Merge pull request #41627 from sharwell/get-option

Update GetOption to return the default option value

src/Analyzers/Core/Analyzers/AbstractBuiltInCodeStyleDiagnosticAnalyzer.cs

@@ -2,11 +2,14 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Options;
+using Roslyn.Utilities;
 
 #if CODE_STYLE
 using OptionSet = Microsoft.CodeAnalysis.Diagnostics.AnalyzerConfigOptions;
@@ -22,20 +25,20 @@ internal abstract class AbstractBuiltInCodeStyleDiagnosticAnalyzer : AbstractCod
         /// </summary>
         /// <param name="diagnosticId">Diagnostic ID reported by this analyzer</param>
         /// <param name="option">
-        /// Per-language option that can be used to configure the given diagnosticId.
-        /// Null, if there is no such unique option.
+        /// Per-language option that can be used to configure the given <paramref name="diagnosticId"/>.
+        /// <see langword="null"/>, if there is no such unique option.
         /// </param>
         /// <param name="title">Title for the diagnostic descriptor</param>
         /// <param name="messageFormat">
         /// Message for the diagnostic descriptor.
-        /// Null if the message is identical to the title.
+        /// <see langword="null"/> if the message is identical to the title.
         /// </param>
         /// <param name="configurable">Flag indicating if the reported diagnostics are configurable by the end users</param>
         protected AbstractBuiltInCodeStyleDiagnosticAnalyzer(
             string diagnosticId,
-            IPerLanguageOption option,
+            IPerLanguageOption? option,
             LocalizableString title,
-            LocalizableString messageFormat = null,
+            LocalizableString? messageFormat = null,
             bool configurable = true)
             : base(diagnosticId, title, messageFormat, configurable)
         {
@@ -48,22 +51,22 @@ internal abstract class AbstractBuiltInCodeStyleDiagnosticAnalyzer : AbstractCod
         /// </summary>
         /// <param name="diagnosticId">Diagnostic ID reported by this analyzer</param>
         /// <param name="option">
-        /// Language specific option that can be used to configure the given diagnosticId.
-        /// Null, if there is no such unique option.
+        /// Language specific option that can be used to configure the given <paramref name="diagnosticId"/>.
+        /// <see langword="null"/>, if there is no such unique option.
         /// </param>
         /// <param name="language">Language for the given language-specific <paramref name="option"/>.</param>
         /// <param name="title">Title for the diagnostic descriptor</param>
         /// <param name="messageFormat">
         /// Message for the diagnostic descriptor.
-        /// Null if the message is identical to the title.
+        /// <see langword="null"/> if the message is identical to the title.
         /// </param>
         /// <param name="configurable">Flag indicating if the reported diagnostics are configurable by the end users</param>
         protected AbstractBuiltInCodeStyleDiagnosticAnalyzer(
             string diagnosticId,
-            ILanguageSpecificOption option,
+            ILanguageSpecificOption? option,
             string language,
             LocalizableString title,
-            LocalizableString messageFormat = null,
+            LocalizableString? messageFormat = null,
             bool configurable = true)
             : base(diagnosticId, title, messageFormat, configurable)
         {
@@ -88,11 +91,11 @@ internal abstract class AbstractBuiltInCodeStyleDiagnosticAnalyzer : AbstractCod
             string diagnosticId,
             ImmutableHashSet<IPerLanguageOption> options,
             LocalizableString title,
-            LocalizableString messageFormat = null,
+            LocalizableString? messageFormat = null,
             bool configurable = true)
             : base(diagnosticId, title, messageFormat, configurable)
         {
-            Debug.Assert(options != null);
+            RoslynDebug.Assert(options != null);
             Debug.Assert(options.Count > 1);
             AddDiagnosticIdToOptionMapping(diagnosticId, options);
         }
@@ -117,11 +120,11 @@ internal abstract class AbstractBuiltInCodeStyleDiagnosticAnalyzer : AbstractCod
             ImmutableHashSet<ILanguageSpecificOption> options,
             string language,
             LocalizableString title,
-            LocalizableString messageFormat = null,
+            LocalizableString? messageFormat = null,
             bool configurable = true)
             : base(diagnosticId, title, messageFormat, configurable)
         {
-            Debug.Assert(options != null);
+            RoslynDebug.Assert(options != null);
             Debug.Assert(options.Count > 1);
             AddDiagnosticIdToOptionMapping(diagnosticId, options, language);
         }
@@ -180,7 +183,7 @@ protected AbstractBuiltInCodeStyleDiagnosticAnalyzer(ImmutableArray<DiagnosticDe
         {
         }
 
-        private static void AddDiagnosticIdToOptionMapping(string diagnosticId, IPerLanguageOption option)
+        private static void AddDiagnosticIdToOptionMapping(string diagnosticId, IPerLanguageOption? option)
         {
             if (option != null)
             {
@@ -188,7 +191,7 @@ private static void AddDiagnosticIdToOptionMapping(string diagnosticId, IPerLang
             }
         }
 
-        private static void AddDiagnosticIdToOptionMapping(string diagnosticId, ILanguageSpecificOption option, string language)
+        private static void AddDiagnosticIdToOptionMapping(string diagnosticId, ILanguageSpecificOption? option, string language)
         {
             if (option != null)
             {

src/Analyzers/Core/Analyzers/AbstractCodeStyleDiagnosticAnalyzer.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis.Diagnostics;
 
@@ -9,7 +11,7 @@ namespace Microsoft.CodeAnalysis.CodeStyle
 {
     internal abstract class AbstractCodeStyleDiagnosticAnalyzer : DiagnosticAnalyzer
     {
-        protected readonly string DescriptorId;
+        protected readonly string? DescriptorId;
 
         protected readonly DiagnosticDescriptor Descriptor;
 
