more nullable error cleanups

.editorconfig

@@ -153,6 +153,7 @@ dotnet_diagnostic.IDE2003.severity = silent
 
 #cleared null error types
 dotnet_diagnostic.CS8612.severity = error
+dotnet_diagnostic.CS8622.severity = error
 dotnet_diagnostic.CS8714.severity = error
 dotnet_diagnostic.CS8762.severity = error
 dotnet_diagnostic.CS8765.severity = error

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -777,7 +777,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			ArgumentWithAlignmentAndFormat,
 		}
 
-		private IEnumerable<(TokenKind, string)> TokenizeFormatString(string value)
+		private IEnumerable<(TokenKind, string?)> TokenizeFormatString(string value)
 		{
 			int pos = -1;
 
@@ -1395,7 +1395,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		OverloadResolutionErrors IsUnambiguousCall(ExpectedTargetDetails expectedTargetDetails, IMethod method,
-			ResolveResult target, IType[] typeArguments, ResolveResult[] arguments,
+			ResolveResult? target, IType[] typeArguments, ResolveResult[] arguments,
 			string[]? argumentNames, int firstOptionalArgumentIndex,
 			out IParameterizedMember? foundMember, out bool bestCandidateIsExpandedForm)
 		{
@@ -1676,7 +1676,7 @@ 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())
 			{
@@ -1823,7 +1823,7 @@ 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;
@@ -1850,11 +1850,11 @@ 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));
 			}

ICSharpCode.Decompiler/CSharp/RecordDecompiler.cs

@@ -267,7 +267,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		/// <summary>
 		/// Gets the detected primary constructor. Returns null, if there was no primary constructor detected.
 		/// </summary>
-		public IMethod PrimaryConstructor => primaryCtor;
+		public IMethod? PrimaryConstructor => primaryCtor;
 
 		public bool IsInheritedRecord => isInheritedRecord;
 

ICSharpCode.Decompiler/CSharp/Resolver/MethodGroupResolveResult.cs

@@ -79,10 +79,10 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 	{
 		readonly IReadOnlyList<MethodListWithDeclaringType> methodLists;
 		readonly IReadOnlyList<IType> typeArguments;
-		readonly ResolveResult targetResult;
+		readonly ResolveResult? targetResult;
 		readonly string methodName;
 
-		public MethodGroupResolveResult(ResolveResult targetResult, string methodName,
+		public MethodGroupResolveResult(ResolveResult? targetResult, string methodName,
 			IReadOnlyList<MethodListWithDeclaringType> methods, IReadOnlyList<IType> typeArguments)
 			: base(SpecialType.NoType)
 		{

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

@@ -106,7 +106,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public int Alignment { get; }
 
-		public string Suffix { get; }
+		public string? Suffix { get; }
 
 		public CSharpTokenNode RBraceToken {
 			get { return GetChildByRole(RBrace); }

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

@@ -86,12 +86,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				visitor.VisitPatternPlaceholder(this, child);
 			}
 
-			public override T? AcceptVisitor<T>(IAstVisitor<T> visitor)
+			public override T AcceptVisitor<T>(IAstVisitor<T> visitor)
 			{
 				return visitor.VisitPatternPlaceholder(this, child);
 			}
 
-			public override S? AcceptVisitor<T, S>(IAstVisitor<T, S> visitor, T data)
+			public override S AcceptVisitor<T, S>(IAstVisitor<T, S> visitor, T data)
 			{
 				return visitor.VisitPatternPlaceholder(this, child, data);
 			}

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

@@ -89,7 +89,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		public BlockStatement Body {
+		public BlockStatement? Body {
 			get { return GetChildByRole(Roles.Body); }
 			set { SetChildByRole(Roles.Body, value); }
 		}

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

@@ -40,7 +40,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			get { return GetChildByRole(Roles.Assign); }
 		}
 
-		public Expression Initializer {
+		public Expression? Initializer {
 			get { return GetChildByRole(InitializerRole); }
 			set { SetChildByRole(InitializerRole, value); }
 		}

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -766,7 +766,7 @@ 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

@@ -50,7 +50,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					if (fieldDecl.Variables.Count != 1)
 						continue;
 					string oldName = fieldDecl.Variables.Single().Name;
-					ISymbol symbol = fieldDecl.GetSymbol();
+					ISymbol? symbol = fieldDecl.GetSymbol();
 					if (memberNames.Contains(oldName) && ((IField)symbol).Accessibility == Accessibility.Private)
 					{
 						string newName = PickNewName(memberNames, oldName);

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs

@@ -80,7 +80,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		void DecompileQueries(AstNode node)
 		{
-			Expression query = DecompileQuery(node as InvocationExpression);
+			Expression? query = DecompileQuery(node as InvocationExpression);
 			if (query != null)
 			{
 				if (node.Parent is ExpressionStatement && CanUseDiscardAssignment())
@@ -351,13 +351,13 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		/// <summary>
 		/// Ensure that all ThenBy's are correct, and that the list of ThenBy's is terminated by an 'OrderBy' invocation.
 		/// </summary>
-		bool ValidateThenByChain(InvocationExpression invocation, string expectedParameterName)
+		bool ValidateThenByChain(InvocationExpression? invocation, string expectedParameterName)
 		{
 			if (invocation == null || invocation.Arguments.Count != 1)
 				return false;
 			if (!(invocation.Target is MemberReferenceExpression mre))
 				return false;
-			if (!MatchSimpleLambda(invocation.Arguments.Single(), out ParameterDeclaration parameter, out _))
+			if (!MatchSimpleLambda(invocation.Arguments.Single(), out ParameterDeclaration? parameter, out _))
 				return false;
 			if (parameter.Name != expectedParameterName)
 				return false;

ICSharpCode.Decompiler/CSharp/Transforms/TransformFieldAndConstructorInitializers.cs

@@ -184,7 +184,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (instanceCtorsNotChainingWithThis.Length > 0)
 			{
 				var ctorMethodDef = instanceCtorsNotChainingWithThis[0].GetSymbol() as IMethod;
-				ITypeDefinition declaringTypeDefinition = ctorMethodDef?.DeclaringTypeDefinition;
+				ITypeDefinition? declaringTypeDefinition = ctorMethodDef?.DeclaringTypeDefinition;
 				if (ctorMethodDef != null && declaringTypeDefinition?.IsReferenceType == false && !declaringTypeDefinition.IsRecord)
 					return;
 

