dotnet-csharp-source-generators

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

dotnet-csharp-source-generators

Guidance for both creating and consuming Roslyn source generators in .NET. Creating: IIncrementalGenerator, syntax providers, semantic analysis, emit patterns, diagnostic reporting, testing with CSharpGeneratorDriver. Consuming: [GeneratedRegex], [LoggerMessage], System.Text.Json source generation, [JsonSerializable].

Cross-references: [skill:dotnet-csharp-modern-patterns] for partial properties and related C# features, [skill:dotnet-csharp-coding-standards] for naming conventions.

Creating Source Generators

Project Setup

Source generators are shipped as analyzers targeting netstandard2.0.

<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.Analyzers" Version="3.3.4" PrivateAssets="all" />
    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.12.0" PrivateAssets="all" />
  </ItemGroup>
</Project>

Always target netstandard2.0. Generators load into the compiler process, which requires this TFM for compatibility. Use LangVersion>latest to write modern C# in the generator itself.

IIncrementalGenerator (Preferred)

Always use IIncrementalGenerator over the legacy ISourceGenerator. Incremental generators are cache-aware and only re-run when inputs change, making them significantly faster in IDE scenarios.

[Generator]
public sealed class AutoNotifyGenerator : IIncrementalGenerator
{
    public void Initialize(IncrementalGeneratorInitializationContext context)
    {
        // Step 1: Filter syntax nodes to candidate fields
        var fieldDeclarations = context.SyntaxProvider
            .ForAttributeWithMetadataName(
                "MyLib.AutoNotifyAttribute",
                predicate: static (node, _) => node is FieldDeclarationSyntax,
                transform: static (ctx, _) => GetFieldInfo(ctx))
            .Where(static info => info is not null)
            .Select(static (info, _) => info!.Value);

        // Step 2: Group fields by containing type, then emit one file per type
        context.RegisterSourceOutput(fieldDeclarations.Collect(),
            static (spc, fields) => Execute(fields, spc));
    }

    private static FieldInfo? GetFieldInfo(
        GeneratorAttributeSyntaxContext context)
    {
        var fieldSymbol = context.TargetSymbol as IFieldSymbol;
        if (fieldSymbol is null)
            return null;

        var containingType = fieldSymbol.ContainingType;

        // Use fully qualified type name to handle generic and nested types
        var fullTypeName = containingType.ToDisplayString(
            SymbolDisplayFormat.FullyQualifiedFormat
                .WithGlobalNamespaceStyle(SymbolDisplayGlobalNamespaceStyle.Omitted));

        return new FieldInfo(
            fieldSymbol.ContainingNamespace.IsGlobalNamespace
                ? ""
                : fieldSymbol.ContainingNamespace.ToDisplayString(),
            containingType.Name,
            fullTypeName,
            fieldSymbol.Name,
            fieldSymbol.Type.ToDisplayString());
    }

    private static void Execute(
        ImmutableArray<FieldInfo> fields,
        SourceProductionContext context)
    {
        // Group by fully qualified type name to emit one file per class
        foreach (var group in fields.GroupBy(f => f.FullTypeName))
        {
            var first = group.First();
            var ns = first.Namespace;
            var className = first.ClassName;
            var properties = new StringBuilder();

            foreach (var field in group)
            {
                var propertyName = GetPropertyName(field.FieldName);
                properties.AppendLine($$"""
                        public {{field.FieldType}} {{propertyName}}
                        {
                            get => {{field.FieldName}};
                            set
                            {
                                if (!global::System.Collections.Generic.EqualityComparer<{{field.FieldType}}>.Default.Equals({{field.FieldName}}, value))
                                {
                                    {{field.FieldName}} = value;
                                    PropertyChanged?.Invoke(this,
                                        new global::System.ComponentModel.PropertyChangedEventArgs(nameof({{propertyName}})));
                                }
                            }
                        }
                    """);
            }

            // Handle global namespace (no namespace declaration)
            var nsBlock = string.IsNullOrEmpty(ns) ? "" : $"namespace {ns};\n\n";

            var source = $$"""
                // <auto-generated/>
                #nullable enable

                {{nsBlock}}partial class {{className}}
                    : global::System.ComponentModel.INotifyPropertyChanged
                {
                    public event global::System.ComponentModel.PropertyChangedEventHandler? PropertyChanged;

                {{properties}}
                }
                """;

            // Include namespace in hint name to avoid collisions across namespaces
            var hintPrefix = string.IsNullOrEmpty(ns) ? className : $"{ns}.{className}";
            context.AddSource($"{hintPrefix}.AutoNotify.g.cs", source);
        }
    }

    private static string GetPropertyName(string fieldName)
        => fieldName.TrimStart('_') is [var first, .. var rest]
            ? $"{char.ToUpperInvariant(first)}{rest}"
            : fieldName;
}

internal readonly record struct FieldInfo(
    string Namespace,
    string ClassName,
    string FullTypeName,
    string FieldName,
    string FieldType);

Scope note: This example targets top-level, non-generic classes for clarity. A production generator should also handle generic type parameters (emitting matching partial class Foo<T> declarations) and nested types (emitting nested partial class hierarchies). Report a diagnostic for unsupported shapes rather than emitting invalid code.

