dotnet-roslyn-analyzers

Imported from wshaddix/dotnet-skills (skills/dotnet-roslyn-analyzers/SKILL.md). Install upstream with npx skills add wshaddix/dotnet-skills --skill dotnet-roslyn-analyzers. Copyright stays with the author.

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.

Scope boundary: This skill covers writing analyzers. For consuming and configuring existing analyzers (CA rules, EditorConfig severity, third-party packages), see [skill:dotnet-add-analyzers]. For authoring source generators (IIncrementalGenerator, syntax providers, code emission), see [skill:dotnet-csharp-source-generators]. Analyzers and source generators share the same NuGet packaging layout (analyzers/dotnet/cs/) and Microsoft.CodeAnalysis.CSharp dependency, but serve different purposes: analyzers report diagnostics, generators emit code.

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>
    <!-- NuGet: Microsoft.CodeAnalysis.CSharp -->
    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.12.0" PrivateAssets="all" />
    <!-- NuGet: Microsoft.CodeAnalysis.Analyzers (meta-diagnostics for analyzer authors) -->
    <PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.11.0" PrivateAssets="all" />
  </ItemGroup>
</Project>

• EnforceExtendedAnalyzerRules enables RS-series meta-diagnostics that catch common analyzer authoring mistakes (see Meta-Diagnostics section below).
• 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)].

using System.Collections.Immutable;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Diagnostics;

[DiagnosticAnalyzer(LanguageNames.CSharp)]
public sealed class NoPublicFieldsAnalyzer : DiagnosticAnalyzer
{
    // Diagnostic ID uses project prefix + sequential number
    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}");

    // Return an ImmutableArray -- allocating a new array per call is RS1030-adjacent waste
    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; }
        = ImmutableArray.Create(Rule);

    public override void Initialize(AnalysisContext context)
    {
        // Required: enable concurrent execution and generated code analysis config
        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)
        {
            var diagnostic = Diagnostic.Create(
                Rule,
                field.Locations[0],
                field.Name);

            context.ReportDiagnostic(diagnostic);
        }
    }
}

Analysis Context Registration

Choose the most appropriate registration method for your analysis:

// RegisterSyntaxNodeAction -- analyze specific syntax nodes
context.RegisterSyntaxNodeAction(
    AnalyzeInvocation,
    SyntaxKind.InvocationExpression);

// RegisterOperationAction -- analyze semantic operations
context.RegisterOperationAction(
    AnalyzeAssignment,
    OperationKind.SimpleAssignment);

// RegisterCompilationStartAction -- accumulate state, then report at compilation end
context.RegisterCompilationStartAction(compilationContext =>
{
    // Resolve types once at compilation start
    var disposableType = compilationContext.Compilation
        .GetTypeByMetadataName("System.IDisposable");

    if (disposableType is null)
        return;

    compilationContext.RegisterSymbolAction(
        ctx => AnalyzeTypeDisposal(ctx, disposableType),
        SymbolKind.NamedType);
});

DiagnosticDescriptor Conventions

Follow these conventions for all custom analyzers:

ID Prefix Patterns

Use a short, unique prefix derived from your project or library name, followed by a sequential number:

Avoid prefixes reserved by the .NET platform: CA (code analysis), CS (compiler), RS (Roslyn SDK), IDE (code style), IL (linker), SYSLIB (runtime). Include the namespace in the ID constant to prevent collisions when multiple analyzer packages are installed.

Category Naming

Use standard .NET analysis categories where applicable:

Design, Globalization, Interoperability, Maintainability, Naming, Performance, Reliability, Security, Style, Usage

For domain-specific categories, use a clear, titlecase name (e.g., EntityFramework, AspNetCore).

