public async Task OverwriteHandler()
        {
            var services = ServiceProviderHelper.GetServiceCollection();

            services.Replace(new ServiceDescriptor(typeof(IRequestHandler <CreateCommand, CreateCommandResult>), typeof(CustomCreateHandler), ServiceLifetime.Transient));

            var serviceProvider = services.BuildServiceProvider();
            var mediator        = serviceProvider.GetService <IMediator>();

            await Assert.ThrowsAsync <NotImplementedException>(() => mediator.Send(new CreateCommand()));
        }