@@ -17,7 +19,7 @@ internal abstract class AbstractCodeStyleDiagnosticAnalyzer : DiagnosticAnalyzer
         /// Diagnostic descriptor for code you want to fade out *and* want to have a smart-tag
         /// appear for.  This is the common descriptor for code that is being faded out
         /// </summary>
-        protected readonly DiagnosticDescriptor UnnecessaryWithSuggestionDescriptor;
+        protected readonly DiagnosticDescriptor? UnnecessaryWithSuggestionDescriptor;
 
         /// <summary>
         /// Diagnostic descriptor for code you want to fade out and do *not* want to have a smart-tag
@@ -31,7 +33,7 @@ internal abstract class AbstractCodeStyleDiagnosticAnalyzer : DiagnosticAnalyzer
         /// same issue when they bring up the code action on that line.  Using these two descriptors
         /// helps ensure that there will not be useless code-action overload.
         /// </summary>
-        protected readonly DiagnosticDescriptor UnnecessaryWithoutSuggestionDescriptor;
+        protected readonly DiagnosticDescriptor? UnnecessaryWithoutSuggestionDescriptor;
 
         protected readonly LocalizableString _localizableTitle;
         protected readonly LocalizableString _localizableMessageFormat;
@@ -40,7 +42,7 @@ internal abstract class AbstractCodeStyleDiagnosticAnalyzer : DiagnosticAnalyzer
 
         protected AbstractCodeStyleDiagnosticAnalyzer(
             string descriptorId, LocalizableString title,
-            LocalizableString messageFormat = null,
+            LocalizableString? messageFormat = null,
             bool configurable = true)
         {
             DescriptorId = descriptorId;
@@ -48,8 +50,8 @@ internal abstract class AbstractCodeStyleDiagnosticAnalyzer : DiagnosticAnalyzer
             _localizableMessageFormat = messageFormat ?? title;
             _configurable = configurable;
 
-            Descriptor = CreateDescriptor();
-            UnnecessaryWithSuggestionDescriptor = CreateUnnecessaryDescriptor();
+            Descriptor = CreateDescriptorWithId(DescriptorId, _localizableTitle, _localizableMessageFormat);
+            UnnecessaryWithSuggestionDescriptor = CreateUnnecessaryDescriptor(DescriptorId);
             UnnecessaryWithoutSuggestionDescriptor = CreateUnnecessaryDescriptor(descriptorId + "WithoutSuggestion");
 
             SupportedDiagnostics = ImmutableArray.Create(
@@ -59,10 +61,11 @@ internal abstract class AbstractCodeStyleDiagnosticAnalyzer : DiagnosticAnalyzer
         protected AbstractCodeStyleDiagnosticAnalyzer(ImmutableArray<DiagnosticDescriptor> supportedDiagnostics)
         {
             SupportedDiagnostics = supportedDiagnostics;
-        }
 
-        protected DiagnosticDescriptor CreateUnnecessaryDescriptor()
-            => CreateUnnecessaryDescriptor(DescriptorId);
+            Descriptor = SupportedDiagnostics[0];
+            _localizableTitle = Descriptor.Title;
+            _localizableMessageFormat = Descriptor.MessageFormat;
+        }
 
         protected DiagnosticDescriptor CreateUnnecessaryDescriptor(string descriptorId)
             => CreateDescriptorWithId(
@@ -71,19 +74,13 @@ protected DiagnosticDescriptor CreateUnnecessaryDescriptor(string descriptorId)
 
         public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; }
 
-        protected DiagnosticDescriptor CreateDescriptor(params string[] customTags)
-            => CreateDescriptorWithId(DescriptorId, _localizableTitle, _localizableMessageFormat, customTags: customTags);
-
-        protected DiagnosticDescriptor CreateDescriptorWithTitle(LocalizableString title, params string[] customTags)
-            => CreateDescriptorWithId(DescriptorId, title, title, customTags: customTags);
-
         protected static DiagnosticDescriptor CreateDescriptorWithId(
             string id,
             LocalizableString title,
             LocalizableString messageFormat,
             bool isUnnecessary = false,
             bool isConfigurable = true,
-            LocalizableString description = null,
+            LocalizableString? description = null,
             params string[] customTags)
             => new DiagnosticDescriptor(
                     id, title, messageFormat,

src/Compilers/Core/Portable/DiagnosticAnalyzer/DiagnosticAnalyzer.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using Roslyn.Utilities;
 
@@ -23,9 +25,9 @@ public abstract class DiagnosticAnalyzer
         /// <param name="context"></param>
         public abstract void Initialize(AnalysisContext context);
 
-        public sealed override bool Equals(object obj)
+        public sealed override bool Equals(object? obj)
         {
-            return (object)this == obj;
+            return (object?)this == obj;
         }
 
         public sealed override int GetHashCode()

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

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace Roslyn.Utilities
+{
+    [AttributeUsage(AttributeTargets.Struct | AttributeTargets.GenericParameter)]
+    internal sealed class NonDefaultableAttribute : Attribute
+    {
+    }
+}

src/EditorFeatures/CSharp/SplitStringLiteral/SplitStringLiteralOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Composition;
 using Microsoft.CodeAnalysis.Options;

src/EditorFeatures/Core/Options/BraceCompletionOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Composition;
 using Microsoft.CodeAnalysis.Options;

src/EditorFeatures/Core/Options/NavigationBarOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Composition;
 using Microsoft.CodeAnalysis.Options;

src/EditorFeatures/Core/Options/SignatureHelpOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Composition;
 using Microsoft.CodeAnalysis.Options;

src/EditorFeatures/Core/Shared/Options/FeatureOnOffOptions.cs

@@ -2,11 +2,12 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Composition;
 using Microsoft.CodeAnalysis.Options;
 using Microsoft.CodeAnalysis.Options.Providers;
-using Microsoft.CodeAnalysis.Utilities;
 
 namespace Microsoft.CodeAnalysis.Editor.Shared.Options
 {

src/EditorFeatures/Core/Tagging/AbstractAsynchronousTaggerProvider.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 #if DEBUG
 using System.Diagnostics;
@@ -100,7 +102,7 @@ internal abstract partial class AbstractAsynchronousTaggerProvider<TTag> : Foreg
 #endif
         }
 
