public void UpdateBook_NonExistingBook_ReturnsNotFound() { mockService.Setup(s => s.UpdateBook(It.IsAny <UpdateBookDto>())) .Returns(BookTestData.UpdateBookServiceResponseNullData()); BookController bookController = new BookController(mockService.Object); UpdateBookDto updatedBook = BookTestData.UpdateBookDto(); var result = bookController.UpdateBook(updatedBook); Assert.That(result, Is.InstanceOf <NotFoundObjectResult>()); }