ICSharpCode.Decompiler/DecompilerException.cs

@@ -36,15 +36,15 @@ namespace ICSharpCode.Decompiler
 	/// </summary>
 	public class DecompilerException : Exception, ISerializable
 	{
-		public string AssemblyName => File.Name;
+		public string? AssemblyName => File?.Name;
 
-		public string FileName => File.FileName;
+		public string? FileName => File?.FileName;
 
-		public IEntity DecompiledEntity { get; }
-		public IModule Module { get; }
-		public MetadataFile File { get; }
+		public IEntity? DecompiledEntity { get; }
+		public IModule? Module { get; }
+		public MetadataFile? File { get; }
 
-		public DecompilerException(MetadataModule module, IEntity decompiledEntity,
+		public DecompilerException(MetadataModule module, IEntity? decompiledEntity,
 			Exception innerException, string? message = null)
 			: base(message ?? GetDefaultMessage(decompiledEntity), innerException)
 		{
@@ -115,7 +115,7 @@ namespace ICSharpCode.Decompiler
 			for (int i = 0; i < stackTrace.FrameCount; i++)
 			{
 				StackFrame? frame = stackTrace.GetFrame(i);
-				MethodBase? method = frame.GetMethod();
+				MethodBase? method = frame?.GetMethod();
 				if (method == null)
 					continue;
 
@@ -126,7 +126,7 @@ namespace ICSharpCode.Decompiler
 				Type? declaringType = method.DeclaringType;
 				if (declaringType != null)
 				{
-					b.Append(declaringType.FullName.Replace('+', '.'));
+					b.Append(declaringType.FullName?.Replace('+', '.'));
 					b.Append('.');
 				}
 				b.Append(method.Name);
@@ -168,7 +168,7 @@ namespace ICSharpCode.Decompiler
 				b.Append(')');
 
 				// source location
-				if (frame.GetILOffset() >= 0)
+				if (frame?.GetILOffset() >= 0)
 				{
 					string? filename = null;
 					try

ICSharpCode.Decompiler/Disassembler/ReflectionDisassembler.cs

@@ -1188,7 +1188,7 @@ namespace ICSharpCode.Decompiler.Disassembler
 					break;
 				default:
 					var blob = metadata.GetBlobReader(constant.Value);
-					object value;
+					object? value;
 					try
 					{
 						value = blob.ReadConstant(constant.TypeCode);

ICSharpCode.Decompiler/IL/ControlFlow/AsyncAwaitDecompiler.cs

@@ -74,29 +74,29 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		ILTransformContext context;
 
 		// These fields are set by MatchTaskCreationPattern() or MatchEnumeratorCreationNewObj()
-		IType taskType; // return type of the async method; or IAsyncEnumerable{T}/IAsyncEnumerator{T}
-		IType underlyingReturnType; // return type of the method (only the "T" for Task{T}), for async enumerators this is the type being yielded
+		IType? taskType; // return type of the async method; or IAsyncEnumerable{T}/IAsyncEnumerator{T}
+		IType? underlyingReturnType; // return type of the method (only the "T" for Task{T}), for async enumerators this is the type being yielded
 		AsyncMethodType methodType;
-		ITypeDefinition stateMachineType;
-		IType builderType;
-		IField builderField;
-		IField stateField;
+		ITypeDefinition? stateMachineType;
+		IType? builderType;
+		IField? builderField;
+		IField? stateField;
 		int initialState;
 		Dictionary<IField, ILVariable> fieldToParameterMap = new Dictionary<IField, ILVariable>();
 		Dictionary<ILVariable, ILVariable> cachedFieldToParameterMap = new Dictionary<ILVariable, ILVariable>();
-		IField disposeModeField; // 'disposeMode' field (IAsyncEnumerable/IAsyncEnumerator only)
+		IField? disposeModeField; // 'disposeMode' field (IAsyncEnumerable/IAsyncEnumerator only)
 
 		// These fields are set by AnalyzeMoveNext():
-		ILFunction moveNextFunction;
-		ILVariable cachedStateVar; // variable in MoveNext that caches the stateField.
-		TryCatch mainTryCatch;
-		Block setResultReturnBlock; // block that is jumped to for return statements
-									// Note: for async enumerators, a jump to setResultReturnBlock is a 'yield break;'
+		ILFunction? moveNextFunction;
+		ILVariable? cachedStateVar; // variable in MoveNext that caches the stateField.
+		TryCatch? mainTryCatch;
+		Block? setResultReturnBlock; // block that is jumped to for return statements
+									 // Note: for async enumerators, a jump to setResultReturnBlock is a 'yield break;'
 		int finalState;       // final state after the setResultAndExitBlock
 		bool finalStateKnown;
-		ILVariable resultVar; // the variable that gets returned by the setResultAndExitBlock
-		Block setResultYieldBlock; // block that is jumped to for 'yield return' statements
-		ILVariable doFinallyBodies;
+		ILVariable? resultVar; // the variable that gets returned by the setResultAndExitBlock
+		Block? setResultYieldBlock; // block that is jumped to for 'yield return' statements
+		ILVariable? doFinallyBodies;
 
 		// These fields are set by AnalyzeStateMachine():
 		int smallestAwaiterVarIndex;
@@ -295,15 +295,15 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			}
 			if (startCall.Arguments.Count != 2)
 				return false;
-			ILInstruction loadBuilderExpr = startCall.Arguments[0];
-			if (!startCall.Arguments[1].MatchLdLoca(out ILVariable stateMachineVar))
+			ILInstruction? loadBuilderExpr = startCall.Arguments[0];
+			if (!startCall.Arguments[1].MatchLdLoca(out ILVariable? stateMachineVar))
 				return false;
 			stateMachineType = stateMachineVar.Type.GetDefinition();
 			if (stateMachineType == null)
 				return false;
 			pos--;
 
