Add annotations for extension methods classes

src/Compilers/Core/Portable/InternalUtilities/RoslynString.cs

@@ -11,5 +11,11 @@ internal static class RoslynString
         /// <inheritdoc cref="string.IsNullOrEmpty(string)"/>
         public static bool IsNullOrEmpty([NotNullWhen(returnValue: false)] string? value)
             => string.IsNullOrEmpty(value);
+
+#if !NET20
+        /// <inheritdoc cref="string.IsNullOrWhiteSpace(string)"/>
+        public static bool IsNullOrWhiteSpace([NotNullWhen(returnValue: false)] string? value)
+            => string.IsNullOrWhiteSpace(value);
+#endif
     }
 }

src/Features/CSharp/Portable/ExtractMethod/CSharpSelectionResult.ExpressionResult.cs

@@ -36,7 +36,7 @@ public override bool ContainingScopeHasAsyncKeyword()
                 return false;
             }
 
-            public override SyntaxNode GetContainingScope()
+            public override SyntaxNode? GetContainingScope()
             {
                 Contract.ThrowIfNull(this.SemanticDocument);
                 Contract.ThrowIfFalse(this.SelectionInExpression);

src/Features/CSharp/Portable/ExtractMethod/Extensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp.Extensions;
-using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.ExtractMethod;
 using Microsoft.CodeAnalysis.Shared.Extensions;
@@ -16,7 +18,7 @@ namespace Microsoft.CodeAnalysis.CSharp.ExtractMethod
 {
     internal static class Extensions
     {
-        public static ExpressionSyntax GetUnparenthesizedExpression(this SyntaxNode node)
+        public static ExpressionSyntax? GetUnparenthesizedExpression(this SyntaxNode? node)
         {
             if (!(node is ParenthesizedExpressionSyntax parenthesizedExpression))
             {
@@ -26,19 +28,17 @@ public static ExpressionSyntax GetUnparenthesizedExpression(this SyntaxNode node
             return GetUnparenthesizedExpression(parenthesizedExpression.Expression);
         }
 
-        public static StatementSyntax GetStatementUnderContainer(this SyntaxNode node)
+        public static StatementSyntax? GetStatementUnderContainer(this SyntaxNode node)
         {
             Contract.ThrowIfNull(node);
 
-            while (node != null)
+            for (SyntaxNode? current = node; current is object; current = current.Parent)
             {
-                if (node.Parent != null &&
-                    node.Parent.IsStatementContainerNode())
+                if (current.Parent != null &&
+                    current.Parent.IsStatementContainerNode())
                 {
-                    return node as StatementSyntax;
+                    return current as StatementSyntax;
                 }
-
-                node = node.Parent;
             }
 
             return null;
@@ -49,12 +49,12 @@ public static StatementSyntax GetParentLabeledStatementIfPossible(this SyntaxNod
             return (StatementSyntax)((node.Parent is LabeledStatementSyntax) ? node.Parent : node);
         }
 
-        public static bool IsStatementContainerNode(this SyntaxNode node)
+        public static bool IsStatementContainerNode([NotNullWhen(returnValue: true)] this SyntaxNode? node)
         {
             return node is BlockSyntax || node is SwitchSectionSyntax;
         }
 
-        public static BlockSyntax GetBlockBody(this SyntaxNode node)
+        public static BlockSyntax? GetBlockBody(this SyntaxNode? node)
         {
             switch (node)
             {
@@ -246,22 +246,22 @@ public static bool HasHybridTriviaBetween(this SyntaxToken token1, SyntaxToken t
             return false;
         }
 
-        public static bool IsArrayInitializer(this SyntaxNode node)
+        public static bool IsArrayInitializer([NotNullWhen(returnValue: true)] this SyntaxNode? node)
         {
             return node is InitializerExpressionSyntax && node.Parent is EqualsValueClauseSyntax;
         }
 
-        public static bool IsExpressionInCast(this SyntaxNode node)
+        public static bool IsExpressionInCast([NotNullWhen(returnValue: true)] this SyntaxNode? node)
         {
             return node is ExpressionSyntax && node.Parent is CastExpressionSyntax;
         }
 
-        public static bool IsExpression(this SyntaxNode node)
+        public static bool IsExpression([NotNullWhen(returnValue: true)] this SyntaxNode? node)
         {
             return node is ExpressionSyntax;
         }
 
-        public static bool IsObjectType(this ITypeSymbol type)
+        public static bool IsObjectType(this ITypeSymbol? type)
         {
             return type == null || type.SpecialType == SpecialType.System_Object;
         }

src/Features/CSharp/Portable/GenerateMember/GenerateParameterizedMember/CSharpGenerateParameterizedMemberService.cs

@@ -48,7 +48,7 @@ protected override ITypeSymbol DetermineReturnTypeWorker(CancellationToken cance
                 var typeInference = this.Document.Document.GetLanguageService<ITypeInferenceService>();
                 var inferredType = typeInference.InferType(
                     this.Document.SemanticModel, _invocationExpression, objectAsDefault: true,
-                    nameOpt: this.State.IdentifierToken.ValueText, cancellationToken: cancellationToken);
+                    name: this.State.IdentifierToken.ValueText, cancellationToken: cancellationToken);
                 return inferredType;
             }
 

src/Features/Core/Portable/ChangeSignature/AbstractChangeSignatureService.cs

@@ -330,6 +330,10 @@ internal ChangeSignatureOptionsResult GetChangeSignatureOptions(ChangeSignatureA
                 var doc = originalSolution.GetDocument(docId)!;
                 var updater = doc.Project.LanguageServices.GetRequiredService<AbstractChangeSignatureService>();
                 var root = doc.GetSyntaxRootSynchronously(CancellationToken.None);
+                if (root is null)
+                {
+                    throw new NotSupportedException(WorkspacesResources.Document_does_not_support_syntax_trees);
+                }
 
                 var nodes = nodesToUpdate[docId];
 

src/Features/Core/Portable/GenerateMember/GenerateVariable/AbstractGenerateVariableService.State.cs

@@ -378,7 +378,7 @@ private int GetStatementIndex(ChildSyntaxList children, SyntaxNode statement)
                 var typeInference = semanticDocument.Document.GetLanguageService<ITypeInferenceService>();
                 var inferredType = typeInference.InferType(
                     semanticDocument.SemanticModel, SimpleNameOrMemberAccessExpressionOpt, objectAsDefault: true,
-                    nameOpt: IdentifierToken.ValueText, cancellationToken: cancellationToken);
+                    name: IdentifierToken.ValueText, cancellationToken: cancellationToken);
 
                 var compilation = semanticDocument.SemanticModel.Compilation;
                 inferredType = inferredType.SpecialType == SpecialType.System_Void

src/Features/VisualBasic/Portable/GenerateMember/GenerateParameterizedMember/VisualBasicGenerateParameterizedMemberService.vb

