focus on CS8603, return null from method

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -541,7 +541,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			return typeSystemAstBuilder;
 		}
 
-		IDocumentationProvider CreateDefaultDocumentationProvider()
+		IDocumentationProvider? CreateDefaultDocumentationProvider()
 		{
 			try
 			{
@@ -1096,7 +1096,7 @@ 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;

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -265,7 +265,7 @@ 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;

ICSharpCode.Decompiler/CSharp/OutputVisitor/TextWriterTokenWriter.cs

@@ -437,7 +437,7 @@ 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)
 			{

ICSharpCode.Decompiler/CSharp/ProjectDecompiler/TargetFramework.cs

@@ -79,7 +79,7 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 		/// </summary>
 		public bool IsPortableClassLibrary { get; }
 
-		static string GetTargetFrameworkMoniker(string frameworkIdentifier, int version)
+		static string? GetTargetFrameworkMoniker(string frameworkIdentifier, int version)
 		{
 			// Reference: https://docs.microsoft.com/en-us/dotnet/standard/frameworks
 			switch (frameworkIdentifier)

ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs

@@ -488,7 +488,7 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 		const int RT_ICON = 3;
 		const int RT_GROUP_ICON = 14;
 
-		unsafe static byte[] CreateApplicationIcon(Win32ResourceDirectory resources)
+		unsafe static byte[]? CreateApplicationIcon(Win32ResourceDirectory resources)
 		{
 			var iconGroup = resources.Find(new Win32ResourceName(RT_GROUP_ICON))?.FirstDirectory()?.FirstData()?.Data;
 			if (iconGroup == null)

ICSharpCode.Decompiler/CSharp/Resolver/CSharpConversions.cs

@@ -554,7 +554,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		/// For IList{T}, ICollection{T}, IEnumerable{T} and IReadOnlyList{T}, returns T.
 		/// Otherwise, returns null.
 		/// </summary>
-		IType UnpackGenericArrayInterface(IType interfaceType)
+		IType? UnpackGenericArrayInterface(IType interfaceType)
 		{
 			ParameterizedType? pt = interfaceType as ParameterizedType;
 			if (pt != null)
@@ -938,7 +938,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				&& (StandardImplicitConversion(a, b).IsValid || StandardImplicitConversion(b, a).IsValid);
 		}
 
-		IType FindMostEncompassedType(IEnumerable<IType> candidates)
+		IType? FindMostEncompassedType(IEnumerable<IType> candidates)
 		{
 			IType? best = null;
 			foreach (var current in candidates)
@@ -951,7 +951,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			return best;
 		}
 
-		IType FindMostEncompassingType(IEnumerable<IType> candidates)
+		IType? FindMostEncompassingType(IEnumerable<IType> candidates)
 		{
 			IType? best = null;
 			foreach (var current in candidates)
@@ -1482,7 +1482,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		/// <summary>
 		/// Unpacks the generic Task[T]. Returns null if the input is not Task[T].
 		/// </summary>
-		static IType UnpackTask(IType type)
+		static IType? UnpackTask(IType type)
 		{
 			ParameterizedType? pt = type as ParameterizedType;
 			if (pt != null && pt.TypeParameterCount == 1 && pt.Name == "Task" && pt.Namespace == "System.Threading.Tasks")

ICSharpCode.Decompiler/CSharp/Resolver/CSharpResolver.cs

@@ -565,7 +565,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		#endregion
 
 		#region GetOverloadableOperatorName
-		static string GetOverloadableOperatorName(UnaryOperatorType op)
+		static string? GetOverloadableOperatorName(UnaryOperatorType op)
 		{
 			switch (op)
 			{
@@ -980,7 +980,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		#endregion
 
 		#region Enum helper methods
-		IType GetEnumUnderlyingType(IType enumType)
+		IType? GetEnumUnderlyingType(IType enumType)
 		{
 			ITypeDefinition? def = enumType.GetDefinition();
 			return def != null ? def.EnumUnderlyingType : SpecialType.UnknownType;
@@ -1203,7 +1203,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		#endregion
 
 		#region GetOverloadableOperatorName
-		static string GetOverloadableOperatorName(BinaryOperatorType op)
+		static string? GetOverloadableOperatorName(BinaryOperatorType op)
 		{
 			switch (op)
 			{

ICSharpCode.Decompiler/CSharp/Resolver/OverloadResolution.cs

@@ -891,7 +891,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		#endregion
 
 		#region Output Properties
-		public IParameterizedMember BestCandidate {
+		public IParameterizedMember? BestCandidate {
 			get { return bestCandidate != null ? bestCandidate.Member : null; }
 		}
 
@@ -919,7 +919,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			get { return bestCandidate != null && IsApplicable(bestCandidate.Errors); }
 		}
 
-		public IParameterizedMember BestCandidateAmbiguousWith {
+		public IParameterizedMember? BestCandidateAmbiguousWith {
 			get { return bestCandidateAmbiguousWith != null ? bestCandidateAmbiguousWith.Member : null; }
 		}
 
@@ -958,7 +958,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		/// 
 		/// parameterIndex = GetArgumentToParameterMap()[argumentIndex]
 		/// </summary>
-		public IReadOnlyList<int> GetArgumentToParameterMap()
+		public IReadOnlyList<int>? GetArgumentToParameterMap()
 		{
 			if (bestCandidate != null)
 				return bestCandidate.ArgumentToParameterMap;
@@ -1037,7 +1037,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			return args;
 		}
 
-		public IParameterizedMember GetBestCandidateWithSubstitutedTypeArguments()
+		public IParameterizedMember? GetBestCandidateWithSubstitutedTypeArguments()
 		{
 			if (bestCandidate == null)
 				return null;

ICSharpCode.Decompiler/CSharp/Resolver/TypeInference.cs

@@ -698,7 +698,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			}
 		}
 
-		TP GetTPForType(IType v)
+		TP? GetTPForType(IType v)
 		{
 			if (v is NullabilityAnnotatedTypeParameter natp)
 			{

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -624,7 +624,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		Statement TransformToForeach(UsingInstruction inst, Expression resource)
+		Statement? TransformToForeach(UsingInstruction inst, Expression resource)
 		{
 			if (!settings.ForEachStatement)
 			{

ICSharpCode.Decompiler/CSharp/Syntax/DepthFirstAstVisitor.cs

@@ -713,7 +713,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	/// </summary>
 	public abstract class DepthFirstAstVisitor<T> : IAstVisitor<T>
 	{
-		protected virtual T VisitChildren(AstNode node)
+		protected virtual T? VisitChildren(AstNode node)
 		{
 			AstNode? next;
 			for (var child = node.FirstChild; child != null; child = next)
@@ -726,7 +726,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return default(T);
 		}
 
-		public virtual T VisitNullNode(AstNode nullNode)
+		public virtual T? VisitNullNode(AstNode nullNode)
 		{
 			// Should we call VisitChildren here?
 			// We usually want to ignore null nodes.

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

@@ -94,7 +94,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				&& this.Expression.DoMatch(o.Expression, match);
 		}
 
-		public static TokenRole GetOperatorRole(UnaryOperatorType op)
+		public static TokenRole? GetOperatorRole(UnaryOperatorType op)
 		{
 			switch (op)
 			{

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

@@ -293,7 +293,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// <summary>
 		/// Gets the method name for the operator type. ("op_Addition", "op_Implicit", etc.)
 		/// </summary>
-		public static string GetName(OperatorType? type)
+		public static string? GetName(OperatorType? type)
 		{
 			if (type == null)
 				return null;

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -1443,7 +1443,7 @@ 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()
 			{
@@ -1464,7 +1464,7 @@ 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();
 
@@ -2400,7 +2400,7 @@ 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.NullabilityConstraint != Nullability.NotNullable && tp.DirectBaseTypes.All(IsObjectOrValueType))
 			{

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUnsafeModifier.cs

@@ -40,8 +40,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		protected override bool VisitChildren(AstNode node)
 		{
 			bool result = false;
-			AstNode next;
+			AstNode? next;
-			for (AstNode child = node.FirstChild; child != null; child = next)
+			for (AstNode? child = node.FirstChild; child != null; child = next)
 			{
 				// Store next to allow the loop to continue
 				// if the visitor removes/replaces child.

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -77,7 +77,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		public override AstNode VisitExpressionStatement(ExpressionStatement expressionStatement)
 		{
-			AstNode result = TransformForeachOnMultiDimArray(expressionStatement);
+			AstNode? result = TransformForeachOnMultiDimArray(expressionStatement);
 			if (result != null)
 				return result;
 			result = TransformFor(expressionStatement);
@@ -182,7 +182,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (!m1.Success)
 				return null;
 			var variable = m1.Get<IdentifierExpression>("variable").Single().GetILVariable();
-			AstNode next = node.NextSibling;
+			AstNode? next = node.NextSibling;
 			if (next is ForStatement forStatement && ForStatementUsesVariable(forStatement, variable))
 			{
 				node.Remove();
@@ -477,7 +477,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		Statement TransformForeachOnMultiDimArray(ExpressionStatement expressionStatement)
+		Statement? TransformForeachOnMultiDimArray(ExpressionStatement expressionStatement)
 		{
 			if (!context.Settings.ForEachStatement)
 				return null;
@@ -609,7 +609,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		PropertyDeclaration TransformAutomaticProperty(PropertyDeclaration propertyDeclaration)
+		PropertyDeclaration? TransformAutomaticProperty(PropertyDeclaration propertyDeclaration)
 		{
 			IProperty? property = propertyDeclaration.GetSymbol() as IProperty;
 			if (!CanTransformToAutomaticProperty(property, !property.DeclaringTypeDefinition.Fields.Any(f => f.Name == "_" + property.Name && f.IsCompilerGenerated())))
@@ -736,7 +736,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		Identifier ReplaceBackingFieldUsage(Identifier identifier)
+		Identifier? ReplaceBackingFieldUsage(Identifier identifier)
 		{
 			if (NameCouldBeBackingFieldOfAutomaticProperty(identifier.Name, out _))
 			{
@@ -757,7 +757,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return null;
 		}
 
-		Identifier ReplaceEventFieldAnnotation(Identifier identifier)
+		Identifier? ReplaceEventFieldAnnotation(Identifier identifier)
 		{
 			var parent = identifier.Parent;
 			var mrr = parent.Annotation<MemberResolveResult>();
@@ -995,7 +995,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return true;
 		}
 
-		EventDeclaration TransformAutomaticEvents(CustomEventDeclaration ev)
+		EventDeclaration? TransformAutomaticEvents(CustomEventDeclaration ev)
 		{
 			if (!ev.PrivateImplementationType.IsNull)
 				return null;
@@ -1055,7 +1055,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			Body = destructorBodyPattern
 		};
 
