Returning Result<T> from a behaviour.

MediatR version: 9.0.0 .NET target framework: net5.0 dotnet version: 5.0.201

I’m trying to implement FluentValidation without throwing an exception, and to do that I’m using a Result<T> type. I have the following Query:

public class TestQuery
{
    public record Query(string? TestString) : IRequest<Result<string>>;

    public class QueryValidator : AbstractValidator<Query>
    {
        public QueryValidator()
        {
            RuleFor(x => x.TestString)
                .NotEmpty();
        }
    }
    
    public class Handler : RequestHandler<Query, Result<string>>
    {
        protected override Result<string> Handle(Query request)
        {
            return request.TestString!;
        }
    }

and the behaviour written like so:

public class ValidationBehaviour<TRequest, TResponse> : IPipelineBehavior<TRequest, Result<TResponse>>
    where TRequest : IRequest<Result<TResponse>>
{
    private readonly IEnumerable<IValidator<TRequest>> _validators;

    public ValidationBehaviour(IEnumerable<IValidator<TRequest>> validators)
    {
        _validators = validators;
    }

    public Task<Result<TResponse>> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<Result<TResponse>> next)
    {
        var context = new ValidationContext<TRequest>(request);
        var failures = _validators
            .Select(x => x.Validate(context))
            .SelectMany(x => x.Errors)
            .Where(x => x is not null)
            .ToList();

        if (failures.Any())
        {
            return Task.FromResult(Result<TResponse>.FromError(new GenericError("test")));
        }

        return next();
    }
}

and I register the behaviour with DI like so:

services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationBehaviour<,>));

However the behaviour never runs when I run the query (I have verified with the debugger). What gives?

[defunky]

Is there no way around this aside from reflection? How do people normally handle validation errors without throwing exceptions?

[kevbry]

I think the issue is right here: https://github.com/jbogard/MediatR/blob/50af3de7fa89d56067952581179fa22a1b66de0b/src/MediatR/Wrappers/RequestHandlerWrapper.cs#L63

When mediatr is trying to locate your behaviours, it’s asking whichever DI adapter you’re using for the set of IPipelineBehavior<TRequest,TResponse> implementations it has registered. In this case, in RequestHandlerWrapperImpl, TRequest will be of type Query and TResponse will be of type Result<string>. This is an issue, because when RequestHandlerWrapperImpl asks DI for an IBehavior, it’s going to try to instantiate ValidationBehaviour using the generic parameters <Query, Result<string>> and not the <Query,string> you were probably expecting it to ask for. If you turn on a first chance exception handler or look in the visual studio exception timeline you’ll see an exception that looks like this (I changed the names of TRequest and TResponse in your behavior to TReq and TResp to differentiate them from the names in RequestHandlerWrapperImpl):

Exception thrown: ‘System.ArgumentException’ in System.Private.CoreLib.dll (“GenericArguments[0], TestQuery+Query’, on ‘ValidationBehaviour2[TReq,TResp]’ violates the constraint of type ‘TReq’.”) Exception thrown: ‘System.ArgumentException’ in System.Private.CoreLib.dll (“GenericArguments[0], ‘TestQuery+Query’, on ‘ValidationBehaviour`2[TReq,TResp]’ violates the constraint of type ‘TReq’.”)

I’m guessing your behavior registration looked like this (assuming you’re using the microsoft DI container): services.AddTransient(typeof(IPipelineBehavior<,>),typeof(ValidationBehaviour<,>));

If you change it to the following, you’ll probably see it start working: services.AddTransient(typeof(IPipelineBehavior<Query,Result<string>>), typeof(ValidationBehaviour<Query,string>));

If you want to not have to manually register every possible Result<T> => ValidationBehavior<?,T> combination, I think you’re going to have to resort to reflection (what I did), or find a container that does the work of creating those closed implementation types for you. If there’s another way to do it, I’d love to hear it. I’m fairly sure I remember seeing a bug in here a couple of years back that had some gnarly reflection code to accomplish this - I’ll update if I can find it.

Edit: There are probably a dozen bugs here that are all asking variations of this question (just search for IPipelineBehavior). Most are in the simple case (TResponse isn’t generic) that’s been solved in .net 5.0 now that open generic registration is allowed, but quite a few are in your (and my) category where TResponse is generic. Here are a few of the latter case: https://github.com/jbogard/MediatR/issues/562 https://github.com/jbogard/MediatR/issues/583 https://github.com/jbogard/MediatR/issues/466 https://github.com/jbogard/MediatR/issues/305 https://github.com/jbogard/MediatR/issues/200