public async Task AddAuthor_WhenCallingValidRequest_ShouldReturnOk()
        {
            // Arrange
            var authorId = Guid.NewGuid();

            var locationUrl = "http://location";

            _authorServiceMock.Setup(a => a.AddAuthor(It.IsAny <AuthorDto>())).Returns(authorId);
            _urlHelperMock.Setup(m => m.Link(It.IsAny <string>(), It.IsAny <object>()))
            .Returns(locationUrl);

            // Act
            IActionResult actionResult = await _authorsApiController.AddAuthor(new AuthorDto());

            CreatedResult result = actionResult as CreatedResult; // null if cast fails

            // Assert
            Assert.IsNotNull(actionResult);
            Assert.IsNotNull(result);
            Assert.AreEqual(locationUrl, result.Location);
            Assert.IsInstanceOf <AuthorDto>(result.Value);
        }