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())); }