-		DestructorDeclaration TransformDestructor(MethodDeclaration methodDef)
+		DestructorDeclaration? TransformDestructor(MethodDeclaration methodDef)
 		{
 			Match m = destructorPattern.Match(methodDef);
 			if (m.Success)
@@ -1072,7 +1072,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return null;
 		}
 
-		DestructorDeclaration TransformDestructorBody(DestructorDeclaration dtorDef)
+		DestructorDeclaration? TransformDestructorBody(DestructorDeclaration dtorDef)
 		{
 			Match m = destructorBodyPattern.Match(dtorDef.Body);
 			if (m.Success)
@@ -1099,7 +1099,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		/// Simplify nested 'try { try {} catch {} } finally {}'.
 		/// This transformation must run after the using/lock tranformations.
 		/// </summary>
-		TryCatchStatement TransformTryCatchFinally(TryCatchStatement tryFinally)
+		TryCatchStatement? TransformTryCatchFinally(TryCatchStatement tryFinally)
 		{
 			if (tryCatchFinallyPattern.IsMatch(tryFinally))
 			{
@@ -1130,7 +1130,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		};
 
-		AstNode SimplifyCascadingIfElseStatements(IfElseStatement node)
+		AstNode? SimplifyCascadingIfElseStatements(IfElseStatement node)
 		{
 			Match m = cascadingIfElsePattern.Match(node);
 			if (m.Success)

ICSharpCode.Decompiler/CSharp/TypeSystem/ResolvedUsingScope.cs

@@ -213,12 +213,12 @@ namespace ICSharpCode.Decompiler.CSharp.TypeSystem
 				get { return parentNamespace.Compilation; }
 			}
 
-			INamespace INamespace.GetChildNamespace(string name)
+			INamespace? INamespace.GetChildNamespace(string name)
 			{
 				return null;
 			}
 
-			ITypeDefinition INamespace.GetTypeDefinition(string name, int typeParameterCount)
+			ITypeDefinition? INamespace.GetTypeDefinition(string name, int typeParameterCount)
 			{
 				return null;
 			}

ICSharpCode.Decompiler/CSharp/TypeSystem/TypeOrNamespaceReference.cs

@@ -43,7 +43,7 @@ namespace ICSharpCode.Decompiler.CSharp.TypeSystem
 		/// <summary>
 		/// Returns the namespace that is referenced; or null if no such namespace is found.
 		/// </summary>
-		public INamespace ResolveNamespace(CSharpResolver resolver)
+		public INamespace? ResolveNamespace(CSharpResolver resolver)
 		{
 			NamespaceResolveResult? nrr = Resolve(resolver) as NamespaceResolveResult;
 			return nrr != null ? nrr.Namespace : null;

ICSharpCode.Decompiler/Disassembler/DisassemblerHelpers.cs

@@ -150,7 +150,7 @@ namespace ICSharpCode.Decompiler.Disassembler
 				writer.WriteLocalReference(name, "param_" + index);
 			}
 
-			string GetParameterName(int parameterNumber)
+			string? GetParameterName(int parameterNumber)
 			{
 				var methodDefinition = metadata.GetMethodDefinition(handle);
 				if ((methodDefinition.Attributes & System.Reflection.MethodAttributes.Static) != 0)
@@ -339,7 +339,7 @@ namespace ICSharpCode.Decompiler.Disassembler
 			}
 			return sb.ToString();
 		}
-		public static string PrimitiveTypeName(string fullName)
+		public static string? PrimitiveTypeName(string fullName)
 		{
 			switch (fullName)
 			{

ICSharpCode.Decompiler/Documentation/IdStringMemberReference.cs

@@ -61,7 +61,7 @@ namespace ICSharpCode.Decompiler.Documentation
 			get { return declaringTypeReference; }
 		}
 
-		public IMember Resolve(ITypeResolveContext context)
+		public IMember? Resolve(ITypeResolveContext context)
 		{
 			IType declaringType = declaringTypeReference.Resolve(context);
 			foreach (var member in declaringType.GetMembers(CanMatch, GetMemberOptions.IgnoreInheritedMembers))

ICSharpCode.Decompiler/Documentation/XmlDocLoader.cs

@@ -34,7 +34,7 @@ namespace ICSharpCode.Decompiler.Documentation
 		static readonly Lazy<XmlDocumentationProvider> mscorlibDocumentation = new Lazy<XmlDocumentationProvider>(LoadMscorlibDocumentation);
 		static readonly ConditionalWeakTable<MetadataFile, XmlDocumentationProvider> cache = new();
 
-		static XmlDocumentationProvider LoadMscorlibDocumentation()
+		static XmlDocumentationProvider? LoadMscorlibDocumentation()
 		{
 			string xmlDocFile = FindXmlDocumentation("mscorlib.dll", TargetRuntime.Net_4_0)
 				?? FindXmlDocumentation("mscorlib.dll", TargetRuntime.Net_2_0);
@@ -120,7 +120,7 @@ namespace ICSharpCode.Decompiler.Documentation
 		/// Given the assembly file name, looks up the XML documentation file name.
 		/// Returns null if no XML documentation file is found.
 		/// </summary>
-		internal static string LookupLocalizedXmlDoc(string fileName)
+		internal static string? LookupLocalizedXmlDoc(string fileName)
 		{
 			if (string.IsNullOrEmpty(fileName))
 				return null;

ICSharpCode.Decompiler/Documentation/XmlDocumentationProvider.cs

@@ -81,7 +81,7 @@ namespace ICSharpCode.Decompiler.Documentation
 				return false;
 			}
 
-			internal void Add(string key, string value)
+			internal void Add(string key, string? value)
 			{
 				entries[pos++] = new KeyValuePair<string, string>(key, value);
 				if (pos == entries.Length)
@@ -279,7 +279,7 @@ namespace ICSharpCode.Decompiler.Documentation
 						if (reader.LocalName == "member")
 						{
 							int pos = linePosMapper.GetPositionForLine(reader.LineNumber) + Math.Max(reader.LinePosition - 2, 0);
-							string memberAttr = reader.GetAttribute("name");
+							string? memberAttr = reader.GetAttribute("name");
 							if (memberAttr != null)
 								indexList.Add(new IndexEntry(GetHashCode(memberAttr), pos));
 							reader.Skip();
@@ -330,7 +330,7 @@ namespace ICSharpCode.Decompiler.Documentation
 			return GetDocumentation(entity.GetIdString());
 		}
 
-		string GetDocumentation(string key, bool allowReload)
+		string? GetDocumentation(string key, bool allowReload)
 		{
 			int hashcode = GetHashCode(key);
 			var index = this.index; // read volatile field
@@ -347,7 +347,7 @@ namespace ICSharpCode.Decompiler.Documentation
 			XmlDocumentationCache cache = this.cache;
 			lock (cache)
 			{
-				if (!cache.TryGet(key, out string val))
+				if (!cache.TryGet(key, out string? val))
 				{
 					try
 					{
@@ -376,7 +376,7 @@ namespace ICSharpCode.Decompiler.Documentation
 			}
 		}
 
-		string ReloadAndGetDocumentation(string key)
+		string? ReloadAndGetDocumentation(string key)
 		{
 			try
 			{
@@ -407,7 +407,7 @@ namespace ICSharpCode.Decompiler.Documentation
 		#endregion
 
 		#region Load / Read XML
-		string LoadDocumentation(string key, int positionInFile)
+		string? LoadDocumentation(string key, int positionInFile)
 		{
 			using (FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read | FileShare.Delete))
 			{
@@ -420,7 +420,7 @@ namespace ICSharpCode.Decompiler.Documentation
 					{
 						if (r.NodeType == XmlNodeType.Element)
 						{
-							string memberAttr = r.GetAttribute("name");
+							string? memberAttr = r.GetAttribute("name");
 							if (memberAttr == key)
 							{
 								return r.ReadInnerXml();

ICSharpCode.Decompiler/FlowAnalysis/DataFlowVisitor.cs

@@ -237,7 +237,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 #if DEBUG
 			Debug.Assert(initialized, "Initialize() was not called");
 
-			if (debugDict.TryGetValue(inst, out State previousState))
+			if (debugDict.TryGetValue(inst, out State? previousState))
 			{
 				Debug.Assert(previousState.LessThanOrEqual(state));
 				previousState.JoinWith(state);
@@ -350,7 +350,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 		/// </remarks>
 		State GetBlockInputState(Block block)
 		{
-			if (stateOnBranch.TryGetValue(block, out State s))
+			if (stateOnBranch.TryGetValue(block, out State? s))
 			{
 				return s;
 			}
@@ -398,7 +398,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 				state.ReplaceWith(stateOnBranch[block]);
 				block.AcceptVisitor(this);
 			}
-			if (stateOnLeave.TryGetValue(container, out State stateOnExit))
+			if (stateOnLeave.TryGetValue(container, out State? stateOnExit))
 			{
 				state.ReplaceWith(stateOnExit);
 			}
@@ -463,7 +463,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 
 		void MergeBranchStateIntoStateOnLeave(Leave inst, State branchState)
 		{
-			if (stateOnLeave.TryGetValue(inst.TargetContainer, out State targetState))
+			if (stateOnLeave.TryGetValue(inst.TargetContainer, out State? targetState))
 			{
 				targetState.JoinWith(branchState);
 			}
@@ -505,7 +505,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 		protected State HandleTryBlock(TryInstruction inst)
 		{
 			State oldStateOnException = currentStateOnException;
-			if (stateOnException.TryGetValue(inst, out State newStateOnException))
+			if (stateOnException.TryGetValue(inst, out State? newStateOnException))
 			{
 				newStateOnException.JoinWith(state);
 			}

ICSharpCode.Decompiler/Humanizer/Vocabulary.cs

@@ -119,7 +119,7 @@ namespace Humanizer.Inflections
 			return result ?? word;
 		}
 
-		private string ApplyRules(IList<Rule> rules, string word, bool skipFirstRule)
+		private string? ApplyRules(IList<Rule> rules, string word, bool skipFirstRule)
 		{
 			if (word == null)
 			{
@@ -164,7 +164,7 @@ namespace Humanizer.Inflections
 				_replacement = replacement;
 			}
 
-			public string Apply(string word)
+			public string? Apply(string word)
 			{
 				if (!_regex.IsMatch(word))
 				{