Severity Selection

Default to Warning for most rules. Use Error sparingly -- users cannot suppress errors via EditorConfig without disabling the rule entirely.

helpLinkUri

Always provide a non-null helpLinkUri (RS1015 enforces this). Point to stable documentation:

helpLinkUri: $"https://github.com/myorg/mylib/blob/main/docs/rules/{DiagnosticId}.md"

CodeFixProvider

Code fix providers offer automated corrections for diagnostics. They inherit from CodeFixProvider and register fixes for specific diagnostic IDs.

using System.Collections.Immutable;
using System.Composition;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CodeActions;
using Microsoft.CodeAnalysis.CodeFixes;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

[ExportCodeFixProvider(LanguageNames.CSharp, Name = nameof(NoPublicFieldsCodeFixProvider))]
[Shared]
public sealed class NoPublicFieldsCodeFixProvider : CodeFixProvider
{
    public override ImmutableArray<string> FixableDiagnosticIds { get; }
        = ImmutableArray.Create(NoPublicFieldsAnalyzer.DiagnosticId);

    // Enable FixAll support -- RS1016 flags missing FixAllProvider
    public override FixAllProvider GetFixAllProvider()
        => WellKnownFixAllProviders.BatchFixer;

    public override async Task RegisterCodeFixesAsync(CodeFixContext context)
    {
        var root = await context.Document
            .GetSyntaxRootAsync(context.CancellationToken)
            .ConfigureAwait(false);

        var diagnostic = context.Diagnostics[0];
        var diagnosticSpan = diagnostic.Location.SourceSpan;

        var fieldDeclaration = root?.FindToken(diagnosticSpan.Start)
            .Parent?
            .AncestorsAndSelf()
            .OfType<FieldDeclarationSyntax>()
            .FirstOrDefault();

        if (fieldDeclaration is null)
            return;

        // EquivalenceKey enables FixAll grouping -- RS1010/RS1011 require unique keys
        context.RegisterCodeFix(
            CodeAction.Create(
                title: "Convert to property",
                createChangedDocument: ct =>
                    ConvertToPropertyAsync(context.Document, fieldDeclaration, ct),
                equivalenceKey: "ConvertToProperty"),
            diagnostic);
    }

    private static async Task<Document> ConvertToPropertyAsync(
        Document document,
        FieldDeclarationSyntax fieldDeclaration,
        CancellationToken cancellationToken)
    {
        var root = await document
            .GetSyntaxRootAsync(cancellationToken)
            .ConfigureAwait(false);

        var variable = fieldDeclaration.Declaration.Variables[0];
        var propertyName = variable.Identifier.Text;

        // Build auto-property with same type and accessibility
        var property = SyntaxFactory.PropertyDeclaration(
                fieldDeclaration.Declaration.Type,
                propertyName)
            .WithModifiers(fieldDeclaration.Modifiers)
            .WithAccessorList(
                SyntaxFactory.AccessorList(
                    SyntaxFactory.List(new[]
                    {
                        SyntaxFactory.AccessorDeclaration(SyntaxKind.GetAccessorDeclaration)
                            .WithSemicolonToken(SyntaxFactory.Token(SyntaxKind.SemicolonToken)),
                        SyntaxFactory.AccessorDeclaration(SyntaxKind.SetAccessorDeclaration)
                            .WithSemicolonToken(SyntaxFactory.Token(SyntaxKind.SemicolonToken))
                    })))
            .WithLeadingTrivia(fieldDeclaration.GetLeadingTrivia())
            .WithTrailingTrivia(fieldDeclaration.GetTrailingTrivia());

        var newRoot = root!.ReplaceNode(fieldDeclaration, property);
        return document.WithSyntaxRoot(newRoot);
    }
}

Key CodeFixProvider Patterns

• EquivalenceKey: Every CodeAction must have a unique equivalenceKey for FixAll support (RS1010, RS1011). Use the fix description or diagnostic ID as the key.
• Document vs. Solution modification: Use createChangedDocument when the fix modifies a single file. Use createChangedSolution when the fix must rename symbols across files or add new files.
• Trivia preservation: Always transfer leading/trailing trivia from replaced nodes to maintain formatting and comments.
• FixAllProvider: Return WellKnownFixAllProviders.BatchFixer for batch-applicable fixes. Omit only for fixes that require user interaction or have cross-fix dependencies.