-        internal IAccurateTagger<T> CreateTaggerWorker<T>(ITextView textViewOpt, ITextBuffer subjectBuffer) where T : ITag
+        internal IAccurateTagger<T>? CreateTaggerWorker<T>(ITextView textViewOpt, ITextBuffer subjectBuffer) where T : ITag
         {
             if (!subjectBuffer.GetFeatureOnOffOption(EditorComponentOnOffOptions.Tagger))
             {
@@ -252,11 +254,11 @@ private struct DiffResult
             public NormalizedSnapshotSpanCollection Removed { get; }
 
             public DiffResult(List<SnapshotSpan> added, List<SnapshotSpan> removed)
-                : this(added?.Count == 0 ? null : (IEnumerable<SnapshotSpan>)added, removed?.Count == 0 ? null : (IEnumerable<SnapshotSpan>)removed)
+                : this(added?.Count == 0 ? null : (IEnumerable<SnapshotSpan>?)added, removed?.Count == 0 ? null : (IEnumerable<SnapshotSpan>?)removed)
             {
             }
 
-            public DiffResult(IEnumerable<SnapshotSpan> added, IEnumerable<SnapshotSpan> removed)
+            public DiffResult(IEnumerable<SnapshotSpan>? added, IEnumerable<SnapshotSpan>? removed)
             {
                 Added = added != null ? new NormalizedSnapshotSpanCollection(added) : NormalizedSnapshotSpanCollection.Empty;
                 Removed = removed != null ? new NormalizedSnapshotSpanCollection(removed) : NormalizedSnapshotSpanCollection.Empty;

src/Features/CSharp/Portable/UseInferredMemberName/CSharpUseInferredMemberNameDiagnosticAnalyzer.cs

@@ -12,6 +12,7 @@
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.CodeAnalysis.UseInferredMemberName;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp.UseInferredMemberName
 {
@@ -61,6 +62,7 @@ private void ReportDiagnosticsIfNeeded(NameColonSyntax nameColon, SyntaxNodeAnal
                     properties: null));
 
             // Also fade out the part of the name-colon syntax
+            RoslynDebug.AssertNotNull(UnnecessaryWithoutSuggestionDescriptor);
             var fadeSpan = TextSpan.FromBounds(nameColon.Name.SpanStart, nameColon.ColonToken.Span.End);
             context.ReportDiagnostic(
                 Diagnostic.Create(
@@ -93,6 +95,7 @@ private void ReportDiagnosticsIfNeeded(NameEqualsSyntax nameEquals, SyntaxNodeAn
                     properties: null));
 
             // Also fade out the part of the name-equals syntax
+            RoslynDebug.AssertNotNull(UnnecessaryWithoutSuggestionDescriptor);
             var fadeSpan = TextSpan.FromBounds(nameEquals.Name.SpanStart, nameEquals.EqualsToken.Span.End);
             context.ReportDiagnostic(
                 Diagnostic.Create(

src/Features/Core/Portable/Completion/CompletionOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis.Options;
 

src/Features/Core/Portable/ExtractMethod/ExtractMethodOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.ExtractMethod

src/Features/Core/Portable/GenerateConstructorFromMembers/GenerateConstructorFromMembersOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.GenerateConstructorFromMembers

src/Features/Core/Portable/GenerateEqualsAndGetHashCodeFromMembers/GenerateEqualsAndGetHashCodeFromMembersOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.GenerateEqualsAndGetHashCodeFromMembers

src/Features/Core/Portable/ImplementType/ImplementTypeOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.ImplementType

src/Features/Core/Portable/PreferFrameworkType/PreferFrameworkTypeDiagnosticAnalyzerBase.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Immutable;
 using System.Threading;

src/Features/Core/Portable/RemoveUnnecessaryCast/AbstractRemoveUnnecessaryCastDiagnosticAnalyzer.cs

@@ -10,6 +10,7 @@
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.RemoveUnnecessaryCast
 {
@@ -65,6 +66,7 @@ private void AnalyzeSyntax(SyntaxNodeAnalysisContext context)
                 return null;
             }
 
+            RoslynDebug.AssertNotNull(UnnecessaryWithSuggestionDescriptor);
             return Diagnostic.Create(
                 UnnecessaryWithSuggestionDescriptor,
                 node.SyntaxTree.GetLocation(GetFadeSpan(node)),

src/Features/Core/Portable/Shared/Options/ServiceFeatureOnOffOptions.cs

-using System;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#nullable enable
+
+using System;
 using Microsoft.CodeAnalysis.Options;
 using Microsoft.CodeAnalysis.SolutionCrawler;
 

src/Features/Core/Portable/SimplifyThisOrMe/AbstractSimplifyThisOrMeDiagnosticAnalyzer.cs

@@ -13,6 +13,7 @@
 using Microsoft.CodeAnalysis.QualifyMemberAccess;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.SimplifyThisOrMe
 {
@@ -96,12 +97,8 @@ private void AnalyzeNode(SyntaxNodeAnalysisContext context)
 
             var severity = optionValue.Notification.Severity;
 
+            RoslynDebug.AssertNotNull(DescriptorId);
             var descriptor = CreateUnnecessaryDescriptor(DescriptorId);
-            if (descriptor == null)
-            {
-                return;
-            }
-
             var tree = model.SyntaxTree;
             var builder = ImmutableDictionary.CreateBuilder<string, string>();
 

src/Features/Core/Portable/Structure/BlockStructureOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.Structure

src/Features/Core/Portable/UseCollectionInitializer/AbstractUseCollectionInitializerDiagnosticAnalyzer.cs