@@ -55,7 +55,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.GenerateMember.GenerateMethod
                 Dim typeInference = Document.Project.LanguageServices.GetService(Of ITypeInferenceService)()
                 Dim inferredType = typeInference.InferType(
                     Document.SemanticModel, Me.InvocationExpression, objectAsDefault:=True,
-                    nameOpt:=Me.State.IdentifierToken.ValueText, cancellationToken:=cancellationToken)
+                    name:=Me.State.IdentifierToken.ValueText, cancellationToken:=cancellationToken)
                 Return inferredType
             End Function
 

src/Workspaces/Core/Portable/AddImports/AbstractAddImportsService.cs

@@ -165,7 +165,7 @@ private void GetContainers(SyntaxNode root, SyntaxNode contextLocation, out Synt
             aliasContainer = contextSpine.FirstOrDefault(HasAliases) ?? fallbackNode;
         }
 
-        private static SyntaxNode GetFirstApplicableContainer(SyntaxNode contextNode)
+        private static SyntaxNode? GetFirstApplicableContainer(SyntaxNode contextNode)
         {
             var usingDirective = contextNode.GetAncestor<TUsingOrAliasSyntax>();
             if (usingDirective != null)
@@ -174,7 +174,7 @@ private static SyntaxNode GetFirstApplicableContainer(SyntaxNode contextNode)
             }
 
             return contextNode.GetAncestor<TNamespaceDeclarationSyntax>() ??
-                   (SyntaxNode)contextNode.GetAncestorOrThis<TCompilationUnitSyntax>();
+                   (SyntaxNode?)contextNode.GetAncestorOrThis<TCompilationUnitSyntax>();
         }
     }
 }

src/Workspaces/Core/Portable/Editing/ImportAdderService.cs

@@ -212,8 +212,7 @@ CancellationToken cancellationToken
             SyntaxGenerator generator,
             CancellationToken cancellationToken)
         {
-            SyntaxNode? first = null;
-            SyntaxNode? last = null;
+            (SyntaxNode first, SyntaxNode last)? nodes = null;
             var importsToAdd = ArrayBuilder<SyntaxNode>.GetInstance();
 
             var annotatedNodes = syntaxNodes.Where(x => x.HasAnnotations(SymbolAnnotation.Kind));
@@ -246,8 +245,7 @@ CancellationToken cancellationToken
                             continue;
                         }
 
-                        first ??= annotatedNode;
-                        last = annotatedNode;
+                        nodes = (first: nodes?.first ?? annotatedNode, last: annotatedNode);
 
                         if (addedSymbols.Contains(namespaceSymbol))
                         {
@@ -276,7 +274,7 @@ CancellationToken cancellationToken
             // since whatever added the symbol annotation probably also added simplifier annotations,
             // and if not they probably didn't for a reason
 
-            return (importsToAdd.ToImmutableAndFree(), addedSymbols, first?.GetCommonRoot(last));
+            return (importsToAdd.ToImmutableAndFree(), addedSymbols, nodes is var (first, last) ? first.GetCommonRoot(last) : null);
         }
 
         /// <summary>

src/Workspaces/Core/Portable/ExtensionManager/IExtensionManagerExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
@@ -61,7 +63,7 @@ public static void PerformAction(this IExtensionManager extensionManager, object
         public static async Task PerformActionAsync(
             this IExtensionManager extensionManager,
             object extension,
-            Func<Task> function)
+            Func<Task?> function)
         {
             try
             {
@@ -84,7 +86,7 @@ public static void PerformAction(this IExtensionManager extensionManager, object
         public static async Task<T> PerformFunctionAsync<T>(
             this IExtensionManager extensionManager,
             object extension,
-            Func<Task<T>> function,
+            Func<Task<T>?> function,
             T defaultValue)
         {
             try

src/Workspaces/Core/Portable/Shared/Extensions/FileLinePositionSpanExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Shared.Extensions
 {
     internal static class FileLinePositionSpanExtensions
@@ -7,7 +9,7 @@ internal static class FileLinePositionSpanExtensions
         /// <summary>
         /// Get mapped file path if exist, otherwise return null.
         /// </summary>
-        public static string GetMappedFilePathIfExist(this FileLinePositionSpan fileLinePositionSpan)
+        public static string? GetMappedFilePathIfExist(this FileLinePositionSpan fileLinePositionSpan)
         {
             return fileLinePositionSpan.HasMappedPath ? fileLinePositionSpan.Path : null;
         }

src/Workspaces/Core/Portable/Shared/Extensions/IFindReferencesResultExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
@@ -87,15 +89,15 @@ internal static partial class IFindReferencesResultExtensions
 
         public static IEnumerable<ReferencedSymbol> FilterToAliasMatches(
             this IEnumerable<ReferencedSymbol> result,
-            IAliasSymbol aliasSymbolOpt)
+            IAliasSymbol? aliasSymbol)
         {
-            if (aliasSymbolOpt == null)
+            if (aliasSymbol == null)
             {
                 return result;
             }
 
             return from r in result
-                   let aliasLocations = r.Locations.Where(loc => SymbolEquivalenceComparer.Instance.Equals(loc.Alias, aliasSymbolOpt))
+                   let aliasLocations = r.Locations.Where(loc => SymbolEquivalenceComparer.Instance.Equals(loc.Alias, aliasSymbol))
                    where aliasLocations.Any()
                    select new ReferencedSymbol(r.DefinitionAndProjectId, aliasLocations);
         }
@@ -117,7 +119,7 @@ where aliasLocations.Any()
         {
             foreach (var reference in result)
             {
-                var isCaseSensitive = solution.Workspace.Services.GetLanguageServices(reference.Definition.Language).GetService<ISyntaxFactsService>().IsCaseSensitive;
+                var isCaseSensitive = solution.Workspace.Services.GetLanguageServices(reference.Definition.Language).GetRequiredService<ISyntaxFactsService>().IsCaseSensitive;
                 var comparer = isCaseSensitive ? StringComparer.Ordinal : StringComparer.OrdinalIgnoreCase;
                 if (reference.Definition.IsOrdinaryMethod() &&
                     !comparer.Equals(reference.Definition.Name, symbol.Name))

src/Workspaces/Core/Portable/Shared/Extensions/ILanguageMetadataExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Microsoft.CodeAnalysis.Host.Mef;
 using Roslyn.Utilities;
@@ -10,6 +13,7 @@ namespace Microsoft.CodeAnalysis.Shared.Extensions
 {
     internal static class ILanguageMetadataExtensions
     {
+        [return: MaybeNull]
         public static TInterface ToSpecificLanguage<TInterface, TMetadata>(this IEnumerable<Lazy<TInterface, TMetadata>> services, string languageName)
             where TMetadata : ILanguageMetadata
         {

src/Workspaces/Core/Portable/Shared/Extensions/ILanguageServiceProviderExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -13,7 +15,7 @@ internal static class ILanguageServiceProviderExtensions
     {
         public static IEnumerable<Lazy<T, TMetadata>> SelectMatchingExtensions<T, TMetadata>(
             this HostLanguageServices serviceProvider,
-            IEnumerable<Lazy<T, TMetadata>> items)
+            IEnumerable<Lazy<T, TMetadata>>? items)
             where TMetadata : ILanguageMetadata
         {
             if (items == null)

src/Workspaces/Core/Portable/Shared/Extensions/ISolutionExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Threading;
@@ -22,18 +25,20 @@ internal static partial class ISolutionExtensions
 
             foreach (var projectId in solution.ProjectIds)
             {
-                var project = solution.GetProject(projectId);
+                var project = solution.GetProject(projectId)!;
                 if (project.SupportsCompilation)
                 {
                     var compilation = await project.GetCompilationAsync(cancellationToken).ConfigureAwait(false);
+#nullable disable // Can 'compilation' be null here?
                     results.Add(compilation.Assembly.GlobalNamespace);
+#nullable enable
                 }
             }
 
             return results.ToImmutableAndFree();
         }
 
-        public static IEnumerable<DocumentId> GetChangedDocuments(this Solution newSolution, Solution oldSolution)
+        public static IEnumerable<DocumentId> GetChangedDocuments(this Solution? newSolution, Solution oldSolution)
         {
             if (newSolution != null)
             {
@@ -49,14 +54,14 @@ public static IEnumerable<DocumentId> GetChangedDocuments(this Solution newSolut
             }
         }
 
