node)
 		{
 			if (node is null)
 				return;
@@ -240,7 +242,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		public override void VisitQueryFromClause(QueryFromClause queryFromClause)
 		{
-			if (queryFromClause.Parent.FirstChild != queryFromClause)
+			if (queryFromClause.Parent!.FirstChild != queryFromClause)
 			{
 				AddToSequencePoint(queryFromClause);
 				VisitAsSequencePoint(queryFromClause.Expression);
@@ -459,7 +461,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			current = outerStates.Pop();
 		}
 
-		void AddToSequencePointRaw(ILFunction function, IEnumerable<Interval> ranges)
+		void AddToSequencePointRaw(ILFunction? function, IEnumerable<Interval> ranges)
 		{
 			current.Intervals.AddRange(ranges);
 			Debug.Assert(current.Function == null || current.Function == function);
@@ -489,7 +491,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (HasUsableILRange(inst) && current.Intervals != null)
 			{
 				current.Intervals.AddRange(inst.ILRanges);
-				var function = inst.Parent.Ancestors.OfType<ILFunction>().FirstOrDefault();
+				var function = inst.Parent!.Ancestors.OfType<ILFunction>().FirstOrDefault();
 				Debug.Assert(current.Function == null || current.Function == function);
 				current.Function = function;
 			}
@@ -570,7 +572,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					newList.Add(hidden);
 				}
 
-				List<int> sequencePointCandidates = function.SequencePointCandidates;
+				List<int> sequencePointCandidates = function.SequencePointCandidates!;
 				int currSPCandidateIndex = 0;
 
 				for (int i = 0; i < newList.Count - 1; i++)

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -19,6 +19,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 
@@ -30,6 +31,8 @@ using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.Util;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	sealed class StatementBuilder : ILVisitor<TranslatedStatement>
@@ -65,7 +68,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			this.currentReturnContainer = (BlockContainer)currentFunction.Body;
 			this.currentIsIterator = currentFunction.IsIterator;
 			this.currentResultType = currentFunction.IsAsync
-				? currentFunction.AsyncReturnType
+				? currentFunction.AsyncReturnType!
 				: currentFunction.ReturnType;
 			this.typeSystem = typeSystem;
 			this.settings = settings;
@@ -150,7 +153,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return new IfElseStatement(condition, trueStatement, falseStatement).WithILInstruction(inst);
 		}
 
-		internal IEnumerable<ConstantResolveResult> CreateTypedCaseLabel(long i, IType type, List<(string Key, int Value)> map = null)
+		internal IEnumerable<ConstantResolveResult> CreateTypedCaseLabel(long i, IType type, List<(string? Key, int Value)>? map = null)
 		{
 			object value;
 			// unpack nullable type, if necessary:
@@ -171,7 +174,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 			else if (type.Kind == TypeKind.Enum)
 			{
-				var enumType = type.GetDefinition().EnumUnderlyingType;
+				var enumType = type.GetDefinition()!.EnumUnderlyingType;
 				TypeCode typeCode = ReflectionHelper.GetTypeCode(enumType);
 				if (typeCode != TypeCode.Empty)
 				{
@@ -202,12 +205,12 @@ namespace ICSharpCode.Decompiler.CSharp
 			return TranslateSwitch(null, inst).WithILInstruction(inst);
 		}
 
-		SwitchStatement TranslateSwitch(BlockContainer switchContainer, SwitchInstruction inst)
+		SwitchStatement TranslateSwitch(BlockContainer? switchContainer, SwitchInstruction inst)
 		{
 			var oldBreakTarget = breakTarget;
 			breakTarget = switchContainer; // 'break' within a switch would only leave the switch
 			var oldCaseLabelMapping = caseLabelMapping;
-			caseLabelMapping = new Dictionary<Block, ConstantResolveResult>();
+			caseLabelMapping = new Dictionary<Block, ConstantResolveResult?>();
 
 			var (value, type, strToInt) = exprBuilder.TranslateSwitchValue(inst, false);
 
@@ -219,7 +222,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			foreach (var section in inst.Sections)
 			{
 				// This is used in the block-label mapping.
-				ConstantResolveResult firstValueResolveResult;
+				ConstantResolveResult? firstValueResolveResult;
 				var astSection = new Syntax.SwitchSection();
 				// Create case labels:
 				if (section == defaultSection)
@@ -246,7 +249,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				{
 					case Branch br:
 						// we can only inline the block, if all branches are in the switchContainer.
-						if (br.TargetContainer == switchContainer && switchContainer.Descendants.OfType<Branch>().Where(b => b.TargetBlock == br.TargetBlock).All(b => BlockContainer.FindClosestSwitchContainer(b) == switchContainer))
+						if (br.TargetContainer == switchContainer && switchContainer!.Descendants.OfType<Branch>().Where(b => b.TargetBlock == br.TargetBlock).All(b => BlockContainer.FindClosestSwitchContainer(b) == switchContainer))
 							caseLabelMapping.Add(br.TargetBlock, firstValueResolveResult);
 						break;
 					default:
@@ -262,7 +265,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				{
 					case Branch br:
 						// we can only inline the block, if all branches are in the switchContainer.
-						if (br.TargetContainer == switchContainer && switchContainer.Descendants.OfType<Branch>().Where(b => b.TargetBlock == br.TargetBlock).All(b => BlockContainer.FindClosestSwitchContainer(b) == switchContainer))
+						if (br.TargetContainer == switchContainer && switchContainer!.Descendants.OfType<Branch>().Where(b => b.TargetBlock == br.TargetBlock).All(b => BlockContainer.FindClosestSwitchContainer(b) == switchContainer))
 							ConvertSwitchSectionBody(astSection, br.TargetBlock);
 						else
 							ConvertSwitchSectionBody(astSection, section.Body);
@@ -303,7 +306,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 					Debug.Assert(block.FinalInstruction.OpCode == OpCode.Nop);
 				}
-				if (endContainerLabels.TryGetValue(switchContainer, out string label))
+				if (endContainerLabels.TryGetValue(switchContainer, out string? label))
 				{
 					lastSectionStatements.Add(new LabelStatement { Label = label });
 					lastSectionStatements.Add(new BreakStatement());
@@ -322,7 +325,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (!bodyInst.HasFlag(InstructionFlags.EndPointUnreachable))
 			{
 				// we need to insert 'break;'
-				BlockStatement block = body as BlockStatement;
+				BlockStatement? block = body as BlockStatement;
 				if (block != null)
 				{
 					block.Add(new BreakStatement());
@@ -335,11 +338,11 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		/// <summary>Target block that a 'continue;' statement would jump to</summary>
-		Block continueTarget;
+		Block? continueTarget;
 		/// <summary>Number of ContinueStatements that were created for the current continueTarget</summary>
 		int continueCount;
-		/// <summary>Maps blocks to cases.</summary>
-		Dictionary<Block, ConstantResolveResult> caseLabelMapping;
+		/// <summary>Maps blocks to cases. A null value marks the default case.</summary>
+		Dictionary<Block, ConstantResolveResult?>? caseLabelMapping;
 