DiagnosticSuppressor

A DiagnosticSuppressor conditionally suppresses diagnostics from other analyzers. Use this when your codebase has a pattern that a third-party analyzer flags incorrectly, and EditorConfig cannot express the suppression condition.

using System.Collections.Immutable;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.Diagnostics;

[DiagnosticAnalyzer(LanguageNames.CSharp)]
public sealed class CustomNullCheckSuppressor : DiagnosticSuppressor
{
    private static readonly SuppressionDescriptor SuppressCA1062 = new(
        id: "MYLIB_SUP001",
        suppressedDiagnosticId: "CA1062",
        justification: "Parameter is validated by custom Guard.NotNull helper");

    public override ImmutableArray<SuppressionDescriptor> SupportedSuppressions { get; }
        = ImmutableArray.Create(SuppressCA1062);

    public override void ReportSuppressions(SuppressionAnalysisContext context)
    {
        foreach (var diagnostic in context.ReportedDiagnostics)
        {
            var tree = diagnostic.Location.SourceTree;
            if (tree is null)
                continue;

            var root = tree.GetRoot(context.CancellationToken);
            var node = root.FindNode(diagnostic.Location.SourceSpan);

            // Walk up to the containing method and check for Guard.NotNull call
            var method = node.FirstAncestorOrSelf<Microsoft.CodeAnalysis.CSharp.Syntax.MethodDeclarationSyntax>();
            if (method is null)
                continue;

            var methodText = method.ToFullString();

            // Simplified check -- production code should use semantic analysis
            if (methodText.Contains("Guard.NotNull"))
            {
                context.ReportSuppression(
                    Suppression.Create(SuppressCA1062, diagnostic));
            }
        }
    }
}

When to Use DiagnosticSuppressor vs. EditorConfig

Suppressors cannot report new diagnostics -- they can only suppress existing ones. They participate in the same analyzer pipeline and follow the same netstandard2.0 targeting requirements.

Version gate: DiagnosticSuppressor requires Roslyn 3.8+. If your analyzer package targets older Roslyn versions via multi-version packaging, guard suppressor registration behind #if ROSLYN_3_8_OR_GREATER (see Multi-Roslyn-Version Targeting below).

CodeRefactoringProvider

A CodeRefactoringProvider offers code transformations triggered by the user (lightbulb menu) without requiring a diagnostic. Use this for structural improvements, pattern applications, or code generation that are not defects.

using System.Composition;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CodeActions;
using Microsoft.CodeAnalysis.CodeRefactorings;
using Microsoft.CodeAnalysis.CSharp.Syntax;

[ExportCodeRefactoringProvider(LanguageNames.CSharp, Name = nameof(ExtractInterfaceRefactoring))]
[Shared]
public sealed class ExtractInterfaceRefactoring : CodeRefactoringProvider
{
    public override async Task ComputeRefactoringsAsync(CodeRefactoringContext context)
    {
        var root = await context.Document
            .GetSyntaxRootAsync(context.CancellationToken)
            .ConfigureAwait(false);

        var node = root?.FindNode(context.Span);
        var classDecl = node?.FirstAncestorOrSelf<ClassDeclarationSyntax>();

        if (classDecl is null)
            return;

        // Only offer when cursor is on the class identifier
        if (!classDecl.Identifier.Span.IntersectsWith(context.Span))
            return;

        context.RegisterRefactoring(
            CodeAction.Create(
                title: $"Extract interface I{classDecl.Identifier.Text}",
                createChangedSolution: ct =>
                    ExtractInterfaceAsync(context.Document, classDecl, ct),
                equivalenceKey: "ExtractInterface"));
    }

    private static async Task<Solution> ExtractInterfaceAsync(
        Document document,
        ClassDeclarationSyntax classDecl,
        CancellationToken cancellationToken)
    {
        // ... build interface from public method signatures, add base type
        // See details.md for complete implementation
        throw new NotImplementedException();
    }
}

See details.md for the complete ExtractInterfaceAsync implementation (which adds a methods parameter for the extracted method list) and CSharpCodeRefactoringVerifier<T> test examples.

