ValidatorHandler.cs

    public class ValidatorHandler<TRequest, TResponse>
    : IRequestHandler<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
    {

        private readonly IRequestHandler<TRequest, TResponse> _inner;
        private readonly IValidator<TRequest> _validator;


        public ValidatorHandler(IRequestHandler<TRequest, TResponse> inner,
            IValidator<TRequest> validator)
        {
            _inner = inner;
            _validator = validator;
        }

        public TResponse Handle(TRequest request)
        {
            var validationResult = _validator.Validate(request);
            if (!validationResult.IsValid)
            {
                throw new ValidationException(validationResult.Errors);
            }

            return _inner.Handle(request);
        }
    }