public async Task CreatesNewMessagePassValidationWithoutId() { // Arrange var message = new Message() { Content = "Hello World" }; var mockService = new Mock <IMessageDbService>(); mockService.Setup(service => service.CreateAsync(message)) .Callback(() => message.Id = 1); var controller = new MessagesController(mockService.Object); var result = await controller.CreateAsync(message); Assert.That(message.Id, Is.GreaterThan(0)); }