Add "Make type partial" codefix

src/Analyzers/CSharp/CodeFixes/CSharpCodeFixes.projitems

@@ -34,6 +34,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)HideBase\HideBaseCodeFixProvider.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Iterator\CSharpAddYieldCodeFixProvider.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Iterator\CSharpChangeToIEnumerableCodeFixProvider.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)MakeDeclarationPartial\CSharpMakeDeclarationPartialCodeFixProvider.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MakeMemberStatic\CSharpMakeMemberStaticCodeFixProvider.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MakeStatementAsynchronous\CSharpMakeStatementAsynchronousCodeFixProvider.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MakeStructReadOnly\CSharpMakeStructReadOnlyCodeFixProvider.cs" />

src/Analyzers/CSharp/CodeFixes/MakeDeclarationPartial/CSharpMakeDeclarationPartialCodeFixProvider.cs

@@ -0,0 +1,28 @@
+// 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;
+using System.Collections.Immutable;
+using System.Composition;
+using Microsoft.CodeAnalysis.CodeFixes;
+using Microsoft.CodeAnalysis.Host.Mef;
+using Microsoft.CodeAnalysis.MakeDeclarationPartial;
+
+namespace Microsoft.CodeAnalysis.CSharp.MakeDeclarationPartial
+{
+    [ExportCodeFixProvider(LanguageNames.CSharp, Name = PredefinedCodeFixProviderNames.MakeDeclarationPartial), Shared]
+    internal sealed class CSharpMakeDeclarationPartialCodeFixProvider : AbstractMakeDeclarationPartialCodeFixProvider
+    {
+        private const string CS0260 = nameof(CS0260);
+
+        [ImportingConstructor]
+        [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
+        public CSharpMakeDeclarationPartialCodeFixProvider()
+        {
+        }
+
+        public override ImmutableArray<string> FixableDiagnosticIds { get; } =
+            ImmutableArray.Create(CS0260);
+    }
+}

src/Analyzers/CSharp/Tests/CSharpAnalyzers.UnitTests.projitems

@@ -24,6 +24,8 @@
     <Compile Include="$(MSBuildThisFileDirectory)HideBase\HideBaseTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Iterator\AddYieldTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Iterator\ChangeToIEnumerableTests.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)MakeDeclarationPartial\MakeDeclarationPartialFixAllTests.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)MakeDeclarationPartial\MakeDeclarationPartialTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MakeMemberStatic\MakeMemberStaticTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MakeStatementAsynchronous\CSharpMakeStatementAsynchronousCodeFixTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MakeTypeAbstract\MakeTypeAbstractTests.cs" />

src/Analyzers/CSharp/Tests/MakeDeclarationPartial/MakeDeclarationPartialFixAllTests.cs

@@ -0,0 +1,174 @@
+// 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;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.CSharp.MakeDeclarationPartial;
+using Microsoft.CodeAnalysis.Editor.UnitTests.CodeActions;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Microsoft.CodeAnalysis.Testing;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.CSharp.UnitTests.MakeDeclarationPartial
+{
+    using VerifyCS = CSharpCodeFixVerifier<
+        EmptyDiagnosticAnalyzer,
+        CSharpMakeDeclarationPartialCodeFixProvider>;
+
+    [Trait(Traits.Feature, Traits.Features.CodeActionsMakeDeclarationPartial)]
+    public sealed class MakeDeclarationPartialFixAllTests
+    {
+        public static IEnumerable<object[]> AllValidDeclarationTypes()
+        {
+            yield return new[] { "class" };
+            yield return new[] { "struct" };
+            yield return new[] { "interface" };
+            yield return new[] { "record" };
+            yield return new[] { "record class" };
+            yield return new[] { "record struct" };
+        }
+
+        [Theory]
+        [MemberData(nameof(AllValidDeclarationTypes))]
+        public async Task SeveralDeclarationsOfSingleClass(string declarationType)
+        {
+            await new VerifyCS.Test
+            {
+                TestCode = $$"""
+                partial {{declarationType}} Declaration
+                {
+                }
+
+                {{declarationType}} {|CS0260:Declaration|}
+                {
+                }
+
+                {{declarationType}} {|CS0260:Declaration|}
+                {
+                }
+                """,
+                FixedCode = $$"""
+                partial {{declarationType}} Declaration
+                {
+                }
+
+                partial {{declarationType}} Declaration
+                {
+                }
+
+                partial {{declarationType}} Declaration
+                {
+                }
+                """,
+                LanguageVersion = LanguageVersion.CSharp10
+            }.RunAsync();
+        }
+
+        [Theory]
+        [MemberData(nameof(AllValidDeclarationTypes))]
+        public async Task SingleDeclarationOfSeveralClasses(string declarationType)
+        {
+            await new VerifyCS.Test
+            {
+                TestCode = $$"""
+                partial {{declarationType}} Declaration1
+                {
+                }
+
+                {{declarationType}} {|CS0260:Declaration1|}
+                {
+                }
+
+                partial {{declarationType}} Declaration2
+                {
+                }
+
+                {{declarationType}} {|CS0260:Declaration2|}
+                {
+                }
+                """,
+                FixedCode = $$"""
+                partial {{declarationType}} Declaration1
+                {
+                }
+
+                partial {{declarationType}} Declaration1
+                {
+                }
+
+                partial {{declarationType}} Declaration2
+                {
+                }
+
+                partial {{declarationType}} Declaration2
+                {
+                }
+                """,
+                LanguageVersion = LanguageVersion.CSharp10
+            }.RunAsync();
+        }
+
+        [Theory]
+        [MemberData(nameof(AllValidDeclarationTypes))]
+        public async Task SeveralDeclarationsOfSeveralClasses(string declarationType)
+        {
+            await new VerifyCS.Test
+            {
+                TestCode = $$"""
+                partial {{declarationType}} Declaration1
+                {
+                }
+
+                {{declarationType}} {|CS0260:Declaration1|}
+                {
+                }
+
+                {{declarationType}} {|CS0260:Declaration1|}
+                {
+                }
+
+                partial {{declarationType}} Declaration2
+                {
+                }
+
+                {{declarationType}} {|CS0260:Declaration2|}
+                {
+                }
+
+                {{declarationType}} {|CS0260:Declaration2|}
+                {
+                }
+                """,
+                FixedCode = $$"""
+                partial {{declarationType}} Declaration1
+                {
+                }
+
+                partial {{declarationType}} Declaration1
+                {
+                }
+
+                partial {{declarationType}} Declaration1
+                {
+                }
+
+                partial {{declarationType}} Declaration2
+                {
+                }
+
+                partial {{declarationType}} Declaration2
+                {
+                }
+
+                partial {{declarationType}} Declaration2
+                {
+                }
+                """,
+                LanguageVersion = LanguageVersion.CSharp10
+            }.RunAsync();
+        }
+    }
+}