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Getting Started with Source Generators

The Dispatch source generators enable compile-time service registration with full AOT (Ahead-of-Time) compatibility. Instead of relying on runtime reflection to discover and register services, the source generator analyzes your code at compile time and generates explicit registration code.

Before You Start​

  • .NET 8.0+ (or .NET 9/10 for latest features)
  • Install the source generator package: 
    dotnet add package Excalibur.Dispatch.SourceGenerators
  • Familiarity with dependency injection and actions and handlers

Why Use Source Generators?​

BenefitDescription
No Runtime ReflectionAll service discovery happens at compile time
Faster StartupNo assembly scanning required at application start
Native AOT SupportCompatible with PublishAot=true for .NET 8+
Trimming SafeNo types unexpectedly removed by IL trimmer
Explicit ControlOpt-in registration prevents surprises

Quick Start​

1. Install the Package​

dotnet add package Excalibur.Dispatch.SourceGenerators

The [AutoRegister] attribute is included in Excalibur.Dispatch.Abstractions, which is automatically referenced.

2. Mark Your Services​

Add the [AutoRegister] attribute to classes you want automatically registered:

using Excalibur.Dispatch.Abstractions;
using Microsoft.Extensions.DependencyInjection;

// Basic usage - registers as Scoped by default
[AutoRegister]
public class OrderHandler : IDispatchHandler<CreateOrderCommand>
{
    public Task<IMessageResult> HandleAsync(
        CreateOrderCommand message,
        IMessageContext context,
        CancellationToken ct)
    {
        // Handle the command
        return Task.FromResult(MessageResult.Success());
    }
}

3. Call the Generated Extension​

In your Program.cs or startup configuration, call the generated extension method:

var builder = WebApplication.CreateBuilder(args);

// Register all services marked with [AutoRegister]
builder.Services.AddGeneratedServices();

// Your other service registrations...
builder.Services.AddDispatch();

var app = builder.Build();

That's it! The source generator discovers all [AutoRegister] types at compile time and generates the AddGeneratedServices() extension method with explicit registrations.

Attribute Options​

The [AutoRegister] attribute provides full control over how services are registered:

Service Lifetime​

// Default: Scoped (recommended for request-scoped services)
[AutoRegister]
public class ScopedService : IScopedService { }

// Singleton: One instance for the entire application
[AutoRegister(Lifetime = ServiceLifetime.Singleton)]
public class CacheService : ICacheService { }

// Transient: New instance every time resolved
[AutoRegister(Lifetime = ServiceLifetime.Transient)]
public class HelperService : IHelperService { }

Registration Mode​

Control whether the service is registered by its concrete type, interfaces, or both:

// Default: Register as both concrete type AND all interfaces
[AutoRegister]
public class MyService : IFirst, ISecond { }
// Generates:
//   services.AddScoped<MyService>();
//   services.AddScoped<IFirst, MyService>();
//   services.AddScoped<ISecond, MyService>();

// Concrete type only (no interface registration)
[AutoRegister(AsSelf = true, AsInterfaces = false)]
public class InternalHelper { }
// Generates:
//   services.AddScoped<InternalHelper>();

// Interfaces only (no concrete type registration)
[AutoRegister(AsSelf = false, AsInterfaces = true)]
public class MultiImplementation : IReader, IWriter { }
// Generates:
//   services.AddScoped<IReader, MultiImplementation>();
//   services.AddScoped<IWriter, MultiImplementation>();

Common Patterns​

Handlers​

// Command handler (Scoped is appropriate for request-scoped handlers)
[AutoRegister]
public class CreateOrderHandler : IDispatchHandler<CreateOrderCommand>
{
    private readonly IOrderRepository _repository;

    public CreateOrderHandler(IOrderRepository repository)
    {
        _repository = repository;
    }

    public async Task<IMessageResult> HandleAsync(
        CreateOrderCommand command,
        IMessageContext context,
        CancellationToken ct)
    {
        var order = Order.Create(command.CustomerId, command.Items);
        await _repository.SaveAsync(order, ct);
        return MessageResult.Success();
    }
}