Key Pipeline Design Rules

1. Filter early -- Use ForAttributeWithMetadataName or CreateSyntaxProvider with a tight predicate to minimize work.
2. Transform to simple data -- Extract only the data you need (strings, records) in the transform step. Never pass ISymbol or SyntaxNode through the pipeline (they hold the compilation alive and break caching).
3. Use value equality -- Pipeline outputs are compared by value. Use record struct or implement IEquatable<T> for custom types.
4. Emit deterministic output -- Same inputs must produce identical source. Use // <auto-generated/> and #nullable enable headers.

Syntax Providers

// ForAttributeWithMetadataName -- most common, filters by attribute
var candidates = context.SyntaxProvider.ForAttributeWithMetadataName(
    "MyLib.GenerateMapperAttribute",
    predicate: static (node, _) => node is ClassDeclarationSyntax,
    transform: static (ctx, _) => /* extract info */);

// CreateSyntaxProvider -- general-purpose, any syntax predicate
var candidates = context.SyntaxProvider.CreateSyntaxProvider(
    predicate: static (node, _) => node is MethodDeclarationSyntax m
        && m.Modifiers.Any(SyntaxKind.PartialKeyword),
    transform: static (ctx, _) => /* extract info */);

Diagnostic Reporting

Report errors and warnings through SourceProductionContext rather than throwing exceptions. To report location-specific diagnostics, include a Location in your pipeline data (captured from the syntax node in the transform step).

private static readonly DiagnosticDescriptor InvalidFieldType = new(
    id: "AN001",
    title: "Invalid field type for AutoNotify",
    messageFormat: "Field '{0}' must be a non-pointer type",
    category: "AutoNotify",
    defaultSeverity: DiagnosticSeverity.Error,
    isEnabledByDefault: true);

// In the transform step, capture location:
var location = context.TargetNode.GetLocation();

// In the Execute method, report with location:
context.ReportDiagnostic(Diagnostic.Create(
    InvalidFieldType,
    location,       // captured from syntax node, not from projected data
    fieldName));

Note: Location is not value-equatable, so including it in your pipeline record breaks incremental caching. A common pattern is to carry it as a separate field that you exclude from equality, or report diagnostics in a CreateSyntaxProvider step before projecting to value types.

Emit Patterns

// Prefer raw string literals for templates (C# 11+, in the generator project)
var source = $$"""
    // <auto-generated/>
    #nullable enable

    namespace {{ns}};

    partial class {{className}}
    {
        {{generatedMembers}}
    }
    """;

context.AddSource($"{className}.g.cs", source);

File naming convention: {TypeName}.{Feature}.g.cs -- the .g.cs suffix signals generated code and is excluded by many linters.

Post-Init Output (Static Source)

Use RegisterPostInitializationOutput for marker attributes and helper types that do not depend on user code:

context.RegisterPostInitializationOutput(static ctx =>
{
    ctx.AddSource("AutoNotifyAttribute.g.cs", """
        // <auto-generated/>
        namespace MyLib;

        [System.AttributeUsage(System.AttributeTargets.Field)]
        internal sealed class AutoNotifyAttribute : System.Attribute { }
        """);
});

Testing Source Generators

Use CSharpGeneratorDriver to run generators in-memory and verify output.

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;

[Fact]
public void Generator_ProducesExpectedOutput()
{
    // Arrange
    var source = """
        using MyLib;

        namespace TestApp;

        public partial class ViewModel
        {
            [AutoNotify]
            private string _name = "";
        }
        """;

    var syntaxTree = CSharpSyntaxTree.ParseText(source);
    var references = AppDomain.CurrentDomain.GetAssemblies()
        .Where(a => !a.IsDynamic && !string.IsNullOrEmpty(a.Location))
        .Select(a => MetadataReference.CreateFromFile(a.Location))
        .Cast<MetadataReference>()
        .ToList();

    var compilation = CSharpCompilation.Create("TestAssembly",
        [syntaxTree],
        references,
        new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));

    var generator = new AutoNotifyGenerator();

    // Act
    GeneratorDriver driver = CSharpGeneratorDriver.Create(generator);
    driver = driver.RunGeneratorsAndUpdateCompilation(
        compilation, out var outputCompilation, out var diagnostics);

    // Assert
    Assert.Empty(diagnostics.Where(d => d.Severity == DiagnosticSeverity.Error));

    var runResult = driver.GetRunResult();
    Assert.Single(runResult.GeneratedTrees);

    var generatedSource = runResult.GeneratedTrees[0].GetText().ToString();
    Assert.Contains("public string Name", generatedSource);
}

Snapshot Testing (Verify)

For more robust testing, use the Verify.SourceGenerators package to snapshot-test generated output:

[Fact]
public Task Generator_SnapshotTest()
{
    var source = """
        using MyLib;
        namespace TestApp;
        public partial class ViewModel
        {
            [AutoNotify]
            private string _name = "";
        }
        """;

    return TestHelper.Verify(source);
}

Consuming Built-In Source Generators

[GeneratedRegex] (net7.0+)