-        public static TextDocument GetTextDocument(this Solution solution, DocumentId documentId)
+        public static TextDocument? GetTextDocument(this Solution solution, DocumentId? documentId)
         {
             return solution.GetDocument(documentId) ?? solution.GetAdditionalDocument(documentId) ?? solution.GetAnalyzerConfigDocument(documentId);
         }
 
-        public static TextDocumentKind GetDocumentKind(this Solution solution, DocumentId documentId)
+        public static TextDocumentKind? GetDocumentKind(this Solution solution, DocumentId documentId)
         {
-            return solution.GetTextDocument(documentId).Kind;
+            return solution.GetTextDocument(documentId)?.Kind;
         }
 
         public static Solution WithTextDocumentText(this Solution solution, DocumentId documentId, SourceText text, PreservationMode mode = PreservationMode.PreserveIdentity)
@@ -73,6 +78,9 @@ public static Solution WithTextDocumentText(this Solution solution, DocumentId d
                 case TextDocumentKind.AdditionalDocument:
                     return solution.WithAdditionalDocumentText(documentId, text, mode);
 
+                case null:
+                    throw new InvalidOperationException(WorkspacesResources.The_solution_does_not_contain_the_specified_document);
+
                 default:
                     throw ExceptionUtilities.UnexpectedValue(documentKind);
             }

src/Workspaces/Core/Portable/Shared/Extensions/ISymbolExtensions.cs

@@ -511,7 +511,7 @@ public static bool IsUnsafe([NotNullWhen(returnValue: true)] this ISymbol? membe
         }
 
         public static ITypeSymbol ConvertToType(
-            this ISymbol symbol,
+            this ISymbol? symbol,
             Compilation compilation,
             bool extensionUsedAsInstance = false)
         {

src/Workspaces/Core/Portable/Shared/Extensions/ITypeInferenceServiceExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Linq;
 using System.Threading;
@@ -22,7 +24,7 @@ public static ImmutableArray<TypeInferenceInfo> GetTypeInferenceInfo(this ITypeI
         public static ImmutableArray<TypeInferenceInfo> GetTypeInferenceInfo(this ITypeInferenceService service, SemanticModel semanticModel, SyntaxNode expression, CancellationToken cancellationToken)
             => service.GetTypeInferenceInfo(semanticModel, expression, nameOpt: null, cancellationToken: cancellationToken);
 
-        public static INamedTypeSymbol InferDelegateType(
+        public static INamedTypeSymbol? InferDelegateType(
             this ITypeInferenceService typeInferenceService,
             SemanticModel semanticModel,
             SyntaxNode expression,
@@ -32,7 +34,7 @@ public static ImmutableArray<TypeInferenceInfo> GetTypeInferenceInfo(this ITypeI
             return GetFirstDelegateType(semanticModel, types);
         }
 
-        public static INamedTypeSymbol InferDelegateType(
+        public static INamedTypeSymbol? InferDelegateType(
            this ITypeInferenceService typeInferenceService,
            SemanticModel semanticModel,
            int position,
@@ -42,13 +44,13 @@ public static ImmutableArray<TypeInferenceInfo> GetTypeInferenceInfo(this ITypeI
             return GetFirstDelegateType(semanticModel, types);
         }
 
-        private static INamedTypeSymbol GetFirstDelegateType(SemanticModel semanticModel, ImmutableArray<ITypeSymbol> types)
+        private static INamedTypeSymbol? GetFirstDelegateType(SemanticModel semanticModel, ImmutableArray<ITypeSymbol> types)
         {
             var delegateTypes = types.Select(t => t.GetDelegateType(semanticModel.Compilation));
             return delegateTypes.WhereNotNull().FirstOrDefault();
         }
 
-        public static ITypeSymbol InferType(
+        public static ITypeSymbol? InferType(
             this ITypeInferenceService typeInferenceService,
             SemanticModel semanticModel,
             SyntaxNode expression,
@@ -57,28 +59,28 @@ private static INamedTypeSymbol GetFirstDelegateType(SemanticModel semanticModel
         {
             return InferType(
                 typeInferenceService, semanticModel, expression, objectAsDefault,
-                nameOpt: null, cancellationToken: cancellationToken);
+                name: null, cancellationToken: cancellationToken);
         }
 
-        public static ITypeSymbol InferType(
+        public static ITypeSymbol? InferType(
             this ITypeInferenceService typeInferenceService,
             SemanticModel semanticModel,
             SyntaxNode expression,
             bool objectAsDefault,
-            string nameOpt,
+            string? name,
             CancellationToken cancellationToken)
         {
-            var types = typeInferenceService.InferTypes(semanticModel, expression, nameOpt, cancellationToken);
+            var types = typeInferenceService.InferTypes(semanticModel, expression, name, cancellationToken);
 
             if (types.Length == 0)
             {
                 return objectAsDefault ? semanticModel.Compilation.ObjectType : null;
             }
 
-            return types.FirstOrDefault();
+            return types.First();
         }
 
-        public static ITypeSymbol InferType(
+        public static ITypeSymbol? InferType(
             this ITypeInferenceService typeInferenceService,
             SemanticModel semanticModel,
             int position,
@@ -87,18 +89,18 @@ private static INamedTypeSymbol GetFirstDelegateType(SemanticModel semanticModel
         {
             return InferType(
                 typeInferenceService, semanticModel, position, objectAsDefault,
-                nameOpt: null, cancellationToken: cancellationToken);
+                name: null, cancellationToken: cancellationToken);
         }
 
-        public static ITypeSymbol InferType(
+        public static ITypeSymbol? InferType(
             this ITypeInferenceService typeInferenceService,
             SemanticModel semanticModel,
             int position,
             bool objectAsDefault,
-            string nameOpt,
+            string? name,
             CancellationToken cancellationToken)
         {
-            var types = typeInferenceService.InferTypes(semanticModel, position, nameOpt, cancellationToken);
+            var types = typeInferenceService.InferTypes(semanticModel, position, name, cancellationToken);
 
             if (types.Length == 0)
             {

src/Workspaces/Core/Portable/Shared/Extensions/ObjectExtensions.TypeSwitch.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Microsoft.CodeAnalysis.Shared.Extensions
 {
     internal static partial class ObjectExtensions
     {
-        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TBaseType, TResult> defaultFunc = null)
+        [return: MaybeNull]
+        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TBaseType, TResult>? defaultFunc = null)
             where TDerivedType1 : TBaseType
             where TDerivedType2 : TBaseType
             where TDerivedType3 : TBaseType
@@ -29,11 +33,12 @@ internal static partial class ObjectExtensions
             }
             else
             {
-                return default;
+                return default!;
             }
         }
 