 		protected internal override TranslatedStatement VisitBranch(Branch inst)
 		{
@@ -359,7 +362,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		/// <summary>Target container that a 'break;' statement would break out of</summary>
-		BlockContainer breakTarget;
+		BlockContainer? breakTarget;
 		/// <summary>Dictionary from BlockContainer to label name for 'goto of_container';</summary>
 		readonly Dictionary<BlockContainer, string> endContainerLabels = new Dictionary<BlockContainer, string>();
 
@@ -401,7 +404,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				else
 					return new ReturnStatement().WithILInstruction(inst);
 			}
-			if (!endContainerLabels.TryGetValue(inst.TargetContainer, out string label))
+			if (!endContainerLabels.TryGetValue(inst.TargetContainer, out string? label))
 			{
 				label = "end_" + inst.TargetLabel;
 				if (!duplicateLabels.TryGetValue(label, out int count))
@@ -463,7 +466,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					catchClause.AddAnnotation(new ILVariableResolveResult(v, v.Type));
 					if (v.StoreCount > 1 || v.LoadCount > 0 || v.AddressCount > 0)
 					{
-						catchClause.VariableName = v.Name;
+						catchClause.VariableName = v.Name!;
 						catchClause.Type = exprBuilder.ConvertType(v.Type);
 					}
 					else if (!v.Type.IsKnownType(KnownTypeCode.Object))
@@ -602,7 +605,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		Statement TransformToForeach(UsingInstruction inst, Expression resource)
+		Statement? TransformToForeach(UsingInstruction inst, Expression resource)
 		{
 			if (!settings.ForEachStatement)
 			{
@@ -680,7 +683,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			// Handle explicit casts:
 			// This is the case if an explicit type different from the collection-item-type was used.
 			// For example: foreach (ClassA item in nonGenericEnumerable)
-			var type = singleGetter.Method.ReturnType;
+			var type = singleGetter!.Method.ReturnType;
 			ILInstruction instToReplace = singleGetter;
 			bool useVar = false;
 			switch (instToReplace.Parent)
@@ -707,13 +710,13 @@ namespace ICSharpCode.Decompiler.CSharp
 					break;
 			}
 
-			VariableDesignation designation = null;
+			VariableDesignation? designation = null;
 
 			// Handle the required foreach-variable transformation:
 			switch (transformation)
 			{
 				case RequiredGetCurrentTransformation.UseExistingVariable:
-					if (foreachVariable.Type.Kind != TypeKind.Dynamic)
+					if (foreachVariable!.Type.Kind != TypeKind.Dynamic)
 						foreachVariable.Type = type;
 					foreachVariable.Kind = VariableKind.ForeachLocal;
 					foreachVariable.Name = AssignVariableNames.GenerateForeachVariableName(currentFunction, collectionExpr.Annotation<ILInstruction>(), decompileRun.UsingScope, foreachVariable);
@@ -735,7 +738,7 @@ namespace ICSharpCode.Decompiler.CSharp
 						VariableKind.Local, type,
 						AssignVariableNames.GenerateVariableName(currentFunction, type, decompileRun.UsingScope)
 					);
-					instToReplace.Parent.ReplaceWith(new LdLoca(localCopyVariable));
+					instToReplace.Parent!.ReplaceWith(new LdLoca(localCopyVariable));
 					body.Instructions.Insert(0, new StLoc(localCopyVariable, new LdLoc(foreachVariable)));
 					body.Instructions.Insert(0, new StLoc(foreachVariable, instToReplace));
 					break;
@@ -747,7 +750,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 			if (designation == null)
 			{
-				designation = new SingleVariableDesignation { Identifier = foreachVariable.Name };
+				designation = new SingleVariableDesignation { Identifier = foreachVariable!.Name };
 				// Add the variable annotation for highlighting
 				designation.AddAnnotation(new ILVariableResolveResult(foreachVariable, foreachVariable.Type));
 			}
@@ -773,7 +776,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			// Construct the foreach loop.
 			var foreachStmt = new ForeachStatement {
 				IsAsync = isAsync,
-				VariableType = useVar ? new SimpleType("var") : exprBuilder.ConvertType(foreachVariable.Type),
+				VariableType = useVar ? new SimpleType("var") : exprBuilder.ConvertType(foreachVariable!.Type),
 				VariableDesignation = designation,
 				InExpression = collectionExpr.Detach(),
 				EmbeddedStatement = foreachBody
@@ -851,7 +854,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return NormalizeTypeVisitor.TypeErasure.EquivalentTypes(a, b);
 		}
 