@@ -6,11 +6,11 @@
 
 using System.Collections;
 using System.Collections.Immutable;
-using System.Threading;
 using Microsoft.CodeAnalysis.CodeStyle;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.LanguageServices;
 using Microsoft.CodeAnalysis.Text;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.UseCollectionInitializer
 {
@@ -148,9 +148,11 @@ private void AnalyzeNode(SyntaxNodeAnalysisContext context, INamedTypeSymbol ien
                     var location1 = Location.Create(syntaxTree, TextSpan.FromBounds(
                         match.SpanStart, arguments[0].SpanStart));
 
+                    RoslynDebug.AssertNotNull(UnnecessaryWithSuggestionDescriptor);
                     context.ReportDiagnostic(Diagnostic.Create(
                         UnnecessaryWithSuggestionDescriptor, location1, additionalLocations: locations));
 
+                    RoslynDebug.AssertNotNull(UnnecessaryWithoutSuggestionDescriptor);
                     context.ReportDiagnostic(Diagnostic.Create(
                         UnnecessaryWithoutSuggestionDescriptor,
                         Location.Create(syntaxTree, TextSpan.FromBounds(

src/Features/Core/Portable/UseConditionalExpression/AbstractUseConditionalExpressionDiagnosticAnalyzer.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis.CodeStyle;
 using Microsoft.CodeAnalysis.Diagnostics;

src/Features/Core/Portable/UseConditionalExpression/UseConditionalExpressionOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.UseConditionalExpression

src/Features/Core/Portable/UseObjectInitializer/AbstractUseObjectInitializerDiagnosticAnalyzer.cs

@@ -11,6 +11,7 @@
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.LanguageServices;
 using Microsoft.CodeAnalysis.Text;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.UseObjectInitializer
 {
@@ -125,11 +126,13 @@ private void AnalyzeNode(SyntaxNodeAnalysisContext context)
                 var location1 = Location.Create(syntaxTree, TextSpan.FromBounds(
                     match.MemberAccessExpression.SpanStart, end));
 
+                RoslynDebug.AssertNotNull(UnnecessaryWithSuggestionDescriptor);
                 context.ReportDiagnostic(Diagnostic.Create(
                     UnnecessaryWithSuggestionDescriptor, location1, additionalLocations: locations));
 
                 if (match.Statement.Span.End > match.Initializer.FullSpan.End)
                 {
+                    RoslynDebug.AssertNotNull(UnnecessaryWithoutSuggestionDescriptor);
                     context.ReportDiagnostic(Diagnostic.Create(
                         UnnecessaryWithoutSuggestionDescriptor,
                         Location.Create(syntaxTree, TextSpan.FromBounds(

src/Features/Core/Portable/ValidateFormatString/ValidateFormatStringOption.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Composition;
 using Microsoft.CodeAnalysis.Options;

src/Tools/ExternalAccess/FSharp/Completion/FSharpCompletionOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.ExternalAccess.FSharp.Completion

src/Tools/ExternalAccess/FSharp/Shared/Options/FSharpServiceFeatureOnOffOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using Microsoft.CodeAnalysis.Options;
 using Microsoft.CodeAnalysis.SolutionCrawler;

src/VisualStudio/Core/Def/Implementation/Workspace/VisualStudioNavigationOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.VisualStudio.LanguageServices.Implementation

src/Workspaces/Core/Portable/CodeStyle/CodeStyleOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Diagnostics;
 using Microsoft.CodeAnalysis.Options;

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

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis.CodeStyle;
 using Microsoft.CodeAnalysis.Options;

src/Workspaces/Core/Portable/EmbeddedLanguages/RegularExpressions/RegularExpressionsOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Composition;
 using Microsoft.CodeAnalysis.Options;

src/Workspaces/Core/Portable/NamingStyles/EditorConfig/EditorConfigNamingStyleParser.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -20,10 +22,10 @@ internal static partial class EditorConfigNamingStyleParser
         // TODO: revisit this cache. The assumption that the dictionary doesn't change in the exact instance is terribly fragile,
         // and with the new .editorconfig support won't hold as well as we'd like: a single tree will have a stable instance but
         // that won't necessarily be the same across files and projects.
-        private static readonly ConditionalWeakTable<IReadOnlyDictionary<string, string>, NamingStylePreferences> _cache = new ConditionalWeakTable<IReadOnlyDictionary<string, string>, NamingStylePreferences>();
+        private static readonly ConditionalWeakTable<IReadOnlyDictionary<string, string?>, NamingStylePreferences> _cache = new ConditionalWeakTable<IReadOnlyDictionary<string, string?>, NamingStylePreferences>();
         private static readonly object _cacheLock = new object();
 
-        public static NamingStylePreferences GetNamingStylesFromDictionary(IReadOnlyDictionary<string, string> rawOptions)
+        public static NamingStylePreferences GetNamingStylesFromDictionary(IReadOnlyDictionary<string, string?> rawOptions)
         {
             if (_cache.TryGetValue(rawOptions, out var value))
             {
@@ -42,7 +44,7 @@ public static NamingStylePreferences GetNamingStylesFromDictionary(IReadOnlyDict
             }
         }
 
-        public static NamingStylePreferences ParseDictionary(IReadOnlyDictionary<string, string> allRawConventions)
+        public static NamingStylePreferences ParseDictionary(IReadOnlyDictionary<string, string?> allRawConventions)
         {
             var symbolSpecifications = ArrayBuilder<SymbolSpecification>.GetInstance();
             var namingStyles = ArrayBuilder<NamingStyle>.GetInstance();
@@ -127,14 +129,14 @@ public static NamingStylePreferences ParseDictionary(IReadOnlyDictionary<string,
                     }));
         }
 
