RequestValidationDecorator.cs

using System.Threading.Tasks;
using MediatR;

namespace ThinControllers.Demo
{
    public interface IValidationHandler<in TRequest>
    {
        void Validate(TRequest request);
    }

    public class RequestValidationDecorator<TRequest> : 
        IAsyncRequestHandler<TRequest, Unit> where TRequest : IAsyncRequest
    {
        private readonly IAsyncRequestHandler<TRequest, Unit> _requestHandler;
        private readonly IValidationHandler<TRequest> _validationHandler;

        public RequestValidationDecorator(IAsyncRequestHandler<TRequest, Unit> requestHandler, IValidationHandler<TRequest> validationHandler)
        {
            _requestHandler = requestHandler;
            _validationHandler = validationHandler;
        }

        public async Task<Unit> Handle(TRequest message)
        {
            _validationHandler.Validate(message);
            return await _requestHandler.Handle(message);
        }
    }
}