-		private bool IsDynamicCastToIEnumerable(Expression expr, out Expression dynamicExpr)
+		private bool IsDynamicCastToIEnumerable(Expression expr, [NotNullWhen(true)] out Expression? dynamicExpr)
 		{
 			if (!(expr is CastExpression cast))
 			{
@@ -874,7 +877,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// Otherwise returns the unmodified container.
 		/// </summary>
 		/// <param name="optionalLeaveInst">If the leave is a return/break and has no side-effects, we can move the return out of the using-block and put it after the loop, otherwise returns null.</param>
-		BlockContainer UnwrapNestedContainerIfPossible(BlockContainer container, out Leave optionalLeaveInst)
+		BlockContainer UnwrapNestedContainerIfPossible(BlockContainer container, out Leave? optionalLeaveInst)
 		{
 			optionalLeaveInst = null;
 			// Check block structure:
@@ -954,7 +957,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// <param name="singleGetter">Returns the call instruction invoking Current's getter.</param>
 		/// <param name="foreachVariable">Returns the the foreach variable, if a suitable was found. This variable is only assigned once and its assignment is the first statement in <paramref name="loopBody"/>.</param>
 		/// <returns><see cref="RequiredGetCurrentTransformation"/> for details.</returns>
-		RequiredGetCurrentTransformation DetectGetCurrentTransformation(BlockContainer usingContainer, Block loopBody, BlockContainer loopContainer, ILVariable enumerator, ILInstruction moveNextUsage, out CallInstruction singleGetter, out ILVariable foreachVariable)
+		RequiredGetCurrentTransformation DetectGetCurrentTransformation(BlockContainer usingContainer, Block loopBody, BlockContainer loopContainer, ILVariable enumerator, ILInstruction moveNextUsage, out CallInstruction? singleGetter, out ILVariable? foreachVariable)
 		{
 			singleGetter = null;
 			foreachVariable = null;
@@ -966,7 +969,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			// => no foreach
 			if (loads.Length != 1 || !ParentIsCurrentGetter(loads[0]))
 				return RequiredGetCurrentTransformation.NoForeach;
-			singleGetter = (CallInstruction)loads[0].Parent;
+			singleGetter = (CallInstruction)loads[0].Parent!;
 			// singleGetter is not part of the first instruction in body or cannot be uninlined
 			// => no foreach
 			if (!(singleGetter.IsDescendantOf(loopBody.Instructions[0])
@@ -1089,7 +1092,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				return false;
 			if (targetType.IsReferenceType ?? false)
 				return false;
-			switch (inst.Parent.OpCode)
+			switch (inst.Parent!.OpCode)
 			{
 				case OpCode.Call:
 				case OpCode.CallVirt:
@@ -1173,7 +1176,7 @@ namespace ICSharpCode.Decompiler.CSharp
 							.WithRR(new ResolveResult(inst.Variable.Type));
 				}
 			}
-			fixedStmt.Variables.Add(new VariableInitializer(inst.Variable.Name, initExpr).WithILVariable(inst.Variable));
+			fixedStmt.Variables.Add(new VariableInitializer(inst.Variable.Name!, initExpr).WithILVariable(inst.Variable));
 			fixedStmt.EmbeddedStatement = Convert(inst.Body);
 			return fixedStmt.WithILInstruction(inst);
 		}
@@ -1236,8 +1239,8 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		Statement ConvertLoop(BlockContainer container)
 		{
-			ILInstruction condition;
-			Block loopBody;
+			ILInstruction? condition;
+			Block? loopBody;
 			BlockStatement blockStatement;
 			continueCount = 0;
 			breakTarget = container;
@@ -1387,19 +1390,19 @@ namespace ICSharpCode.Decompiler.CSharp
 			LocalFunctionDeclarationStatement TranslateFunction(ILFunction function)
 			{
 				var astBuilder = exprBuilder.astBuilder;
-				var method = (MethodDeclaration)astBuilder.ConvertEntity(function.ReducedMethod);
+				var method = (MethodDeclaration)astBuilder.ConvertEntity(function.ReducedMethod!);
 
-				var variables = function.Variables.Where(v => v.Kind == VariableKind.Parameter).ToDictionary(v => v.Index);
+				var variables = function.Variables.Where(v => v.Kind == VariableKind.Parameter).ToDictionary(v => v.Index!.Value);
 
 				foreach (var (i, p) in method.Parameters.WithIndex())
 				{
 					if (variables.TryGetValue(i, out var v))
 					{
-						p.Name = v.Name;
+						p.Name = v.Name!;
 					}
 				}
 
-				if (function.Method.HasBody)
+				if (function.Method!.HasBody)
 				{
 					var nestedBuilder = new StatementBuilder(
 						typeSystem,
@@ -1428,7 +1431,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					method.Modifiers |= Modifiers.Extern;
 				}
 
-				CSharpDecompiler.AddAnnotationsToDeclaration(function.ReducedMethod, method, function);
+				CSharpDecompiler.AddAnnotationsToDeclaration(function.ReducedMethod!, method, function);
 				CSharpDecompiler.CleanUpMethodDeclaration(method, method.Body, function, function.Method.HasBody);
 				CSharpDecompiler.RemoveAttribute(method, KnownAttribute.CompilerGenerated);
 				var stmt = new LocalFunctionDeclarationStatement(method);
@@ -1471,7 +1474,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					blockStatement.Add(Convert(block.FinalInstruction));
 				}
 			}
-			if (endContainerLabels.TryGetValue(container, out string label))
+			if (endContainerLabels.TryGetValue(container, out string? label))
 			{
 				if (isLoop && !(blockStatement.LastOrDefault() is ContinueStatement))
 				{
@@ -1491,7 +1494,7 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		string EnsureUniqueLabel(Block block)
 		{
-			if (labels.TryGetValue(block, out string label))
+			if (labels.TryGetValue(block, out string? label))
 				return label;
 			if (!duplicateLabels.TryGetValue(block.Label, out int count))
 			{
@@ -1509,10 +1512,10 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			if (!leave.Value.MatchNop())
 				return false;
-			Block block = (Block)leave.Parent;
+			Block block = (Block)leave.Parent!;
 			if (leave.ChildIndex != block.Instructions.Count - 1 || block.FinalInstruction.OpCode != OpCode.Nop)
 				return false;
-			BlockContainer container = (BlockContainer)block.Parent;
+			BlockContainer container = (BlockContainer)block.Parent!;
 			return block.ChildIndex == container.Blocks.Count - 1
 				&& container == leave.TargetContainer;
 		}

