Mediator — lightweight, fast, assembly-scoped mediator for ASP.NET Core

A strongly-typed mediator implementation for .NET — similar to MediatR but explicitly designed for high performance, assembly-scoped behaviors, module isolation, and a clean extensibility model.

Supports:

• Requests (Commands / Queries)
• Notifications
• Pipeline behaviors (validation, idempotence, logging, transactions…)
• Assembly-filtered behaviors
• DI integration without reflection-heavy scanning
• Simple ISender / IPublisher API

---

Table of contents

• Install
• Quick Start
• API Overview
• Configuration
• Using Requests (Commands / Queries)
• Using Notifications
• Pipeline Behaviors
• Advanced: Behavior Assembly Filtering
• Endpoint Example
• FAQ
• License

---

Install

dotnet add package RmToolkit.Mediator

or:

<PackageReference Include="RmToolkit.Mediator" Version="*" />

---

Quick Start

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddMediator(conf =>
{
    conf.HandlerAssembliesToRegister = new[]
    {
        typeof(Program).Assembly,
        MyModule.AssemblyInfo.Assembly
    };

    conf.RequestHandlerLifetime = ServiceLifetime.Transient;
    conf.NotificationPublisherLifetime = ServiceLifetime.Transient;

    conf.AddBehavior(typeof(ValidationBehavior<,>));
});

var app = builder.Build();
app.Run();

---

API Overview

ISender

public interface ISender
{
    Task<TResponse> SendAsync<TRequest, TResponse>(TRequest request, CancellationToken cancellationToken = default)
        where TRequest : IRequest<TResponse>;

    Task<TResponse> SendAsync<TResponse>(IRequest<TResponse> request, CancellationToken cancellationToken = default);

    Task<Result> SendAsync(ICommand command, CancellationToken cancellationToken = default);

    Task<Result<TResponse>> SendAsync<TResponse>(ICommand<TResponse> command, CancellationToken cancellationToken = default);
}

IPublisher

public interface IPublisher
{
    Task PublishAsync<TNotification>(TNotification notification, CancellationToken cancellationToken = default)
        where TNotification : INotification;
}

Marker Interfaces

public interface IRequest<TResponse> { }
public interface ICommand : IRequest<Result> { }
public interface ICommand<TResponse> : IRequest<Result<TResponse>> { }
public interface IQuery<TResponse> : IRequest<Result<TResponse>> { }

public interface INotification { }

---

Configuration

Your mediator is configured via:

builder.Services.AddMediator(conf =>
{
    conf.HandlerAssembliesToRegister = [...];
    conf.RequestHandlerLifetime = ServiceLifetime.Transient;
    conf.NotificationPublisherLifetime = ServiceLifetime.Transient;

    conf.AddBehavior(typeof(LoggingBehavior<,>));
});

✔ Updated AddBehavior API

public IMediatorConfigurator AddBehavior<TBehavior>(
    Action<BehaviorBuilder>? configure = null,
    ServiceLifetime lifetime = ServiceLifetime.Transient
)
    => AddBehavior(typeof(TBehavior), configure, lifetime);

public IMediatorConfigurator AddBehavior(
    Type behaviorType,
    Action<BehaviorBuilder>? configure = null,
    ServiceLifetime lifetime = ServiceLifetime.Transient
)
{
    _services.AddBehavior(behaviorType, configure, lifetime);
    return this;
}

---

Using Requests (Commands / Queries)

Define a Command

public sealed record CreateOrder(int OrderId) : ICommand<int>;

Define a Handler

internal sealed class CreateOrderHandler : ICommandHandler<CreateOrder, int>
{
    public async Task<Result<int>> Handle(CreateOrder request, CancellationToken ct)
    {
        return Result.Success(request.OrderId);
    }
}

Send a Command

var result = await sender.SendAsync(new CreateOrder(55));

---

Using Notifications

Notification type

public sealed record UserRegistered(string UserId, string Email) : INotification;

Handler

internal class SendWelcomeEmailHandler : INotificationHandler<UserRegistered>
{
    public Task Handle(UserRegistered notification, CancellationToken ct)
    {
        Console.WriteLine($"Sending welcome email to {notification.Email}");
        return Task.CompletedTask;
    }
}

Publishing

await publisher.PublishAsync(new UserRegistered("u1", "a@example.com"));

---

Pipeline Behaviors

A pipeline behavior wraps the request handler:

public interface IPipelineBehavior<TRequest, TResponse>
{
    Task<TResponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TRequest, TResponse> next,
        CancellationToken ct
    );
}

Example: Validation Behavior

public sealed class ValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
{
    public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TRequest, TResponse> next, CancellationToken ct)
    {
        Console.WriteLine("Validation...");
        return await next(request, ct);
    }
}

Register

conf.AddBehavior(typeof(ValidationBehavior<,>));

---

Advanced: Behavior Assembly Filtering

You can apply a behavior only to handlers in specific assemblies.

conf.AddBehavior(typeof(ModuleAIdempotentBehavior<,>),
    b => b.ForAssemblyHandlers(ModuleA.AssemblyInfo.Assembly));

conf.AddBehavior(typeof(ModuleBIdempotentBehavior<,>),
    b => b.ForAssemblyHandlers(ModuleB.AssemblyInfo.Assembly));

This lets you:

• isolate module behaviors
• enforce module-level cross-cutting rules
• avoid global behavior pollution

---

Endpoint Example

app.MapGet("/orders/{id:int}", async (int id, ISender sender) =>
{
    var result = await sender.SendAsync(new CreateOrder(id));
    return result;
});

---

FAQ

Do behaviors apply globally?

Yes, unless you configure assembly filtering using BehaviorBuilder.

Can I completely isolate module behaviors?

Yes — module-level behaviors execute only for handlers in the module’s assembly.

Are handlers auto-registered?

Yes, from assemblies listed in:

conf.HandlerAssembliesToRegister

---

License

MIT License — free for personal and commercial use.