Add a base type for all code style analyzers to help reduce duplicated code.

src/Features/Core/Portable/CodeStyle/AbstractCodeStyleDiagnosticAnalyzer.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.
+
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace Microsoft.CodeAnalysis.CodeStyle
+{
+    internal abstract class AbstractCodeStyleDiagnosticAnalyzer : DiagnosticAnalyzer
+    {
+        private readonly string _id;
+        private readonly string _title;
+        private readonly DiagnosticDescriptor _descriptor;
+        private readonly DiagnosticDescriptor _unnecessaryWithSuggestionDescriptor;
+        private readonly DiagnosticDescriptor _unnecessaryWithoutSuggestionDescriptor;
+
+        protected AbstractCodeStyleDiagnosticAnalyzer(string id, string title)
+        {
+            _id = id;
+            _title = title;
+            _descriptor = CreateDescriptor(id, DiagnosticSeverity.Hidden);
+            _unnecessaryWithSuggestionDescriptor = CreateDescriptor(
+                id, DiagnosticSeverity.Hidden, DiagnosticCustomTags.Unnecessary);
+            _unnecessaryWithoutSuggestionDescriptor = CreateDescriptor(id + "WithoutSuggestion",
+                DiagnosticSeverity.Hidden, DiagnosticCustomTags.Unnecessary);
+            SupportedDiagnostics = ImmutableArray.Create(
+                _descriptor, _unnecessaryWithoutSuggestionDescriptor, _unnecessaryWithSuggestionDescriptor);
+        }
+
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; }
+
+        private DiagnosticDescriptor CreateDescriptor(string id, DiagnosticSeverity severity, params string[] customTags)
+            => new DiagnosticDescriptor(
+                id,
+                _title,
+                _title,
+                DiagnosticCategory.Style,
+                severity,
+                isEnabledByDefault: true,
+                customTags: customTags);
+    }
+}
\ No newline at end of file

src/Features/Core/Portable/Features.csproj

@@ -92,6 +92,7 @@
     <Compile Include="..\..\..\Compilers\Shared\DesktopShim.cs">
       <Link>Shared\Utilities\DesktopShim.cs</Link>
     </Compile>
+    <Compile Include="CodeStyle\AbstractCodeStyleDiagnosticAnalyzer.cs" />
     <Compile Include="NavigateTo\AbstractNavigateToSearchService.SearchResult.cs" />
     <Compile Include="AddMissingReference\AbstractAddMissingReferenceCodeFixProvider.cs" />
     <Compile Include="AddMissingReference\CodeAction.cs" />