How to register generic behavior correctly using MediatR?

Question

I saw a couple of videos on youtube with implementation of preprocessing validation/logging or whatever through MediatR pipeline behaviors and decided to try this.

I've ended up with this generic implementation of behavior:

public class LoggingPipeLineBehavior<TRequest, TResult>: IPipelineBehavior<TRequest, Result<TResult>>
    where TRequest: IRequest<Result<TResult>>
{
    private readonly ILogger<LoggingPipeLineBehavior<TRequest, TResult>> _logger;

    public LoggingPipeLineBehavior(ILogger<LoggingPipeLineBehavior<TRequest, TResult>> logger)
    {
        _logger = logger;
    }

    public async Task<Result<TResult>> Handle(
        TRequest request,
        RequestHandlerDelegate<Result<TResult>> next,
        CancellationToken cancellationToken)
    {
        _logger.LogInformation("Starting request {@RequestName}, {@DateTimeUtc}",
            typeof(TRequest).Name,
            DateTime.UtcNow);
        
        var result = await next();

        if (result.IsError)
        {
            _logger.LogError("Requst failure {@RequestName}, {@Error} {@DateTimeUtc}",
                typeof(TRequest).Name,
                result.FirstError,
                DateTime.UtcNow);
        }
        
        _logger.LogInformation("Completed request {@RequestName}, {@DateTimeUtc}",
            typeof(TRequest).Name,
            DateTime.UtcNow);
        
        return result;
    }
}

I use result type pattern in all of my requests. For example:

public record CreateArtistCommand(string Name, string? Description) : IRequest<Result<Guid>>;

So i've registered this behavior as i had seen on wiki:

var assembly = typeof(DependencyInjection).Assembly;
        
        services.AddMediatR(configuration =>
            {
                configuration.RegisterServicesFromAssembly(assembly);
                configuration.AddOpenBehavior(typeof(LoggingPipeLineBehavior<,>));
            });

And nothing happened... MediatR sends requests straight to their handlers bypassing this behavior. I've been debugging it for a hour removing these or those dependencies from container and still no result.

i've also tried this method of registration and nothing works:

configuration.AddBehavior(typeof(IPipelineBehavior<,>), typeof(LoggingPipeLineBehavior<,>);

Am i doing something wrong? How should i register this behavior?

Answer 1

Why do you need to wrap the response in Result? If you get rid of that, and your endpoints will return non-generic classes, everything will work.

Then your class will look like this:

public class LoggingPipeLineBehavior<TRequest, TResult>: IPipelineBehavior<TRequest, TResult>
    where TRequest: IRequest<TResult>

Or if you need specific responses to be handled, then:

public class LoggingPipeLineBehavior<TRequest, TResult>: IPipelineBehavior<TRequest, SpecificResponse>
        where TRequest: IRequest<TResult>