-        private static Dictionary<string, string> TrimDictionary(IReadOnlyDictionary<string, string> allRawConventions)
+        private static Dictionary<string, string?> TrimDictionary(IReadOnlyDictionary<string, string?> allRawConventions)
         {
             // Keys have been lowercased, but values have not. Because values here reference key
             // names we need any comparisons to ignore case.
             // For example, to make a naming style called "Pascal_Case_style" match up correctly
             // with the key "dotnet_naming_style.pascal_case_style.capitalization", we have to
             // ignore casing for that lookup.
-            var trimmedDictionary = new Dictionary<string, string>(allRawConventions.Count, AnalyzerConfigOptions.KeyComparer);
+            var trimmedDictionary = new Dictionary<string, string?>(allRawConventions.Count, AnalyzerConfigOptions.KeyComparer);
             foreach (var item in allRawConventions)
             {
                 var key = item.Key.Trim();
@@ -145,7 +147,7 @@ public static NamingStylePreferences ParseDictionary(IReadOnlyDictionary<string,
             return trimmedDictionary;
         }
 
-        private static IEnumerable<string> GetRuleTitles(IReadOnlyDictionary<string, string> allRawConventions)
+        private static IEnumerable<string> GetRuleTitles(IReadOnlyDictionary<string, string?> allRawConventions)
             => (from kvp in allRawConventions
                 where kvp.Key.Trim().StartsWith("dotnet_naming_rule.", StringComparison.Ordinal)
                 let nameSplit = kvp.Key.Split('.')

src/Workspaces/Core/Portable/Options/DocumentOptionSet.cs

@@ -27,13 +27,11 @@ internal DocumentOptionSet(OptionSet backingOptionSet, string language)
             _language = language;
         }
 
-        [return: MaybeNull]
-        public override object GetOption(OptionKey optionKey)
+        public override object? GetOption(OptionKey optionKey)
         {
             return _backingOptionSet.GetOption(optionKey);
         }
 
-        [return: MaybeNull]
         public T GetOption<T>(PerLanguageOption<T> option)
         {
             return _backingOptionSet.GetOption(option, _language);

src/Workspaces/Core/Portable/Options/EditorConfig/EditorConfigDocumentOptionsProviderFactory.cs

@@ -11,6 +11,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.ErrorLogger;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.Options.EditorConfig
 {
@@ -83,7 +84,7 @@ public bool TryGetDocumentOption(OptionKey option, out object? value)
 
                     try
                     {
-                        return editorConfigPersistence.TryGetOption(_options, option.Option.Type, out value);
+                        return editorConfigPersistence.TryGetOption(_options.AsNullable(), option.Option.Type, out value);
                     }
                     catch (Exception ex)
                     {

src/Workspaces/Core/Portable/Options/EditorConfig/EditorConfigFileGenerator.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;

src/Workspaces/Core/Portable/Options/EditorConfig/EditorConfigStorageLocation.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using Microsoft.CodeAnalysis.CodeStyle;
 

src/Workspaces/Core/Portable/Options/EditorConfig/EditorConfigStorageLocation`1.cs

@@ -2,9 +2,12 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 #if CODE_STYLE
@@ -21,7 +24,7 @@ internal sealed class EditorConfigStorageLocation<T> : OptionStorageLocation, IE
         public string KeyName { get; }
 
         private readonly Func<string, Type, Optional<T>> _parseValue;
-        private readonly Func<T, OptionSet, string> _getEditorConfigStringForValue;
+        private readonly Func<T, OptionSet, string?> _getEditorConfigStringForValue;
 
         public EditorConfigStorageLocation(string keyName, Func<string, Optional<T>> parseValue, Func<T, string> getEditorConfigStringForValue)
             : this(keyName, parseValue, (value, optionSet) => getEditorConfigStringForValue(value))
@@ -56,9 +59,10 @@ public EditorConfigStorageLocation(string keyName, Func<string, Optional<T>> par
             _getEditorConfigStringForValue = getEditorConfigStringForValue ?? throw new ArgumentNullException(nameof(getEditorConfigStringForValue));
         }
 
-        public bool TryGetOption(IReadOnlyDictionary<string, string> rawOptions, Type type, out object result)
+        public bool TryGetOption(IReadOnlyDictionary<string, string?> rawOptions, Type type, out object? result)
         {
-            if (rawOptions.TryGetValue(KeyName, out var value))
+            if (rawOptions.TryGetValue(KeyName, out var value)
+                && value is object)
             {
                 var ret = TryGetOption(value, type, out var typedResult);
                 result = typedResult;
@@ -69,7 +73,7 @@ public bool TryGetOption(IReadOnlyDictionary<string, string> rawOptions, Type ty
             return false;
         }
 
-        internal bool TryGetOption(string value, Type type, out T result)
+        internal bool TryGetOption(string value, Type type, [MaybeNullWhen(false)] out T result)
         {
             var optionalValue = _parseValue(value, type);
             if (optionalValue.HasValue)
@@ -79,7 +83,7 @@ internal bool TryGetOption(string value, Type type, out T result)
             }
             else
             {
-                result = default;
+                result = default!;
                 return false;
             }
         }
@@ -95,7 +99,7 @@ public string GetEditorConfigString(T value, OptionSet optionSet)
             return $"{KeyName} = {editorConfigStringForValue}";
         }
 
-        string IEditorConfigStorageLocation2.GetEditorConfigString(object value, OptionSet optionSet)
-            => GetEditorConfigString((T)value, optionSet);
+        string IEditorConfigStorageLocation2.GetEditorConfigString(object? value, OptionSet optionSet)
+            => GetEditorConfigString((T)value!, optionSet);
     }
 }

src/Workspaces/Core/Portable/Options/EditorConfig/IEditorConfigStorageLocation.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 
@@ -9,6 +11,6 @@ namespace Microsoft.CodeAnalysis.Options
 {
     internal interface IEditorConfigStorageLocation
     {
-        bool TryGetOption(IReadOnlyDictionary<string, string> rawOptions, Type type, out object value);
+        bool TryGetOption(IReadOnlyDictionary<string, string?> rawOptions, Type type, out object? value);
     }
 }

