Skills
skills/rudironsoni/dotnet-harness-plugin/dotnet-roslyn-analyzers

dotnet-roslyn-analyzers

SKILL.md

dotnet-roslyn-analyzers

Guidance for authoring custom Roslyn analyzers, code fix providers, code refactoring providers, and diagnostic suppressors. Covers project setup, DiagnosticDescriptor conventions, analysis context registration, code fix actions, code refactoring actions, multi-Roslyn-version targeting (3.8 through 4.14), testing with Microsoft.CodeAnalysis.Testing, NuGet packaging, and performance best practices.

For extended code examples (CodeRefactoringProvider implementation, multi-version project structure, test matrix configuration), see details.md in this skill directory.

Scope

  • DiagnosticAnalyzer authoring and analysis context registration
  • CodeFixProvider and CodeRefactoringProvider implementation
  • Multi-Roslyn-version targeting (3.8 through 4.14)
  • Testing with Microsoft.CodeAnalysis.Testing
  • NuGet packaging for analyzer assemblies

Out of scope

  • Consuming and configuring existing analyzers (CA rules, severity) -- see [skill:dotnet-add-analyzers]
  • Authoring source generators (IIncrementalGenerator) -- see [skill:dotnet-csharp-source-generators]
  • Naming conventions -- see [skill:dotnet-csharp-coding-standards]

Cross-references: [skill:dotnet-csharp-source-generators] for shared Roslyn packaging concepts and IIncrementalGenerator patterns, [skill:dotnet-add-analyzers] for consuming and configuring analyzers, [skill:dotnet-testing-strategy] for general test organization and framework selection, [skill:dotnet-csharp-coding-standards] for naming conventions applied to analyzer code.

Project Setup

Analyzer projects must target netstandard2.0. The compiler loads analyzers into various host processes (Visual Studio on .NET Framework/Mono, MSBuild on .NET Core, dotnet build CLI) -- targeting net8.0+ breaks compatibility with hosts that do not run on that runtime.


<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <EnforceExtendedAnalyzerRules>true</EnforceExtendedAnalyzerRules>
    <IsRoslynComponent>true</IsRoslynComponent>
    <LangVersion>latest</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.12.0" PrivateAssets="all" />
    <PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.11.0" PrivateAssets="all" />
  </ItemGroup>
</Project>

```csharp

- `EnforceExtendedAnalyzerRules` enables RS-series meta-diagnostics that catch common analyzer authoring mistakes.
- `IsRoslynComponent` enables IDE tooling support for the project.
- `LangVersion>latest` lets you write modern C# in the analyzer itself while still targeting `netstandard2.0`.
- All Roslyn SDK packages must use `PrivateAssets="all"` to avoid shipping them as transitive dependencies.

---

## DiagnosticAnalyzer

Every analyzer inherits from `DiagnosticAnalyzer` and must be decorated with `[DiagnosticAnalyzer(LanguageNames.CSharp)]`.

```csharp

[DiagnosticAnalyzer(LanguageNames.CSharp)]
public sealed class NoPublicFieldsAnalyzer : DiagnosticAnalyzer
{
    public const string DiagnosticId = "MYLIB001";

    private static readonly DiagnosticDescriptor Rule = new(
        id: DiagnosticId,
        title: "Public fields should be properties",
        messageFormat: "Field '{0}' is public; use a property instead",
        category: "Design",
        defaultSeverity: DiagnosticSeverity.Warning,
        isEnabledByDefault: true,
        helpLinkUri: $"https://example.com/docs/rules/{DiagnosticId}");

    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; }
        = ImmutableArray.Create(Rule);

    public override void Initialize(AnalysisContext context)
    {
        context.EnableConcurrentExecution();
        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
        context.RegisterSymbolAction(AnalyzeField, SymbolKind.Field);
    }