ICSharpCode.Decompiler/CSharp/Syntax/AstNodeCollection.cs

@@ -310,9 +310,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			Insert(IndexOf(existingItem!) + 1, newItem);
 		}
 
-		public void InsertBefore(T existingItem, T newItem)
+		public void InsertBefore(T? existingItem, T newItem)
 		{
-			int index = IndexOf(existingItem);
+			// A null existingItem yields IndexOf == -1, so the new item is appended.
+			int index = IndexOf(existingItem!);
 			Insert(index < 0 ? list.Count : index, newItem);
 		}
 

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -20,6 +20,7 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Reflection;
 using System.Runtime.CompilerServices;
@@ -31,6 +32,8 @@ using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -38,7 +41,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	/// </summary>
 	public class TypeSystemAstBuilder
 	{
-		readonly CSharpResolver resolver;
+		readonly CSharpResolver? resolver;
 
 		#region Constructor
 		/// <summary>
@@ -479,13 +482,13 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		AstType ConvertTypeHelper(IType genericType, IReadOnlyList<IType> typeArguments)
 		{
-			ITypeDefinition typeDef = genericType.GetDefinition();
+			ITypeDefinition? typeDef = genericType.GetDefinition();
 			Debug.Assert(typeDef != null || genericType.Kind == TypeKind.Unknown);
 			Debug.Assert(typeArguments.Count >= genericType.TypeParameterCount);
 
 			if (UseKeywordsForBuiltinTypes && typeDef != null)
 			{
-				string keyword = KnownTypeReference.GetCSharpNameByTypeCode(typeDef.KnownTypeCode);
+				string? keyword = KnownTypeReference.GetCSharpNameByTypeCode(typeDef.KnownTypeCode);
 				if (keyword != null)
 				{
 					return new PrimitiveType(keyword);
@@ -527,10 +530,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					localTypeArguments = Empty<IType>.Array;
 				}
 				ResolveResult rr = resolver.LookupSimpleNameOrTypeName(typeDef.Name, localTypeArguments, NameLookupMode);
-				TypeResolveResult trr = rr as TypeResolveResult;
+				TypeResolveResult? trr = rr as TypeResolveResult;
 				if (trr != null || (localTypeArguments.Length == 0 && resolver.IsVariableReferenceWithSameType(rr, typeDef.Name, out trr)))
 				{
-					if (!trr.IsError && TypeMatches(trr.Type, typeDef, typeArguments))
+					if (!trr!.IsError && TypeMatches(trr.Type, typeDef, typeArguments))
 					{
 						// We can use the short type name
 						SimpleType shortResult = MakeSimpleType(typeDef.Name);
@@ -591,7 +594,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			{
 				if (!TypeDefMatches(typeDef, type.GetDefinition()))
 					return false;
-				ParameterizedType pt = type as ParameterizedType;
+				ParameterizedType? pt = type as ParameterizedType;
 				if (pt == null)
 				{
 					return typeArguments.All(t => t.Kind == TypeKind.UnboundTypeArgument);
@@ -606,14 +609,14 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		bool TypeDefMatches(ITypeDefinition typeDef, IType type)
+		bool TypeDefMatches(ITypeDefinition typeDef, IType? type)
 		{
 			if (type == null || type.Name != typeDef.Name || type.Namespace != typeDef.Namespace || type.TypeParameterCount != typeDef.TypeParameterCount)
 				return false;
 			bool defIsNested = typeDef.DeclaringTypeDefinition != null;
 			bool typeIsNested = type.DeclaringType != null;
 			if (defIsNested && typeIsNested)
-				return TypeDefMatches(typeDef.DeclaringTypeDefinition, type.DeclaringType);
+				return TypeDefMatches(typeDef.DeclaringTypeDefinition!, type.DeclaringType);
 			else
 				return defIsNested == typeIsNested;
 		}
@@ -642,12 +645,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		public AstType ConvertNamespace(string namespaceName, out NamespaceResolveResult nrr)
+		public AstType ConvertNamespace(string namespaceName, out NamespaceResolveResult? nrr)
 		{
 			return ConvertNamespace(namespaceName, out nrr, requiresGlobalPrefix: false);
 		}
 
-		AstType ConvertNamespace(string namespaceName, out NamespaceResolveResult nrr, bool requiresGlobalPrefix)
+		AstType ConvertNamespace(string namespaceName, out NamespaceResolveResult? nrr, bool requiresGlobalPrefix)
 		{
 			if (resolver != null)
 			{
@@ -702,7 +705,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					};
 					if (AddResolveResultAnnotations)
 					{
-						var @namespace = resolver.Compilation.RootNamespace.GetChildNamespace(namespaceName);
+						var @namespace = resolver!.Compilation.RootNamespace.GetChildNamespace(namespaceName);
 						if (@namespace != null)
 							ns.AddAnnotation(nrr = new NamespaceResolveResult(@namespace));
 					}
@@ -731,7 +734,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		bool IsValidNamespace(string firstNamespacePart, out NamespaceResolveResult nrr)
+		bool IsValidNamespace(string firstNamespacePart, out NamespaceResolveResult? nrr)
 		{
 			nrr = null;
 			if (resolver == null)
@@ -795,7 +798,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				InitializedObjectResolveResult targetResult = new InitializedObjectResolveResult(attribute.AttributeType);
 				foreach (var namedArg in attribute.NamedArguments)
 				{
-					NamedExpression namedArgument = new NamedExpression(namedArg.Name, ConvertConstantValue(namedArg.Type, namedArg.Value));
+					NamedExpression namedArgument = new NamedExpression(namedArg.Name!, ConvertConstantValue(namedArg.Type, namedArg.Value));
 					if (AddResolveResultAnnotations)
 					{
 						IMember member = CustomAttribute.MemberForNamedArgument(attribute.AttributeType, namedArg);
@@ -817,10 +820,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return attr;
 		}
 
