IPipelineBehaviour with custom type for TResponse (ie. Result<TResponse>)

I have several commands for which I want to apply cross-cutting concerns. The thing is, my handlers return Result<TResponse> and not just TResponse.

public readonly struct Result<T> { }
public class LoggingBehaviour<TRequest, TResponse> : IPipelineBehavior<TRequest, Result<TResponse>> 
{
    public async Task<Result<TResponse>> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<Result<TResponse>> next)
    {
        Log.Information($"BEFORE");
        var response = await next();
        Log.Information($"AFTER");

        return response;
    }
}

// in Startup.cs
services.AddTransient(typeof(IPipelineBehavior<,>), typeof(LoggingBehaviour<,>))

but I receive a run-time error when my commands are executed

An unhandled exception has occurred while executing the request.
System.ArgumentException: Implementation type LoggingBehaviour`2[Command1,Result[Response]]' can't be converted to service type 'MediatR.IPipelineBehavior`2[Command1,Result[Response]]'

I suppose my registration is not right, but can’t figure out how to do it properly.

Can you help me? Thanks!

[lilasquared]

I think there are issues with registering open generics on the default DI container for dotnet core - you may need to use a more advanced container. Here is my code using Simple Injector

class Program
{
    static async Task Main(string[] args)
    {
        var container = new Container();

        object Get(Type requestedType) => container.GetInstance(requestedType);

        container.Options.DefaultScopedLifestyle = new SimpleInjector.Lifestyles.AsyncScopedLifestyle();
        container.Options.DefaultLifestyle = Lifestyle.Transient;
        container.RegisterSingleton<IMediator>(() => new Mediator(Get));
        container.Collection.Register(typeof(IPipelineBehavior<,>), new[] 
        { 
            typeof(LoggingPipelineBehavior<,>) 
        });
        container.Register(typeof(IRequestHandler<,>), Assembly.GetExecutingAssembly());
        var bus = container.GetInstance<IMediator>();
        container.Verify();

        var res = await bus.Send(new WhatsTheTime());

        Console.WriteLine(res.Payload.Time);
        Console.ReadLine();
    }
}

public class LoggingPipelineBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, Result<TResponse>>
{
    public async Task<Result<TResponse>> Handle(
        TRequest request,
        CancellationToken cancellationToken,
        RequestHandlerDelegate<Result<TResponse>> next)
    {
        Console.WriteLine($"BEFORE");
        var response = await next();
        Console.WriteLine($"AFTER");

        return response;
    }
}

public class ThisIsTheTime
{
    public DateTime Time { get; }
    public ThisIsTheTime(DateTime dt) => Time = dt;
}

public class WhatsTheTime : IRequest<Result<ThisIsTheTime>> { }

public class WhatsTheTimeHandler : IRequestHandler<WhatsTheTime, Result<ThisIsTheTime>>
{
    public Task<Result<ThisIsTheTime>> Handle(WhatsTheTime request, CancellationToken cancellationToken)
    {
        return Task.FromResult(new Result<ThisIsTheTime>(new ThisIsTheTime(DateTime.UtcNow)));
    }
}

public readonly struct Result<T> {
    public readonly T Payload { get; }

    public Result(T payload)
    {
        Payload = payload;
    }
}

[lilasquared]

here is the comment from jimmy on the issue - its not open generics but constrained open generics https://github.com/jbogard/MediatR/issues/305#issuecomment-417002645

not sure if this is the same as the exception you’re seeing though