Key CodeRefactoringProvider Patterns

• Span check: Only offer refactorings when the cursor/selection intersects the relevant node. Broad span matching clutters the lightbulb menu.
• No diagnostic required: Unlike CodeFixProvider, refactoring providers do not fix a diagnostic -- they offer optional transformations.
• Solution-level changes: Use createChangedSolution when the refactoring adds files, renames symbols, or modifies multiple documents.
• Attribute requirements: Decorate with [ExportCodeRefactoringProvider(LanguageNames.CSharp)] and [Shared] (MEF). The provider is not registered via SupportedDiagnostics.
• Testing: Use CSharpCodeRefactoringVerifier<T> from Microsoft.CodeAnalysis.CSharp.CodeRefactoring.Testing (see details.md).

Multi-Roslyn-Version Targeting

Analyzer NuGet packages can ship multiple DLLs targeting different Roslyn versions, allowing the analyzer to use newer APIs when available while maintaining compatibility with older compilers.

Version Boundaries

The Roslyn SDK uses these version boundaries for multi-targeting:

Project Configuration (Meziantou.Analyzer Pattern)

Define a $(RoslynVersion) MSBuild property and reference Microsoft.CodeAnalysis.CSharp using Version="$(RoslynVersion).0". For multi-version builds, use Directory.Build.targets to parameterize the Roslyn version across build configurations (see details.md for the full project structure).

Conditional Compilation Constants

Define constants following the ROSLYN_X_Y and ROSLYN_X_Y_OR_GREATER pattern in Directory.Build.targets:

<PropertyGroup Condition="'$(RoslynVersion)' >= '3.8'">
  <DefineConstants>$(DefineConstants);ROSLYN_3_8;ROSLYN_3_8_OR_GREATER</DefineConstants>
</PropertyGroup>
<!-- Repeat for 4.2, 4.4, 4.6, 4.8, 4.14 -- see details.md for all six -->

Use these constants to guard version-specific code:

#if ROSLYN_4_8_OR_GREATER
    // CollectionExpression operation kind available in Roslyn 4.8+
    context.RegisterOperationAction(AnalyzeCollectionExpression,
        OperationKind.CollectionExpression);
#endif

NuGet Packaging Paths

Multi-version analyzers use version-specific NuGet paths: analyzers/dotnet/roslyn{version}/cs/ for each version, plus analyzers/dotnet/cs/ as the fallback for hosts below 3.8. The host selects the DLL from the highest matching roslyn{version} directory. Use <None Include="..." Pack="true" PackagePath="analyzers/dotnet/roslyn{version}/cs" /> items to place each build in its correct path. See details.md for the complete packaging .csproj and pack verification commands.

Multi-Version Test Matrix

Test each Roslyn version build independently by parameterizing $(RoslynVersion) in the test project. Use xUnit v3 with Microsoft.Testing.Platform v2 (MTP2) for the test runner:

# Build and test each Roslyn version (xUnit v3 + MTP2)
for version in 3.8 4.2 4.4; do
  dotnet test -p:RoslynVersion=$version
done

See details.md for a complete multi-version project structure (Directory.Build.props, Directory.Build.targets, packaging .csproj, and GitHub Actions CI matrix with xUnit v3).

Testing Analyzers

Use the Microsoft.CodeAnalysis.Testing infrastructure for ergonomic, high-level analyzer testing. This is preferred over raw CSharpCompilation testing.