Repository Services​

// Repository with explicit Scoped lifetime (matches DbContext scope)
[AutoRegister(Lifetime = ServiceLifetime.Scoped)]
public class OrderRepository : IOrderRepository
{
    private readonly IEventStore _eventStore;

    public OrderRepository(IEventStore eventStore)
    {
        _eventStore = eventStore;
    }

    // Implementation...
}

Singleton Caches​

// Singleton for application-wide caching
[AutoRegister(Lifetime = ServiceLifetime.Singleton)]
public class HandlerMetadataCache : IHandlerMetadataCache
{
    private readonly ConcurrentDictionary<Type, HandlerMetadata> _cache = new();

    // Implementation...
}

Middleware​

// Middleware typically uses Scoped lifetime
[AutoRegister]
public class ValidationMiddleware : IDispatchMiddleware
{
    public DispatchMiddlewareStage? Stage => DispatchMiddlewareStage.Validation;

    public async ValueTask<IMessageResult> InvokeAsync(
        IDispatchMessage message,
        IMessageContext context,
        DispatchRequestDelegate nextDelegate,
        CancellationToken ct)
    {
        // Validation logic...
        return await nextDelegate(message, context, ct);
    }
}

Generated Code​

The source generator produces a single file with an extension method. You can inspect it by enabling generated file output:

<!-- In your .csproj -->
<PropertyGroup>
    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
    <CompilerGeneratedFilesOutputPath>$(BaseIntermediateOutputPath)\GeneratedFiles</CompilerGeneratedFilesOutputPath>
</PropertyGroup>

Example generated output:

// <auto-generated/>
namespace Microsoft.Extensions.DependencyInjection;

public static class GeneratedServiceCollectionExtensions
{
    /// <summary>
    /// Registers all services discovered at compile time via [AutoRegister] attribute.
    /// </summary>
    public static IServiceCollection AddGeneratedServices(this IServiceCollection services)
    {
        services.AddScoped<MyApp.Services.OrderHandler>();
        services.AddScoped<MyApp.Services.IDispatchHandler<CreateOrderCommand>, MyApp.Services.OrderHandler>();
        services.AddSingleton<MyApp.Services.CacheService>();
        services.AddSingleton<MyApp.Services.ICacheService, MyApp.Services.CacheService>();

        return services;
    }

    /// <summary>
    /// Gets the count of services discovered at compile time.
    /// </summary>
    public static int GeneratedServiceCount => 2;
}

Coexistence with Manual Registration​

Generated registrations work alongside manual registrations:

var builder = WebApplication.CreateBuilder(args);

// Generated registrations from [AutoRegister]
builder.Services.AddGeneratedServices();

// Manual registrations for special cases
builder.Services.AddSingleton<ISpecialService>(sp =>
    new SpecialService(configuration["SpecialKey"]));

// Excalibur framework
builder.Services.AddDispatch();

var app = builder.Build();

Services without [AutoRegister] must be registered manually. This explicit control prevents unexpected registrations.

Excluded Types​

The generator automatically skips:

  • Abstract classes - Cannot be instantiated
  • Static classes - Cannot be registered as services
  • System interfaces - IDisposable, IAsyncDisposable, etc. are not registered

Troubleshooting​

Generated Method Not Found​

If AddGeneratedServices() is not available:

  1. Ensure Excalibur.Dispatch.SourceGenerators package is referenced
  2. Clean and rebuild the solution
  3. Check the build output for analyzer errors

Build Diagnostic SRG001​

When services are discovered, you'll see an informational diagnostic:

info SRG001: Discovered 5 type(s) with [AutoRegister] attribute for service registration

This confirms the generator is working correctly.

No Services Discovered​

If GeneratedServiceCount is 0:

  1. Verify classes have [AutoRegister] attribute
  2. Ensure classes are not abstract or static
  3. Check that the attribute namespace Excalibur.Dispatch.Abstractions is imported

Next Steps​

See Also​