Make fields readonly

src/EditorFeatures/CSharpTest/CodeActions/Preview/PreviewTests.cs

@@ -40,7 +40,7 @@ public sealed override Task ComputeRefactoringsAsync(CodeRefactoringContext cont
 
             private class MyCodeAction : CodeAction
             {
-                private Document _oldDocument;
+                private readonly Document _oldDocument;
 
                 public MyCodeAction(Document document)
                 {

src/EditorFeatures/CSharpTest/Diagnostics/MockDiagnosticAnalyzerTests.cs

@@ -16,7 +16,7 @@ public partial class MockDiagnosticAnalyzerTests : AbstractCSharpDiagnosticProvi
         private class MockDiagnosticAnalyzer : DiagnosticAnalyzer
         {
             public const string Id = "MockDiagnostic";
-            private DiagnosticDescriptor _descriptor = new DiagnosticDescriptor(Id, "MockDiagnostic", "MockDiagnostic", "InternalCategory", DiagnosticSeverity.Warning, isEnabledByDefault: true);
+            private readonly DiagnosticDescriptor _descriptor = new DiagnosticDescriptor(Id, "MockDiagnostic", "MockDiagnostic", "InternalCategory", DiagnosticSeverity.Warning, isEnabledByDefault: true);
 
             public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics
             {

src/EditorFeatures/CSharpTest/Diagnostics/Suppression/SuppressionTests.cs

@@ -512,7 +512,7 @@ public class UserErrorDiagnosticSuppressionTests : CSharpPragmaWarningDisableSup
             {
                 private class UserDiagnosticAnalyzer : DiagnosticAnalyzer
                 {
-                    private DiagnosticDescriptor _descriptor =
+                    private readonly DiagnosticDescriptor _descriptor =
                         new DiagnosticDescriptor("ErrorDiagnostic", "ErrorDiagnostic", "ErrorDiagnostic", "ErrorDiagnostic", DiagnosticSeverity.Error, isEnabledByDefault: true);
 
                     public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics
@@ -577,7 +577,7 @@ public class DiagnosticWithBadIdSuppressionTests : CSharpPragmaWarningDisableSup
 
                 private class UserDiagnosticAnalyzer : DiagnosticAnalyzer
                 {
-                    private DiagnosticDescriptor _descriptor =
+                    private readonly DiagnosticDescriptor _descriptor =
                         new DiagnosticDescriptor("@~DiagnosticWithBadId", "DiagnosticWithBadId", "DiagnosticWithBadId", "DiagnosticWithBadId", DiagnosticSeverity.Info, isEnabledByDefault: true);
 
                     public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics
@@ -1557,7 +1557,7 @@ public class UserInfoDiagnosticSuppressionTests : CSharpLocalSuppressMessageSupp
             {
                 private class UserDiagnosticAnalyzer : DiagnosticAnalyzer
                 {
-                    private DiagnosticDescriptor _descriptor =
+                    private readonly DiagnosticDescriptor _descriptor =
                         new DiagnosticDescriptor("InfoDiagnostic", "InfoDiagnostic", "InfoDiagnostic", "InfoDiagnostic", DiagnosticSeverity.Info, isEnabledByDefault: true);
 
                     public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics

src/EditorFeatures/CSharpTest/Diagnostics/UseImplicitOrExplicitType/UseImplicitTypeTests.cs

@@ -1768,7 +1768,7 @@ public async Task SuggestVarOnLocalWithNonApparentCustomArrayType()
             await TestMissingInRegularAndScriptAsync(before, new TestParameters(options: ImplicitTypeWhereApparent()));
         }
 