ICSharpCode.Decompiler/IL/ControlFlow/LoopDetection.cs

@@ -311,7 +311,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		/// 3) otherwise (exit point unknown, heuristically extend loop): null
 		/// </returns>
 		/// <remarks>This method must not write to the Visited flags on the CFG.</remarks>
-		internal ControlFlowNode FindExitPoint(ControlFlowNode loopHead, IReadOnlyList<ControlFlowNode> naturalLoop)
+		internal ControlFlowNode? FindExitPoint(ControlFlowNode loopHead, IReadOnlyList<ControlFlowNode> naturalLoop)
 		{
 			bool hasReachableExit = HasReachableExit(loopHead);
 			if (!hasReachableExit)

ICSharpCode.Decompiler/IL/ControlFlow/SwitchAnalysis.cs

@@ -126,7 +126,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				trueValues = trueValues.IntersectWith(inputValues);
 				if (trueValues.SetEquals(inputValues) || trueValues.IsEmpty)
 					return false;
-				Block trueBlock;
+				Block? trueBlock;
 				if (trueInst.MatchBranch(out trueBlock) && AnalyzeBlock(trueBlock, trueValues))
 				{
 					// OK, true block was further analyzed.
@@ -159,7 +159,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 
 			var remainingValues = inputValues.ExceptWith(trueValues);
 			ILInstruction falseInst = block.Instructions.Last();
-			Block falseBlock;
+			Block? falseBlock;
 			if (falseInst.MatchBranch(out falseBlock) && AnalyzeBlock(falseBlock, remainingValues))
 			{
 				// OK, false block was further analyzed.
@@ -237,7 +237,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			{
 				return;
 			}
-			if (inst.MatchBranch(out Block targetBlock))
+			if (inst.MatchBranch(out Block? targetBlock))
 			{
 				if (targetBlockToSectionIndex.TryGetValue(targetBlock, out int index))
 				{
@@ -252,7 +252,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 					Sections.Add(new KeyValuePair<LongSet, ILInstruction>(values, inst));
 				}
 			}
-			else if (inst.MatchLeave(out BlockContainer targetContainer))
+			else if (inst.MatchLeave(out BlockContainer? targetContainer))
 			{
 				if (targetContainerToSectionIndex.TryGetValue(targetContainer, out int index))
 				{

ICSharpCode.Decompiler/IL/ControlFlow/SwitchDetection.cs

@@ -233,9 +233,9 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			var dict = new Dictionary<Block, SwitchSection>(); // branch target -> switch section
 			sw.Sections.RemoveAll(
 				section => {
-					if (section.Body.MatchBranch(out Block target))
+					if (section.Body.MatchBranch(out Block? target))
 					{
-						if (dict.TryGetValue(target, out SwitchSection primarySection))
+						if (dict.TryGetValue(target, out SwitchSection? primarySection))
 						{
 							primarySection.Labels = primarySection.Labels.UnionWith(section.Labels);
 							primarySection.HasNullLabel |= section.HasNullLabel;
@@ -438,13 +438,13 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 
 			// if (comp(logic.not(call get_HasValue(ldloca nullableVar))) br NullCase
 			// br RootBlock
-			var nullableBlock = (Block)controlFlowGraph.GetNode(analysis.RootBlock).Predecessors.SingleOrDefault()?.UserData;
+			var nullableBlock = (Block?)controlFlowGraph.GetNode(analysis.RootBlock).Predecessors.SingleOrDefault()?.UserData;
 			if (nullableBlock == null ||
 				nullableBlock.Instructions.Count < 2 ||
 				!nullableBlock.Instructions.Last().MatchBranch(analysis.RootBlock) ||
 				!nullableBlock.Instructions.SecondToLastOrDefault().MatchIfInstruction(out var cond, out var trueInst) ||
 				!cond.MatchLogicNot(out var getHasValue) ||
-				!NullableLiftingTransform.MatchHasValueCall(getHasValue, out ILInstruction nullableInst))
+				!NullableLiftingTransform.MatchHasValueCall(getHasValue, out ILInstruction? nullableInst))
 				return;
 
 			// could check that nullableInst is ldloc or ldloca and that the switch variable matches a GetValueOrDefault

ICSharpCode.Decompiler/IL/ControlFlow/YieldReturnDecompiler.cs

@@ -269,7 +269,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				return false;
 			}
 
-			ILInstruction newObj;
+			ILInstruction? newObj;
 			if (body.Instructions.Count == 1)
 			{
 				// No parameters passed to enumerator (not even 'this'):
@@ -943,7 +943,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 
 			void ConvertBranchAfterYieldReturn(Block newBlock, Block oldBlock, int pos)
 			{
-				Block targetBlock;
+				Block? targetBlock;
 				if (isCompiledWithMono && disposingField != null)
 				{
 					// Mono skips over the state assignment if 'this.disposing' is set:
@@ -1233,7 +1233,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			{
 				context.CancellationToken.ThrowIfCancellationRequested();
 				int oldState = blockState[block.ChildIndex];
-				BlockContainer container; // new container for the block
+				BlockContainer? container; // new container for the block
 				if (GetNewState(block) is int newState)
 				{
 					// OK, state change
@@ -1436,7 +1436,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 					return false;
 				if (!call.Method.Name.StartsWith("<>__Finally"))
 					return false;
-				ITypeDefinition declaringTypeDefinition = call.Method.DeclaringTypeDefinition;
+				ITypeDefinition? declaringTypeDefinition = call.Method.DeclaringTypeDefinition;
 				if (declaringTypeDefinition.MetadataToken != this.enumeratorType)
 					return false;
 				if (declaringTypeDefinition.ParentModule.MetadataFile.Metadata != metadata)

ICSharpCode.Decompiler/IL/PointerArithmeticOffset.cs

@@ -35,7 +35,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <param name="pointerElementType">The target type of the pointer type.</param>
 		/// <param name="checkForOverflow">Whether the pointer arithmetic operation checks for overflow.</param>
 		/// <param name="unwrapZeroExtension">Whether to allow zero extensions in the mul argument.</param>
-		public static ILInstruction Detect(ILInstruction byteOffsetInst, IType pointerElementType,
+		public static ILInstruction? Detect(ILInstruction byteOffsetInst, IType pointerElementType,
 			bool checkForOverflow,
 			bool unwrapZeroExtension = false)
 		{

ICSharpCode.Decompiler/IL/Transforms/AssignVariableNames.cs

@@ -227,7 +227,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				{
 					var v = i.Variable;
 					// if there is already a valid name for the variable slot, just use it
-					if (variableMapping.TryGetValue(v, out string name))
+					if (variableMapping.TryGetValue(v, out string? name))
 					{
 						v.Name = name;
 						continue;
@@ -463,7 +463,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return SplitName(proposedName, out _);
 		}
 
-		static string GetNameFromInstruction(ILInstruction inst)
+		static string? GetNameFromInstruction(ILInstruction inst)
 		{
 			switch (inst)
 			{
@@ -506,7 +506,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return null;
 		}
 
-		static string GetNameForArgument(ILInstruction parent, int i)
+		static string? GetNameForArgument(ILInstruction parent, int i)
 		{
 			switch (parent)
 			{
@@ -566,7 +566,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				type = NullableType.GetUnderlyingType(((TypeWithElementType)type).ElementType);
 			}
 
-			string name = type.Kind switch {
+			string? name = type.Kind switch {
 				TypeKind.Array => "array",
 				TypeKind.Pointer => "ptr",
 				TypeKind.TypeParameter => "val",
@@ -637,7 +637,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return name;
 		}
 
-		static string CleanUpVariableName(string name)
+		static string? CleanUpVariableName(string name)
 		{
 			// remove the backtick (generics)
 			int pos = name.IndexOf('`');

ICSharpCode.Decompiler/IL/Transforms/CombineExitsTransform.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			ExpressionTransforms.RunOnSingleStatement(combinedExit, context);
 		}
 
-		static Leave CombineExits(Block block)
+		static Leave? CombineExits(Block block)
 		{
 			if (!(block.Instructions.SecondToLastOrDefault() is IfInstruction ifInst && block.Instructions.LastOrDefault() is Leave leaveElse))
 				return null;

ICSharpCode.Decompiler/IL/Transforms/DeconstructionTransform.cs

@@ -191,8 +191,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		{
 			int startPos = pos;
 			Action<DeconstructInstruction>? delayedActions = null;
-			if (MatchDeconstruction(block.Instructions[pos], out IMethod deconstructMethod,
+			if (MatchDeconstruction(block.Instructions[pos], out IMethod? deconstructMethod,
-				out ILInstruction rootTestedOperand))
+				out ILInstruction? rootTestedOperand))
 			{
 				pos++;
 			}

ICSharpCode.Decompiler/IL/Transforms/DelegateConstruction.cs

@@ -80,7 +80,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
-		internal static bool MatchDelegateConstruction(ILInstruction inst, out IMethod? targetMethod,
+		internal static bool MatchDelegateConstruction(ILInstruction? inst, out IMethod? targetMethod,
 			[NotNullWhen(true)] out ILInstruction? target, [NotNullWhen(true)] out IType? delegateType, bool allowTransformed = false)
 		{
 			targetMethod = null;
@@ -165,7 +165,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return new GenericContext(classTypeParameters, methodTypeParameters);
 		}
 
-		ILFunction TransformDelegateConstruction(
+		ILFunction? TransformDelegateConstruction(
 			ILInstruction value, IMethod targetMethod,
 			ILInstruction target, IType delegateType)
 		{

ICSharpCode.Decompiler/IL/Transforms/DynamicCallSiteTransform.cs

@@ -147,7 +147,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				container.SortBlocks(deleteUnreachableBlocks: true);
 		}
 
-		ILInstruction MakeDynamicInstruction(CallSiteInfo callsite, CallVirt targetInvokeCall, List<ILInstruction> deadArguments)
+		ILInstruction? MakeDynamicInstruction(CallSiteInfo callsite, CallVirt targetInvokeCall, List<ILInstruction> deadArguments)
 		{
 			switch (callsite.Kind)
 			{

