Return response with errors instead of throwing exception in validation pipeline mediatr 3

Question

I am currently working with Pipeline behavior in Mediatr 3 for request validation. All the examples that I came across were throwing ValidationException if any failures happening, instead of doing that I want to return the response with the error. Anyone has idea on how to do it?

Below is the code for the validation pipeline:

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

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

    public Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next)
    {
        var failures = _validators
            .Select(v => v.Validate(request))
            .SelectMany(result => result.Errors)
            .Where(f => f != null)
            .ToList();

        if (failures.Any())
        {
            throw new ValidationException(failures);
        }

        return next();
    }
}

Note: I found this question Handling errors/exceptions in a mediator pipeline using CQRS? and I am interested in the 1st option on the answer, but no clear example on how to do that.

This is my response class:

public class ResponseBase : ValidationResult
{
    public ResponseBase() : base() { }

    public ResponseBase(IEnumerable<ValidationFailure> failures) : base(failures)  {
    }
}

and I added below signature in the validation pipeline class:

public class ValidationPipeline<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse> where TRequest : IRequest<TResponse> 
where TResponse : ResponseBase

I did this then in the Handle method:

var response = new ResponseBase(failures);
return Task.FromResult<TResponse>(response);

But that gave me error 'cannot convert to TResponse'.

Answer 1

Simply don't call next if there's any failures:

public Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next)
{
    var failures = _validators
        .Select(v => v.Validate(request))
        .SelectMany(result => result.Errors)
        .Where(f => f != null)
        .ToList();

    if (failures.Any())
    {
        var response = new Thing(); //obviously a type conforming to TResponse
        response.Failures = failures; //I'm making an assumption on the property name here.

        return Task.FromResult(response);
    }
    else
    {
        return next();
    }
}

Note:
Your class (Thing in my example) must be of type TResponse