-			if (loadBuilderExpr.MatchLdLocRef(out ILVariable builderVar))
+			if (loadBuilderExpr.MatchLdLocRef(out ILVariable? builderVar))
 			{
 				// Check third-to-last instruction (copy of builder)
 				// stloc builder(ldfld StateMachine::<>t__builder(ldloc stateMachine))
@@ -340,8 +340,8 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 					return false;
 				if (!MatchCall(returnValue, "get_Task", out var getTaskArgs) || getTaskArgs.Count != 1)
 					return false;
-				ILInstruction target;
-				IField builderField2;
+				ILInstruction? target;
+				IField? builderField2;
 				if (builderType.IsReferenceType == true)
 				{
 					if (!getTaskArgs[0].MatchLdFld(out target, out builderField2))
@@ -804,7 +804,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			return block.Instructions[1].MatchLeave(blockContainer);
 		}
 
-		private Block CheckSetResultReturnBlock(BlockContainer blockContainer, int setResultReturnBlockIndex, bool[] blocksAnalyzed)
+		private Block? CheckSetResultReturnBlock(BlockContainer blockContainer, int setResultReturnBlockIndex, bool[] blocksAnalyzed)
 		{
 			if (setResultReturnBlockIndex >= blockContainer.Blocks.Count)
 			{
@@ -1931,7 +1931,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			context.StepEndGroup(keepIfEmpty: true);
 		}
 
-		internal static Block GetBodyEntryPoint(BlockContainer body)
+		internal static Block? GetBodyEntryPoint(BlockContainer? body)
 		{
 			if (body == null)
 				return null;

ICSharpCode.Decompiler/IL/ControlFlow/DetectPinnedRegions.cs

@@ -163,7 +163,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			for (int i = 0; i < container.Blocks.Count; i++)
 			{
 				var block = container.Blocks[i];
-				if (IsNullSafeArrayToPointerPattern(block, out ILVariable v, out ILVariable p, out Block targetBlock))
+				if (IsNullSafeArrayToPointerPattern(block, out ILVariable? v, out ILVariable? p, out Block? targetBlock))
 				{
 					context.Step("NullSafeArrayToPointerPattern", block);
 					ILInstruction arrayToPointer = new GetPinnableReference(new LdLoc(v), null);
@@ -176,7 +176,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 					((Branch)block.Instructions.Last()).TargetBlock = targetBlock;
 					modified = true;
 				}
-				else if (IsCustomRefPinPattern(block, out ILInstruction ldlocMem, out var callGPR, out v, out var stlocPtr,
+				else if (IsCustomRefPinPattern(block, out ILInstruction? ldlocMem, out var callGPR, out v, out var stlocPtr,
 					out targetBlock, out var nullBlock, out var notNullBlock))
 				{
 					context.Step("CustomRefPinPattern", block);
@@ -432,7 +432,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				}
 			}
 			condition = condition.UnwrapConv(ConversionKind.Truncate);
-			if (condition.MatchLdLen(StackType.I, out ILInstruction array))
+			if (condition.MatchLdLen(StackType.I, out ILInstruction? array))
 			{
 				// OK
 			}
@@ -451,7 +451,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			}
 			if (!array.MatchLdLoc(v))
 				return false;
-			if (!trueInst.MatchBranch(out Block notNullAndNotEmptyBlock))
+			if (!trueInst.MatchBranch(out Block? notNullAndNotEmptyBlock))
 				return false;
 			if (notNullAndNotEmptyBlock.Parent != block.Parent)
 				return false;
@@ -468,7 +468,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			// }
 			if (block.Instructions.Count != 2)
 				return false;
-			if (!block.Instructions[0].MatchStLoc(out var p2, out ILInstruction value))
+			if (!block.Instructions[0].MatchStLoc(out var p2, out ILInstruction? value))
 				return false;
 			if (p != p2)
 			{
@@ -950,8 +950,8 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			}
 			pinnedRegion.Init = new GetPinnableReference(pinnedRegion.Init, null);
 
-			ILVariable otherVar;
-			ILInstruction otherVarInit;
+			ILVariable? otherVar;
+			ILInstruction? otherVarInit;
 			// In optimized builds, the 'nativeVar' may end up being a stack slot,
 			// and only gets assigned to a real variable after the offset adjustment.
 			if (nativeVar.Kind == VariableKind.StackSlot && nativeVar.LoadCount == 1

ICSharpCode.Decompiler/IL/Transforms/EarlyExpressionTransforms.cs

@@ -132,7 +132,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 		internal static bool LdObjToLdLoc(LdObj inst, ILTransformContext context)
 		{
-			if (inst.Target.MatchLdLoca(out ILVariable v)
+			if (inst.Target.MatchLdLoca(out ILVariable? v)
 				&& TypeUtils.IsCompatibleTypeForMemoryAccess(v.Type, inst.Type)
 				&& inst.UnalignedPrefix == 0
 				&& !inst.IsVolatile)

ICSharpCode.Decompiler/IL/Transforms/IndexRangeTransform.cs

@@ -275,8 +275,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 			void TransformSlicing(bool sliceLengthWasMisdetectedAsStartOffset = false)
 			{
-				ILVariable sliceLengthVar;
-				ILInstruction sliceLengthVarInit;
+				ILVariable? sliceLengthVar;
+				ILInstruction? sliceLengthVarInit;
 				if (sliceLengthWasMisdetectedAsStartOffset)
 				{
 					// Special case: when slicing without a start point, the slice length calculation is mis-detected as the start offset,
@@ -303,7 +303,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 				if (!(sliceLengthVar.IsSingleDefinition && sliceLengthVar.LoadCount == 1))
 					return;
-				if (!MatchSliceLength(sliceLengthVarInit, out IndexKind endIndexKind, out ILInstruction endIndexLoad, containerLengthVar, ref containerVar, startOffsetVar))
+				if (!MatchSliceLength(sliceLengthVarInit, out IndexKind endIndexKind, out ILInstruction? endIndexLoad, containerLengthVar, ref containerVar, startOffsetVar))
 					return;
 				if (!CheckContainerLengthVariableUseCount(containerLengthVar, startIndexKind, endIndexKind))
 				{

ICSharpCode.Decompiler/Metadata/DotNetCorePathFinder.cs

@@ -262,7 +262,7 @@ namespace ICSharpCode.Decompiler.Metadata
 			return version.ToString();
 		}
 