-        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TBaseType, TResult> defaultFunc = null)
+        [return: MaybeNull]
+        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TBaseType, TResult>? defaultFunc = null)
             where TDerivedType1 : TBaseType
             where TDerivedType2 : TBaseType
             where TDerivedType3 : TBaseType
@@ -61,11 +66,12 @@ internal static partial class ObjectExtensions
             }
             else
             {
-                return default;
+                return default!;
             }
         }
 
-        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TBaseType, TResult> defaultFunc = null)
+        [return: MaybeNull]
+        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TBaseType, TResult>? defaultFunc = null)
             where TDerivedType1 : TBaseType
             where TDerivedType2 : TBaseType
             where TDerivedType3 : TBaseType
@@ -98,11 +104,12 @@ internal static partial class ObjectExtensions
             }
             else
             {
-                return default;
+                return default!;
             }
         }
 
-        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TBaseType, TResult> defaultFunc = null)
+        [return: MaybeNull]
+        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TBaseType, TResult>? defaultFunc = null)
             where TDerivedType1 : TBaseType
             where TDerivedType2 : TBaseType
             where TDerivedType3 : TBaseType
@@ -140,11 +147,12 @@ internal static partial class ObjectExtensions
             }
             else
             {
-                return default;
+                return default!;
             }
         }
 
-        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TDerivedType7, TDerivedType8, TDerivedType9, TDerivedType10, TDerivedType11, TDerivedType12, TDerivedType13, TDerivedType14, TDerivedType15, TDerivedType16, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TDerivedType7, TResult> matchFunc7, Func<TDerivedType8, TResult> matchFunc8, Func<TDerivedType9, TResult> matchFunc9, Func<TDerivedType10, TResult> matchFunc10, Func<TDerivedType11, TResult> matchFunc11, Func<TDerivedType12, TResult> matchFunc12, Func<TDerivedType13, TResult> matchFunc13, Func<TDerivedType14, TResult> matchFunc14, Func<TDerivedType15, TResult> matchFunc15, Func<TDerivedType16, TResult> matchFunc16, Func<TBaseType, TResult> defaultFunc = null)
+        [return: MaybeNull]
+        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TDerivedType7, TDerivedType8, TDerivedType9, TDerivedType10, TDerivedType11, TDerivedType12, TDerivedType13, TDerivedType14, TDerivedType15, TDerivedType16, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TDerivedType7, TResult> matchFunc7, Func<TDerivedType8, TResult> matchFunc8, Func<TDerivedType9, TResult> matchFunc9, Func<TDerivedType10, TResult> matchFunc10, Func<TDerivedType11, TResult> matchFunc11, Func<TDerivedType12, TResult> matchFunc12, Func<TDerivedType13, TResult> matchFunc13, Func<TDerivedType14, TResult> matchFunc14, Func<TDerivedType15, TResult> matchFunc15, Func<TDerivedType16, TResult> matchFunc16, Func<TBaseType, TResult>? defaultFunc = null)
             where TDerivedType1 : TBaseType
             where TDerivedType2 : TBaseType
             where TDerivedType3 : TBaseType
@@ -232,11 +240,12 @@ internal static partial class ObjectExtensions
             }
             else
             {
-                return default;
+                return default!;
             }
         }
 
-        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TDerivedType7, TDerivedType8, TDerivedType9, TDerivedType10, TDerivedType11, TDerivedType12, TDerivedType13, TDerivedType14, TDerivedType15, TDerivedType16, TDerivedType17, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TDerivedType7, TResult> matchFunc7, Func<TDerivedType8, TResult> matchFunc8, Func<TDerivedType9, TResult> matchFunc9, Func<TDerivedType10, TResult> matchFunc10, Func<TDerivedType11, TResult> matchFunc11, Func<TDerivedType12, TResult> matchFunc12, Func<TDerivedType13, TResult> matchFunc13, Func<TDerivedType14, TResult> matchFunc14, Func<TDerivedType15, TResult> matchFunc15, Func<TDerivedType16, TResult> matchFunc16, Func<TDerivedType17, TResult> matchFunc17, Func<TBaseType, TResult> defaultFunc = null)
+        [return: MaybeNull]
+        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TDerivedType7, TDerivedType8, TDerivedType9, TDerivedType10, TDerivedType11, TDerivedType12, TDerivedType13, TDerivedType14, TDerivedType15, TDerivedType16, TDerivedType17, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TDerivedType7, TResult> matchFunc7, Func<TDerivedType8, TResult> matchFunc8, Func<TDerivedType9, TResult> matchFunc9, Func<TDerivedType10, TResult> matchFunc10, Func<TDerivedType11, TResult> matchFunc11, Func<TDerivedType12, TResult> matchFunc12, Func<TDerivedType13, TResult> matchFunc13, Func<TDerivedType14, TResult> matchFunc14, Func<TDerivedType15, TResult> matchFunc15, Func<TDerivedType16, TResult> matchFunc16, Func<TDerivedType17, TResult> matchFunc17, Func<TBaseType, TResult>? defaultFunc = null)
             where TDerivedType1 : TBaseType
             where TDerivedType2 : TBaseType
             where TDerivedType3 : TBaseType
@@ -329,11 +338,12 @@ internal static partial class ObjectExtensions
             }
             else
             {
-                return default;
+                return default!;
             }
         }
 
-        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TDerivedType7, TDerivedType8, TDerivedType9, TDerivedType10, TDerivedType11, TDerivedType12, TDerivedType13, TDerivedType14, TDerivedType15, TDerivedType16, TDerivedType17, TDerivedType18, TDerivedType19, TDerivedType20, TDerivedType21, TDerivedType22, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TDerivedType7, TResult> matchFunc7, Func<TDerivedType8, TResult> matchFunc8, Func<TDerivedType9, TResult> matchFunc9, Func<TDerivedType10, TResult> matchFunc10, Func<TDerivedType11, TResult> matchFunc11, Func<TDerivedType12, TResult> matchFunc12, Func<TDerivedType13, TResult> matchFunc13, Func<TDerivedType14, TResult> matchFunc14, Func<TDerivedType15, TResult> matchFunc15, Func<TDerivedType16, TResult> matchFunc16, Func<TDerivedType17, TResult> matchFunc17, Func<TDerivedType18, TResult> matchFunc18, Func<TDerivedType19, TResult> matchFunc19, Func<TDerivedType20, TResult> matchFunc20, Func<TDerivedType21, TResult> matchFunc21, Func<TDerivedType22, TResult> matchFunc22, Func<TBaseType, TResult> defaultFunc = null)
+        [return: MaybeNull]
+        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TDerivedType7, TDerivedType8, TDerivedType9, TDerivedType10, TDerivedType11, TDerivedType12, TDerivedType13, TDerivedType14, TDerivedType15, TDerivedType16, TDerivedType17, TDerivedType18, TDerivedType19, TDerivedType20, TDerivedType21, TDerivedType22, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TDerivedType7, TResult> matchFunc7, Func<TDerivedType8, TResult> matchFunc8, Func<TDerivedType9, TResult> matchFunc9, Func<TDerivedType10, TResult> matchFunc10, Func<TDerivedType11, TResult> matchFunc11, Func<TDerivedType12, TResult> matchFunc12, Func<TDerivedType13, TResult> matchFunc13, Func<TDerivedType14, TResult> matchFunc14, Func<TDerivedType15, TResult> matchFunc15, Func<TDerivedType16, TResult> matchFunc16, Func<TDerivedType17, TResult> matchFunc17, Func<TDerivedType18, TResult> matchFunc18, Func<TDerivedType19, TResult> matchFunc19, Func<TDerivedType20, TResult> matchFunc20, Func<TDerivedType21, TResult> matchFunc21, Func<TDerivedType22, TResult> matchFunc22, Func<TBaseType, TResult>? defaultFunc = null)
             where TDerivedType1 : TBaseType
             where TDerivedType2 : TBaseType
             where TDerivedType3 : TBaseType
@@ -451,11 +461,12 @@ internal static partial class ObjectExtensions
             }
             else
             {
-                return default;
+                return default!;
             }
         }
 