-        private static string trivial2uple =
+        private static readonly string trivial2uple =
                     @"
 namespace System
 {

src/EditorFeatures/CSharpTest/EncapsulateField/EncapsulateFieldTestState.cs

@@ -22,7 +22,7 @@ namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.EncapsulateField
 {
     internal class EncapsulateFieldTestState : IDisposable
     {
-        private TestHostDocument _testDocument;
+        private readonly TestHostDocument _testDocument;
         public TestWorkspace Workspace { get; }
         public Document TargetDocument { get; }
         public string NotificationMessage { get; private set; }

src/EditorFeatures/CSharpTest/MakeLocalFunctionStatic/MakeLocalFunctionStaticTests.cs

@@ -18,8 +18,8 @@ public partial class MakeLocalFunctionStaticTests : AbstractCSharpDiagnosticProv
         internal override (DiagnosticAnalyzer, CodeFixProvider) CreateDiagnosticProviderAndFixer(Workspace workspace)
             => (new MakeLocalFunctionStaticDiagnosticAnalyzer(), new MakeLocalFunctionStaticCodeFixProvider());
 
-        private static ParseOptions CSharp72ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_2);
-        private static ParseOptions CSharp8ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp8);
+        private static readonly ParseOptions CSharp72ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_2);
+        private static readonly ParseOptions CSharp8ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp8);
 
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsMakeLocalFunctionStatic)]
         public async Task TestAboveCSharp8()

src/EditorFeatures/CSharpTest/SymbolId/SymbolKeyTestBase.cs

@@ -271,7 +271,7 @@ private static void GetSourceAliasSymbols(CSharpCompilation comp, List<ISymbol>
 
         private class LocalSymbolDumper
         {
-            private CSharpCompilation _compilation;
+            private readonly CSharpCompilation _compilation;
             public LocalSymbolDumper(CSharpCompilation compilation)
             {
                 _compilation = compilation;

src/EditorFeatures/CSharpTest/UseLocalFunction/UseLocalFunctionTests.cs

@@ -18,8 +18,8 @@ public partial class UseLocalFunctionTests : AbstractCSharpDiagnosticProviderBas
         internal override (DiagnosticAnalyzer, CodeFixProvider) CreateDiagnosticProviderAndFixer(Workspace workspace)
             => (new CSharpUseLocalFunctionDiagnosticAnalyzer(), new CSharpUseLocalFunctionCodeFixProvider());
 
-        private static ParseOptions CSharp72ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_2);
-        private static ParseOptions CSharp8ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp8);
+        private static readonly ParseOptions CSharp72ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_2);
+        private static readonly ParseOptions CSharp8ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp8);
 
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseLocalFunction)]
         public async Task TestMissingBeforeCSharp7()

src/EditorFeatures/CSharpTest/UseNamedArguments/UseNamedArgumentsTests.cs

@@ -14,7 +14,7 @@ namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.UseNamedArguments
 {
     public class UseNamedArgumentsTests : AbstractCSharpCodeActionTest
     {
-        private static ParseOptions CSharp72 = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_2);
+        private static readonly ParseOptions CSharp72 = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_2);
 
         protected override CodeRefactoringProvider CreateCodeRefactoringProvider(Workspace workspace, TestParameters parameters)
             => new CSharpUseNamedArgumentsCodeRefactoringProvider();

src/EditorFeatures/CSharpTest/UseSimpleUsingStatement/UseSimpleUsingStatementTests.cs

@@ -19,8 +19,8 @@ public partial class UseSimpleUsingStatementTests : AbstractCSharpDiagnosticProv
         internal override (DiagnosticAnalyzer, CodeFixProvider) CreateDiagnosticProviderAndFixer(Workspace workspace)
             => (new UseSimpleUsingStatementDiagnosticAnalyzer(), new UseSimpleUsingStatementCodeFixProvider());
 
-        private static ParseOptions CSharp72ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_2);
-        private static ParseOptions CSharp8ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp8);
+        private static readonly ParseOptions CSharp72ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_2);
+        private static readonly ParseOptions CSharp8ParseOptions = CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp8);
 
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseSimpleUsingStatement)]
         public async Task TestAboveCSharp8()