How do I register and use a MediatR pipeline handler, for ASP.NET Core?

Question

I'm using ASP.NET Core, the latest MediatR, and MediatR extension for Core's DI.

I'm trying to set up a pipeline with validation, using the official blog post. The example is here.

I don't understand how to register/use that pipeline class. Another blog post shows how to do this, but I think it's for AutoFac.

How do I do this for the built-in container?

Answer 1

The posts you mention use MediatR 2.x.
MediatR 3.0 has been released not long ago, and has built-in support for pipelines. I would suggest you read the associated documentation.

In short, MediatR now exposes a IPipelineBehavior<TRequest, TResponse>, and the instances you register in your container will be discovered automatically by MediatR when constructing the handler.

Here's what it could look like in ASP.NET Core:

public class MyRequest : IRequest<string>
{
}

public class MyRequestHandler : IRequestHandler<MyRequest, string>
{
    public string Handle(MyRequest message)
    {
        return "Hello!";
    }
}

public class TracingBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
{
    public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next)
    {
        Trace.WriteLine("Before");
        var response = await next();
        Trace.WriteLine("After");

        return response;
    }
}

Very simple, a request, a handler and a behavior that does some "logging".

Registration is very easy, too:

var services = new ServiceCollection();
services.AddMediatR(typeof(Program));
services.AddTransient(typeof(IPipelineBehavior<,>), typeof(TracingBehaviour<,>));
var provider = services.BuildServiceProvider();

var mediator = provider.GetRequiredService<IMediator>();

var response = await mediator.Send(new MyRequest());

It's just a matter of registering the open-generic TracingBehavior as a generic implementation of IPipelineBehavior.