-		internal static (Version version, DirectoryInfo directory) ConvertToVersion(DirectoryInfo directory)
+		internal static (Version? version, DirectoryInfo? directory) ConvertToVersion(DirectoryInfo directory)
 		{
 			string RemoveTrailingVersionInfo()
 			{
@@ -289,7 +289,7 @@ namespace ICSharpCode.Decompiler.Metadata
 		public static string? FindDotNetExeDirectory()
 		{
 			string dotnetExeName = (Environment.OSVersion.Platform == PlatformID.Unix) ? "dotnet" : "dotnet.exe";
-			foreach (var item in Environment.GetEnvironmentVariable("PATH").Split(Path.PathSeparator))
+			foreach (var item in Environment.GetEnvironmentVariable("PATH")!.Split(Path.PathSeparator))
 			{
 				try
 				{

ICSharpCode.Decompiler/TypeSystem/ApplyAttributeTypeVisitor.cs

@@ -48,7 +48,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			bool[]? dynamicAttributeData = null;
 			bool hasNativeIntegersAttribute = (options & TypeSystemOptions.NativeIntegersWithoutAttribute) != 0;
 			bool[]? nativeIntegersAttributeData = null;
-			string[]? tupleElementNames = null;
+			string?[]? tupleElementNames = null;
 			Nullability nullability;
 			Nullability[]? nullableAttributeData = null;
 			if ((options & TypeSystemOptions.NullabilityAnnotations) != 0)

ICSharpCode.Decompiler/TypeSystem/DecompilerTypeSystem.cs

@@ -245,7 +245,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			// Load referenced assemblies and type-forwarder references.
 			// This is necessary to make .NET Core/PCL binaries work better.
 			var referencedAssemblies = new List<MetadataFile>();
-			var assemblyReferenceQueue = new Queue<(bool IsAssembly, MetadataFile MainModule, object Reference, Task<MetadataFile> ResolveTask)>();
+			var assemblyReferenceQueue = new Queue<(bool IsAssembly, MetadataFile MainModule, object Reference, Task<MetadataFile?> ResolveTask)>();
 			var comparer = KeyComparer.Create(((bool IsAssembly, MetadataFile MainModule, object Reference) reference) =>
 				reference.IsAssembly ? "A:" + ((IAssemblyReference)reference.Reference).FullName :
 									   "M:" + reference.Reference);
@@ -348,7 +348,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				{
 					// Immediately start loading the referenced module as we add the entry to the queue.
 					// This allows loading multiple modules in parallel.
-					Task<MetadataFile> asm;
+					Task<MetadataFile?> asm;
 					if (isAssembly)
 					{
 						asm = assemblyResolver.ResolveAsync((IAssemblyReference)reference);

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataEvent.cs

@@ -99,7 +99,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		{
 			if (method == null)
 				return EmptyList<IMember>.Instance;
-			return method.ExplicitlyImplementedInterfaceMembers.Select(m => ((IMethod)m).AccessorOwner).Where(m => m != null);
+			return method.ExplicitlyImplementedInterfaceMembers.Select(m => ((IMethod)m).AccessorOwner).OfType<IMember>();
 		}
 
 		public ITypeDefinition DeclaringTypeDefinition => AnyAccessor?.DeclaringTypeDefinition;

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataProperty.cs

@@ -188,7 +188,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		{
 			if (method == null)
 				return EmptyList<IMember>.Instance;
-			return method.ExplicitlyImplementedInterfaceMembers.Select(m => ((IMethod)m).AccessorOwner).Where(m => m != null);
+			return method.ExplicitlyImplementedInterfaceMembers.Select(m => ((IMethod)m).AccessorOwner).OfType<IMember>();
 		}
 
 		public ITypeDefinition DeclaringTypeDefinition => AnyAccessor?.DeclaringTypeDefinition;

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

@@ -83,7 +83,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			if (type == null)
 				throw new ArgumentNullException(nameof(type));
 
-			return type.GetAllBaseTypes().Select(t => t.GetDefinition()).Where(d => d != null).Distinct();
+			return type.GetAllBaseTypes().Select(t => t.GetDefinition()).OfType<ITypeDefinition>().Distinct();
 		}
 
 		/// <summary>

ICSharpCode.ILSpyX/Abstractions/ILanguage.cs

@@ -27,7 +27,7 @@ namespace ICSharpCode.ILSpyX.Abstractions
 	{
 		bool ShowMember(IEntity member);
 		CodeMappingInfo GetCodeMappingInfo(MetadataFile module, EntityHandle member);
-		string GetEntityName(MetadataFile module, System.Reflection.Metadata.EntityHandle handle, bool fullName, bool omitGenerics);
+		string? GetEntityName(MetadataFile module, System.Reflection.Metadata.EntityHandle handle, bool fullName, bool omitGenerics);
 		string GetTooltip(IEntity entity);
 
 		string TypeToString(IType type, bool includeNamespace);

ILSpy.ReadyToRun/ReadyToRunLanguage.cs

@@ -216,7 +216,7 @@ namespace ICSharpCode.ILSpy.ReadyToRun
 			}
 		}
 
-		public override RichText GetRichTextTooltip(IEntity entity)
+		public override RichText? GetRichTextTooltip(IEntity entity)
 		{
 			return Languages.ILLanguage.GetRichTextTooltip(entity);
 		}

ILSpy/Analyzers/AnalyzerEntityTreeNode.cs

@@ -22,8 +22,8 @@ using System.Windows;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.ILSpy.TreeNodes;
 using ICSharpCode.ILSpyX;
-using ICSharpCode.ILSpyX.TreeView.PlatformAbstractions;
 using ICSharpCode.ILSpyX.TreeView;
+using ICSharpCode.ILSpyX.TreeView.PlatformAbstractions;
 
 namespace ICSharpCode.ILSpy.Analyzers
 {
@@ -32,7 +32,7 @@ namespace ICSharpCode.ILSpy.Analyzers
 	/// </summary>
 	public abstract class AnalyzerEntityTreeNode : AnalyzerTreeNode, IMemberTreeNode
 	{
-		public abstract IEntity Member { get; }
+		public abstract IEntity? Member { get; }
 
 		public override void ActivateItem(IPlatformRoutedEventArgs e)
 		{

ILSpy/Analyzers/TreeNodes/AnalyzedEventTreeNode.cs

@@ -40,7 +40,7 @@ namespace ICSharpCode.ILSpy.Analyzers.TreeNodes
 			this.LazyLoading = true;
 		}
 