src/Workspaces/Core/Portable/Options/EditorConfig/IEditorConfigStorageLocation2.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 #if CODE_STYLE
 using OptionSet = Microsoft.CodeAnalysis.Diagnostics.AnalyzerConfigOptions;
 #endif
@@ -15,6 +17,6 @@ internal interface IEditorConfigStorageLocation2 : IEditorConfigStorageLocation
         /// <summary>
         /// Gets the editorconfig string representation for this storage location.
         /// </summary>
-        string GetEditorConfigString(object value, OptionSet optionSet);
+        string GetEditorConfigString(object? value, OptionSet optionSet);
     }
 }

src/Workspaces/Core/Portable/Options/EditorConfig/NamingStylePreferenceEditorConfigStorageLocation.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -12,7 +14,7 @@ namespace Microsoft.CodeAnalysis.Options
 {
     internal sealed class NamingStylePreferenceEditorConfigStorageLocation : OptionStorageLocation, IEditorConfigStorageLocation
     {
-        public bool TryGetOption(IReadOnlyDictionary<string, string> rawOptions, Type type, out object result)
+        public bool TryGetOption(IReadOnlyDictionary<string, string?> rawOptions, Type type, out object result)
         {
             var tuple = ParseDictionary(rawOptions, type);
             result = tuple.result;
@@ -20,7 +22,7 @@ public bool TryGetOption(IReadOnlyDictionary<string, string> rawOptions, Type ty
         }
 
         private static (object result, bool succeeded) ParseDictionary(
-            IReadOnlyDictionary<string, string> allRawConventions, Type type)
+            IReadOnlyDictionary<string, string?> allRawConventions, Type type)
         {
             if (type == typeof(NamingStylePreferences))
             {

src/Workspaces/Core/Portable/Options/GlobalOptionService.cs

@@ -250,13 +250,11 @@ public SerializableOptionSet GetSerializableOptionsSnapshot(ImmutableHashSet<str
             }
         }
 
-        [return: MaybeNull]
         public T GetOption<T>(Option<T> option)
         {
             return (T)GetOption(new OptionKey(option))!;
         }
 
-        [return: MaybeNull]
         public T GetOption<T>(PerLanguageOption<T> option, string? language)
         {
             return (T)GetOption(new OptionKey(option, language))!;

src/Workspaces/Core/Portable/Options/IOptionWithGroup.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Options
 {
     /// <summary>

src/Workspaces/Core/Portable/Options/IWorkspaceOptionService.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Options
 {
     /// <summary>

src/Workspaces/Core/Portable/Options/LocalUserProfileStorageLocation.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Options
 {
     /// <summary>

src/Workspaces/Core/Portable/Options/OptionGroup.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 
 namespace Microsoft.CodeAnalysis.Options

src/Workspaces/Core/Portable/Options/OptionKey.cs

@@ -2,7 +2,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
+using Roslyn.Utilities;
 
 #if CODE_STYLE
 using WorkspacesResources = Microsoft.CodeAnalysis.CodeStyleResources;
@@ -10,12 +13,13 @@
 
 namespace Microsoft.CodeAnalysis.Options
 {
+    [NonDefaultable]
     public readonly struct OptionKey : IEquatable<OptionKey>
     {
         public IOption Option { get; }
-        public string Language { get; }
+        public string? Language { get; }
 
-        public OptionKey(IOption option, string language = null)
+        public OptionKey(IOption option, string? language = null)
         {
             if (language != null && !option.IsPerLanguage)
             {
@@ -30,7 +34,7 @@ public OptionKey(IOption option, string language = null)
             this.Language = language;
         }
 
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             return obj is OptionKey key &&
                    Equals(key);

src/Workspaces/Core/Portable/Options/OptionSet.cs

@@ -7,7 +7,6 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
-using System.Diagnostics.CodeAnalysis;
 using Microsoft.CodeAnalysis.Diagnostics;
 
 namespace Microsoft.CodeAnalysis.Options
@@ -24,23 +23,21 @@ public abstract partial class OptionSet
         private ImmutableDictionary<string, AnalyzerConfigOptions> _lazyAnalyzerConfigOptions = s_emptyAnalyzerConfigOptions;
 
         /// <summary>
-        /// Gets the value of the option.
+        /// Gets the value of the option, or the default value if not otherwise set.
         /// </summary>
         public abstract object? GetOption(OptionKey optionKey);
 
         /// <summary>
-        /// Gets the value of the option.
+        /// Gets the value of the option, or the default value if not otherwise set.
         /// </summary>
-        [return: MaybeNull]
         public T GetOption<T>(Option<T> option)
         {
             return (T)GetOption(new OptionKey(option, language: null))!;
         }
 
         /// <summary>
-        /// Gets the value of the option.
+        /// Gets the value of the option, or the default value if not otherwise set.
         /// </summary>
-        [return: MaybeNull]
         public T GetOption<T>(PerLanguageOption<T> option, string? language)
         {
             return (T)GetOption(new OptionKey(option, language))!;
@@ -54,7 +51,7 @@ public T GetOption<T>(PerLanguageOption<T> option, string? language)
         /// <summary>
         /// Creates a new <see cref="OptionSet" /> that contains the changed value.
         /// </summary>
-        public OptionSet WithChangedOption<T>(Option<T> option, [MaybeNull] T value)
+        public OptionSet WithChangedOption<T>(Option<T> option, T value)
         {
             return WithChangedOption(new OptionKey(option), value);
         }
@@ -62,7 +59,7 @@ public OptionSet WithChangedOption<T>(Option<T> option, [MaybeNull] T value)
         /// <summary>
         /// Creates a new <see cref="OptionSet" /> that contains the changed value.
         /// </summary>
-        public OptionSet WithChangedOption<T>(PerLanguageOption<T> option, string? language, [MaybeNull] T value)
+        public OptionSet WithChangedOption<T>(PerLanguageOption<T> option, string? language, T value)
         {
             return WithChangedOption(new OptionKey(option, language), value);
         }

src/Workspaces/Core/Portable/Options/OptionStorageLocation.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Options
 {
     /// <summary>

src/Workspaces/Core/Portable/Options/Option`1.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Immutable;
 
