Issue: "No service for type 'MediatR.IRequestHandler'"

[AlbertoVPersonal]

Environment

• Windows 11 Pro
• .NET8
• MediatR 12.4.1
• Console app
• All classes are in the same assembly

Issue description

I want to use generic handlers with notifications. Due to I was not able to do it, I have changed to generic requests.
And now I'm getting the below error message.

Issue error message

System.InvalidOperationException: No service for type 'MediatR.IRequestHandler`1[NotificationsMediatR.Notifications.GenericBasicTypeNotification`1[System.String]]' has been registered.

Source code

Handlers

public class LocalMediatR
{
    private static LocalMediatR _instance;

    private IMediator _mediator;

    // ...
}

public class HandlerBaseProperties {
  // ...
}

public class BaseNotification : IRequest {
 // ...
}

internal class GenericBasicTypeNotification<T> : BaseNotification { 
 // ...
}

internal class GenericBaseTypeNoResponseHandler<T> : HandlerBaseProperties, IRequestHandler<GenericBasicTypeNotification<T>>
{
    public async Task Handle(GenericBasicTypeNotification<T> request, CancellationToken cancellationToken) {
       // ...
   }
}

Configure services

//...

// Inject MediatR
services.AddMediatR(cfg => {
    cfg.MaxTypesClosing = 0;
    cfg.RegisterGenericHandlers = true;
    cfg.RegisterServicesFromAssembly(typeof(Startup).Assembly);
});

services.AddSingleton<IMediator, Mediator>();
services.AddTransient<BaseWithoutResponseTopicHandler>();
services.AddTransient(typeof(GenericBaseTypeNoResponseHandler<>));
services.AddTransient<GenericBaseTypeNoResponseHandler<string>>(); // Theoretically it is not needed

//...

Configure method

// set the mediatR
var mediatR = LocalMediatR.GetInstance();
mediatR.SetMediatR(mediator);

Example of use and it generates the error

var notif = new GenericBasicTypeNotification<string>()
{
    Payload = "my value"
};
await LocalMediatR.GetInstance().Mediator.Send(notif);

---

other links read and tests performed

• https://github.com/jbogard/MediatR/wiki
• Question: How to register generic requests #1041
• Handlers for interfaced notifications are not working as expected #1050
• https://stackoverflow.com/questions/73760859/mediatr-generic-handlers/73760945
• https://stackoverflow.com/questions/77862142/how-to-register-generic-type-mediatr-handler/77878919
• https://stackoverflow.com/questions/53895522/add-a-generic-handler-for-send-and-publish-methods-of-the-mediatr-library-in-asp/54041414

[zachpainter77]

I'm guessing that your handlers are in a different assembly than your requests. If that is the case then you just need to add both assemblies in the AddMediatR configuration options.

Also, the types you are passing as generic type parameters must also be available during the registration of mediatR so you'll also need to include those assemblies as well.

Also the internal access modifiers on your handler and requests might also be causing some issues.

[zachpainter77]

namespace Assembly1{
   public class MyDto{
   }
}
namespace Assembly2{
    public class GenericBasicTypeNotification<T> : BaseNotification { 
     // ...
    }
}
namespace Assembly3{
    public class GenericBaseTypeNoResponseHandler<T> : HandlerBaseProperties, RequestHandler<GenericBasicTypeNotification<T>>
    {
        public async Task Handle(GenericBasicTypeNotification<T> request, CancellationToken cancellationToken) {
           // ...
        }
     }
}

//AddMediatR method
services.AddMediatR(cfg => {   
    cfg.RegisterGenericHandlers = true;
    cfg.RegisterServicesFromAssemblies([typeof(MyDto).Assmebly, typeof(GenericBasicTypeNotification<>).Assembly, typeof(GenericBaseTypeNoResponseHandler<>).Assembly]);
});

//remember all assemblies need to be included so that they are available to scan during AddMediatR registration.

[zachpainter77]

Ok so this specific issue is also caused by the fact that the auto registration only scans the assemblies you provide. Your assemblies do not define the primitive types so you'll need to add them.

builder.Services.AddMediatR(cfg =>
{
    //add this to disable the constraint on number of types that can close.
    cfg.MaxTypesClosing = 0; 
    //enable generic handlers                   
    cfg.RegisterGenericHandlers = true;    
    //add typeof(int).Assembly to include primitive types in assembly scanning.
    cfg.RegisterServicesFromAssemblies([typeof(Startup).Assembly, typeof(int).Assembly]); 
});

Request / Handler Definition ->

using MediatR;

namespace TestApplication
{      
    public class GenericRequest<T> : IRequest<T>
    {
        public T Payload { get; set; }
    }


    public class GenericHandler<T> : IRequestHandler<GenericRequest<T>, T>
    {
        public Task<T> Handle(GenericRequest<T> request, CancellationToken cancellationToken)
        {
            return Task.FromResult(request.Payload);
        }
    }
}

MVC controller ->

using MediatR;
using Microsoft.AspNetCore.Mvc;
using TestApplication;

namespace TestMediatRMVC.Controllers
{
    public class HomeController : Controller
    {
      
        private readonly IMediator _mediator;

        public HomeController(IMediator mediator)
        {
            _mediator = mediator;
        }

        public async Task<IActionResult> Index()
        {
            var request = new GenericRequest<string>
            {
                Payload = "Hello World!"
            };
            var result = await _mediator.Send(request);
            ViewBag.Result = result;
            return View();
        }
    }
}

Razor view ->

@{
    ViewData["Title"] = "Home Page";
}

<div class="text-center">
    <h1 class="display-4">Welcome</h1>
    <p>Learn about <a href="https://learn.microsoft.com/aspnet/core">building Web apps with ASP.NET Core</a>.</p>
</div>

<p>@ViewBag.Result</p>

browser view ->

This should fix your issue. Cheers!

[AlbertoVPersonal]

Thanks for your comments. I will test it. Last week I have been very busy.