-		public override IEntity Member => analyzedEvent;
+		public override IEntity? Member => analyzedEvent;
 
 		public override object Icon => EventTreeNode.GetIcon(analyzedEvent);
 

ILSpy/Analyzers/TreeNodes/AnalyzedFieldTreeNode.cs

@@ -52,6 +52,6 @@ namespace ICSharpCode.ILSpy.Analyzers.TreeNodes
 			}
 		}
 
-		public override IEntity Member => analyzedField;
+		public override IEntity? Member => analyzedField;
 	}
 }

ILSpy/Analyzers/TreeNodes/AnalyzedMethodTreeNode.cs

@@ -54,6 +54,6 @@ namespace ICSharpCode.ILSpy.Analyzers.TreeNodes
 			}
 		}
 
-		public override IEntity Member => analyzedMethod;
+		public override IEntity? Member => analyzedMethod;
 	}
 }

ILSpy/Analyzers/TreeNodes/AnalyzedPropertyTreeNode.cs

@@ -62,6 +62,6 @@ namespace ICSharpCode.ILSpy.Analyzers.TreeNodes
 			}
 		}
 
-		public override IEntity Member => analyzedProperty;
+		public override IEntity? Member => analyzedProperty;
 	}
 }

ILSpy/Analyzers/TreeNodes/AnalyzedTypeTreeNode.cs

@@ -52,6 +52,6 @@ namespace ICSharpCode.ILSpy.Analyzers.TreeNodes
 			}
 		}
 
-		public override IEntity Member => analyzedType;
+		public override IEntity? Member => analyzedType;
 	}
 }

ILSpy/App.xaml.cs

@@ -18,6 +18,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel.Composition.Hosting;
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
@@ -34,17 +35,15 @@ using ICSharpCode.ILSpy.AppEnv;
 using ICSharpCode.ILSpy.Options;
 using ICSharpCode.ILSpyX.Analyzers;
 using ICSharpCode.ILSpyX.Settings;
+using ICSharpCode.ILSpyX.TreeView;
 
 using Medo.Application;
 
 using Microsoft.VisualStudio.Composition;
 
-using TomsToolbox.Wpf.Styles;
-using ICSharpCode.ILSpyX.TreeView;
-
 using TomsToolbox.Composition;
 using TomsToolbox.Wpf.Composition;
-using System.ComponentModel.Composition.Hosting;
+using TomsToolbox.Wpf.Styles;
 
 namespace ICSharpCode.ILSpy
 {
@@ -115,7 +114,7 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		private static void SingleInstance_NewInstanceDetected(object sender, NewInstanceEventArgs e)
+		private static void SingleInstance_NewInstanceDetected(object? sender, NewInstanceEventArgs e)
 		{
 #pragma warning disable CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
 			ICSharpCode.ILSpy.MainWindow.Instance.HandleSingleInstanceCommandLineArguments(e.Args);
@@ -210,7 +209,7 @@ namespace ICSharpCode.ILSpy
 			base.OnStartup(e);
 		}
 
-		void DotNet40_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
+		void DotNet40_UnobservedTaskException(object? sender, UnobservedTaskExceptionEventArgs e)
 		{
 			// On .NET 4.0, an unobserved exception in a task terminates the process unless we mark it as observed
 			e.SetObserved();

ILSpy/AvalonEdit/TextMarkerService.cs

@@ -34,7 +34,7 @@ namespace ICSharpCode.ILSpy.AvalonEdit
 	/// </summary>
 	sealed class TextMarkerService : DocumentColorizingTransformer, IBackgroundRenderer, ITextMarkerService
 	{
-		TextSegmentCollection<TextMarker> markers;
+		TextSegmentCollection<TextMarker>? markers;
 		TextView textView;
 
 		public TextMarkerService(TextView textView)
@@ -46,7 +46,7 @@ namespace ICSharpCode.ILSpy.AvalonEdit
 			OnDocumentChanged(null, null);
 		}
 
-		void OnDocumentChanged(object sender, EventArgs e)
+		void OnDocumentChanged(object? sender, EventArgs? e)
 		{
 			if (textView.Document != null)
 				markers = new TextSegmentCollection<TextMarker>(textView.Document);

ILSpy/Commands/CommandWrapper.cs

@@ -39,7 +39,7 @@ namespace ICSharpCode.ILSpy
 				return command;
 		}
 
-		public event EventHandler CanExecuteChanged {
+		public event EventHandler? CanExecuteChanged {
 			add { wrappedCommand.CanExecuteChanged += value; }
 			remove { wrappedCommand.CanExecuteChanged -= value; }
 		}

ILSpy/Commands/DelegateCommand.cs

@@ -12,7 +12,7 @@ namespace ICSharpCode.ILSpy.Commands
 		private readonly Action action;
 		private readonly Func<bool> canExecute;
 
-		public event EventHandler CanExecuteChanged {
+		public event EventHandler? CanExecuteChanged {
 			add { CommandManager.RequerySuggested += value; }
 			remove { CommandManager.RequerySuggested -= value; }
 		}
@@ -44,7 +44,7 @@ namespace ICSharpCode.ILSpy.Commands
 		private readonly Action<T> action;
 		private readonly Func<T, bool> canExecute;
 
-		public event EventHandler CanExecuteChanged {
+		public event EventHandler? CanExecuteChanged {
 			add { CommandManager.RequerySuggested += value; }
 			remove { CommandManager.RequerySuggested -= value; }
 		}

ILSpy/Commands/IProtocolHandler.cs

@@ -25,6 +25,6 @@ namespace ICSharpCode.ILSpy
 {
 	public interface IProtocolHandler
 	{
-		ILSpyTreeNode Resolve(string protocol, MetadataFile module, Handle handle, out bool newTabPage);
+		ILSpyTreeNode? Resolve(string protocol, MetadataFile module, Handle handle, out bool newTabPage);
 	}
 }