ICSharpCode.Decompiler/IL/Transforms/ExpressionTransforms.cs

@@ -141,7 +141,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 			else if (rightWithoutConv.MatchLdcI4(0) && inst.Kind.IsEqualityOrInequality())
 			{
-				if (inst.Left.MatchLdLen(StackType.I, out ILInstruction array))
+				if (inst.Left.MatchLdLen(StackType.I, out ILInstruction? array))
 				{
 					// comp.unsigned(ldlen array == conv i4->i(ldc.i4 0))
 					// => comp(ldlen.i4 array == ldc.i4 0)
@@ -167,7 +167,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		protected internal override void VisitConv(Conv inst)
 		{
 			inst.Argument.AcceptVisitor(this);
-			if (inst.Argument.MatchLdLen(StackType.I, out ILInstruction array) && inst.TargetType.IsIntegerType()
+			if (inst.Argument.MatchLdLen(StackType.I, out ILInstruction? array) && inst.TargetType.IsIntegerType()
 				&& (!inst.CheckForOverflow || context.Settings.AssumeArrayLengthFitsIntoInt32))
 			{
 				context.Step("conv.i4(ldlen array) => ldlen.i4(array)", inst);
@@ -303,7 +303,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		protected internal override void VisitNewObj(NewObj inst)
 		{
 			Block? block;
-			if (TransformSpanTCtorContainingStackAlloc(inst, out ILInstruction locallocSpan))
+			if (TransformSpanTCtorContainingStackAlloc(inst, out ILInstruction? locallocSpan))
 			{
 				context.Step("new Span<T>(stackalloc) -> stackalloc Span<T>", inst);
 				inst.ReplaceWith(locallocSpan);
@@ -329,7 +329,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				inst.ReplaceWith(replacement);
 				return;
 			}
-			if (TransformDelegateCtorLdVirtFtnToLdVirtDelegate(inst, out LdVirtDelegate ldVirtDelegate))
+			if (TransformDelegateCtorLdVirtFtnToLdVirtDelegate(inst, out LdVirtDelegate? ldVirtDelegate))
 			{
 				context.Step("new Delegate(target, ldvirtftn Method) -> ldvirtdelegate Delegate Method(target)", inst);
 				inst.ReplaceWith(ldVirtDelegate);
@@ -462,7 +462,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			EarlyExpressionTransforms.AddressOfLdLocToLdLoca(inst, context);
 			if (EarlyExpressionTransforms.LdObjToLdLoc(inst, context))
 				return;
-			if (TransformDecimalFieldToConstant(inst, out LdcDecimal decimalConstant))
+			if (TransformDecimalFieldToConstant(inst, out LdcDecimal? decimalConstant))
 			{
 				context.Step("TransformDecimalFieldToConstant", inst);
 				inst.ReplaceWith(decimalConstant);
@@ -549,8 +549,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			Block? falseInst = inst.FalseInst as Block;
 			if (falseInst == null || falseInst.Instructions.Count != 1)
 				return inst;
-			ILVariable v;
+			ILVariable? v;
-			ILInstruction value1, value2;
+			ILInstruction? value1, value2;
 			if (trueInst.Instructions[0].MatchStLoc(out v, out value1) && falseInst.Instructions[0].MatchStLoc(v, out value2))
 			{
 				context.Step("conditional operator", inst);
@@ -629,7 +629,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			{
 				// validate that each integer is used for exactly one value
 				var integersUsed = new HashSet<int>();
-				foreach ((string key, int val) in str2int.Map)
+				foreach ((string? key, int val) in str2int.Map)
 				{
 					if (!integersUsed.Add(val))
 						return;

ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs

@@ -156,21 +156,21 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// </summary>
 		bool MatchDoWhileLoop(BlockContainer loop)
 		{
-			(List<IfInstruction>? conditions, ILInstruction exit, bool swap, bool split, bool unwrap) = AnalyzeDoWhileConditions(loop);
+			(List<IfInstruction>? conditions, ILInstruction? exit, bool swap, bool split, bool unwrap) = AnalyzeDoWhileConditions(loop);
 			// not a do-while loop, exit.
 			if (conditions == null || conditions.Count == 0)
 				return false;
 			context.Step("Transform to do-while loop: " + loop.EntryPoint.Label, loop);
 			Block conditionBlock;
 			// first we remove all extracted instructions from the original block.
-			var originalBlock = (Block)exit.Parent;
+			var originalBlock = exit.Parent as Block;
 			if (unwrap)
 			{
 				// we found a condition block nested in a condition that is followed by a return statement:
 				// we flip the condition and swap the blocks
 				Debug.Assert(originalBlock.Parent is IfInstruction);
 				var returnCondition = (IfInstruction)originalBlock.Parent;
-				var topLevelBlock = (Block)returnCondition.Parent;
+				var topLevelBlock = returnCondition.Parent as Block;
 				Debug.Assert(topLevelBlock.Parent == loop);
 				var leaveFunction = topLevelBlock.Instructions[returnCondition.ChildIndex + 1];
 				Debug.Assert(leaveFunction.MatchReturn(out _));

ICSharpCode.Decompiler/IL/Transforms/ILExtraction.cs

@@ -97,7 +97,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// 
 		/// May return null if extraction is not possible.
 		/// </summary>
-		public static ILVariable Extract(ILInstruction instToExtract, ILTransformContext context)
+		public static ILVariable? Extract(ILInstruction instToExtract, ILTransformContext context)
 		{
 			var function = instToExtract.Ancestors.OfType<ILFunction>().First();
 			ExtractionContext ctx = new ExtractionContext(function, context);

ICSharpCode.Decompiler/IL/Transforms/ILInlining.cs

@@ -577,7 +577,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		{
 			if (findResult.Type == FindResultType.NamedArgument)
 			{
-				var originalStore = (StLoc)inlinedExpression.Parent;
+				var originalStore = inlinedExpression.Parent as StLoc;
 				return !originalStore.ILStackWasEmpty;
 			}
 			Debug.Assert(findResult.Type == FindResultType.Found);
@@ -784,7 +784,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			if (expr.MatchLdLoc(v) || expr.MatchLdLoca(v))
 			{
 				// Match found, we can inline
-				if (expr.SlotInfo == StObj.TargetSlot && !((StObj)expr.Parent).CanInlineIntoTargetSlot(expressionBeingMoved))
+				if (expr.SlotInfo == StObj.TargetSlot && !(expr.Parent as StObj).CanInlineIntoTargetSlot(expressionBeingMoved))
 				{
 					if ((options & InliningOptions.AllowChangingOrderOfEvaluationForExceptions) != 0)
 					{
@@ -867,7 +867,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// <summary>
 		/// Finds the first call instruction within the instructions that were inlined into inst.
 		/// </summary>
-		internal static CallInstruction FindFirstInlinedCall(ILInstruction inst)
+		internal static CallInstruction? FindFirstInlinedCall(ILInstruction inst)
 		{
 			foreach (var child in inst.Children)
 			{

ICSharpCode.Decompiler/IL/Transforms/InterpolatedStringTransform.cs

@@ -32,7 +32,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				return;
 			int interpolationStart = pos;
 			int interpolationEnd;
-			ILInstruction insertionPoint;
+			ILInstruction? insertionPoint;
 			// stloc v(newobj DefaultInterpolatedStringHandler..ctor(ldc.i4 literalLength, ldc.i4 formattedCount))
 			if (block.Instructions[pos] is StLoc {
 				Variable: ILVariable { Kind: VariableKind.Local } v,

ICSharpCode.Decompiler/IL/Transforms/LocalFunctionDecompiler.cs

@@ -308,7 +308,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
-		private ILInstruction FindCompatibleArgument(LocalFunctionInfo info, IList<ILInstruction> arguments, bool ignoreStructure = false)
+		private ILInstruction? FindCompatibleArgument(LocalFunctionInfo info, IList<ILInstruction> arguments, bool ignoreStructure = false)
 		{
 			foreach (var arg in arguments)
 			{
@@ -321,7 +321,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return null;
 		}
 
-		private ILVariable ResolveAncestorScopeReference(ILInstruction inst)
+		private ILVariable? ResolveAncestorScopeReference(ILInstruction inst)
 		{
 			if (!inst.MatchLdFld(out var target, out var field))
 				return null;
@@ -338,7 +338,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return null;
 		}
 
-		private ILFunction GetDeclaringFunction(ILFunction localFunction)
+		private ILFunction? GetDeclaringFunction(ILFunction localFunction)
 		{
 			if (localFunction.DeclarationScope == null)
 				return null;
@@ -446,7 +446,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
-		ILFunction ReadLocalFunctionDefinition(ILFunction rootFunction, IMethod targetMethod, int skipCount)
+		ILFunction? ReadLocalFunctionDefinition(ILFunction rootFunction, IMethod targetMethod, int skipCount)
 		{
 			var methodDefinition = context.PEFile.Metadata.GetMethodDefinition((MethodDefinitionHandle)targetMethod.MetadataToken);
 			var genericContext = GenericContextFromTypeArguments(targetMethod, skipCount);
@@ -723,7 +723,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				function.DeclarationScope = FindCommonAncestorInstruction<BlockContainer>(function.DeclarationScope, closureVar.CaptureScope);
 			return true;
 
-			ILInstruction GetClosureInitializer(ILVariable variable)
+			ILInstruction? GetClosureInitializer(ILVariable variable)
 			{
 				var type = variable.Type.UnwrapByRef().GetDefinition();
 				if (type == null)

ICSharpCode.Decompiler/IL/Transforms/NullPropagationTransform.cs

@@ -71,7 +71,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// Check if "condition ? trueInst : falseInst" can be simplified using the null-conditional operator.
 		/// Returns the replacement instruction, or null if no replacement is possible.
 		/// </summary>
-		internal ILInstruction Run(ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst)
+		internal ILInstruction? Run(ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst)
 		{
 			Debug.Assert(context.Settings.NullPropagation);
 			Debug.Assert(!condition.MatchLogicNot(out _), "Caller should pass in positive condition");
@@ -107,7 +107,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// <summary>
 		/// testedVar != null ? nonNullInst : nullInst
 		/// </summary>
-		ILInstruction TryNullPropagation(ILVariable testedVar, ILInstruction nonNullInst, ILInstruction nullInst,
+		ILInstruction? TryNullPropagation(ILVariable testedVar, ILInstruction nonNullInst, ILInstruction nullInst,
 			Mode mode)
 		{
 			bool removedRewrapOrNullableCtor = false;

ICSharpCode.Decompiler/IL/Transforms/NullableLiftingTransform.cs

@@ -176,7 +176,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// Main entry point for lifting; called by both the expression-transform
 		/// and the block transform.
 		/// </summary>
-		ILInstruction Lift(ILInstruction ifInst, ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst)
+		ILInstruction? Lift(ILInstruction ifInst, ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst)
 		{
 			// ifInst is usually the IfInstruction to which condition belongs;
 			// but can also be a BinaryNumericInstruction.
@@ -479,7 +479,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				}
 			}
 
-			internal ILInstruction MakeLifted(ComparisonKind newComparisonKind, ILInstruction left, ILInstruction right)
+			internal ILInstruction? MakeLifted(ComparisonKind newComparisonKind, ILInstruction left, ILInstruction right)
 			{
 				if (Instruction is Comp comp)
 				{

ICSharpCode.Decompiler/IL/Transforms/ReduceNestingTransform.cs

@@ -451,7 +451,7 @@ namespace ICSharpCode.Decompiler.IL
 		///
 		/// [else-]if (parent-cond) else { ifInst }
 		/// </summary>
-		private IfInstruction GetElseIfParent(IfInstruction ifInst)
+		private IfInstruction? GetElseIfParent(IfInstruction ifInst)
 		{
 			Debug.Assert(ifInst.Parent is Block);
 			if (Block.Unwrap(ifInst.Parent) == ifInst && // only instruction in block