Required NuGet Packages

<PropertyGroup>
  <!-- Enable Microsoft.Testing.Platform v2 runner -->
  <UseMicrosoftTestingPlatformRunner>true</UseMicrosoftTestingPlatformRunner>
</PropertyGroup>

<ItemGroup>
  <!-- NuGet: Microsoft.CodeAnalysis.CSharp.Analyzer.Testing (framework-agnostic, uses DefaultVerifier) -->
  <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Analyzer.Testing" Version="1.1.3" />
  <!-- NuGet: Microsoft.CodeAnalysis.CSharp.CodeFix.Testing -->
  <PackageReference Include="Microsoft.CodeAnalysis.CSharp.CodeFix.Testing" Version="1.1.3" />
  <!-- NuGet: Microsoft.CodeAnalysis.CSharp.Workspaces (dependency) -->
  <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="4.12.0" />
  <!-- NuGet: xunit.v3 (xUnit v3 test framework) -->
  <PackageReference Include="xunit.v3" Version="3.2.2" />
</ItemGroup>

Migration note: The framework-specific packages (e.g., Microsoft.CodeAnalysis.CSharp.Analyzer.Testing.XUnit) are obsolete. Use the generic packages with DefaultVerifier instead. This decouples Roslyn testing infrastructure from the test framework, making it compatible with xUnit v3 and Microsoft.Testing.Platform v2 (MTP2).

Analyzer-Only Testing

Use CSharpAnalyzerVerifier<T> to test that diagnostics are reported (or not reported) correctly:

using Microsoft.CodeAnalysis.Testing;
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);
    }

    [Fact]
    public async Task PrivateField_NoDiagnostic()
    {
        var test = """
            public class MyClass
            {
                private int _value;
            }
            """;

        await Verify.VerifyAnalyzerAsync(test);
    }

    // Also test: public const fields (no diagnostic), public readonly fields (no diagnostic)
}

Diagnostic Markup Syntax

The testing framework uses markup to indicate expected diagnostic locations:

Analyzer + CodeFix Testing

Use CSharpCodeFixVerifier<TAnalyzer, TCodeFix> to test the full analyzer-to-fix pipeline:

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

public class NoPublicFieldsCodeFixTests
{
    [Fact]
    public async Task PublicField_FixConvertsToProperty()
    {
        var test = """
            public class MyClass
            {
                public int {|MYLIB001:Value|};
            }
            """;

        var fixedCode = """
            public class MyClass
            {
                public int Value { get; set; }
            }
            """;

        await Verify.VerifyCodeFixAsync(test, fixedCode);
    }
}

Multi-File Test Scenarios

For multi-file tests, use the Verify.Test class with TestState.Sources to add multiple named source files. Set expected diagnostics in each file using the standard markup syntax. Use test.RunAsync() instead of the static VerifyAnalyzerAsync shorthand.

NuGet Packaging

Analyzers are shipped as NuGet packages with a specific directory layout. The assemblies go under analyzers/dotnet/cs/, not lib/.

Package Layout

MyAnalyzers.nupkg
  analyzers/
    dotnet/
      cs/
        MyAnalyzers.dll           # Analyzer assembly
        MyAnalyzers.CodeFixes.dll # Code fix assembly (optional, separate)
  lib/
    netstandard2.0/
      _._                        # Empty placeholder (no runtime dependency)

Project Configuration

<!-- Analyzer .csproj -->
<PropertyGroup>
  <IncludeBuildOutput>false</IncludeBuildOutput>
  <DevelopmentDependency>true</DevelopmentDependency>
  <SuppressDependenciesWhenPacking>true</SuppressDependenciesWhenPacking>
</PropertyGroup>

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

Separate Analyzer and Code Fix Assemblies

For multi-analyzer NuGet packages, separating analyzers from code fixes improves IDE load time. The IDE loads code fix assemblies lazily (only when the user requests a fix), while analyzer assemblies load immediately. Create two projects (MyAnalyzers and MyAnalyzers.CodeFixes), then pack both DLLs into analyzers/dotnet/cs/ using <None Include="..." Pack="true" PackagePath="analyzers/dotnet/cs" /> items.

Pack Verification

After packing, verify the layout with unzip -l ./bin/Release/MyAnalyzers.1.0.0.nupkg | grep analyzers/ (nupkg files are zip archives). See [skill:dotnet-csharp-source-generators] for additional shared packaging concepts (the analyzers/dotnet/cs/ layout is identical for both analyzers and source generators).