ILSpy/Commands/SimpleCommand.cs

@@ -24,7 +24,7 @@ namespace ICSharpCode.ILSpy
 {
 	public abstract class SimpleCommand : ICommand
 	{
-		public event EventHandler CanExecuteChanged {
+		public event EventHandler? CanExecuteChanged {
 			add { CommandManager.RequerySuggested += value; }
 			remove { CommandManager.RequerySuggested -= value; }
 		}
@@ -41,12 +41,12 @@ namespace ICSharpCode.ILSpy
 	{
 		private bool isChecked;
 
-		public event EventHandler CanExecuteChanged {
+		public event EventHandler? CanExecuteChanged {
 			add { CommandManager.RequerySuggested += value; }
 			remove { CommandManager.RequerySuggested -= value; }
 		}
 
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 
 		void ICommand.Execute(object? parameter)
 		{

ILSpy/Controls/SearchBox.cs

@@ -110,7 +110,7 @@ namespace ICSharpCode.ILSpy.Controls
 				wl.Visibility = HasText ? Visibility.Hidden : Visibility.Visible;
 		}
 
-		void timer_Tick(object sender, EventArgs e)
+		void timer_Tick(object? sender, EventArgs e)
 		{
 			timer.Stop();
 			timer = null;

ILSpy/Controls/TreeView/SharpTreeNodeView.cs

@@ -103,7 +103,7 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 			}
 		}
 
-		void Node_PropertyChanged(object sender, PropertyChangedEventArgs e)
+		void Node_PropertyChanged(object? sender, PropertyChangedEventArgs e)
 		{
 			if (e.PropertyName == "IsEditing")
 			{

ILSpy/Controls/TreeView/SharpTreeView.cs

@@ -171,7 +171,7 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 			}
 		}
 
-		void flattener_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+		void flattener_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
 		{
 			// Deselect nodes that are being hidden, if any remain in the tree
 			if (e.Action == NotifyCollectionChangedAction.Remove && Items.Count > 0)

ILSpy/Controls/TreeView/SharpTreeViewItemAutomationPeer.cs

@@ -32,7 +32,7 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 			: base(owner)
 		{
 			SharpTreeViewItem.DataContextChanged += OnDataContextChanged;
-			SharpTreeNode node = SharpTreeViewItem.DataContext as SharpTreeNode;
+			SharpTreeNode? node = SharpTreeViewItem.DataContext as SharpTreeNode;
 			if (node == null)
 				return;
 
@@ -61,18 +61,18 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 
 		public ExpandCollapseState ExpandCollapseState {
 			get {
-				SharpTreeNode node = SharpTreeViewItem.DataContext as SharpTreeNode;
+				SharpTreeNode? node = SharpTreeViewItem.DataContext as SharpTreeNode;
 				if (node == null || !node.ShowExpander)
 					return ExpandCollapseState.LeafNode;
 				return node.IsExpanded ? ExpandCollapseState.Expanded : ExpandCollapseState.Collapsed;
 			}
 		}
 
-		private void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
+		private void OnPropertyChanged(object? sender, PropertyChangedEventArgs e)
 		{
 			if (e.PropertyName != "IsExpanded")
 				return;
-			SharpTreeNode node = sender as SharpTreeNode;
+			SharpTreeNode? node = sender as SharpTreeNode;
 			if (node == null || node.Children.Count == 0)
 				return;
 			bool newValue = node.IsExpanded;
@@ -83,12 +83,12 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 				newValue ? ExpandCollapseState.Expanded : ExpandCollapseState.Collapsed);
 		}
 
-		private void OnDataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
+		private void OnDataContextChanged(object? sender, DependencyPropertyChangedEventArgs e)
 		{
-			SharpTreeNode oldNode = e.OldValue as SharpTreeNode;
+			SharpTreeNode? oldNode = e.OldValue as SharpTreeNode;
 			if (oldNode != null)
 				oldNode.PropertyChanged -= OnPropertyChanged;
-			SharpTreeNode newNode = e.NewValue as SharpTreeNode;
+			SharpTreeNode? newNode = e.NewValue as SharpTreeNode;
 			if (newNode != null)
 				newNode.PropertyChanged += OnPropertyChanged;
 		}

ILSpy/Docking/DockWorkspace.cs

@@ -52,7 +52,7 @@ namespace ICSharpCode.ILSpy.Docking
 			MessageBus<CurrentAssemblyListChangedEventArgs>.Subscribers += (sender, e) => CurrentAssemblyList_Changed(sender, e);
 		}
 
-		private void CurrentAssemblyList_Changed(object sender, NotifyCollectionChangedEventArgs e)
+		private void CurrentAssemblyList_Changed(object? sender, NotifyCollectionChangedEventArgs e)
 		{
 			if (e.OldItems == null)
 			{
@@ -163,7 +163,7 @@ namespace ICSharpCode.ILSpy.Docking
 			}
 		}
 
-		void LayoutSerializationCallback(object sender, LayoutSerializationCallbackEventArgs e)
+		void LayoutSerializationCallback(object? sender, LayoutSerializationCallbackEventArgs e)
 		{
 			switch (e.Model)
 			{

ILSpy/Docking/PaneCollection.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.ILSpy.Docking
 	{
 		private ObservableCollection<T> observableCollection = new ObservableCollection<T>();
 
-		public event NotifyCollectionChangedEventHandler CollectionChanged;
+		public event NotifyCollectionChangedEventHandler? CollectionChanged;
 
 		public PaneCollection()
 		{

ILSpy/LanguageSettings.cs

@@ -173,9 +173,9 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 
-		protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
 		{
 			if (PropertyChanged != null)
 			{

ILSpy/Languages/Language.cs

@@ -364,7 +364,7 @@ namespace ICSharpCode.ILSpy
 		/// Converts a member signature to a string.
 		/// This is used for displaying the tooltip on a member reference.
 		/// </summary>
-		public virtual RichText GetRichTextTooltip(IEntity entity)
+		public virtual RichText? GetRichTextTooltip(IEntity entity)
 		{
 			return GetTooltip(entity);
 		}