-        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TDerivedType7, TDerivedType8, TDerivedType9, TDerivedType10, TDerivedType11, TDerivedType12, TDerivedType13, TDerivedType14, TDerivedType15, TDerivedType16, TDerivedType17, TDerivedType18, TDerivedType19, TDerivedType20, TDerivedType21, TDerivedType22, TDerivedType23, TDerivedType24, TDerivedType25, TDerivedType26, TDerivedType27, TDerivedType28, TDerivedType29, TDerivedType30, TDerivedType31, TDerivedType32, TDerivedType33, TDerivedType34, TDerivedType35, TDerivedType36, TDerivedType37, TDerivedType38, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TDerivedType7, TResult> matchFunc7, Func<TDerivedType8, TResult> matchFunc8, Func<TDerivedType9, TResult> matchFunc9, Func<TDerivedType10, TResult> matchFunc10, Func<TDerivedType11, TResult> matchFunc11, Func<TDerivedType12, TResult> matchFunc12, Func<TDerivedType13, TResult> matchFunc13, Func<TDerivedType14, TResult> matchFunc14, Func<TDerivedType15, TResult> matchFunc15, Func<TDerivedType16, TResult> matchFunc16, Func<TDerivedType17, TResult> matchFunc17, Func<TDerivedType18, TResult> matchFunc18, Func<TDerivedType19, TResult> matchFunc19, Func<TDerivedType20, TResult> matchFunc20, Func<TDerivedType21, TResult> matchFunc21, Func<TDerivedType22, TResult> matchFunc22, Func<TDerivedType23, TResult> matchFunc23, Func<TDerivedType24, TResult> matchFunc24, Func<TDerivedType25, TResult> matchFunc25, Func<TDerivedType26, TResult> matchFunc26, Func<TDerivedType27, TResult> matchFunc27, Func<TDerivedType28, TResult> matchFunc28, Func<TDerivedType29, TResult> matchFunc29, Func<TDerivedType30, TResult> matchFunc30, Func<TDerivedType31, TResult> matchFunc31, Func<TDerivedType32, TResult> matchFunc32, Func<TDerivedType33, TResult> matchFunc33, Func<TDerivedType34, TResult> matchFunc34, Func<TDerivedType35, TResult> matchFunc35, Func<TDerivedType36, TResult> matchFunc36, Func<TDerivedType37, TResult> matchFunc37, Func<TDerivedType38, TResult> matchFunc38, Func<TBaseType, TResult> defaultFunc = null)
+        [return: MaybeNull]
+        public static TResult TypeSwitch<TBaseType, TDerivedType1, TDerivedType2, TDerivedType3, TDerivedType4, TDerivedType5, TDerivedType6, TDerivedType7, TDerivedType8, TDerivedType9, TDerivedType10, TDerivedType11, TDerivedType12, TDerivedType13, TDerivedType14, TDerivedType15, TDerivedType16, TDerivedType17, TDerivedType18, TDerivedType19, TDerivedType20, TDerivedType21, TDerivedType22, TDerivedType23, TDerivedType24, TDerivedType25, TDerivedType26, TDerivedType27, TDerivedType28, TDerivedType29, TDerivedType30, TDerivedType31, TDerivedType32, TDerivedType33, TDerivedType34, TDerivedType35, TDerivedType36, TDerivedType37, TDerivedType38, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TDerivedType2, TResult> matchFunc2, Func<TDerivedType3, TResult> matchFunc3, Func<TDerivedType4, TResult> matchFunc4, Func<TDerivedType5, TResult> matchFunc5, Func<TDerivedType6, TResult> matchFunc6, Func<TDerivedType7, TResult> matchFunc7, Func<TDerivedType8, TResult> matchFunc8, Func<TDerivedType9, TResult> matchFunc9, Func<TDerivedType10, TResult> matchFunc10, Func<TDerivedType11, TResult> matchFunc11, Func<TDerivedType12, TResult> matchFunc12, Func<TDerivedType13, TResult> matchFunc13, Func<TDerivedType14, TResult> matchFunc14, Func<TDerivedType15, TResult> matchFunc15, Func<TDerivedType16, TResult> matchFunc16, Func<TDerivedType17, TResult> matchFunc17, Func<TDerivedType18, TResult> matchFunc18, Func<TDerivedType19, TResult> matchFunc19, Func<TDerivedType20, TResult> matchFunc20, Func<TDerivedType21, TResult> matchFunc21, Func<TDerivedType22, TResult> matchFunc22, Func<TDerivedType23, TResult> matchFunc23, Func<TDerivedType24, TResult> matchFunc24, Func<TDerivedType25, TResult> matchFunc25, Func<TDerivedType26, TResult> matchFunc26, Func<TDerivedType27, TResult> matchFunc27, Func<TDerivedType28, TResult> matchFunc28, Func<TDerivedType29, TResult> matchFunc29, Func<TDerivedType30, TResult> matchFunc30, Func<TDerivedType31, TResult> matchFunc31, Func<TDerivedType32, TResult> matchFunc32, Func<TDerivedType33, TResult> matchFunc33, Func<TDerivedType34, TResult> matchFunc34, Func<TDerivedType35, TResult> matchFunc35, Func<TDerivedType36, TResult> matchFunc36, Func<TDerivedType37, TResult> matchFunc37, Func<TDerivedType38, TResult> matchFunc38, Func<TBaseType, TResult>? defaultFunc = null)
             where TDerivedType1 : TBaseType
             where TDerivedType2 : TBaseType
             where TDerivedType3 : TBaseType
@@ -653,7 +664,7 @@ internal static partial class ObjectExtensions
             }
             else
             {
-                return default;
+                return default!;
             }
         }
     }

src/Workspaces/Core/Portable/Shared/Extensions/ObjectExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
 
 namespace Microsoft.CodeAnalysis.Shared.Extensions
 {
     internal static partial class ObjectExtensions
     {
-        public static string GetTypeDisplayName(this object obj)
+        public static string GetTypeDisplayName(this object? obj)
         {
             return obj == null ? "null" : obj.GetType().Name;
         }

src/Workspaces/Core/Portable/Shared/Extensions/ParenthesizedExpressionSyntaxExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
+using System.Diagnostics.CodeAnalysis;
+
 namespace Microsoft.CodeAnalysis.Extensions
 {
     internal static class CommonParenthesizedExpressionSyntaxExtensions
@@ -81,14 +85,14 @@ private static bool AnySymbolIsUserDefinedOperator(SymbolInfo symbolInfo)
             return false;
         }
 