Performance Best Practices

Analyzers run in real-time during editing. Poor performance degrades the IDE experience for every user of the analyzer.

Allocation-Free Callbacks

Avoid allocations in hot-path callbacks. Every RegisterSyntaxNodeAction callback runs per-node, potentially thousands of times per keystroke:

// BAD: resolves type on every symbol callback (per-symbol overhead)
context.RegisterSymbolAction(symbolCtx =>
{
    // GetTypeByMetadataName is called for EVERY symbol in the compilation
    var disposableType = symbolCtx.Compilation
        .GetTypeByMetadataName("System.IDisposable");
    if (disposableType is null) return;

    Analyze(symbolCtx, disposableType);
}, SymbolKind.NamedType);

// GOOD: resolve state once per compilation, closure allocated once
context.RegisterCompilationStartAction(compilationCtx =>
{
    var importantType = compilationCtx.Compilation
        .GetTypeByMetadataName("System.IDisposable");
    if (importantType is null) return;

    // One closure per compilation -- acceptable cost
    compilationCtx.RegisterSymbolAction(
        symbolCtx => Analyze(symbolCtx, importantType),
        SymbolKind.NamedType);
});

Note: A single closure allocated once per compilation inside RegisterCompilationStartAction is acceptable. The anti-pattern is resolving types or allocating closures inside per-node or per-symbol callbacks, where the cost multiplies across thousands of invocations.

Symbol-Based Filtering

Prefer symbol-based analysis over syntax-based analysis when both are viable. Symbol analysis operates on the compiler's resolved model and avoids re-parsing:

// Prefer: RegisterSymbolAction for declared-symbol checks
context.RegisterSymbolAction(ctx =>
{
    var method = (IMethodSymbol)ctx.Symbol;
    if (method.ReturnsVoid && method.Parameters.Length == 0) { /* ... */ }
}, SymbolKind.Method);

// Avoid: RegisterSyntaxNodeAction when symbol analysis suffices
// Syntax analysis requires manually resolving types and handling aliases

ImmutableArray for SupportedDiagnostics

Cache SupportedDiagnostics as an ImmutableArray<DiagnosticDescriptor> field. The runtime calls this property frequently:

// GOOD: single allocation, cached
public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; }
    = ImmutableArray.Create(Rule1, Rule2, Rule3);

// BAD: allocates a new array on every access
public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics
    => ImmutableArray.Create(Rule1, Rule2, Rule3);

Additional Performance Guidelines

• Enable concurrent execution: Always call context.EnableConcurrentExecution() unless your analyzer has shared mutable state (which it should not).
• Avoid per-compilation state in fields: Do not store per-compilation data in analyzer instance fields (RS1008). Use RegisterCompilationStartAction to scope state to a single compilation.
• Filter early in syntax predicates: When using RegisterSyntaxNodeAction, register for the most specific SyntaxKind possible to minimize callback frequency.
• Avoid Compilation.GetSemanticModel(): Use the SemanticModel provided by the analysis context instead (RS1030).

Common Meta-Diagnostics (RS-Series)

The Microsoft.CodeAnalysis.Analyzers package reports RS-series diagnostics on your analyzer code itself. These are invaluable for catching authoring mistakes at compile time. Enable them via EnforceExtendedAnalyzerRules in your project file.

Frequently Encountered RS Diagnostics

Release Tracking (RS2000-Series)

For mature analyzers, enable release tracking to manage diagnostic ID lifecycle:

Release tracking uses AnalyzerReleases.Shipped.md and AnalyzerReleases.Unshipped.md files to track which diagnostic IDs have been published.

References

• Tutorial: Write your first analyzer and code fix
• Roslyn SDK overview
• Microsoft.CodeAnalysis.Testing
• Analyzer NuGet packaging conventions
• dotnet/roslyn-analyzers (RS diagnostic source)
• Meziantou.Analyzer (exemplar project)