如何使用 FakeItEasy 模拟服务？

Question

我有一个通过服务调用访问的简单实体框架存储库（请参阅 this UML diagram 查看相关课程）。我正在尝试按如下方式测试服务

public class ProductServiceTests
{
    private IProductService sut;
    private IProductRepository productRepo;
    [Fact]
    public async Task CanCallGetProductById()
    {
        //Arrange
        productRepo = new Fake<IProductRepository>().FakedObject;
        sut = new ProductService(productRepo);
        var id = 1;
        var expectation = new Product { Id = id };
        A.CallTo(() => sut.GetProductById(id)).Returns(expectation);

        // Act
        var result = await sut.GetProductById(id);

        // Assert
        Assert.Equal(id, result.Id);
    }
}

但是，这会导致以下异常System.ArgumentException : Object 'ProductService' of type ProductService is not recognized as a fake object.

这意味着我可能需要使用以下服务的模拟

 sut = new Fake<IProductService>().FakedObject;

然而，这种方法也存在问题：对函数await sut.GetProductById(id) 的调用实际上并没有调用任何被测代码。确实，如果我抛出异常，测试仍然通过

    public async Task<Product> GetProductById(int id)
    {
        throw new NotImplementedException();
        //return await _productRepository.GetAll().FirstOrDefaultAsync(x => x.Id == id);
    }

编辑 经过一些故障排除后，我意识到，实际上，我试图测试的代码实际上并没有达到。比如下面的测试

A.CallTo(() => productRepo.GetAll()).MustHaveHappened();

失败并显示Product.GetAll() Expected to find it once or more but no calls were made to the fake object.的消息

请有人解释我应该如何解决这个错误？

编辑

IProductRepository 和 IProductService 定义如下：

public interface IProductRepository
{
    IQueryable<Product> GetAll();
}
public interface IProductService
{
    Task<Product> GetProductById(int id);
}

而ProductService的实现方式如下

public class ProductService : IProductService
{
    private readonly IProductRepository _productRepository;

    public ProductService(IProductRepository productRepository)
    {
        _productRepository = productRepository;
    }
   

    public async Task<Product> GetProductById(int id)
    {
        //throw new NotImplementedException();
        return await _productRepository.GetAll()
                                        .FirstOrDefaultAsync(x => x.Id == id);
    }
}

Answer 1

使用假冒（或模拟或其他）来帮助测试服务的一般方法是模拟被测系统的协作者（在本例中为IProductService），并使用真实被测系统（如果您不使用真实的被测系统，则您的系统不是“被测系统”）。因此，采用您的原始代码并进行调整，您会想要这样的东西：

public class ProductServiceTests
{
    private IProductService sut;
    private IProductRepository productRepo;
    [Fact]
    public async Task CanCallGetProductById()
    {
        //Arrange
        productRepo = new Fake<IProductRepository>().FakedObject;
        sut = new ProductService(productRepo);
        var id = 1;
        var expectation = new Product { Id = id };


        // This is different from your example. 
        // The idea is to configure `productRepo`, not `sut`.
        // Also, I don't know the interface for `IProductRepository`,
        // so you'll have to adjust this call to match `GetAll`
        A.CallTo(() => productRepo.GetAll(id)).Returns(new [] {expectation});

        // Act
        var result = await sut.GetProductById(id);

        // Assert
        Assert.Equal(id, result.Id);
    }
}