Compile-time regex generation. Zero runtime compilation cost, AOT-compatible.

public partial class Validators
{
    [GeneratedRegex(@"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$",
        RegexOptions.Compiled | RegexOptions.IgnoreCase)]
    private static partial Regex EmailRegex();

    public static bool IsValidEmail(string email)
        => EmailRegex().IsMatch(email);
}

Key rules:

• Method must be static partial returning Regex
• Place on partial class (or partial struct)
• Replaces new Regex(...) with zero allocation at runtime
• Supports all RegexOptions except RegexOptions.Compiled (which is ignored -- the source generator replaces it)

[LoggerMessage] (net6.0+)

High-performance structured logging with zero-allocation at log-disabled levels.

public static partial class LogMessages
{
    [LoggerMessage(Level = LogLevel.Information,
        Message = "Processing order {OrderId} for customer {CustomerId}")]
    public static partial void OrderProcessing(
        this ILogger logger, int orderId, string customerId);

    [LoggerMessage(Level = LogLevel.Error,
        Message = "Failed to process order {OrderId}")]
    public static partial void OrderProcessingFailed(
        this ILogger logger, int orderId, Exception exception);
}

// Usage
logger.OrderProcessing(order.Id, order.CustomerId);

Key rules:

• Methods must be static partial in a partial class
• Parameters matching {Placeholder} in the message are logged as structured data
• Exception parameter is logged automatically (do not include in message template)
• Event IDs are auto-assigned if not specified; specify explicit IDs for stable telemetry

System.Text.Json Source Generation (net6.0+)

AOT-compatible JSON serialization. Eliminates runtime reflection.

[JsonSerializable(typeof(Order))]
[JsonSerializable(typeof(List<Order>))]
[JsonSerializable(typeof(Customer))]
[JsonSourceGenerationOptions(
    PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull)]
public partial class AppJsonContext : JsonSerializerContext;

Registration in ASP.NET Core

builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.TypeInfoResolverChain.Insert(0, AppJsonContext.Default);
});

// Or for Minimal APIs
app.MapGet("/orders/{id}", async (int id, IOrderService service) =>
{
    var order = await service.GetByIdAsync(id);
    return order is not null
        ? Results.Ok(order)
        : Results.NotFound();
});

Manual Serialization

// Serialize
var json = JsonSerializer.Serialize(order, AppJsonContext.Default.Order);

// Deserialize
var order = JsonSerializer.Deserialize(json, AppJsonContext.Default.Order);

// With stream
await JsonSerializer.SerializeAsync(stream, orders,
    AppJsonContext.Default.ListOrder);

Key rules:

• Register all types that need serialization in [JsonSerializable] attributes
• Use TypeInfoResolverChain (net8.0+) to combine multiple contexts
• Required for Native AOT -- reflection-based serialization is trimmed
• See [skill:dotnet-csharp-modern-patterns] for related C# features used in generated code

[JsonSerializable] with Polymorphism (net7.0+)

[JsonDerivedType(typeof(CreditCardPayment), "credit")]
[JsonDerivedType(typeof(BankTransferPayment), "bank")]
public abstract class Payment
{
    public decimal Amount { get; init; }
}

public class CreditCardPayment : Payment
{
    public required string CardLast4 { get; init; }
}

public class BankTransferPayment : Payment
{
    public required string AccountNumber { get; init; }
}

[JsonSerializable(typeof(Payment))]
public partial class PaymentJsonContext : JsonSerializerContext;

Generator Reference: Packaging and Consumption

Referencing a Generator in a Consuming Project

<ItemGroup>
  <ProjectReference Include="..\MyGenerator\MyGenerator.csproj"
                    OutputItemType="Analyzer"
                    ReferenceOutputAssembly="false" />
</ItemGroup>

NuGet Package Layout

When shipping a generator as a NuGet package, place the assembly under analyzers/dotnet/cs/:

MyGenerator.nupkg
  analyzers/
    dotnet/
      cs/
        MyGenerator.dll
  lib/
    netstandard2.0/
      _._   (empty placeholder if no runtime dependency)

<!-- In the generator .csproj -->
<PropertyGroup>
  <IncludeBuildOutput>false</IncludeBuildOutput>
  <DevelopmentDependency>true</DevelopmentDependency>
</PropertyGroup>

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

Debugging Source Generators

// Add to Initialize() for attach-debugger workflow
#if DEBUG
if (!System.Diagnostics.Debugger.IsAttached)
{
    System.Diagnostics.Debugger.Launch();
}
#endif

Alternatively, emit generated files to disk for inspection:

<!-- In the consuming project -->
<PropertyGroup>
  <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
  <CompilerGeneratedFilesOutputPath>Generated</CompilerGeneratedFilesOutputPath>
</PropertyGroup>

Add Generated/ to .gitignore.

References

• Source Generator Cookbook
• Incremental Generators
• GeneratedRegex source generator
• Compile-time logging source generation
• System.Text.Json source generation
• .NET Framework Design Guidelines