-		private IEnumerable<AttributeSection> ConvertAttributes(IEnumerable<IAttribute> attributes, string target = null)
+		private IEnumerable<AttributeSection> ConvertAttributes(IEnumerable<IAttribute> attributes, string? target = null)
 		{
 			if (SortAttributes)
-				attributes = attributes.OrderBy(a => a, new DelegateComparer<IAttribute>(CompareAttribute));
+				attributes = attributes.OrderBy(a => a, new DelegateComparer<IAttribute>((a, b) => CompareAttribute(a!, b!)));
 			return attributes.Select(a => {
 				var section = new AttributeSection(ConvertAttribute(a));
 				if (target != null)
@@ -863,7 +866,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				{
 					var argA = a.NamedArguments[i];
 					var argB = b.NamedArguments[i];
-					result = argA.Name.CompareTo(argB.Name);
+					result = argA.Name!.CompareTo(argB.Name);
 					if (result != 0)
 						return result;
 					result = CompareType(argA.Type, argB.Type);
@@ -893,7 +896,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				throw new ArgumentNullException(nameof(type));
 			AstType astType = ConvertTypeHelper(type);
 
-			string shortName = null;
+			string? shortName = null;
 			if (type.Name.Length > 9 && type.Name.EndsWith("Attribute", StringComparison.Ordinal))
 			{
 				shortName = type.Name.Remove(type.Name.Length - 9);
@@ -919,16 +922,16 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return astType;
 		}
 
-		private void ApplyShortAttributeNameIfPossible(IType type, AstType astType, string shortName)
+		private void ApplyShortAttributeNameIfPossible(IType type, AstType astType, string? shortName)
 		{
 			switch (astType)
 			{
 				case SimpleType st:
-					ResolveResult shortRR = null;
-					ResolveResult withExtraAttrSuffix = resolver.LookupSimpleNameOrTypeName(type.Name + "Attribute", EmptyList<IType>.Instance, NameLookupMode.Type);
+					ResolveResult? shortRR = null;
+					ResolveResult withExtraAttrSuffix = resolver!.LookupSimpleNameOrTypeName(type.Name + "Attribute", EmptyList<IType>.Instance, NameLookupMode.Type);
 					if (shortName != null)
 					{
-						shortRR = resolver.LookupSimpleNameOrTypeName(shortName, EmptyList<IType>.Instance, NameLookupMode.Type);
+						shortRR = resolver!.LookupSimpleNameOrTypeName(shortName, EmptyList<IType>.Instance, NameLookupMode.Type);
 					}
 					// short type is either unknown or not an attribute type -> we can use the short name.
 					if (shortRR != null && (shortRR is UnknownIdentifierResolveResult || !IsAttributeType(shortRR)))
@@ -972,7 +975,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		private bool IsAttributeType(IType type)
+		private bool IsAttributeType(IType? type)
 		{
 			return type != null && type.GetNonInterfaceBaseTypes().Any(t => t.IsKnownType(KnownTypeCode.Attribute));
 		}
@@ -1066,7 +1069,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// which is of type <c>int</c>.
 		/// However, the returned expression will always be implicitly convertible to <paramref name="type"/>.
 		/// </summary>
-		public Expression ConvertConstantValue(IType type, object constantValue)
+		public Expression ConvertConstantValue(IType type, object? constantValue)
 		{
 			return ConvertConstantValue(type, type, constantValue);
 		}
@@ -1074,7 +1077,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// <summary>
 		/// Creates an Expression for the given constant value.
 		/// </summary>
-		public Expression ConvertConstantValue(IType expectedType, IType type, object constantValue)
+		public Expression ConvertConstantValue(IType expectedType, IType type, object? constantValue)
 		{
 			if (type == null)
 				throw new ArgumentNullException(nameof(type));
@@ -1132,7 +1135,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					}
 					if (underlyingType.IsKnownType(KnownTypeCode.Double) || underlyingType.IsKnownType(KnownTypeCode.Single))
 						return ConvertFloatingPointLiteral(underlyingType, constantValue);
-					IType literalType = underlyingType;
+					IType? literalType = underlyingType;
 					bool integerTypeMismatch = underlyingType.IsCSharpSmallIntegerType() || underlyingType.IsCSharpNativeIntegerType();
 					if (integerTypeMismatch)
 					{
@@ -1161,7 +1164,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		bool IsSpecialConstant(IType expectedType, object constant, out Expression expression)
+		bool IsSpecialConstant(IType expectedType, object constant, [NotNullWhen(true)] out Expression? expression)
 		{
 			expression = null;
 			if (!specialConstants.TryGetValue(constant, out var info))
@@ -1292,7 +1295,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		Expression ConvertEnumValue(IType type, long val)
 		{
-			ITypeDefinition enumDefinition = type.GetDefinition();
+			ITypeDefinition enumDefinition = type.GetDefinition()!;
 			TypeCode enumBaseTypeCode = ReflectionHelper.GetTypeCode(enumDefinition.EnumUnderlyingType);
 			var fields = enumDefinition.Fields
 				.Select(PrepareConstant)
@@ -1311,7 +1314,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			if (IsFlagsEnum(enumDefinition))
 			{
 				long enumValue = val;
-				Expression expr = null;
+				Expression? expr = null;
 				long negatedEnumValue = ~val;
 				// limit negatedEnumValue to the appropriate range
 				switch (enumBaseTypeCode)
@@ -1329,7 +1332,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 						negatedEnumValue &= uint.MaxValue;
 						break;
 				}
-				Expression negatedExpr = null;
+				Expression? negatedExpr = null;
 				foreach (var (fieldValue, field) in fields.OrderByDescending(f => CalculateHammingWeight(unchecked((ulong)f.value))))
 				{
 					if (fieldValue == 0)
@@ -1373,10 +1376,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			(long value, IField field) PrepareConstant(IField field)
 			{
 				if (!field.IsConst)
-					return (-1, null);
-				object constantValue = field.GetConstantValue();
+					return (-1, null!);
+				object? constantValue = field.GetConstantValue();
 				if (constantValue == null)
-					return (-1, null);
+					return (-1, null!);
 				return ((long)CSharpPrimitiveCast.Cast(TypeCode.Int64, constantValue, checkForOverflow: false), field);
 			}
 
