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()));
}