ILSpy/MainWindow.xaml.cs

@@ -132,7 +132,7 @@ namespace ICSharpCode.ILSpy
 			this.Loaded += MainWindow_Loaded;
 		}
 
-		private void DockWorkspace_ActiveTabPageChanged(object sender, EventArgs e)
+		private void DockWorkspace_ActiveTabPageChanged(object? sender, EventArgs e)
 		{
 			DockWorkspace dock = DockWorkspace.Instance;
 
@@ -350,7 +350,7 @@ namespace ICSharpCode.ILSpy
 			ToolsChanged(dock.ToolPanes, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
 			TabsChanged(dock.TabPages, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
 
-			void ToolsChanged(object sender, NotifyCollectionChangedEventArgs e)
+			void ToolsChanged(object? sender, NotifyCollectionChangedEventArgs e)
 			{
 				int endIndex = windowMenuItem.Items.IndexOf(separatorBeforeDocuments);
 				int startIndex = windowMenuItem.Items.IndexOf(separatorBeforeTools) + 1;
@@ -429,7 +429,7 @@ namespace ICSharpCode.ILSpy
 				}
 			}
 
-			void TabsChanged(object sender, NotifyCollectionChangedEventArgs e)
+			void TabsChanged(object? sender, NotifyCollectionChangedEventArgs e)
 			{
 				int endIndex = windowMenuItem.Items.Count;
 				int startIndex = windowMenuItem.Items.IndexOf(separatorBeforeDocuments) + 1;
@@ -498,7 +498,7 @@ namespace ICSharpCode.ILSpy
 				}
 			}
 
-			static void TabPageChanged(object sender, PropertyChangedEventArgs e)
+			static void TabPageChanged(object? sender, PropertyChangedEventArgs e)
 			{
 				var windowMenuItem = Instance.mainMenu.Items.OfType<MenuItem>().First(m => (string)m.Tag == nameof(Properties.Resources._Window));
 				foreach (MenuItem menuItem in windowMenuItem.Items.OfType<MenuItem>())
@@ -965,7 +965,7 @@ namespace ICSharpCode.ILSpy
 #endif
 		}
 
-		void assemblyList_Assemblies_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+		void assemblyList_Assemblies_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
 		{
 			if (e.Action == NotifyCollectionChangedAction.Reset)
 			{

ILSpy/Metadata/Helpers.cs

@@ -89,13 +89,13 @@ namespace ICSharpCode.ILSpy.Metadata
 			return view;
 		}
 
-		internal static void View_AutoGeneratedColumns(object sender, EventArgs e)
+		internal static void View_AutoGeneratedColumns(object? sender, EventArgs e)
 		{
 			((DataGrid)sender).AutoGeneratedColumns -= View_AutoGeneratedColumns;
 			((DataGrid)sender).AutoGeneratingColumn -= View_AutoGeneratingColumn;
 		}
 
-		internal static void View_AutoGeneratingColumn(object sender, DataGridAutoGeneratingColumnEventArgs e)
+		internal static void View_AutoGeneratingColumn(object? sender, DataGridAutoGeneratingColumnEventArgs e)
 		{
 			var binding = new Binding(e.PropertyName) { Mode = BindingMode.OneWay };
 			e.Column = GetColumn();

ILSpy/Metadata/MetaDataGrid.cs

@@ -43,7 +43,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			this.hoverLogic.MouseHoverStopped += HoverLogic_MouseHoverStopped;
 		}
 
-		private void HoverLogic_MouseHoverStopped(object sender, System.Windows.Input.MouseEventArgs e)
+		private void HoverLogic_MouseHoverStopped(object? sender, System.Windows.Input.MouseEventArgs e)
 		{
 			// Non-popup tooltips get closed as soon as the mouse starts moving again
 			if (toolTip != null)
@@ -53,7 +53,7 @@ namespace ICSharpCode.ILSpy.Metadata
 			}
 		}
 
-		private void HoverLogic_MouseHover(object sender, System.Windows.Input.MouseEventArgs e)
+		private void HoverLogic_MouseHover(object? sender, System.Windows.Input.MouseEventArgs e)
 		{
 			var position = e.GetPosition(this);
 			var hit = VisualTreeHelper.HitTest(this, position);

ILSpy/Options/DecompilerSettingsViewModel.cs

@@ -92,7 +92,7 @@ namespace ICSharpCode.ILSpy.Options
 
 		public DecompilerSettingsItemViewModel[] Settings { get; }
 
-		private void Item_PropertyChanged(object sender, PropertyChangedEventArgs e)
+		private void Item_PropertyChanged(object? sender, PropertyChangedEventArgs e)
 		{
 			if (e.PropertyName == nameof(DecompilerSettingsItemViewModel.IsEnabled))
 			{

ILSpy/Options/DisplaySettings.cs

@@ -42,14 +42,14 @@ namespace ICSharpCode.ILSpy.Options
 		}
 
 		#region INotifyPropertyChanged implementation
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 
 		protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
 		{
 			PropertyChanged?.Invoke(this, e);
 		}
 
-		protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		protected void OnPropertyChanged([CallerMemberName] string? propertyName = null)
 		{
 			OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
 		}

ILSpy/Options/MiscSettingsViewModel.cs

@@ -119,14 +119,14 @@ namespace ICSharpCode.ILSpy.Options
 
 		#region INotifyPropertyChanged Implementation
 
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 
 		protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
 		{
 			PropertyChanged?.Invoke(this, e);
 		}
 
-		protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		protected void OnPropertyChanged([CallerMemberName] string? propertyName = null)
 		{
 			OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
 		}

ILSpy/Search/SearchPane.xaml.cs

@@ -203,7 +203,7 @@ namespace ICSharpCode.ILSpy.Search
 			listBox.SelectedIndex = 0;
 		}
 
-		void UpdateResults(object sender, EventArgs e)
+		void UpdateResults(object? sender, EventArgs e)
 		{
 			if (currentSearch == null)
 				return;

ILSpy/SessionSettings.cs

@@ -69,9 +69,9 @@ namespace ICSharpCode.ILSpy
 			this.DockLayout = new DockLayoutSettings(doc.Element("DockLayout"));
 		}
 