ICSharpCode.Decompiler/IL/Transforms/SplitVariables.cs

@@ -175,7 +175,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// Given 'ldloc ref_local' and 'ldloca target; stloc ref_local', returns the ldloca.
 		/// This function must return a non-null LdLoca for every use of a SupportedRefLocal.
 		/// </summary>
-		static LdLoca GetAddressLoadForRefLocalUse(LdLoc ldloc)
+		static LdLoca? GetAddressLoadForRefLocalUse(LdLoc ldloc)
 		{
 			if (!ldloc.Variable.IsSingleDefinition)
 				return null; // only single-definition variables can be supported ref locals

ICSharpCode.Decompiler/IL/Transforms/TransformDisplayClassUsage.cs

@@ -246,7 +246,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
-		private DisplayClass AnalyzeVariable(ILVariable v)
+		private DisplayClass? AnalyzeVariable(ILVariable v)
 		{
 			switch (v.Kind)
 			{
@@ -265,7 +265,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
-		DisplayClass DetectDisplayClass(ILVariable v)
+		DisplayClass? DetectDisplayClass(ILVariable v)
 		{
 			ITypeDefinition? definition;
 			if (v.Kind != VariableKind.StackSlot)
@@ -385,7 +385,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
-		DisplayClass DetectDisplayClassInitializer(ILVariable v)
+		DisplayClass? DetectDisplayClassInitializer(ILVariable v)
 		{
 			if (v.StoreInstructions.Count != 1 || !(v.StoreInstructions[0] is StLoc store && store.Parent is Block initializerBlock && initializerBlock.Kind == BlockKind.ObjectInitializer))
 				return null;
@@ -520,7 +520,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// If a value does not match either LdLoc or a LdObj LdLdFlda* LdLoc chain, null is returned.
 		/// The if any of the variables/fields in the chain cannot be propagated, null is returned.
 		/// </summary>
-		ILVariable ResolveVariableToPropagate(ILInstruction value, IType? expectedType = null)
+		ILVariable? ResolveVariableToPropagate(ILInstruction value, IType? expectedType = null)
 		{
 			ILVariable v;
 			switch (value)
@@ -634,7 +634,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// mcs likes to optimize closures in yield state machines away by moving the captured variables' fields into the state machine type,
 		/// We construct a <see cref="DisplayClass"/> that spans the whole method body.
 		/// </summary>
-		DisplayClass HandleMonoStateMachine(ILFunction function, ILVariable thisVariable)
+		DisplayClass? HandleMonoStateMachine(ILFunction function, ILVariable thisVariable)
 		{
 			if (!(function.StateMachineCompiledWithMono && thisVariable.IsThis()))
 				return null;

ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs

@@ -459,7 +459,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			if (!MatchArgumentList(invocation.Arguments[1], out var arguments))
 				arguments = new[] { invocation.Arguments[1] };
 
-			ILInstruction Convert()
+			ILInstruction? Convert()
 			{
 				Func<ILInstruction>[] toBeConverted = new Func<ILInstruction>[arguments.Count];
 				for (int i = 0; i < arguments.Count; i++)
@@ -683,7 +683,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return call.Arguments[0];
 		}
 
-		Func<ILInstruction>[] ConvertCallArguments(IList<ILInstruction> arguments, IMethod method)
+		Func<ILInstruction>[]? ConvertCallArguments(IList<ILInstruction> arguments, IMethod method)
 		{
 			var converted = new Func<ILInstruction>[arguments.Count];
 			Debug.Assert(arguments.Count == method.Parameters.Count);
@@ -1359,7 +1359,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return (null, SpecialType.UnknownType);
 		}
 
-		ILInstruction ConvertValue(ILInstruction value, ILInstruction context)
+		ILInstruction? ConvertValue(ILInstruction value, ILInstruction context)
 		{
 			switch (value)
 			{

ICSharpCode.Decompiler/IL/Transforms/UserDefinedLogicTransform.cs

@@ -147,7 +147,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		///       if (call op_True(ldloc lhsVar)) ldloc lhsVar else call op_BitwiseOr(ldloc lhsVar, rhsInst)
 		///    -> user.logic op_BitwiseOr(ldloc lhsVar, rhsInst)
 		/// </summary>
-		public static ILInstruction Transform(ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst)
+		public static ILInstruction? Transform(ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst)
 		{
 			if (!MatchCondition(condition, out var lhsVar, out var conditionMethodName))
 				return null;
@@ -164,7 +164,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return result;
 		}
 
-		public static ILInstruction TransformDynamic(ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst)
+		public static ILInstruction? TransformDynamic(ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst)
 		{
 			// Check condition:
 			System.Linq.Expressions.ExpressionType unaryOp;

ICSharpCode.Decompiler/Metadata/DotNetCorePathFinder.cs

@@ -211,7 +211,7 @@ namespace ICSharpCode.Decompiler.Metadata
 			}
 		}
 
-		public string TryResolveDotNetCoreShared(IAssemblyReference name, out string? runtimePack)
+		public string? TryResolveDotNetCoreShared(IAssemblyReference name, out string? runtimePack)
 		{
 			if (dotnetBasePath == null)
 			{
@@ -286,7 +286,7 @@ namespace ICSharpCode.Decompiler.Metadata
 			}
 		}
 
-		public static string FindDotNetExeDirectory()
+		public static string? FindDotNetExeDirectory()
 		{
 			string dotnetExeName = (Environment.OSVersion.Platform == PlatformID.Unix) ? "dotnet" : "dotnet.exe";
 			foreach (var item in Environment.GetEnvironmentVariable("PATH").Split(Path.PathSeparator))
@@ -312,7 +312,7 @@ namespace ICSharpCode.Decompiler.Metadata
 			return null;
 		}
 
-		static unsafe string GetRealPath(string path, Encoding encoding)
+		static unsafe string? GetRealPath(string path, Encoding encoding)
 		{
 			var bytes = encoding.GetBytes(path);
 			fixed (byte* input = bytes)

ICSharpCode.Decompiler/Metadata/LightJson/JsonValue.cs

@@ -329,7 +329,7 @@ namespace LightJson
 		/// Gets this value as a String type.
 		/// </summary>
 		/// <value>This value as a String type.</value>
-		public string AsString {
+		public string? AsString {
 			get {
 				switch (this.Type)
 				{
@@ -354,7 +354,7 @@ namespace LightJson
 		/// Gets this value as an JsonObject.
 		/// </summary>
 		/// <value>This value as an JsonObject.</value>
-		public JsonObject AsJsonObject {
+		public JsonObject? AsJsonObject {
 			get {
 				return this.IsJsonObject
 					? (JsonObject)this.reference
@@ -366,7 +366,7 @@ namespace LightJson
 		/// Gets this value as an JsonArray.
 		/// </summary>
 		/// <value>This value as an JsonArray.</value>
-		public JsonArray AsJsonArray {
+		public JsonArray? AsJsonArray {
 			get {
 				return this.IsJsonArray
 					? (JsonArray)this.reference
@@ -397,7 +397,7 @@ namespace LightJson
 		/// Gets this (inner) value as a System.object.
 		/// </summary>
 		/// <value>This (inner) value as a System.object.</value>
-		public object AsObject {
+		public object? AsObject {
 			get {
 				switch (this.Type)
 				{
@@ -808,7 +808,7 @@ namespace LightJson
 			}
 
 			[DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
-			public JsonObject ObjectView {
+			public JsonObject? ObjectView {
 				get {
 					if (this.jsonValue.IsJsonObject)
 					{
@@ -822,7 +822,7 @@ namespace LightJson
 			}
 
 			[DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
-			public JsonArray ArrayView {
+			public JsonArray? ArrayView {
 				get {
 					if (this.jsonValue.IsJsonArray)
 					{

ICSharpCode.Decompiler/NRExtensions.cs

@@ -92,7 +92,7 @@ namespace ICSharpCode.Decompiler
 			}
 		}
 
-		internal static string GetDocumentation(this IEntity entity)
+		internal static string? GetDocumentation(this IEntity entity)
 		{
 			var docProvider = XmlDocLoader.LoadDocumentation(entity.ParentModule.MetadataFile);
 			if (docProvider == null)

ICSharpCode.Decompiler/Output/TextTokenWriter.cs

@@ -108,7 +108,7 @@ namespace ICSharpCode.Decompiler
 			output.Write(name);
 		}
 
-		ISymbol GetCurrentMemberReference()
+		ISymbol? GetCurrentMemberReference()
 		{
 			AstNode node = nodeStack.Peek();
 			var symbol = node.GetSymbol();
@@ -130,7 +130,7 @@ namespace ICSharpCode.Decompiler
 			return FilterMember(symbol);
 		}
 
-		ISymbol FilterMember(ISymbol symbol)
+		ISymbol? FilterMember(ISymbol symbol)
 		{
 			if (symbol == null)
 				return null;
@@ -141,7 +141,7 @@ namespace ICSharpCode.Decompiler
 			return symbol;
 		}
 
-		object GetCurrentLocalReference()
+		object? GetCurrentLocalReference()
 		{
 			AstNode node = nodeStack.Peek();
 			ILVariable variable = node.Annotation<ILVariableResolveResult>()?.Variable;
@@ -169,7 +169,7 @@ namespace ICSharpCode.Decompiler
 			return null;
 		}
 
-		object GetCurrentLocalDefinition(Identifier id)
+		object? GetCurrentLocalDefinition(Identifier id)
 		{
 			AstNode node = nodeStack.Peek();
 			if (node is Identifier && node.Parent != null)
@@ -213,7 +213,7 @@ namespace ICSharpCode.Decompiler
 			return null;
 		}
 
-		ISymbol GetCurrentDefinition()
+		ISymbol? GetCurrentDefinition()
 		{
 			if (nodeStack == null || nodeStack.Count == 0)
 				return null;

ICSharpCode.Decompiler/Semantics/LocalResolveResult.cs

@@ -62,7 +62,7 @@ namespace ICSharpCode.Decompiler.Semantics
 			get { return variable.IsConst; }
 		}
 
-		public override object ConstantValue {
+		public override object? ConstantValue {
 			get { return IsParameter ? null : variable.GetConstantValue(); }
 		}
 

ICSharpCode.Decompiler/TypeSystem/FunctionPointerType.cs

@@ -123,7 +123,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 
 		public override TypeKind Kind => ((module.TypeSystemOptions & TypeSystemOptions.FunctionPointers) != 0) ? TypeKind.FunctionPointer : TypeKind.Struct;
 