-        private static bool IsUserDefinedOperator(ISymbol symbol)
+        private static bool IsUserDefinedOperator([NotNullWhen(returnValue: true)] ISymbol? symbol)
             => symbol is IMethodSymbol methodSymbol &&
                methodSymbol.MethodKind == MethodKind.UserDefinedOperator;
 
         private static bool IsFloatingPoint(TypeInfo typeInfo)
             => IsFloatingPoint(typeInfo.Type) || IsFloatingPoint(typeInfo.ConvertedType);
 
-        private static bool IsFloatingPoint(ITypeSymbol type)
+        private static bool IsFloatingPoint([NotNullWhen(returnValue: true)] ITypeSymbol? type)
             => type?.SpecialType == SpecialType.System_Single || type?.SpecialType == SpecialType.System_Double;
     }
 }

src/Workspaces/Core/Portable/Shared/Extensions/SemanticModelExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using Microsoft.CodeAnalysis.LanguageServices;
@@ -32,8 +35,8 @@ public static SymbolInfo GetSymbolInfo(this SemanticModel semanticModel, SyntaxT
             return semanticModel.GetSymbolInfo(token.Parent, cancellationToken);
         }
 
-        public static TSymbol GetEnclosingSymbol<TSymbol>(this SemanticModel semanticModel, int position, CancellationToken cancellationToken)
-            where TSymbol : ISymbol
+        public static TSymbol? GetEnclosingSymbol<TSymbol>(this SemanticModel semanticModel, int position, CancellationToken cancellationToken)
+            where TSymbol : class, ISymbol
         {
             for (var symbol = semanticModel.GetEnclosingSymbol(position, cancellationToken);
                  symbol != null;
@@ -54,12 +57,12 @@ public static ISymbol GetEnclosingNamedTypeOrAssembly(this SemanticModel semanti
                 (ISymbol)semanticModel.Compilation.Assembly;
         }
 
-        public static INamedTypeSymbol GetEnclosingNamedType(this SemanticModel semanticModel, int position, CancellationToken cancellationToken)
+        public static INamedTypeSymbol? GetEnclosingNamedType(this SemanticModel semanticModel, int position, CancellationToken cancellationToken)
         {
             return semanticModel.GetEnclosingSymbol<INamedTypeSymbol>(position, cancellationToken);
         }
 
-        public static INamespaceSymbol GetEnclosingNamespace(this SemanticModel semanticModel, int position, CancellationToken cancellationToken)
+        public static INamespaceSymbol? GetEnclosingNamespace(this SemanticModel semanticModel, int position, CancellationToken cancellationToken)
         {
             return semanticModel.GetEnclosingSymbol<INamespaceSymbol>(position, cancellationToken);
         }
@@ -84,7 +87,7 @@ public static INamespaceSymbol GetEnclosingNamespace(this SemanticModel semantic
             return symbolInfo.GetAnySymbol().ConvertToType(semanticModel.Compilation);
         }
 
-        private static ISymbol MapSymbol(ISymbol symbol, ITypeSymbol type)
+        private static ISymbol? MapSymbol(ISymbol symbol, ITypeSymbol? type)
         {
             if (symbol.IsConstructor() && symbol.ContainingType.IsAnonymousType)
             {
@@ -139,19 +142,19 @@ private static ISymbol MapSymbol(ISymbol symbol, ITypeSymbol type)
             CancellationToken cancellationToken)
         {
             var languageServices = workspace.Services.GetLanguageServices(token.Language);
-            var syntaxFacts = languageServices.GetService<ISyntaxFactsService>();
+            var syntaxFacts = languageServices.GetRequiredService<ISyntaxFactsService>();
             if (!syntaxFacts.IsBindableToken(token))
             {
                 return TokenSemanticInfo.Empty;
             }
 
-            var semanticFacts = languageServices.GetService<ISemanticFactsService>();
+            var semanticFacts = languageServices.GetRequiredService<ISemanticFactsService>();
 
-            IAliasSymbol aliasSymbol;
-            ITypeSymbol type;
-            ITypeSymbol convertedType;
-            ISymbol declaredSymbol;
-            ImmutableArray<ISymbol> allSymbols;
+            IAliasSymbol? aliasSymbol;
+            ITypeSymbol? type;
+            ITypeSymbol? convertedType;
+            ISymbol? declaredSymbol;
+            ImmutableArray<ISymbol?> allSymbols;
 
             var overriddingIdentifier = syntaxFacts.GetDeclarationIdentifierIfOverride(token);
             if (overriddingIdentifier.HasValue)
@@ -165,7 +168,7 @@ private static ISymbol MapSymbol(ISymbol symbol, ITypeSymbol type)
                 type = null;
                 convertedType = null;
                 declaredSymbol = null;
-                allSymbols = overriddenSymbol is null ? ImmutableArray<ISymbol>.Empty : ImmutableArray.Create(overriddenSymbol);
+                allSymbols = overriddenSymbol is null ? ImmutableArray<ISymbol?>.Empty : ImmutableArray.Create<ISymbol?>(overriddenSymbol);
             }
             else
             {
@@ -178,7 +181,7 @@ private static ISymbol MapSymbol(ISymbol symbol, ITypeSymbol type)
 
                 var skipSymbolInfoLookup = declaredSymbol.IsKind(SymbolKind.RangeVariable);
                 allSymbols = skipSymbolInfoLookup
-                    ? ImmutableArray<ISymbol>.Empty
+                    ? ImmutableArray<ISymbol?>.Empty
                     : semanticFacts
                         .GetBestOrAllSymbols(semanticModel, bindableParent, token, cancellationToken)
                         .WhereAsArray(s => !s.Equals(declaredSymbol))
@@ -203,7 +206,7 @@ private static ISymbol MapSymbol(ISymbol symbol, ITypeSymbol type)
                     if (namedType.TypeKind == TypeKind.Delegate ||
                         namedType.AssociatedSymbol != null)
                     {
-                        allSymbols = ImmutableArray.Create<ISymbol>(type);
+                        allSymbols = ImmutableArray.Create<ISymbol?>(type);
                         type = null;
                     }
                 }
@@ -232,7 +235,7 @@ public static SemanticModel GetOriginalSemanticModel(this SemanticModel semantic
         }
 
         public static HashSet<ISymbol> GetAllDeclaredSymbols(
-            this SemanticModel semanticModel, SyntaxNode container, CancellationToken cancellationToken, Func<SyntaxNode, bool> filter = null)
+            this SemanticModel semanticModel, SyntaxNode? container, CancellationToken cancellationToken, Func<SyntaxNode, bool>? filter = null)
         {
             var symbols = new HashSet<ISymbol>();
             if (container != null)
@@ -244,7 +247,7 @@ public static SemanticModel GetOriginalSemanticModel(this SemanticModel semantic
         }
 
         public static IEnumerable<ISymbol> GetExistingSymbols(
-            this SemanticModel semanticModel, SyntaxNode container, CancellationToken cancellationToken, Func<SyntaxNode, bool> descendInto = null)
+            this SemanticModel semanticModel, SyntaxNode? container, CancellationToken cancellationToken, Func<SyntaxNode, bool>? descendInto = null)
         {
             // Ignore an anonymous type property or tuple field.  It's ok if they have a name that
             // matches the name of the local we're introducing.
@@ -254,7 +257,7 @@ public static SemanticModel GetOriginalSemanticModel(this SemanticModel semantic
 
         private static void GetAllDeclaredSymbols(
             SemanticModel semanticModel, SyntaxNode node,
-            HashSet<ISymbol> symbols, CancellationToken cancellationToken, Func<SyntaxNode, bool> descendInto = null)
+            HashSet<ISymbol> symbols, CancellationToken cancellationToken, Func<SyntaxNode, bool>? descendInto = null)
         {
             var symbol = semanticModel.GetDeclaredSymbol(node, cancellationToken);
 