@@ -1442,8 +1445,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			// even if the expected type is float or double.
 			constantValue = CSharpPrimitiveCast.Cast(type.GetTypeCode(), constantValue, false);
 			bool isDouble = type.IsKnownType(KnownTypeCode.Double);
-			ICompilation compilation = type.GetDefinition().Compilation;
-			Expression expr = null;
+			ICompilation compilation = type.GetDefinition()!.Compilation;
+			Expression? expr = null;
 
 			string str;
 			if (isDouble)
@@ -1522,7 +1525,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		const float MathF_PI = 3.14159274f;
 		const float MathF_E = 2.71828175f;
 
-		Expression TryExtractExpression(IType mathType, IType type, object literalValue, string memberName, bool isDouble)
+		Expression? TryExtractExpression(IType mathType, IType type, object literalValue, string memberName, bool isDouble)
 		{
 			Expression MakeFieldReference()
 			{
@@ -1543,7 +1546,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				return new CastExpression(ConvertType(type), fieldRef);
 			}
 
-			Expression ExtractExpression(long n, long d)
+			Expression? ExtractExpression(long n, long d)
 			{
 				Expression fieldReference = MakeFieldReference();
 
@@ -1777,7 +1780,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				case SymbolKind.TypeParameter:
 					return ConvertTypeParameter((ITypeParameter)symbol);
 				default:
-					IEntity entity = symbol as IEntity;
+					IEntity? entity = symbol as IEntity;
 					if (entity != null)
 						return ConvertEntity(entity);
 					throw new ArgumentException("Invalid value for SymbolKind: " + symbol.SymbolKind);
@@ -1811,7 +1814,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				case SymbolKind.Accessor:
 					IMethod accessor = (IMethod)entity;
 					Accessibility ownerAccessibility = accessor.AccessorOwner?.Accessibility ?? Accessibility.None;
-					return ConvertAccessor(accessor, accessor.AccessorKind, ownerAccessibility, false);
+					return ConvertAccessor(accessor, accessor.AccessorKind, ownerAccessibility, false)!;
 				default:
 					throw new ArgumentException("Invalid value for SymbolKind: " + entity.SymbolKind);
 			}
@@ -1823,7 +1826,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			var subst = new TypeParameterSubstitution(group.TypeParameters, []);
 			ext.TypeParameters.AddRange(group.TypeParameters.Select(ConvertTypeParameter));
 			ext.ReceiverParameters.Add(ConvertParameter(group.MarkerMethod.Specialize(subst).Parameters.Single()));
-			ext.Constraints.AddRange(group.TypeParameters.Select(ConvertTypeParameterConstraint));
+			ext.Constraints.AddRange(group.TypeParameters.Select(ConvertTypeParameterConstraint).OfType<Constraint>());
 			return ext;
 		}
 
@@ -1932,7 +1935,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 						// if the declared type is an enum, replace all references to System.Enum with the enum-underlying type
 						if (!typeDefinition.EnumUnderlyingType.IsKnownType(KnownTypeCode.Int32))
 						{
-							decl.BaseTypes.Add(ConvertType(typeDefinition.EnumUnderlyingType));
+							decl.BaseTypes.Add(ConvertType(typeDefinition.EnumUnderlyingType!));
 						}
 					}
 					else if ((typeDefinition.Kind == TypeKind.Struct || typeDefinition.Kind == TypeKind.Void) && baseType.IsKnownType(KnownTypeCode.ValueType))
@@ -1974,7 +1977,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		DelegateDeclaration ConvertDelegate(IMethod invokeMethod, Modifiers modifiers)
 		{
-			ITypeDefinition d = invokeMethod.DeclaringTypeDefinition;
+			ITypeDefinition d = invokeMethod.DeclaringTypeDefinition!;
 
 			DelegateDeclaration decl = new DelegateDeclaration();
 			decl.Modifiers = modifiers & ~Modifiers.Sealed;
@@ -2055,7 +2058,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			{
 				ct.HasReadOnlySpecifier = true;
 			}
-			Expression initializer = null;
+			Expression? initializer = null;
 			if (field.IsConst && this.ShowConstantValues)
 			{
 				try
@@ -2071,7 +2074,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return decl;
 		}
 
-		BlockStatement GenerateBodyBlock()
+		BlockStatement? GenerateBodyBlock()
 		{
 			if (GenerateBody)
 			{
@@ -2085,7 +2088,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		Accessor ConvertAccessor(IMethod accessor, MethodSemanticsAttributes kind, Accessibility ownerAccessibility, bool addParameterAttribute)
+		Accessor? ConvertAccessor(IMethod? accessor, MethodSemanticsAttributes kind, Accessibility ownerAccessibility, bool addParameterAttribute)
 		{
 			if (accessor == null)
 				return null;
@@ -2155,7 +2158,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return decl;
 		}
 