-		public override ITypeDefinition GetDefinition()
+		public override ITypeDefinition? GetDefinition()
 		{
 			if ((module.TypeSystemOptions & TypeSystemOptions.FunctionPointers) != 0)
 			{

ICSharpCode.Decompiler/TypeSystem/Implementation/AttributeListBuilder.cs

@@ -283,7 +283,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			return false;
 		}
 
-		internal IAttribute GetAttribute(MetadataReader metadata, CustomAttributeHandleCollection customAttributes, KnownAttribute attribute, SymbolKind symbolKind)
+		internal IAttribute? GetAttribute(MetadataReader metadata, CustomAttributeHandleCollection customAttributes, KnownAttribute attribute, SymbolKind symbolKind)
 		{
 			Debug.Assert(attribute.IsCustomAttribute());
 			foreach (var h in customAttributes)

ICSharpCode.Decompiler/TypeSystem/Implementation/CustomAttribute.cs

@@ -109,7 +109,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			}
 		}
 
-		internal static IMember MemberForNamedArgument(IType attributeType, CustomAttributeNamedArgument<IType> namedArgument)
+		internal static IMember? MemberForNamedArgument(IType attributeType, CustomAttributeNamedArgument<IType> namedArgument)
 		{
 			switch (namedArgument.Kind)
 			{

ICSharpCode.Decompiler/TypeSystem/Implementation/DecimalConstantHelper.cs

@@ -32,7 +32,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			return attributeHandles.HasKnownAttribute(module.metadata, KnownAttribute.DecimalConstant);
 		}
 
-		public static object GetDecimalConstantValue(MetadataModule module, CustomAttributeHandleCollection attributeHandles)
+		public static object? GetDecimalConstantValue(MetadataModule module, CustomAttributeHandleCollection attributeHandles)
 		{
 			var metadata = module.metadata;
 			foreach (var attributeHandle in attributeHandles)

ICSharpCode.Decompiler/TypeSystem/Implementation/DefaultAssemblyReference.cs

@@ -39,7 +39,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 				shortName = assemblyName;
 		}
 
-		public IModule Resolve(ITypeResolveContext context)
+		public IModule? Resolve(ITypeResolveContext context)
 		{
 			IModule current = context.CurrentModule;
 			if (current != null && string.Equals(shortName, current.AssemblyName, StringComparison.OrdinalIgnoreCase))

ICSharpCode.Decompiler/TypeSystem/Implementation/DummyTypeParameter.cs

@@ -163,7 +163,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			get { return VarianceModifier.Invariant; }
 		}
 
-		IEntity ITypeParameter.Owner {
+		IEntity? ITypeParameter.Owner {
 			get { return null; }
 		}
 

ICSharpCode.Decompiler/TypeSystem/Implementation/FakeMember.cs

@@ -63,7 +63,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		IEnumerable<IAttribute> IEntity.GetAttributes() => EmptyList<IAttribute>.Instance;
 		bool IEntity.HasAttribute(KnownAttribute attribute) => false;
-		IAttribute IEntity.GetAttribute(KnownAttribute attribute) => null;
+		IAttribute? IEntity.GetAttribute(KnownAttribute attribute) => null;
 
 		public Accessibility Accessibility { get; set; } = Accessibility.Public;
 
@@ -114,7 +114,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		bool IField.IsVolatile => false;
 
 		bool IVariable.IsConst => false;
-		object IVariable.GetConstantValue(bool throwOnInvalidMetadata) => null;
+		object? IVariable.GetConstantValue(bool throwOnInvalidMetadata) => null;
 		IType IVariable.Type => ReturnType;
 
 		public override SymbolKind SymbolKind => SymbolKind.Field;

ICSharpCode.Decompiler/TypeSystem/Implementation/GetClassTypeReference.cs

@@ -84,7 +84,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		/// </summary>
 		public FullTypeName FullTypeName { get { return fullTypeName; } }
 
-		internal static IType ResolveInAllAssemblies(ICompilation compilation, in FullTypeName fullTypeName)
+		internal static IType? ResolveInAllAssemblies(ICompilation compilation, in FullTypeName fullTypeName)
 		{
 			foreach (var asm in compilation.Modules)
 			{

ICSharpCode.Decompiler/TypeSystem/Implementation/MergedNamespace.cs

@@ -108,7 +108,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			get { return GetChildNamespaces().Values; }
 		}
 
-		public INamespace GetChildNamespace(string name)
+		public INamespace? GetChildNamespace(string name)
 		{
 			INamespace ns;
 			if (GetChildNamespaces().TryGetValue(name, out ns))

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataNamespace.cs

@@ -86,7 +86,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		ICompilation ICompilationProvider.Compilation => module.Compilation;
 
-		INamespace INamespace.GetChildNamespace(string name)
+		INamespace? INamespace.GetChildNamespace(string name)
 		{
 			foreach (var ns in ChildNamespaces)
 			{

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataParameter.cs

@@ -156,7 +156,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		bool IVariable.IsConst => false;
 
-		public object GetConstantValue(bool throwOnInvalidMetadata)
+		public object? GetConstantValue(bool throwOnInvalidMetadata)
 		{
 			try
 			{

ICSharpCode.Decompiler/TypeSystem/Implementation/SpecializedMember.cs

@@ -59,7 +59,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			this.substitution = TypeParameterSubstitution.Compose(newSubstitution, this.substitution);
 		}
 
-		internal IMethod WrapAccessor(ref IMethod cachingField, IMethod accessorDefinition)
+		internal IMethod? WrapAccessor(ref IMethod cachingField, IMethod accessorDefinition)
 		{
 			if (accessorDefinition == null)
 				return null;

ICSharpCode.Decompiler/TypeSystem/InheritanceHelper.cs

@@ -109,7 +109,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <summary>
 		/// Finds the member declared in 'derivedType' that has the same signature (could override) 'baseMember'.
 		/// </summary>
-		public static IMember GetDerivedMember(IMember baseMember, ITypeDefinition derivedType)
+		public static IMember? GetDerivedMember(IMember baseMember, ITypeDefinition derivedType)
 		{
 			if (baseMember == null)
 				throw new ArgumentNullException(nameof(baseMember));
@@ -185,7 +185,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			}
 		}
 
-		internal static IAttribute GetAttribute(ITypeDefinition typeDef, KnownAttribute attributeType)
+		internal static IAttribute? GetAttribute(ITypeDefinition typeDef, KnownAttribute attributeType)
 		{
 			foreach (var baseType in typeDef.GetNonInterfaceBaseTypes().Reverse())
 			{
@@ -217,7 +217,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			} while (member.IsOverride && (member = InheritanceHelper.GetBaseMember(member)) != null);
 		}
 
-		internal static IAttribute GetAttribute(IMember member, KnownAttribute attributeType)
+		internal static IAttribute? GetAttribute(IMember member, KnownAttribute attributeType)
 		{
 			HashSet<IMember> visitedMembers = new HashSet<IMember>();
 			do

ICSharpCode.Decompiler/TypeSystem/MetadataModule.cs

@@ -192,7 +192,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return LazyInit.GetOrSet(ref this.internalsVisibleTo, result);
 		}
 
-		static string GetShortName(string fullAssemblyName)
+		static string? GetShortName(string fullAssemblyName)
 		{
 			if (fullAssemblyName == null)
 				return null;
@@ -217,7 +217,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			}
 		}
 
-		public ITypeDefinition GetDefinition(TypeDefinitionHandle handle)
+		public ITypeDefinition? GetDefinition(TypeDefinitionHandle handle)
 		{
 			if (handle.IsNil)
 				return null;
@@ -233,7 +233,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return LazyInit.GetOrSet(ref typeDefs[row], typeDef);
 		}
 
-		public IField GetDefinition(FieldDefinitionHandle handle)
+		public IField? GetDefinition(FieldDefinitionHandle handle)
 		{
 			if (handle.IsNil)
 				return null;
@@ -249,7 +249,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return LazyInit.GetOrSet(ref fieldDefs[row], field);
 		}
 
-		public IMethod GetDefinition(MethodDefinitionHandle handle)
+		public IMethod? GetDefinition(MethodDefinitionHandle handle)
 		{
 			if (handle.IsNil)
 				return null;
@@ -266,7 +266,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return LazyInit.GetOrSet(ref methodDefs[row], method);
 		}
 
-		public IProperty GetDefinition(PropertyDefinitionHandle handle)
+		public IProperty? GetDefinition(PropertyDefinitionHandle handle)
 		{
 			if (handle.IsNil)
 				return null;
@@ -283,7 +283,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return LazyInit.GetOrSet(ref propertyDefs[row], property);
 		}
 
-		public IEvent GetDefinition(EventDefinitionHandle handle)
+		public IEvent? GetDefinition(EventDefinitionHandle handle)
 		{
 			if (handle.IsNil)
 				return null;
@@ -308,7 +308,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 
 		#region Resolve Module
 
-		public IModule ResolveModule(AssemblyReferenceHandle handle)
+		public IModule? ResolveModule(AssemblyReferenceHandle handle)
 		{
 			if (handle.IsNil)
 				return null;
@@ -332,7 +332,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return Compilation.FindModuleByReference(asmRef);
 		}
 
-		public IModule ResolveModule(ModuleReferenceHandle handle)
+		public IModule? ResolveModule(ModuleReferenceHandle handle)
 		{
 			if (handle.IsNil)
 				return null;
@@ -348,7 +348,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return null;
 		}
 
-		public IModule GetDeclaringModule(TypeReferenceHandle handle)
+		public IModule? GetDeclaringModule(TypeReferenceHandle handle)
 		{
 			if (handle.IsNil)
 				return null;
@@ -747,7 +747,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// * May return specialized members, where generics are involved.
 		/// * Other types of handles that don't correspond to TS entities, will return <c>null</c>.
 		/// </remarks>
-		public IEntity ResolveEntity(EntityHandle entityHandle, GenericContext context = default)
+		public IEntity? ResolveEntity(EntityHandle entityHandle, GenericContext context = default)
 		{
 			switch (entityHandle.Kind)
 			{
@@ -904,7 +904,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			}
 			return new UnknownType(typeName);
 
-			IModule ResolveModule(ExportedType type)
+			IModule? ResolveModule(ExportedType type)
 			{
 				switch (type.Implementation.Kind)
 				{

ICSharpCode.Decompiler/TypeSystem/ReflectionHelper.cs

@@ -442,7 +442,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				return new GetClassTypeReference(assemblyReference, typeName.Substring(0, pos), typeName.Substring(pos + 1), tpc);
 		}
 