@@ -275,7 +278,7 @@ public static SemanticModel GetOriginalSemanticModel(this SemanticModel semantic
                 }
             }
 
-            static bool ShouldDescendInto(SyntaxNode node, Func<SyntaxNode, bool> filter)
+            static bool ShouldDescendInto(SyntaxNode node, Func<SyntaxNode, bool>? filter)
                 => filter != null ? filter(node) : true;
         }
     }

src/Workspaces/Core/Portable/Shared/Extensions/StringExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Immutable;
 using System.Diagnostics;
@@ -69,7 +71,7 @@ public static int ConvertTabToSpace(this string textSnippet, int tabSize, int in
             return column - initialColumn;
         }
 
-        public static int IndexOf(this string text, Func<char, bool> predicate)
+        public static int IndexOf(this string? text, Func<char, bool> predicate)
         {
             if (text == null)
             {
@@ -215,9 +217,9 @@ public static int GetLineOffsetFromColumn(this string line, int column, int tabS
             return line.Length;
         }
 
-        public static void AppendToAliasNameSet(this string alias, ImmutableHashSet<string>.Builder builder)
+        public static void AppendToAliasNameSet(this string? alias, ImmutableHashSet<string>.Builder builder)
         {
-            if (string.IsNullOrWhiteSpace(alias))
+            if (RoslynString.IsNullOrWhiteSpace(alias))
             {
                 return;
             }

src/Workspaces/Core/Portable/Shared/Extensions/SyntaxNodeExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -19,7 +22,7 @@ public static IEnumerable<SyntaxNodeOrToken> DepthFirstTraversal(this SyntaxNode
 
         public static IEnumerable<SyntaxNode> GetAncestors(this SyntaxNode node)
         {
-            var current = node.Parent;
+            SyntaxNode? current = node.Parent;
 
             while (current != null)
             {
@@ -32,7 +35,7 @@ public static IEnumerable<SyntaxNode> GetAncestors(this SyntaxNode node)
         public static IEnumerable<TNode> GetAncestors<TNode>(this SyntaxNode node)
             where TNode : SyntaxNode
         {
-            var current = node.Parent;
+            SyntaxNode? current = node.Parent;
             while (current != null)
             {
                 if (current is TNode tNode)
@@ -44,10 +47,10 @@ public static IEnumerable<TNode> GetAncestors<TNode>(this SyntaxNode node)
             }
         }
 
-        public static TNode GetAncestor<TNode>(this SyntaxNode node)
+        public static TNode? GetAncestor<TNode>(this SyntaxNode node)
             where TNode : SyntaxNode
         {
-            var current = node.Parent;
+            SyntaxNode? current = node.Parent;
             while (current != null)
             {
                 if (current is TNode tNode)
@@ -61,13 +64,13 @@ public static TNode GetAncestor<TNode>(this SyntaxNode node)
             return null;
         }
 
-        public static TNode GetAncestorOrThis<TNode>(this SyntaxNode node)
+        public static TNode? GetAncestorOrThis<TNode>(this SyntaxNode? node)
             where TNode : SyntaxNode
         {
             return node?.GetAncestorsOrThis<TNode>().FirstOrDefault();
         }
 
-        public static IEnumerable<TNode> GetAncestorsOrThis<TNode>(this SyntaxNode node)
+        public static IEnumerable<TNode> GetAncestorsOrThis<TNode>(this SyntaxNode? node)
             where TNode : SyntaxNode
         {
             var current = node;
@@ -114,7 +117,7 @@ public static bool HasAncestor<TNode>(this SyntaxNode node)
             }
         }
 
-        public static bool CheckParent<T>(this SyntaxNode node, Func<T, bool> valueChecker) where T : SyntaxNode
+        public static bool CheckParent<T>([NotNullWhen(returnValue: true)] this SyntaxNode? node, Func<T, bool> valueChecker) where T : SyntaxNode
         {
             if (!(node?.Parent is T parentNode))
             {
@@ -184,11 +187,11 @@ public static int FullWidth(this SyntaxNode node)
             return node.FullSpan.Length;
         }
 
-        public static SyntaxNode FindInnermostCommonNode(
+        public static SyntaxNode? FindInnermostCommonNode(
             this IEnumerable<SyntaxNode> nodes,
             Func<SyntaxNode, bool> predicate)
         {
-            IEnumerable<SyntaxNode> blocks = null;
+            IEnumerable<SyntaxNode>? blocks = null;
             foreach (var node in nodes)
             {
                 blocks = blocks == null
@@ -199,10 +202,10 @@ public static int FullWidth(this SyntaxNode node)
             return blocks?.First();
         }
 
-        public static TSyntaxNode FindInnermostCommonNode<TSyntaxNode>(this IEnumerable<SyntaxNode> nodes)
+        public static TSyntaxNode? FindInnermostCommonNode<TSyntaxNode>(this IEnumerable<SyntaxNode> nodes)
             where TSyntaxNode : SyntaxNode
         {
-            return (TSyntaxNode)nodes.FindInnermostCommonNode(n => n is TSyntaxNode);
+            return (TSyntaxNode?)nodes.FindInnermostCommonNode(n => n is TSyntaxNode);
         }
 
         /// <summary>
@@ -250,9 +253,9 @@ public static TextSpan GetContainedSpan(this IEnumerable<SyntaxNode> nodes)
         }
 
         public static IEnumerable<TextSpan> GetContiguousSpans(
-            this IEnumerable<SyntaxNode> nodes, Func<SyntaxNode, SyntaxToken> getLastToken = null)
+            this IEnumerable<SyntaxNode> nodes, Func<SyntaxNode, SyntaxToken>? getLastToken = null)
         {
-            SyntaxNode lastNode = null;
+            SyntaxNode? lastNode = null;
             TextSpan? textSpan = null;
 