@@ -46,8 +48,9 @@ public class Option<T> : ILanguageSpecificOption
 
         public ImmutableArray<OptionStorageLocation> StorageLocations { get; }
 
+        [Obsolete("Use a constructor that specifies an explicit default value.")]
         public Option(string feature, string name)
-            : this(feature, name, default)
+            : this(feature, name, default!)
         {
             // This constructor forwards to the next one; it exists to maintain source-level compatibility with older callers.
         }
@@ -81,7 +84,7 @@ internal Option(string feature, OptionGroup group, string name, T defaultValue,
             this.StorageLocations = storageLocations.ToImmutableArray();
         }
 
-        object IOption.DefaultValue => this.DefaultValue;
+        object? IOption.DefaultValue => this.DefaultValue;
 
         bool IOption.IsPerLanguage => false;
 

src/Workspaces/Core/Portable/Options/PerLanguageOption.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Immutable;
 
@@ -76,7 +78,7 @@ internal PerLanguageOption(string feature, OptionGroup group, string name, T def
             this.StorageLocations = storageLocations.ToImmutableArray();
         }
 
-        object IOption.DefaultValue => this.DefaultValue;
+        object? IOption.DefaultValue => this.DefaultValue;
 
         bool IOption.IsPerLanguage => true;
 

src/Workspaces/Core/Portable/Options/PredefinedDocumentOptionsProviderNames.cs

-using System;
-using System.Collections.Generic;
-using System.Text;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#nullable enable
 
 namespace Microsoft.CodeAnalysis.Options
 {

src/Workspaces/Core/Portable/Options/Providers/IOptionProvider.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Immutable;
 
 namespace Microsoft.CodeAnalysis.Options.Providers

src/Workspaces/Core/Portable/Options/RoamingProfileStorageLocation.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 
 namespace Microsoft.CodeAnalysis.Options

src/Workspaces/Core/Portable/Recommendations/RecommendationOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.Recommendations

src/Workspaces/Core/Portable/Simplification/SimplificationOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using Microsoft.CodeAnalysis.Diagnostics.Analyzers.NamingStyles;
 using Microsoft.CodeAnalysis.Options;

src/Workspaces/Core/Portable/SolutionCrawler/SolutionCrawlerOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using Microsoft.CodeAnalysis.Options;
 

src/Workspaces/Core/Portable/SymbolSearch/SymbolSearchOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.SymbolSearch

src/Workspaces/CoreTest/CodeStyle/EditorConfigCodeStyleParserTests.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Linq;
 using Microsoft.CodeAnalysis.CodeStyle;
@@ -81,10 +83,10 @@ public void TestParseEditorConfigAccessibilityModifiers(string args, int value,
             var storageLocation = CodeStyleOptions.RequireAccessibilityModifiers.StorageLocations
                 .OfType<EditorConfigStorageLocation<CodeStyleOption<AccessibilityModifiersRequired>>>()
                 .Single();
-            var allRawConventions = new Dictionary<string, string> { { storageLocation.KeyName, args } };
+            var allRawConventions = new Dictionary<string, string?> { { storageLocation.KeyName, args } };
 
             Assert.True(storageLocation.TryGetOption(allRawConventions, typeof(CodeStyleOption<AccessibilityModifiersRequired>), out var parsedCodeStyleOption));
-            var codeStyleOption = (CodeStyleOption<AccessibilityModifiersRequired>)parsedCodeStyleOption;
+            var codeStyleOption = (CodeStyleOption<AccessibilityModifiersRequired>)parsedCodeStyleOption!;
             Assert.Equal((AccessibilityModifiersRequired)value, codeStyleOption.Value);
             Assert.Equal(severity, codeStyleOption.Notification.Severity);
         }
@@ -103,10 +105,10 @@ public void TestParseEditorConfigEndOfLine(string configurationString, string ne
             var storageLocation = FormattingOptions.NewLine.StorageLocations
                 .OfType<EditorConfigStorageLocation<string>>()
                 .Single();
-            var allRawConventions = new Dictionary<string, string> { { storageLocation.KeyName, configurationString } };
+            var allRawConventions = new Dictionary<string, string?> { { storageLocation.KeyName, configurationString } };
 
             Assert.True(storageLocation.TryGetOption(allRawConventions, typeof(string), out var parsedNewLine));
-            Assert.Equal(newLine, (string)parsedNewLine);
+            Assert.Equal(newLine, (string?)parsedNewLine);
         }
     }
 }

src/Workspaces/CoreTest/Options/OptionKeyTests.cs

@@ -2,6 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
+using System;
 using Microsoft.CodeAnalysis.Options;
 using Xunit;
 
@@ -10,7 +13,8 @@ namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Options
     public class OptionKeyTests
     {
         [Fact]
-        public void ToStringForOption()
+        [Obsolete]
+        public void ToStringForObsoleteOption()
         {
             var option = new Option<bool>("FooFeature", "BarName");
             var optionKey = new OptionKey(option);
@@ -20,6 +24,17 @@ public void ToStringForOption()
             Assert.Equal("FooFeature - BarName", toStringResult);
         }
 
+        [Fact]
+        public void ToStringForOption()
+        {
+            var option = new Option<bool>("FooFeature", "BarName", false);
+            var optionKey = new OptionKey(option);
+
+            var toStringResult = optionKey.ToString();
+
+            Assert.Equal("FooFeature - BarName", toStringResult);
+        }
+
         [Fact]
         public void ToStringForPerLanguageOption()
         {

src/Workspaces/CoreTest/WorkspaceServiceTests/OptionServiceTests.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 using Microsoft.CodeAnalysis.Test.Utilities;
 using Xunit;
@@ -24,7 +26,7 @@ public void OptionWithNullOrWhitespace()
 