-		static string SkipAheadAndReadAssemblyName(string reflectionTypeName, int pos)
+		static string? SkipAheadAndReadAssemblyName(string reflectionTypeName, int pos)
 		{
 			int nestingLevel = 0;
 			while (pos < reflectionTypeName.Length)

ICSharpCode.Decompiler/TypeSystem/TupleType.cs

@@ -144,7 +144,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Construct a tuple type (without element names) from the given underlying type.
 		/// Returns null if the input is not a valid underlying type.
 		/// </summary>
-		public static TupleType FromUnderlyingType(ICompilation compilation, IType type)
+		public static TupleType? FromUnderlyingType(ICompilation compilation, IType type)
 		{
 			var elementTypes = GetTupleElementTypes(type);
 			if (elementTypes.Length > 0)

ICSharpCode.Decompiler/TypeSystem/TypeProvider.cs

@@ -160,7 +160,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return type ?? new UnknownType(fullTypeName, IsReferenceType(reader, handle, rawTypeKind));
 		}
 
-		public IType GetTypeFromSerializedName(string name)
+		public IType? GetTypeFromSerializedName(string name)
 		{
 			if (name == null)
 			{

ICSharpCode.Decompiler/TypeSystem/TypeSystemExtensions.cs

@@ -354,7 +354,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <remarks>
 		/// Returns null if the type is not a delegate type; or if the invoke method could not be found.
 		/// </remarks>
-		public static IMethod GetDelegateInvokeMethod(this IType type)
+		public static IMethod? GetDelegateInvokeMethod(this IType type)
 		{
 			if (type == null)
 				throw new ArgumentNullException(nameof(type));
@@ -461,7 +461,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the type definition for the specified unresolved type.
 		/// Returns null if the unresolved type does not belong to this assembly.
 		/// </summary>
-		public static ITypeDefinition GetTypeDefinition(this IModule module, FullTypeName fullTypeName)
+		public static ITypeDefinition? GetTypeDefinition(this IModule module, FullTypeName fullTypeName)
 		{
 			if (module == null)
 				throw new ArgumentNullException("assembly");
@@ -481,7 +481,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return typeDef;
 		}
 
-		static ITypeDefinition FindNestedType(ITypeDefinition typeDef, string name, int typeParameterCount)
+		static ITypeDefinition? FindNestedType(ITypeDefinition typeDef, string name, int typeParameterCount)
 		{
 			foreach (var nestedType in typeDef.NestedTypes)
 			{
@@ -626,7 +626,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		#endregion
 
 		#region ResolveResult
-		public static ISymbol GetSymbol(this ResolveResult rr)
+		public static ISymbol? GetSymbol(this ResolveResult rr)
 		{
 			if (rr is LocalResolveResult)
 			{
@@ -713,7 +713,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return false;
 		}
 
-		public static IModule FindModuleByReference(this ICompilation compilation, IAssemblyReference assemblyName)
+		public static IModule? FindModuleByReference(this ICompilation compilation, IAssemblyReference assemblyName)
 		{
 			foreach (var module in compilation.Modules)
 			{
@@ -749,7 +749,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			}
 		}
 
-		public static INamespace GetNamespaceByFullName(this ICompilation compilation, string name)
+		public static INamespace? GetNamespaceByFullName(this ICompilation compilation, string name)
 		{
 			if (string.IsNullOrEmpty(name))
 				return compilation.RootNamespace;

ICSharpCode.Decompiler/Util/CSharpPrimitiveCast.cs

@@ -34,7 +34,7 @@ namespace ICSharpCode.Decompiler.Util
 		/// </summary>
 		/// <exception cref="OverflowException">Overflow checking is enabled and an overflow occurred.</exception>
 		/// <exception cref="InvalidCastException">The cast is invalid, e.g. casting a boolean to an integer.</exception>
-		public static object Cast(TypeCode targetType, object input, bool checkForOverflow)
+		public static object? Cast(TypeCode targetType, object input, bool checkForOverflow)
 		{
 			if (input == null)
 				return null;

ILSpy/Analyzers/TreeNodes/AnalyzedModuleTreeNode.cs

@@ -64,6 +64,6 @@ namespace ICSharpCode.ILSpy.Analyzers.TreeNodes
 			MainWindow.Instance.JumpToReference(analyzedModule.MetadataFile);
 		}
 
-		public override IEntity Member => null;
+		public override IEntity? Member => null;
 	}
 }

ILSpy/App.xaml.cs

@@ -122,7 +122,7 @@ namespace ICSharpCode.ILSpy
 #pragma warning restore CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
 		}
 
-		static Assembly ResolvePluginDependencies(AssemblyLoadContext context, AssemblyName assemblyName)
+		static Assembly? ResolvePluginDependencies(AssemblyLoadContext context, AssemblyName assemblyName)
 		{
 			var rootPath = Path.GetDirectoryName(typeof(App).Assembly.Location);
 			var assemblyFileName = Path.Combine(rootPath, assemblyName.Name + ".dll");

ILSpy/AppEnv/CommandLineTools.cs

@@ -56,7 +56,7 @@ namespace ICSharpCode.ILSpy.AppEnv
 		/// - (2n) + 1 backslashes followed by a quotation mark again produce n backslashes followed by a quotation mark.
 		/// - n backslashes not followed by a quotation mark simply produce n backslashes.
 		/// </remarks>
-		public static string ArgumentArrayToCommandLine(params string[] arguments)
+		public static string? ArgumentArrayToCommandLine(params string[] arguments)
 		{
 			if (arguments == null)
 				return null;

ILSpy/Commands/CopyFullyQualifiedNameContextMenuEntry.cs

@@ -42,7 +42,7 @@ namespace ICSharpCode.ILSpy
 			Clipboard.SetText(member.ReflectionName);
 		}
 
-		private IMemberTreeNode GetMemberNodeFromContext(TextViewContext context)
+		private IMemberTreeNode? GetMemberNodeFromContext(TextViewContext context)
 		{
 			return context.SelectedTreeNodes?.Length == 1 ? context.SelectedTreeNodes[0] as IMemberTreeNode : null;
 		}

ILSpy/Commands/SaveCodeContextMenuEntry.cs

@@ -101,7 +101,7 @@ namespace ICSharpCode.ILSpy.TextView
 		/// will be used.</param>
 		/// 
 		/// <returns>The full path of the selected target file, or <c>null</c> if the user canceled.</returns>
-		static string SelectSolutionFile()
+		static string? SelectSolutionFile()
 		{
 			SaveFileDialog dlg = new SaveFileDialog();
 			dlg.FileName = "Solution.sln";

ILSpy/Commands/ScopeSearchToNamespace.cs

@@ -75,7 +75,7 @@ namespace ICSharpCode.ILSpy
 			return GetNamespace(context) != null;
 		}
 
-		string GetNamespace(TextViewContext context)
+		string? GetNamespace(TextViewContext context)
 		{
 			if (context.Reference?.Reference is IEntity entity)
 				return entity.Namespace;

ILSpy/Controls/ExtensionMethods.cs

@@ -60,7 +60,7 @@ namespace ICSharpCode.ILSpy.Controls
 				this.targetProperty = property;
 			}
 
-			public object GetService(Type serviceType)
+			public object? GetService(Type serviceType)
 			{
 				if (serviceType == typeof(IProvideValueTarget))
 					return this;

ILSpy/Controls/TreeView/LinesRenderer.cs

@@ -32,7 +32,7 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 
 		static Pen pen;
 
-		SharpTreeNodeView NodeView {
+		SharpTreeNodeView? NodeView {
 			get { return TemplatedParent as SharpTreeNodeView; }
 		}
 

ILSpy/Controls/TreeView/SharpTreeNodeView.cs

@@ -43,7 +43,7 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 			set { SetValue(TextBackgroundProperty, value); }
 		}
 
-		public SharpTreeNode Node {
+		public SharpTreeNode? Node {
 			get { return DataContext as SharpTreeNode; }
 		}
 

ILSpy/Controls/TreeView/SharpTreeView.cs

@@ -420,7 +420,7 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 			base.ScrollIntoView(node);
 		}
 
-		object OnFocusItem(object item)
+		object? OnFocusItem(object item)
 		{
 			FrameworkElement element = this.ItemContainerGenerator.ContainerFromItem(item) as FrameworkElement;
 			if (element != null)
@@ -531,7 +531,7 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 			public int Index;
 		}
 
-		DropTarget GetDropTarget(SharpTreeViewItem item, DragEventArgs e)
+		DropTarget? GetDropTarget(SharpTreeViewItem item, DragEventArgs e)
 		{
 			var dropTargets = BuildDropTargets(item, e);
 			var y = e.GetPosition(item).Y;

ILSpy/Controls/TreeView/SharpTreeViewItem.cs

@@ -34,7 +34,7 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 													 new FrameworkPropertyMetadata(typeof(SharpTreeViewItem)));
 		}
 
-		public SharpTreeNode Node {
+		public SharpTreeNode? Node {
 			get { return DataContext as SharpTreeNode; }
 		}
 

ILSpy/Docking/DockLayoutSettings.cs

@@ -77,7 +77,7 @@ namespace ICSharpCode.ILSpy.Docking
 			}
 		}
 
-		public XElement SaveAsXml()
+		public XElement? SaveAsXml()
 		{
 			try
 			{

ILSpy/Languages/CSharpBracketSearcher.cs

@@ -33,7 +33,7 @@ namespace ICSharpCode.ILSpy
 		string openingBrackets = "([{";
 		string closingBrackets = ")]}";
 
-		public BracketSearchResult SearchBracket(IDocument document, int offset)
+		public BracketSearchResult? SearchBracket(IDocument document, int offset)
 		{
 			if (offset > 0)
 			{

ILSpy/Languages/CSharpHighlightingTokenWriter.cs

@@ -474,7 +474,7 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		ISymbol GetCurrentDefinition()
+		ISymbol? GetCurrentDefinition()
 		{
 			if (nodeStack == null || nodeStack.Count == 0)
 				return null;
@@ -488,7 +488,7 @@ namespace ICSharpCode.ILSpy
 			return null;
 		}
 
-		ISymbol GetCurrentMemberReference()
+		ISymbol? GetCurrentMemberReference()
 		{
 			if (nodeStack == null || nodeStack.Count == 0)
 				return null;

ILSpy/Languages/CSharpLanguage.cs

@@ -418,7 +418,7 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		public override ProjectId DecompileAssembly(LoadedAssembly assembly, ITextOutput output, DecompilationOptions options)
+		public override ProjectId? DecompileAssembly(LoadedAssembly assembly, ITextOutput output, DecompilationOptions options)
 		{
 			var module = assembly.GetMetadataFileOrNull();
 			if (module == null)
@@ -702,7 +702,7 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		public override string GetEntityName(MetadataFile module, EntityHandle handle, bool fullName, bool omitGenerics)
+		public override string? GetEntityName(MetadataFile module, EntityHandle handle, bool fullName, bool omitGenerics)
 		{
 			MetadataReader metadata = module.Metadata;
 			switch (handle.Kind)

