AutoDispatch.Generator 1.4.0

AutoDispatch.Generator

AutoDispatch gives you the MediatR-style handler pattern without IRequest<T>, IRequestHandler<,>, reflection, or runtime dispatch overhead. Mark a handler with [Handler], write Handle or HandleAsync, and the generator emits a strongly-typed dispatcher at build time.

Why AutoDispatch?

• Same mental model as MediatR — command/query + handler + dispatcher
• Zero reflection — direct generated calls, no runtime dispatch overhead
• Pipeline behaviors — [Behavior(Order = N)] wraps all async handlers at compile time; no IPipelineBehavior<,> magic at runtime
• No marker interfaces — commands stay as plain POCOs
• AOT-friendly — everything is compile-time generated
• DI-ready — AddAutoDispatch() wires up handlers, behaviors, and IDispatcher

Installation

dotnet add package AutoDispatch.Generator

Then register the generated dispatcher:

builder.Services.AddAutoDispatch();

Before vs After

MediatR-style boilerplate

using MediatR;

public sealed record CreateOrderCommand(string CustomerId) : IRequest<OrderId>;

public sealed class CreateOrderHandler : IRequestHandler<CreateOrderCommand, OrderId>
{
    public Task<OrderId> Handle(CreateOrderCommand request, CancellationToken cancellationToken)
    {
        // ...
    }
}

AutoDispatch

using AutoDispatch;

public sealed record CreateOrderCommand(string CustomerId);

[Handler]
public sealed class CreateOrderHandler
{
    public Task<OrderId> HandleAsync(CreateOrderCommand command, CancellationToken ct = default)
    {
        // ...
    }
}

What gets generated

Given one or more [Handler] classes, AutoDispatch emits:

1. AutoDispatch.HandlerAttribute
2. AutoDispatch.IDispatcher
3. AutoDispatch.Dispatcher
4. AddAutoDispatch() for IServiceCollection

Example generated dispatcher:

#nullable enable
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.DependencyInjection;

namespace AutoDispatch
{
    public interface IDispatcher
    {
        Task<OrderId> SendAsync(CreateOrderCommand command, CancellationToken ct = default);
        void Send(DeleteOrderCommand command);
    }

    internal sealed class Dispatcher : IDispatcher
    {
        private readonly IServiceProvider _sp;

        public Dispatcher(IServiceProvider sp) => _sp = sp;

        public Task<OrderId> SendAsync(CreateOrderCommand command, CancellationToken ct = default)
            => _sp.GetRequiredService<CreateOrderHandler>().HandleAsync(command, ct);

        public void Send(DeleteOrderCommand command)
            => _sp.GetRequiredService<DeleteOrderHandler>().Handle(command);
    }
}

Conventions

AutoDispatch discovers public instance non-static methods on classes marked with [Handler].

Supported signatures:

Rules:

• Only methods named exactly Handle or HandleAsync
• Handle must have exactly one command parameter
• HandleAsync may have one command parameter, or a second CancellationToken
• Methods with zero parameters or more than two parameters are ignored
• Dispatcher is generated as internal sealed
• AddAutoDispatch() registers handlers with AddScoped

Semantic aliases

[CommandHandler] and [QueryHandler] are aliases for [Handler] — use whichever reads best in your codebase.

[CommandHandler]
public sealed class CreateOrderHandler
{
    public Task<OrderId> HandleAsync(CreateOrderCommand command, CancellationToken ct = default)
        => Task.FromResult(new OrderId(Guid.NewGuid()));
}

[QueryHandler]
public sealed class GetOrderHandler
{
    public Task<Order?> HandleAsync(GetOrderQuery query, CancellationToken ct = default)
        => Task.FromResult<Order?>(null);
}

All three attributes are equivalent — the generated code is identical.