             Assert.Throws<System.ArgumentException>(delegate
             {
-                var option2 = new Option<bool>("Test Feature", null, false);
+                var option2 = new Option<bool>("Test Feature", null!, false);
             });
 
             Assert.Throws<System.ArgumentNullException>(delegate
@@ -34,7 +36,7 @@ public void OptionWithNullOrWhitespace()
 
             Assert.Throws<System.ArgumentNullException>(delegate
             {
-                var option4 = new Option<bool>(null, "Test Name", false);
+                var option4 = new Option<bool>(null!, "Test Name", false);
             });
         }
 
@@ -51,7 +53,7 @@ public void OptionPerLanguageOption()
 
             Assert.Throws<System.ArgumentException>(delegate
             {
-                var option2 = new PerLanguageOption<bool>("Test Feature", null, false);
+                var option2 = new PerLanguageOption<bool>("Test Feature", null!, false);
             });
 
             Assert.Throws<System.ArgumentNullException>(delegate
@@ -61,7 +63,7 @@ public void OptionPerLanguageOption()
 
             Assert.Throws<System.ArgumentNullException>(delegate
             {
-                var option4 = new PerLanguageOption<bool>(null, "Test Name", false);
+                var option4 = new PerLanguageOption<bool>(null!, "Test Name", false);
             });
 
             var optionvalid = new PerLanguageOption<bool>("Test Feature", "Test Name", false);
@@ -86,7 +88,7 @@ public void GettingOptionWithChangedOption()
             var key = new OptionKey(option);
             Assert.False(optionSet.GetOption(option));
             optionSet = optionSet.WithChangedOption(key, true);
-            Assert.True((bool)optionSet.GetOption(key));
+            Assert.True((bool?)optionSet.GetOption(key));
         }
 
         [Fact, Trait(Traits.Feature, Traits.Features.Workspace)]
@@ -102,10 +104,10 @@ public void GettingOptionWithoutChangedOption()
             Assert.True(optionSet.GetOption(optionTrue));
 
             var falseKey = new OptionKey(optionFalse);
-            Assert.False((bool)optionSet.GetOption(falseKey));
+            Assert.False((bool?)optionSet.GetOption(falseKey));
 
             var trueKey = new OptionKey(optionTrue);
-            Assert.True((bool)optionSet.GetOption(trueKey));
+            Assert.True((bool?)optionSet.GetOption(trueKey));
         }
 
         [Fact, Trait(Traits.Feature, Traits.Features.Workspace)]
@@ -129,7 +131,7 @@ public void GetKnownOptionsKey()
 
             var optionSet = optionService.GetOptions();
             var optionKey = new OptionKey(option);
-            var value = (bool)optionSet.GetOption(optionKey);
+            var value = (bool?)optionSet.GetOption(optionKey);
             Assert.True(value);
         }
 
@@ -143,7 +145,7 @@ public void SetKnownOptions()
             var optionKey = new OptionKey(option);
             var newOptionSet = optionSet.WithChangedOption(optionKey, false);
             optionService.SetOptions(newOptionSet);
-            var isOptionSet = (bool)optionService.GetOptions().GetOption(optionKey);
+            var isOptionSet = (bool?)optionService.GetOptions().GetOption(optionKey);
             Assert.False(isOptionSet);
         }
 

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/CompilerExtensions.projitems

@@ -14,6 +14,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)..\..\..\..\Compilers\Core\Portable\InternalUtilities\ExceptionUtilities.cs" Link="InternalUtilities\ExceptionUtilities.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)..\..\..\..\Compilers\Core\Portable\InternalUtilities\IReadOnlySet.cs" Link="InternalUtilities\IReadOnlySet.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)..\..\..\..\Compilers\Core\Portable\InternalUtilities\KeyValuePairUtil.cs" Link="InternalUtilities\KeyValuePairUtil.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)..\..\..\..\Compilers\Core\Portable\InternalUtilities\NonDefaultableAttribute.cs" Link="InternalUtilities\NonDefaultableAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)..\..\..\..\Compilers\Core\Portable\InternalUtilities\NullableAttributes.cs" Link="InternalUtilities\NullableAttributes.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)..\..\..\..\Compilers\Core\Portable\InternalUtilities\ReferenceEqualityComparer.cs" Link="InternalUtilities\ReferenceEqualityComparer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)..\..\..\..\Compilers\Core\Portable\InternalUtilities\RoslynString.cs" Link="InternalUtilities\RoslynString.cs" />

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Fading/FadingOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Options;
 
 namespace Microsoft.CodeAnalysis.Fading

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Formatting/FormattingOptions.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis.Options;
@@ -97,7 +99,7 @@ private static Optional<string> ParseEditorConfigEndOfLine(string endOfLineValue
             => s_parenthesesPreferenceMap.TryGetValue(endOfLineValue.Trim(), out var parsedOption) ? parsedOption : NewLine.DefaultValue;
 
         private static string GetEndOfLineEditorConfigString(string option)
-            => s_parenthesesPreferenceMap.TryGetKey(option, out var editorConfigString) ? editorConfigString : null;
+            => s_parenthesesPreferenceMap.TryGetKey(option, out var editorConfigString) ? editorConfigString : "unset";
 
         internal static Option<bool> AllowDisjointSpanMerging { get; } = CreateOption(OptionGroup.Default, nameof(AllowDisjointSpanMerging), defaultValue: false);
 

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Utilities/IReadOnlyDictionaryExtensions.cs

@@ -32,5 +32,13 @@ public static IEnumerable<T> GetEnumerableMetadata<T>(this IReadOnlyDictionary<s
                 default: return SpecializedCollections.EmptyEnumerable<T>();
             }
         }
+
+        public static IReadOnlyDictionary<TKey, TValue?> AsNullable<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> dictionary)
+            where TKey : notnull
+            where TValue : class
+        {
+            // this is a safe cast, even though the language doesn't allow the interface to be 'out TValue'
+            return dictionary!;
+        }
     }
 }