ILSpy/Languages/ILLanguage.cs

@@ -163,7 +163,7 @@ namespace ICSharpCode.ILSpy
 			dis.DisassembleNamespace(nameSpace, module, types.Select(t => (TypeDefinitionHandle)t.MetadataToken));
 		}
 
-		public override ProjectId DecompileAssembly(LoadedAssembly assembly, ITextOutput output, DecompilationOptions options)
+		public override ProjectId? DecompileAssembly(LoadedAssembly assembly, ITextOutput output, DecompilationOptions options)
 		{
 			output.WriteLine("// " + assembly.FileName);
 			output.WriteLine();
@@ -195,7 +195,7 @@ namespace ICSharpCode.ILSpy
 			return null;
 		}
 
-		public override RichText GetRichTextTooltip(IEntity entity)
+		public override RichText? GetRichTextTooltip(IEntity entity)
 		{
 			var output = new AvalonEditTextOutput() { IgnoreNewLineAndIndent = true };
 			var disasm = CreateDisassembler(output, MainWindow.Instance.CreateDecompilationOptions());

ILSpy/Languages/Language.cs

@@ -52,7 +52,7 @@ namespace ICSharpCode.ILSpy
 		/// </summary>
 		public abstract string FileExtension { get; }
 
-		public virtual string ProjectFileExtension {
+		public virtual string? ProjectFileExtension {
 			get { return null; }
 		}
 
@@ -107,7 +107,7 @@ namespace ICSharpCode.ILSpy
 			WriteCommentLine(output, nameSpace);
 		}
 
-		public virtual ProjectId DecompileAssembly(LoadedAssembly assembly, ITextOutput output, DecompilationOptions options)
+		public virtual ProjectId? DecompileAssembly(LoadedAssembly assembly, ITextOutput output, DecompilationOptions options)
 		{
 			WriteCommentLine(output, assembly.FileName);
 			var asm = assembly.GetMetadataFileOrNull();
@@ -478,7 +478,7 @@ namespace ICSharpCode.ILSpy
 		/// <summary>
 		/// This should produce a string representation of the entity for search to match search strings against.
 		/// </summary>
-		public virtual string GetEntityName(MetadataFile module, EntityHandle handle, bool fullName, bool omitGenerics)
+		public virtual string? GetEntityName(MetadataFile module, EntityHandle handle, bool fullName, bool omitGenerics)
 		{
 			MetadataReader metadata = module.Metadata;
 			switch (handle.Kind)

ILSpy/MainWindow.xaml.cs

@@ -83,7 +83,7 @@ namespace ICSharpCode.ILSpy
 			get { return instance; }
 		}
 
-		public SharpTreeView AssemblyTreeView {
+		public SharpTreeView? AssemblyTreeView {
 			get {
 				return FindResource("AssemblyTreeView") as SharpTreeView;
 			}
@@ -312,7 +312,7 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		internal static string GetResourceString(string key)
+		internal static string? GetResourceString(string key)
 		{
 			if (string.IsNullOrEmpty(key))
 			{
@@ -1109,7 +1109,7 @@ namespace ICSharpCode.ILSpy
 		/// <summary>
 		/// Retrieves a node using the .ToString() representations of its ancestors.
 		/// </summary>
-		public SharpTreeNode FindNodeByPath(string[] path, bool returnBestMatch)
+		public SharpTreeNode? FindNodeByPath(string[] path, bool returnBestMatch)
 		{
 			if (path == null)
 				return null;
@@ -1135,7 +1135,7 @@ namespace ICSharpCode.ILSpy
 		/// <summary>
 		/// Gets the .ToString() representation of the node's ancestors.
 		/// </summary>
-		public static string[] GetPathForNode(SharpTreeNode node)
+		public static string[]? GetPathForNode(SharpTreeNode node)
 		{
 			if (node == null)
 				return null;
@@ -1149,7 +1149,7 @@ namespace ICSharpCode.ILSpy
 			return path.ToArray();
 		}
 
-		public ILSpyTreeNode FindTreeNode(object reference)
+		public ILSpyTreeNode? FindTreeNode(object reference)
 		{
 			switch (reference)
 			{
@@ -1586,7 +1586,7 @@ namespace ICSharpCode.ILSpy
 			sessionSettings.Save();
 		}
 
-		private string GetAutoLoadedAssemblyNode(SharpTreeNode node)
+		private string? GetAutoLoadedAssemblyNode(SharpTreeNode node)
 		{
 			if (node == null)
 				return null;

ILSpy/Metadata/CoffHeaderTreeNode.cs

@@ -116,7 +116,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			this.detailsFieldName = detailsFieldName;
 		}
 
-		public override DataTemplate SelectTemplate(object item, DependencyObject container)
+		public override DataTemplate? SelectTemplate(object item, DependencyObject container)
 		{
 			if (((Entry)item).Member == detailsFieldName)
 				return (DataTemplate)MetadataTableViews.Instance["HeaderFlagsDetailsDataGrid"];

ILSpy/Metadata/CorTables/AssemblyRefTableTreeNode.cs

@@ -89,7 +89,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int PublicKeyOrToken => MetadataTokens.GetHeapOffset(assemblyRef.PublicKeyOrToken);
 
-			public string PublicKeyOrTokenTooltip {
+			public string? PublicKeyOrTokenTooltip {
 				get {
 					if (assemblyRef.PublicKeyOrToken.IsNil)
 						return null;

ILSpy/Metadata/CorTables/ConstantTableTreeNode.cs

@@ -97,7 +97,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int Value => MetadataTokens.GetHeapOffset(constant.Value);
 
-			public string ValueTooltip {
+			public string? ValueTooltip {
 				get {
 					return null;
 				}

ILSpy/Metadata/CorTables/CustomAttributeTableTreeNode.cs

@@ -103,7 +103,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int Value => MetadataTokens.GetHeapOffset(customAttr.Value);
 
-			public string ValueTooltip {
+			public string? ValueTooltip {
 				get {
 					return null;
 				}

ILSpy/Metadata/CorTables/DeclSecurityTableTreeNode.cs

@@ -102,7 +102,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int PermissionSet => MetadataTokens.GetHeapOffset(declSecAttr.PermissionSet);
 
-			public string PermissionSetTooltip {
+			public string? PermissionSetTooltip {
 				get {
 					return null;
 				}

ILSpy/Metadata/CorTables/FileTableTreeNode.cs

@@ -90,7 +90,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int HashValue => MetadataTokens.GetHeapOffset(assemblyFile.HashValue);
 
-			public string HashValueTooltip {
+			public string? HashValueTooltip {
 				get {
 					if (assemblyFile.HashValue.IsNil)
 						return null;

ILSpy/Metadata/CorTables/MethodTableTreeNode.cs

@@ -122,7 +122,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			}
 
 			string paramListTooltip;
-			public string ParamListTooltip {
+			public string? ParamListTooltip {
 				get {
 					var param = methodDef.GetParameters().FirstOrDefault();
 					if (param.IsNil)

ILSpy/Metadata/CorTables/ModuleTableTreeNode.cs

@@ -83,12 +83,12 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int GenerationId => MetadataTokens.GetHeapOffset(moduleDef.GenerationId);
 
-			public string GenerationIdTooltip => moduleDef.GenerationId.IsNil ? null : metadataFile.Metadata.GetGuid(moduleDef.GenerationId).ToString();
+			public string? GenerationIdTooltip => moduleDef.GenerationId.IsNil ? null : metadataFile.Metadata.GetGuid(moduleDef.GenerationId).ToString();
 
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int BaseGenerationId => MetadataTokens.GetHeapOffset(moduleDef.BaseGenerationId);
 
-			public string BaseGenerationIdTooltip => moduleDef.BaseGenerationId.IsNil ? null : metadataFile.Metadata.GetGuid(moduleDef.BaseGenerationId).ToString();
+			public string? BaseGenerationIdTooltip => moduleDef.BaseGenerationId.IsNil ? null : metadataFile.Metadata.GetGuid(moduleDef.BaseGenerationId).ToString();
 
 			public ModuleEntry(MetadataFile metadataFile, ModuleDefinitionHandle handle)
 			{

ILSpy/Metadata/CorTables/TypeDefTableTreeNode.cs

@@ -114,7 +114,7 @@ namespace ICSharpCode.ILSpy.Metadata
 				MainWindow.Instance.JumpToReference(new EntityReference(metadataFile, typeDef.BaseType, protocol: "metadata"));
 			}
 
-			public string BaseTypeTooltip {
+			public string? BaseTypeTooltip {
 				get {
 					var output = new PlainTextOutput();
 					var provider = new DisassemblerSignatureTypeProvider(metadataFile, output);
@@ -146,7 +146,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			}
 
 			string fieldListTooltip;
-			public string FieldListTooltip {
+			public string? FieldListTooltip {
 				get {
 					var field = typeDef.GetFields().FirstOrDefault();
 					if (field.IsNil)
@@ -164,7 +164,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			}
 
 			string methodListTooltip;
-			public string MethodListTooltip {
+			public string? MethodListTooltip {
 				get {
 					var method = typeDef.GetMethods().FirstOrDefault();
 					if (method.IsNil)

ILSpy/Metadata/DebugTables/CustomDebugInformationTableTreeNode.cs

@@ -243,7 +243,7 @@ namespace ICSharpCode.ILSpy.Metadata
 
 			object rowDetails;
 
-			public object RowDetails {
+			public object? RowDetails {
 				get {
 					if (rowDetails != null)
 						return rowDetails;

ILSpy/Metadata/DebugTables/DocumentTableTreeNode.cs

@@ -84,7 +84,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int HashAlgorithm => MetadataTokens.GetHeapOffset(document.HashAlgorithm);
 
-			public string HashAlgorithmTooltip {
+			public string? HashAlgorithmTooltip {
 				get {
 					if (document.HashAlgorithm.IsNil)
 						return null;
@@ -100,7 +100,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int Hash => MetadataTokens.GetHeapOffset(document.Hash);
 
-			public string HashTooltip {
+			public string? HashTooltip {
 				get {
 					if (document.Hash.IsNil)
 						return null;
@@ -112,7 +112,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			[ColumnInfo("X8", Kind = ColumnKind.HeapOffset)]
 			public int Language => MetadataTokens.GetHeapOffset(document.Language);
 
-			public string LanguageTooltip {
+			public string? LanguageTooltip {
 				get {
 					if (document.Language.IsNil)
 						return null;