Usage

using AutoDispatch;

public sealed record CreateOrderCommand(string CustomerId);
public sealed record DeleteOrderCommand(Guid OrderId);
public sealed record OrderId(Guid Value);

[Handler]
public sealed class CreateOrderHandler
{
    public Task<OrderId> HandleAsync(CreateOrderCommand command, CancellationToken ct = default)
        => Task.FromResult(new OrderId(Guid.NewGuid()));
}

[Handler]
public sealed class DeleteOrderHandler
{
    public void Handle(DeleteOrderCommand command)
    {
    }
}

Then consume the generated dispatcher:

app.MapPost("/orders", async (CreateOrderCommand command, AutoDispatch.IDispatcher dispatcher, CancellationToken ct) =>
{
    var orderId = await dispatcher.SendAsync(command, ct);
    return Results.Ok(orderId);
});

Generated DI registration

builder.Services.AddAutoDispatch();

Produces code like:

services.AddScoped<CreateOrderHandler>();
services.AddScoped<DeleteOrderHandler>();
services.AddScoped<AutoDispatch.IDispatcher, AutoDispatch.Dispatcher>();

Pipeline behaviors

[Behavior(Order = N)] wraps all async handlers in a compile-time pipeline. Identical mental model to MediatR's IPipelineBehavior<,> — but the chain is emitted as generated code, not resolved via reflection at runtime.

Behavior requirements:

• The behavior class must be public, non-abstract, and open-generic with exactly two type parameters
• It must implement IPipelineBehavior<TCommand, TResult>
• It must expose public Task<TResult> HandleAsync(TCommand command, Func<Task<TResult>> next, CancellationToken ct = default)

Define a behavior

using AutoDispatch;

[Behavior(Order = 0)]
public sealed class LoggingBehavior<TCommand, TResult>
    : IPipelineBehavior<TCommand, TResult>
{
    private readonly ILogger<LoggingBehavior<TCommand, TResult>> _logger;

    public LoggingBehavior(ILogger<LoggingBehavior<TCommand, TResult>> logger)
        => _logger = logger;

    public async Task<TResult> HandleAsync(
        TCommand command,
        Func<Task<TResult>> next,
        CancellationToken ct = default)
    {
        _logger.LogInformation("→ {Command}", typeof(TCommand).Name);
        var result = await next();
        _logger.LogInformation("← {Command}", typeof(TCommand).Name);
        return result;
    }
}

That's all. AddAutoDispatch() registers it automatically.

Multiple behaviors

[Behavior(Order = 0)]  // runs first (outermost)
public sealed class LoggingBehavior<TCmd, TResult> : IPipelineBehavior<TCmd, TResult> { ... }

[Behavior(Order = 1)]  // runs second
public sealed class ValidationBehavior<TCmd, TResult> : IPipelineBehavior<TCmd, TResult> { ... }

[Behavior(Order = 2)]  // runs last (innermost, just before the handler)
public sealed class TimingBehavior<TCmd, TResult> : IPipelineBehavior<TCmd, TResult> { ... }

Execution order: Logging → Validation → Timing → Handler → Timing → Validation → Logging.

When multiple behaviors have the same Order, AutoDispatch preserves declaration order.

What gets generated

For Task<OrderId> SendAsync(CreateOrderCommand) with two behaviors:

// Generated dispatcher method:
public Task<OrderId> SendAsync(CreateOrderCommand command, CancellationToken ct = default)
{
    Func<Task<OrderId>> pipeline =
        () => _sp.GetRequiredService<CreateOrderHandler>().HandleAsync(command, ct);
    var _b1 = _sp.GetRequiredService<TimingBehavior<CreateOrderCommand, OrderId>>();
    var _p1 = pipeline;
    pipeline = () => _b1.HandleAsync(command, _p1, ct);
    var _b0 = _sp.GetRequiredService<LoggingBehavior<CreateOrderCommand, OrderId>>();
    var _p0 = pipeline;
    pipeline = () => _b0.HandleAsync(command, _p0, ct);
    return pipeline();
}

Behaviors and void-async handlers

For Task (no result) handlers, the generator wraps the call in Task<Unit> internally. Unit is emitted by the generator — you never reference it directly; the method signature stays Task SendAsync(...).

Behaviors can also short-circuit by returning a result without calling next().

Behaviors only apply to async handlers

Sync T Send(...) and void Send(...) methods are not wrapped. Add a pipeline when you migrate a sync handler to async, or keep it sync for zero overhead.

Diagnostics

AD001

[Handler] on '{Type}' has no Handle or HandleAsync methods. No dispatch methods will be generated.

