How do I register a MediatR post processor - .net-core

Question

I would like to try the new pipeline feature in MediatR: https://github.com/jbogard/MediatR/wiki/Behaviors
I tried the following, but it does not get executed
services.AddMediatR();
services.AddTransient(typeof(IRequestPostProcessor<,>), typeof(PostHandler<,>));
What am I missing?

Answer 1

You need to register the behavior associated with post-processors, like this unit test shows.
Your registration code would look like:
services.AddMediatR();
services.AddTransient(typeof(IRequestPostProcessor<,>), typeof(PostHandler<,>));
services.AddTransient(typeof(IPipelineBehavior<,>), typeof(RequestPostProcessorBehavior<,>));
That behavior will get all the post-processors you registered and execute them.
Edit
After a comment about the post-processor running twice, I had a look at the code that registers MediatR in the ASP.NET Core built-in DI container, and it turns out instances of IRequestPreProcessor<TRequest, TResponse> and IRequestPostProcessor<TRequest, TResponse> are automatically registered as you can see here. What's left to do to get them running in the pipeline is just register the associated behavior. So the necessary registration is then:
services.AddMediatR();
services.AddTransient(typeof(IPipelineBehavior<,>), typeof(RequestPostProcessorBehavior<,>));

Answer 2

I encountered the same issue as Larsi in the comment above. My code looked like this
services.AddScoped<IPipelineBehavior<AddMessageRequest, MessageResponse>, RequestPostProcessorBehavior<AddMessageRequest, MessageResponse>>();
but the behaviour still executed twice. My solution was to simply not register it manually, seems like the registration is handled otherwise now.
In my case services.AddMediatR(Assembly.GetExecutingAssembly()); was enough.
Would be nice if someone could expand on why this is.