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How to Add Compiler Diagnostic Fix

tip

See Contributing with agent skills and the add-compiler-diagnostic-fix skill in the roslynator repo.

Discuss the Idea​

Start by opening an issue to describe your idea. Once maintainers approve it, you can begin implementation.

Add Diagnostic Metadata​

If the compiler diagnostic is not already listed, add it to Diagnostics.xml:

<Diagnostic
  Id="CS0163"
  Identifier="ControlCannotFallThroughFromOneCaseLabelToAnother"
  Severity="Error"
  Title="Control cannot fall through from one case label to another"
  Message="Control cannot fall through from one case label to another"
  HelpUrl="https://learn.microsoft.com/dotnet/csharp/misc/cs0163" />

Add Code Fix Metadata​

Add a new RCF#### entry to CodeFixes.xml:

<CodeFix Id="RCF0002" Identifier="AddBreakStatementToSwitchSection" Title="Add break statement to switch section">
  <FixableDiagnosticIds>
    <Id>CS0163</Id>
    <Id>CS8070</Id>
  </FixableDiagnosticIds>
</CodeFix>

One RCF fix can target multiple CS ids; one CS id can have multiple RCF fixes.

Generate Code​

From the tools directory:

cd tools && pwsh ./generate_code.ps1

Or open the CodeGenerator project in Visual Studio.

Implement Code Fix Provider​

Add a class in CodeFixes/CSharp/CodeFixes deriving from CompilerDiagnosticCodeFixProvider.

  • Use CompilerDiagnosticIdentifiers.CS####_Identifier constants in FixableDiagnosticIds (not bare "CS####" strings).
  • Call IsEnabled(...) in RegisterCodeFixesAsync so EditorConfig toggles work.

Add Tests​

Add a new file to CodeFixes.Tests. Convention: CS####IdentifierTests.cs.

Use AbstractCSharpCompilerDiagnosticFixVerifier from the in-repo test framework — not XunitCompilerDiagnosticFixVerifier from the NuGet package. See compiler diagnostic fix testing for the public NuGet API.

tip

Compiler fix tests do not use [| and |] span markers — the compiler diagnostic location is implicit.

Update Changelog​

File: CHANGELOG.md at the repository root.

CHANGELOG.md
### Added

- Add code fix "FIX_TITLE" for CS#### ([#PR_ID](https://github.com/dotnet/roslynator/pull/PR_ID))