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 
-		void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		void OnPropertyChanged([CallerMemberName] string? propertyName = null)
 		{
 			var args = new PropertyChangedEventArgs(propertyName);
 

ILSpy/TextView/BracketHighlightRenderer.cs

@@ -33,7 +33,7 @@ namespace ICSharpCode.ILSpy.TextView
 		/// Searches for a matching bracket from the given offset to the start of the document.
 		/// </summary>
 		/// <returns>A BracketSearchResult that contains the positions and lengths of the brackets. Return null if there is nothing to highlight.</returns>
-		BracketSearchResult SearchBracket(IDocument document, int offset);
+		BracketSearchResult? SearchBracket(IDocument document, int offset);
 	}
 
 	public class DefaultBracketSearcher : IBracketSearcher

ILSpy/Themes/WindowStyleManagerBehavior.cs

@@ -65,7 +65,7 @@ namespace ICSharpCode.ILSpy.Themes
 			SettingsService.Instance.DisplaySettings.PropertyChanged -= DisplaySettings_PropertyChanged;
 		}
 
-		private void Color_Changed(object sender, EventArgs e)
+		private void Color_Changed(object? sender, EventArgs e)
 		{
 			ApplyThemeToWindowCaption();
 		}
@@ -83,7 +83,7 @@ namespace ICSharpCode.ILSpy.Themes
 			MessageBox.Show(Properties.Resources.SettingsChangeRestartRequired);
 		}
 
-		private void DisplaySettings_PropertyChanged(object sender, PropertyChangedEventArgs e)
+		private void DisplaySettings_PropertyChanged(object? sender, PropertyChangedEventArgs e)
 		{
 			if (e.PropertyName == nameof(DisplaySettings.StyleWindowTitleBar))
 			{
@@ -114,7 +114,7 @@ namespace ICSharpCode.ILSpy.Themes
 			}
 			else
 			{
-				void Initialized(object o, EventArgs eventArgs)
+				void Initialized(object? o, EventArgs eventArgs)
 				{
 					ApplyThemeToWindowCaption();
 					window.SourceInitialized -= Initialized;

ILSpy/TreeNodes/AssemblyListTreeNode.cs

@@ -26,8 +26,8 @@ using ICSharpCode.Decompiler.Metadata;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.Util;
 using ICSharpCode.ILSpyX;
-using ICSharpCode.ILSpyX.TreeView.PlatformAbstractions;
 using ICSharpCode.ILSpyX.TreeView;
+using ICSharpCode.ILSpyX.TreeView.PlatformAbstractions;
 
 namespace ICSharpCode.ILSpy.TreeNodes
 {
@@ -57,7 +57,7 @@ namespace ICSharpCode.ILSpy.TreeNodes
 		{
 			this.Children.Clear();
 			this.Children.AddRange(collection.GetAssemblies().Select(a => new AssemblyTreeNode(a)));
-			collection.CollectionChanged += delegate (object sender, NotifyCollectionChangedEventArgs e) {
+			collection.CollectionChanged += delegate (object? sender, NotifyCollectionChangedEventArgs e) {
 				switch (e.Action)
 				{
 					case NotifyCollectionChangedAction.Add:

ILSpy/TreeNodes/ILSpyTreeNode.cs

@@ -29,8 +29,8 @@ using ICSharpCode.Decompiler;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.ILSpy.Util;
 using ICSharpCode.ILSpyX.Abstractions;
-using ICSharpCode.ILSpyX.TreeView.PlatformAbstractions;
 using ICSharpCode.ILSpyX.TreeView;
+using ICSharpCode.ILSpyX.TreeView.PlatformAbstractions;
 
 namespace ICSharpCode.ILSpy.TreeNodes
 {
@@ -128,7 +128,7 @@ namespace ICSharpCode.ILSpy.TreeNodes
 			}
 		}
 
-		protected virtual void LanguageSettings_Changed(object sender, EventArgs e)
+		protected virtual void LanguageSettings_Changed(object? sender, EventArgs e)
 		{
 			RaisePropertyChanged(nameof(Text));
 			if (IsVisible)

ILSpy/Updates/NotifyOfUpdatesStrategy.cs

@@ -87,7 +87,7 @@ namespace ICSharpCode.ILSpy.Updates
 			}
 		}
 
-		public static Task<string> CheckForUpdatesAsync(ILSpySettings spySettings)
+		public static Task<string?> CheckForUpdatesAsync(ILSpySettings spySettings)
 		{
 			UpdateSettings s = new UpdateSettings(spySettings);
 			return CheckForUpdateInternal(s);

ILSpy/Updates/UpdateSettings.cs

@@ -78,9 +78,9 @@ namespace ICSharpCode.ILSpy.Updates
 			ILSpySettings.SaveSettings(updateSettings);
 		}
 
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 
-		void OnPropertyChanged(string propertyName)
+		void OnPropertyChanged(string? propertyName)
 		{
 			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
 		}

ILSpy/ViewModels/PaneModel.cs

@@ -39,7 +39,7 @@ namespace ICSharpCode.ILSpy.ViewModels
 				this.model.PropertyChanged += Model_PropertyChanged;
 			}
 
-			private void Model_PropertyChanged(object sender, PropertyChangedEventArgs e)
+			private void Model_PropertyChanged(object? sender, PropertyChangedEventArgs e)
 			{
 				if (e.PropertyName == nameof(model.IsCloseable))
 				{
@@ -47,7 +47,7 @@ namespace ICSharpCode.ILSpy.ViewModels
 				}
 			}
 
-			public event EventHandler CanExecuteChanged;
+			public event EventHandler? CanExecuteChanged;
 
 			public bool CanExecute(object? parameter)
 			{

ILSpy/ViewModels/ViewModelBase.cs

@@ -23,9 +23,9 @@ namespace ICSharpCode.ILSpy.ViewModels
 {
 	public abstract class ViewModelBase : INotifyPropertyChanged
 	{
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 
-		protected void RaisePropertyChanged([CallerMemberName] string propertyName = null)
+		protected void RaisePropertyChanged([CallerMemberName] string? propertyName = null)
 		{
 			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
 		}