Add a valid Handle or HandleAsync method to the handler class.

AD002

Duplicate handler for command '{Command}': both '{HandlerA}' and '{HandlerB}' define a Handle/HandleAsync method for this command type. Remove one handler or rename the method.

Each command/query type must map to exactly one handler method.

AD003

HandleAsync on '{Handler}' for command '{Command}' is missing a CancellationToken parameter. Consider adding CancellationToken ct = default as the second parameter.`

The method still works; the warning helps you preserve cancellation flow.

AD004

[Behavior] on '{Type}' must be a public, non-abstract class with exactly two type parameters so AutoDispatch can close it as <TCommand, TResult>.`

Pipeline behaviors are resolved as closed generics at dispatch time, so [Behavior] types must be declared as open generic classes such as LoggingBehavior<TCommand, TResult>.

AD005

[Behavior] on '{Type}' must implement AutoDispatch.IPipelineBehavior<TCommand, TResult> using its declared type parameters.`

Implement the generated IPipelineBehavior<TCommand, TResult> interface directly on the behavior type.

AD006

[Behavior] on '{Type}' must declare public Task<TResult> HandleAsync(TCommand command, Func<Task<TResult>> next, CancellationToken ct = default).`

Explicit interface implementations are not enough — the generated dispatcher calls the behavior's public HandleAsync method directly.

AutoDispatch vs alternatives

Migrating from MediatR

AutoDispatch follows the same CQRS mental model as MediatR, so migration is mechanical:

1. Install AutoDispatch and remove MediatR

dotnet add package AutoDispatch.Generator
dotnet remove package MediatR
dotnet remove package MediatR.Extensions.Microsoft.DependencyInjection

2. Remove marker interfaces from commands

// Before
public sealed record CreateOrderCommand(string CustomerId) : IRequest<OrderId>;

// After
public sealed record CreateOrderCommand(string CustomerId);

3. Convert handler classes

// Before
public sealed class CreateOrderHandler : IRequestHandler<CreateOrderCommand, OrderId>
{
    public Task<OrderId> Handle(CreateOrderCommand request, CancellationToken cancellationToken)
        => Task.FromResult(new OrderId(Guid.NewGuid()));
}

// After
[Handler]
public sealed class CreateOrderHandler
{
    public Task<OrderId> HandleAsync(CreateOrderCommand command, CancellationToken ct = default)
        => Task.FromResult(new OrderId(Guid.NewGuid()));
}

4. Convert pipeline behaviors

// Before
public sealed class LoggingBehavior<TRequest, TResponse>
    : IPipelineBehavior<TRequest, TResponse> where TRequest : notnull
{
    public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken ct)
    {
        _logger.LogInformation("→ {Request}", typeof(TRequest).Name);
        var result = await next();
        _logger.LogInformation("← {Request}", typeof(TRequest).Name);
        return result;
    }
}

// After
[Behavior(Order = 0)]
public sealed class LoggingBehavior<TCommand, TResult>
    : IPipelineBehavior<TCommand, TResult>
{
    public async Task<TResult> HandleAsync(TCommand command, Func<Task<TResult>> next, CancellationToken ct = default)
    {
        _logger.LogInformation("→ {Command}", typeof(TCommand).Name);
        var result = await next();
        _logger.LogInformation("← {Command}", typeof(TCommand).Name);
        return result;
    }
}

5. Update DI registration

// Before
builder.Services.AddMediatR(cfg => cfg.RegisterServicesFromAssemblyContaining<Program>());

// After
builder.Services.AddAutoDispatch();

6. Update dispatch call sites

// Before (IMediator)
var orderId = await mediator.Send(new CreateOrderCommand(customerId), ct);

// After (IDispatcher)
var orderId = await dispatcher.SendAsync(new CreateOrderCommand(customerId), ct);

Tip: Use the AutoDispatch Migrator Copilot agent to automate the migration across your entire codebase.

Best fit

Use AutoDispatch when you want:

• CQRS-style organization without MediatR ceremony
• Build-time generated dispatch code
• Fast startup and predictable runtime behavior
• Plain C# command/query types with no framework coupling

Also by the same author

🌐 Full suite overview: swevo.github.io

License

MIT