-		static void MergeReadOnlyModifiers(EntityDeclaration decl, Accessor accessor1, Accessor accessor2)
+		static void MergeReadOnlyModifiers(EntityDeclaration decl, Accessor? accessor1, Accessor? accessor2)
 		{
 			if (accessor1 is null)
 				return;
@@ -2164,7 +2167,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				accessor1.Modifiers &= ~Modifiers.Readonly;
 				decl.Modifiers |= Modifiers.Readonly;
 			}
-			else if (accessor1.HasModifier(Modifiers.Readonly) && accessor2.HasModifier(Modifiers.Readonly))
+			else if (accessor1.HasModifier(Modifiers.Readonly) && accessor2!.HasModifier(Modifiers.Readonly))
 			{
 				accessor1.Modifiers &= ~Modifiers.Readonly;
 				accessor2.Modifiers &= ~Modifiers.Readonly;
@@ -2479,7 +2482,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return decl;
 		}
 
-		internal Constraint ConvertTypeParameterConstraint(ITypeParameter tp)
+		internal Constraint? ConvertTypeParameterConstraint(ITypeParameter tp)
 		{
 			if (!tp.HasDefaultConstructorConstraint && !tp.HasReferenceTypeConstraint && !tp.HasValueTypeConstraint && !tp.AllowsRefLikeType && tp.NullabilityConstraint != Nullability.NotNullable && tp.DirectBaseTypes.All(IsObjectOrValueType))
 			{
@@ -2543,7 +2546,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		static bool IsObjectOrValueType(IType type)
 		{
-			ITypeDefinition d = type.GetDefinition();
+			ITypeDefinition? d = type.GetDefinition();
 			return d != null && (d.KnownTypeCode == KnownTypeCode.Object || d.KnownTypeCode == KnownTypeCode.ValueType);
 		}
 		#endregion
@@ -2554,7 +2557,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			VariableDeclarationStatement decl = new VariableDeclarationStatement();
 			decl.Modifiers = v.IsConst ? Modifiers.Const : Modifiers.None;
 			decl.Type = ConvertType(v.Type);
-			Expression initializer = null;
+			Expression? initializer = null;
 			if (v.IsConst)
 			{
 				try
@@ -2576,7 +2579,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return new NamespaceDeclaration(ns.FullName);
 		}
 
-		AstType GetExplicitInterfaceType(IMember member)
+		AstType? GetExplicitInterfaceType(IMember member)
 		{
 			if (member.IsExplicitInterfaceImplementation)
 			{

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -805,7 +805,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			foreach (var node in rootNode.Descendants)
 			{
-				ILVariable ilVar;
+				ILVariable? ilVar;
 				switch (node)
 				{
 					case IdentifierExpression id:

ICSharpCode.Decompiler/CSharp/Transforms/FixNameCollisions.cs

@@ -52,15 +52,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (fieldDecl.Variables.Count != 1)
 						continue;
 					string oldName = fieldDecl.Variables.Single().Name;
-					ISymbol symbol = fieldDecl.GetSymbol();
-					if (memberNames.Contains(oldName) && ((IField)symbol).Accessibility == Accessibility.Private)
+					ISymbol? symbol = fieldDecl.GetSymbol();
+					if (memberNames.Contains(oldName) && symbol is IField { Accessibility: Accessibility.Private })
 					{
 						string newName = PickNewName(memberNames, oldName);
-						if (symbol != null)
-						{
-							fieldDecl.Variables.Single().Name = newName;
-							renamedSymbols[symbol] = newName;
-						}
+						fieldDecl.Variables.Single().Name = newName;
+						renamedSymbols[symbol] = newName;
 					}
 				}
 			}
@@ -69,7 +66,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				if (node is IdentifierExpression || node is MemberReferenceExpression)
 				{
-					ISymbol symbol = node.GetSymbol();
+					ISymbol? symbol = node.GetSymbol();
 					if (symbol != null && renamedSymbols.TryGetValue(symbol, out string? newName))
 					{
 						node.GetChildByRole<Identifier>(SlotKind.Identifier).Name = newName;

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceExtensionMethods.cs

@@ -106,7 +106,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				return;
 			}
-			var method = (IMethod)invocationExpression.GetSymbol();
+			var method = (IMethod)invocationExpression.GetSymbol()!;
 			if (firstArgument is DirectionExpression dirExpr)
 			{
 				if (!context.Settings.RefExtensionMethods || dirExpr.FieldDirection == FieldDirection.Out)

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs

@@ -312,7 +312,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 							if (mre.MemberName == "GroupJoin")
 							{
 								joinClause.IntoIdentifier = p2.Name; // into p2.Name
-								joinClause.IntoIdentifierToken.CopyAnnotationsFrom(p2);
+								joinClause.IntoIdentifierToken!.CopyAnnotationsFrom(p2);
 							}
 							joinClause.AddAnnotation(new QueryJoinClauseAnnotation(outerLambda.Annotation<IL.ILFunction>(), innerLambda.Annotation<IL.ILFunction>()));
 							query.Clauses.Add(joinClause);

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -235,7 +235,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		bool ForStatementUsesVariable(ForStatement statement, IL.ILVariable variable)
+		bool ForStatementUsesVariable(ForStatement statement, IL.ILVariable? variable)
 		{
 			if (statement.Condition?.DescendantsAndSelf.OfType<IdentifierExpression>().Any(ie => ie.GetILVariable() == variable) == true)
 				return true;
@@ -515,7 +515,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					break;
 				if (!int.TryParse(m.Get<PrimitiveExpression>("index").Single().Value?.ToString() ?? "", out int index) || index != i)
 					break;
-				upperBounds[i] = m.Get<IdentifierExpression>("variable").Single().GetILVariable();
+				upperBounds[i] = m.Get<IdentifierExpression>("variable").Single().GetILVariable()!;
 				stmt = stmt.GetNextStatement();
 				i++;
 			} while (stmt != null && upperBounds != null && i < upperBounds.Length);
@@ -948,7 +948,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			// ignore tuple element names, dynamic and nullability
 			if (!NormalizeTypeVisitor.TypeErasure.EquivalentTypes(returnType, eventType))
 				return false;
-			var combineMethod = m.Get<AstNode>("delegateCombine").Single().Parent.GetSymbol() as IMethod;
+			var combineMethod = m.Get<AstNode>("delegateCombine").Single().Parent!.GetSymbol() as IMethod;
 			if (combineMethod == null || combineMethod.Name != (isAddAccessor ? "Combine" : "Remove"))
 				return false;
 			return combineMethod.DeclaringType.FullName == "System.Delegate";

