Covariant IRequest Handler

[jfgauron]

Hi, I would like to have a single IRequestHandler that take cares of all my IRequest commands / queries. I was able to achieve this with INotificationHandler but the very same example with IRequest instead of INotification fails, so I assume there is a technical reason for that? I would need to make it work with IRequest because I need to return a value of a specific type depending on the IRequest sent.

1) Am I correct to assume that it is not possible to do? 2) If yes to previous question, I'd be curious to know what is the reason?

Also, I've found that I was able to achieve what I was looking to do using Behavior, here is an example of what I did:

using MediatR;

namespace MediatRDebug;

public interface ICommand<TResponse> : IRequest<TResponse>
{
    public Task<TResponse> Execute();
}

public class MyPipeline<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : ICommand<TResponse>
{

    public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken cancellationToken)
    {
        return await request.Execute();
    }
}

public class MyCommand : ICommand<string>
{
    public Task<string> Execute()
    {
        return Task.FromResult("This is a test");
    }
}

Now, it works as I would expect, but it feels kind of weird using the pipeline like that. Am I doing something wrong here or this would be the recommended way to achieve what I'm trying to do?

Thanks.

[jbogard]

Probably my best answer here is "don't use MediatR". It was not designed for such a scenario. You're probably better off building your own mediator class and handling the generic funny business yourself.