    private static void AnalyzeField(SymbolAnalysisContext context)
    {
        var field = (IFieldSymbol)context.Symbol;
        if (field.DeclaredAccessibility == Accessibility.Public
            && !field.IsConst && !field.IsReadOnly)
        {
            context.ReportDiagnostic(Diagnostic.Create(Rule, field.Locations[0], field.Name));
        }
    }
}

```text

### Analysis Context Registration

| Method | Granularity | Use When |
|--------|-------------|----------|
| `RegisterSyntaxNodeAction` | Individual syntax nodes | Pattern matching on specific syntax |
| `RegisterSymbolAction` | Declared symbols | Checking symbol-level properties |
| `RegisterOperationAction` | IL-level operations | Analyzing semantic operations |
| `RegisterSyntaxTreeAction` | Entire syntax tree | File-level checks |
| `RegisterCompilationStartAction` | Compilation start | Accumulate state across compilation |
| `RegisterCompilationAction` | Full compilation | One-shot analysis after all files |

---

## DiagnosticDescriptor Conventions

### ID Prefix Patterns

| Pattern | Example | When |
|---------|---------|------|
| `PROJ###` | `MYLIB001` | Single-project analyzers |
| `AREA####` | `PERF0001` | Category-scoped analyzers |
| `XX####` | `MA0042` | Short-prefix convention |

Avoid prefixes reserved by the .NET platform: `CA`, `CS`, `RS`, `IDE`, `IL`, `SYSLIB`.

### Severity Selection

| Severity | Use When |
|----------|----------|
| `Error` | Code will not work correctly at runtime |
| `Warning` | Code works but violates best practices |
| `Info` | Suggestion for improvement |
| `Hidden` | IDE-only refactoring suggestion |

Default to `Warning` for most rules. Always provide a non-null `helpLinkUri` (RS1015 enforces this).

---

## CodeFixProvider

Code fix providers offer automated corrections for diagnostics. Key patterns:

- **EquivalenceKey:** Every `CodeAction` must have a unique `equivalenceKey` for FixAll support (RS1010, RS1011)
- **Document vs. Solution modification:** Use `createChangedDocument` for single-file fixes, `createChangedSolution` for cross-file renames
- **Trivia preservation:** Always transfer leading/trailing trivia from replaced nodes
- **FixAllProvider:** Return `WellKnownFixAllProviders.BatchFixer` for batch-applicable fixes

See `details.md` for the complete CodeFixProvider implementation with property conversion.

---

## DiagnosticSuppressor

Conditionally suppresses diagnostics from other analyzers when EditorConfig cannot express the suppression condition. Requires Roslyn 3.8+.

| Approach | Use When |
|----------|----------|
| EditorConfig severity override | Suppression applies unconditionally |
| `[SuppressMessage]` attribute | Suppression applies to a specific location |
| `DiagnosticSuppressor` | Suppression depends on code structure or patterns |

---

## Multi-Roslyn-Version Targeting

### Version Boundaries

| Roslyn Version | Ships With | Key APIs Added |
|---------------|------------|----------------|
| 3.8 | VS 16.8 / .NET 5 SDK | `DiagnosticSuppressor` |
| 4.2 | VS 17.2 / .NET 6 SDK | Improved incremental analysis |
| 4.4 | VS 17.4 / .NET 7 SDK | `ForAttributeWithMetadataName` |
| 4.8 | VS 17.8 / .NET 8 U1 | `CollectionExpression` support |
| 4.14 | VS 17.14 / .NET 10 SDK | Latest API surface |

Use conditional compilation constants (`ROSLYN_X_Y_OR_GREATER`) and version-specific NuGet paths (`analyzers/dotnet/roslyn{version}/cs/`). See `details.md` for the complete multi-version project structure.

---

## Testing Analyzers

Use `Microsoft.CodeAnalysis.Testing` for ergonomic analyzer testing:

```csharp

using Verify = Microsoft.CodeAnalysis.CSharp.Testing.CSharpAnalyzerVerifier<
    NoPublicFieldsAnalyzer,
    Microsoft.CodeAnalysis.Testing.DefaultVerifier>;

public class NoPublicFieldsAnalyzerTests
{
    [Fact]
    public async Task PublicField_ReportsDiagnostic()
    {
        var test = """
            public class MyClass
            {
                public int {|MYLIB001:Value|};
            }
            """;
        await Verify.VerifyAnalyzerAsync(test);
    }
}

```text

### Diagnostic Markup Syntax

| Markup | Meaning |
|--------|---------|
| `[|text|]` | Diagnostic expected on `text` (single descriptor) |
| `{|DIAG_ID:text|}` | Diagnostic with specific ID expected |

---

## NuGet Packaging

Analyzers ship as NuGet packages with assemblies in `analyzers/dotnet/cs/`, not `lib/`.

```xml

<PropertyGroup>
  <IncludeBuildOutput>false</IncludeBuildOutput>
  <DevelopmentDependency>true</DevelopmentDependency>
  <SuppressDependenciesWhenPacking>true</SuppressDependenciesWhenPacking>
</PropertyGroup>

<ItemGroup>
  <None Include="$(OutputPath)\$(AssemblyName).dll"
        Pack="true"
        PackagePath="analyzers/dotnet/cs" />
</ItemGroup>

```text

---

## Performance Best Practices

- **Resolve types once per compilation** inside `RegisterCompilationStartAction`, not per-node/symbol callbacks
- **Cache `SupportedDiagnostics`** as `ImmutableArray` field, not expression-bodied property
- **Enable concurrent execution** -- always call `context.EnableConcurrentExecution()`
- **Filter early** -- register for the most specific `SyntaxKind` possible
- **Avoid `Compilation.GetSemanticModel()`** -- use the `SemanticModel` from the analysis context (RS1030)

---

## Common Meta-Diagnostics (RS-Series)

| ID | Title | What It Catches |
|----|-------|-----------------|
| RS1001 | Missing `DiagnosticAnalyzerAttribute` | Analyzer class missing attribute |
| RS1008 | Avoid storing per-compilation data | Instance fields with compilation data |
| RS1010 | Create code actions with unique `EquivalenceKey` | Missing equivalence key |
| RS1015 | Provide non-null `helpLinkUri` | Empty help link |
| RS1016 | Code fix providers should provide FixAll support | Missing `GetFixAllProvider()` |
| RS1024 | Symbols should be compared for equality | Using `==` instead of `SymbolEqualityComparer` |
| RS1026 | Enable concurrent execution | Missing `EnableConcurrentExecution()` |
| RS1030 | Do not invoke `Compilation.GetSemanticModel()` | Using wrong semantic model source |
| RS1041 | Compiler extensions should target `netstandard2.0` | Wrong target framework |

---

## References

- [Tutorial: Write your first analyzer and code fix](https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/tutorials/how-to-write-csharp-analyzer-code-fix)
- [Roslyn SDK overview](https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/)
- [Microsoft.CodeAnalysis.Testing](https://github.com/dotnet/roslyn-sdk/tree/main/src/Microsoft.CodeAnalysis.Testing)
- [Analyzer NuGet packaging conventions](https://learn.microsoft.com/en-us/nuget/guides/analyzers-conventions)
- [dotnet/roslyn-analyzers (RS diagnostic source)](https://github.com/dotnet/roslyn-analyzers)
- [Meziantou.Analyzer (exemplar project)](https://github.com/meziantou/Meziantou.Analyzer)
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