             // Sort the nodes in source location order.
@@ -268,12 +271,12 @@ public static TextSpan GetContainedSpan(this IEnumerable<SyntaxNode> nodes)
                     if (lastToken.GetNextToken(includeDirectives: true) == node.GetFirstToken())
                     {
                         // Expand the span
-                        textSpan = TextSpan.FromBounds(textSpan.Value.Start, node.Span.End);
+                        textSpan = TextSpan.FromBounds(textSpan!.Value.Start, node.Span.End);
                     }
                     else
                     {
                         // Return the last span, and start a new one
-                        yield return textSpan.Value;
+                        yield return textSpan!.Value;
                         textSpan = node.Span;
                     }
                 }
@@ -370,12 +373,12 @@ public static bool OverlapsHiddenPosition(this SyntaxNode declaration, SyntaxNod
 
         public static async Task<TRoot> ReplaceSyntaxAsync<TRoot>(
             this TRoot root,
-            IEnumerable<SyntaxNode> nodes,
-            Func<SyntaxNode, SyntaxNode, CancellationToken, Task<SyntaxNode>> computeReplacementNodeAsync,
-            IEnumerable<SyntaxToken> tokens,
-            Func<SyntaxToken, SyntaxToken, CancellationToken, Task<SyntaxToken>> computeReplacementTokenAsync,
-            IEnumerable<SyntaxTrivia> trivia,
-            Func<SyntaxTrivia, SyntaxTrivia, CancellationToken, Task<SyntaxTrivia>> computeReplacementTriviaAsync,
+            IEnumerable<SyntaxNode>? nodes,
+            Func<SyntaxNode, SyntaxNode, CancellationToken, Task<SyntaxNode>>? computeReplacementNodeAsync,
+            IEnumerable<SyntaxToken>? tokens,
+            Func<SyntaxToken, SyntaxToken, CancellationToken, Task<SyntaxToken>>? computeReplacementTokenAsync,
+            IEnumerable<SyntaxTrivia>? trivia,
+            Func<SyntaxTrivia, SyntaxTrivia, CancellationToken, Task<SyntaxTrivia>>? computeReplacementTriviaAsync,
             CancellationToken cancellationToken)
             where TRoot : SyntaxNode
         {
@@ -423,7 +426,7 @@ public static bool OverlapsHiddenPosition(this SyntaxNode declaration, SyntaxNod
                         if (nodesToReplace.TryGetValue(span, out var currentNode))
                         {
                             var original = (SyntaxNode)retryAnnotations.GetAnnotations(currentNode).SingleOrDefault() ?? currentNode;
-                            var newNode = await computeReplacementNodeAsync(original, currentNode, cancellationToken).ConfigureAwait(false);
+                            var newNode = await computeReplacementNodeAsync!(original, currentNode, cancellationToken).ConfigureAwait(false);
                             nodeReplacements[currentNode] = newNode;
                         }
                         else if (tokensToReplace.TryGetValue(span, out var currentToken))
@@ -434,7 +437,7 @@ public static bool OverlapsHiddenPosition(this SyntaxNode declaration, SyntaxNod
                                 original = currentToken;
                             }
 
-                            var newToken = await computeReplacementTokenAsync(original, currentToken, cancellationToken).ConfigureAwait(false);
+                            var newToken = await computeReplacementTokenAsync!(original, currentToken, cancellationToken).ConfigureAwait(false);
                             tokenReplacements[currentToken] = newToken;
                         }
                         else if (triviaToReplace.TryGetValue(span, out var currentTrivia))
@@ -445,7 +448,7 @@ public static bool OverlapsHiddenPosition(this SyntaxNode declaration, SyntaxNod
                                 original = currentTrivia;
                             }
 
-                            var newTrivia = await computeReplacementTriviaAsync(original, currentTrivia, cancellationToken).ConfigureAwait(false);
+                            var newTrivia = await computeReplacementTriviaAsync!(original, currentTrivia, cancellationToken).ConfigureAwait(false);
                             triviaReplacements[currentTrivia] = newTrivia;
                         }
                     }
@@ -756,7 +759,7 @@ private static SyntaxToken FindSkippedTokenBackward(SyntaxTriviaList triviaList,
         }
 
         // Copy of the same function in SyntaxNode.cs
-        public static SyntaxNode GetParent(this SyntaxNode node, bool ascendOutOfTrivia)
+        public static SyntaxNode? GetParent(this SyntaxNode node, bool ascendOutOfTrivia)
         {
             var parent = node.Parent;
             if (parent == null && ascendOutOfTrivia)
@@ -770,7 +773,7 @@ public static SyntaxNode GetParent(this SyntaxNode node, bool ascendOutOfTrivia)
             return parent;
         }
 
-        public static TNode FirstAncestorOrSelfUntil<TNode>(this SyntaxNode node, Func<SyntaxNode, bool> predicate)
+        public static TNode? FirstAncestorOrSelfUntil<TNode>(this SyntaxNode? node, Func<SyntaxNode, bool> predicate)
             where TNode : SyntaxNode
         {
             for (var current = node; current != null; current = current.GetParent(ascendOutOfTrivia: true))

src/Workspaces/Core/Portable/Shared/Extensions/SyntaxTokenExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -10,12 +12,12 @@ namespace Microsoft.CodeAnalysis.Shared.Extensions
 {
     internal static class SyntaxTokenExtensions
     {
-        public static SyntaxNode GetAncestor(this SyntaxToken token, Func<SyntaxNode, bool> predicate)
+        public static SyntaxNode? GetAncestor(this SyntaxToken token, Func<SyntaxNode, bool>? predicate)
         {
             return token.GetAncestor<SyntaxNode>(predicate);
         }
 
-        public static T GetAncestor<T>(this SyntaxToken token, Func<T, bool> predicate = null)
+        public static T? GetAncestor<T>(this SyntaxToken token, Func<T, bool>? predicate = null)
             where T : SyntaxNode
         {
             return token.Parent != null
@@ -38,7 +40,7 @@ public static IEnumerable<SyntaxNode> GetAncestors(this SyntaxToken token, Func<
                 : SpecializedCollections.EmptyEnumerable<SyntaxNode>();
         }
 
-        public static SyntaxNode GetCommonRoot(this SyntaxToken token1, SyntaxToken token2)
+        public static SyntaxNode? GetCommonRoot(this SyntaxToken token1, SyntaxToken token2)
         {
             Contract.ThrowIfTrue(token1.RawKind == 0 || token2.RawKind == 0);
 

src/Workspaces/Core/Portable/Shared/Extensions/TextDocumentExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
 
@@ -34,7 +36,7 @@ public static TextDocument WithText(this TextDocument textDocument, SourceText t
         public static TextDocument WithAdditionalDocumentText(this TextDocument textDocument, SourceText text)
         {
             Contract.ThrowIfFalse(textDocument is AdditionalDocument);
-            return textDocument.Project.Solution.WithAdditionalDocumentText(textDocument.Id, text, PreservationMode.PreserveIdentity).GetTextDocument(textDocument.Id);
+            return textDocument.Project.Solution.WithAdditionalDocumentText(textDocument.Id, text, PreservationMode.PreserveIdentity).GetTextDocument(textDocument.Id)!;
         }
 
         /// <summary>
@@ -43,7 +45,7 @@ public static TextDocument WithAdditionalDocumentText(this TextDocument textDocu
         public static TextDocument WithAnalyzerConfigDocumentText(this TextDocument textDocument, SourceText text)
         {
             Contract.ThrowIfFalse(textDocument is AnalyzerConfigDocument);
-            return textDocument.Project.Solution.WithAnalyzerConfigDocumentText(textDocument.Id, text, PreservationMode.PreserveIdentity).GetTextDocument(textDocument.Id);
+            return textDocument.Project.Solution.WithAnalyzerConfigDocumentText(textDocument.Id, text, PreservationMode.PreserveIdentity).GetTextDocument(textDocument.Id)!;
         }
     }
 }