ICSharpCode.Decompiler/CSharp/Transforms/TransformFieldAndConstructorInitializers.cs

@@ -284,7 +284,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				MemberToDeclaringSyntaxNodeMap = members
 					.Select(m => (symbol: m.GetSymbol(), entity: (EntityDeclaration)m))
 					.Where(_ => _.symbol is IMember)
-					.ToDictionary(_ => (IMember)_.symbol, _ => _.entity);
+					.ToDictionary(_ => (IMember)_.symbol!, _ => _.entity);
 
 				List<ConstructorDeclaration> constructorsNotChainedWithThis = [];
 				List<ConstructorDeclaration> allCtors = [];
@@ -298,7 +298,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 				foreach (var ctor in members.OfType<ConstructorDeclaration>())
 				{
-					var ctorMethod = (IMethod)ctor.GetSymbol();
+					var ctorMethod = (IMethod)ctor.GetSymbol()!;
 					Debug.Assert(ctorMethod.IsConstructor);
 					Debug.Assert(ctorMethod.MetadataToken.IsNil == false);
 
@@ -351,7 +351,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 					// this constructor could be converted to a primary constructor
 					var ctor = constructorsNotChainedWithThis[0];
-					var ctorMethod = (IMethod)constructorsNotChainedWithThis[0].GetSymbol();
+					var ctorMethod = (IMethod)constructorsNotChainedWithThis[0].GetSymbol()!;
 
 					var initializer = InitializerSequence.Analyze(this, ctor, ctorMethod);
 
@@ -425,7 +425,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						bool isPrimaryCtor = constructorsNotChainedWithThis[0] == PrimaryConstructorDecl;
 						var sequence = isPrimaryCtor
 							? PrimaryConstructorInitializers
-							: InitializerSequence.Analyze(this, constructorsNotChainedWithThis[0], (IMethod)constructorsNotChainedWithThis[0].GetSymbol());
+							: InitializerSequence.Analyze(this, constructorsNotChainedWithThis[0], (IMethod)constructorsNotChainedWithThis[0].GetSymbol()!);
 
 						if (sequence == null)
 							return false;
@@ -705,7 +705,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					return;
 
 				var ctor = InstanceConstructors[0];
-				var ctorMethod = (IMethod)ctor.GetSymbol();
+				var ctorMethod = (IMethod)ctor.GetSymbol()!;
 
 				if (TypeDefinition.Kind == TypeKind.Struct && ctorMethod.Parameters.Count == 0 && InstanceInitializers != null)
 				{
@@ -810,7 +810,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 			foreach (var typeDeclaration in node.Descendants.OfType<TypeDeclaration>())
 			{
-				var currentTypeDefinition = (ITypeDefinition)typeDeclaration.GetSymbol();
+				var currentTypeDefinition = (ITypeDefinition)typeDeclaration.GetSymbol()!;
 				TransformDeclaration(currentTypeDefinition, typeDeclaration, typeDeclaration.Members);
 			}
 		}
@@ -838,7 +838,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 			foreach (var constructorDeclaration in members.OfType<ConstructorDeclaration>())
 			{
-				analyzer.MoveConstructorInitializer(constructorDeclaration, (IMethod)constructorDeclaration.GetSymbol());
+				analyzer.MoveConstructorInitializer(constructorDeclaration, (IMethod)constructorDeclaration.GetSymbol()!);
 			}
 
 			analyzer.RemoveImplicitConstructor();

ICSharpCode.Decompiler/CSharp/TranslatedExpression.cs

@@ -29,6 +29,8 @@ using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.Util;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	/// <summary>
@@ -156,7 +158,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			if (descendant == Expression)
 				return this;
-			for (AstNode parent = descendant.Parent; parent != null; parent = parent.Parent)
+			for (AstNode? parent = descendant.Parent; parent != null; parent = parent.Parent)
 			{
 				foreach (var inst in parent.Annotations.OfType<ILInstruction>())
 					descendant.AddAnnotation(inst);
@@ -664,7 +666,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// In conditional contexts, remove the bool-cast emitted when converting
 		/// an "implicit operator bool" invocation.
 		/// </summary>
-		public TranslatedExpression UnwrapImplicitBoolConversion(Func<IType, bool> typeFilter = null)
+		public TranslatedExpression UnwrapImplicitBoolConversion(Func<IType, bool>? typeFilter = null)
 		{
 			if (!this.Type.IsKnownType(KnownTypeCode.Boolean))
 				return this;

ICSharpCode.Decompiler/CSharp/TranslatedStatement.cs

@@ -6,6 +6,8 @@ using System.Linq;
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.IL;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	[DebuggerDisplay("{Statement}")]

ICSharpCode.Decompiler/CSharp/TranslationContext.cs

@@ -18,6 +18,8 @@
 
 using ICSharpCode.Decompiler.TypeSystem;
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.CSharp
 {
 	/// <summary>

ILSpy/Languages/CSharpHighlightingTokenWriter.cs

@@ -437,7 +437,7 @@ namespace ICSharpCode.ILSpy.Languages
 			}
 		}
 
-		public override void WritePrimitiveValue(object value, ICSharpCode.Decompiler.CSharp.Syntax.LiteralFormat format)
+		public override void WritePrimitiveValue(object? value, ICSharpCode.Decompiler.CSharp.Syntax.LiteralFormat format)
 		{
 			HighlightingColor? color = null;
 			if (value is null)