Moq：模拟设置期间奇怪的参数评估

Question

This has caused me half a day of debugging and head-scratching. I'm using Moq 4.2.1408.0717. I have a service, which is mocked. And then I have client which is again mocked, but one of the match attributes will use the (mocked) service for its evaluation. Reduced example below:

[TestFixture]
class PPAP
{
    [Test]
    public void Test_Apple_Pen()
    {
        var fruitMock = new Mock<IFruit>(); // the mocked service
        fruitMock.Setup(x => x.GetFruit()).Returns("apple");

        var fw = new FruitWrapper(fruitMock.Object); // object that uses the (mocked) service behind curtains

        var uhhMock = new Mock<IUhh>();
        uhhMock.Setup(x => x.Uhh(fw.GetFruit())).Returns("apple-pen");

        // Utterly unimportant part below, added for clarity:
        var result = uhhMock.Object.Uhh("apple");
        Assert.That(result, Is.EqualTo("apple-pen"));
    }
}

public interface IFruit
{
    string GetFruit();
}

public interface IUhh
{
    string Uhh(string s);
}

public class FruitWrapper
{
    private IFruit _fruit;

    public FruitWrapper(IFruit fruit)
    {
        _fruit = fruit;
    }

    public string GetFruit()
    {
        var f = _fruit.GetFruit(); // f should never be null since I mocked it already!
        // LOOK HERE *jumps and waves* THIS IS THE PROBLEM:
        if (f == null) {
            throw new Exception("POINT OF THE QUESTION HERE: Service mock doesn't function!!!");
        }
        return f; // yet this method will be called twice with f being null before finally called with f == "apple" during Setup()
    }
}

Problem is, the code using the service (FruitWrapper in the example) is not prepared to receive null from the service (that's the point of mocking the service after all!). Still, during the Setup() of the client mock (uhhMock in the example), FruitWrapper's GetFruit() will be called so that the service will return null (despite the mock already in place). This was killing my test with an exception. I still have no idea why is this happening. Is this normal behavior or is this a Moq bug?

(Now that with the reduced example the test doesn't die on null from service, I see that finally the match parameter is evaluated correctly.) Edit: the example was modified to not survive null from service. If it did, finally there would be a call where the service returns non-null.

(Of course, if I save the parameter (fw.GetFruit()) to a variable before the Setup and use that inside, then everything's fine.)

Edit: since people have trouble understanding what I mean. I added some code:

• a throw in case the service returns null (please, please do manage to understand that THAT is my problem: the service should never return null, it's mocked!
• some unimportant verification at the end to make the test "complete" because the omission seems to irritate people so much that they fail to see anything else. Note: this is not the point. Test result is NOT my problem.

Answer 1

I'm not sure what your test is supposed to be doing. Is it a complete test?

var uhhMock = new Mock<IUhh>();
        uhhMock.Setup(x => x.Uhh(fw.GetFruit())).Returns("apple-pen");

Change this to use the actual value you want to Setup on:

var uhhMock = new Mock<IUhh>();
            uhhMock.Setup(x => x.Uhh("apple")).Returns("apple-pen");

Another problem is that it appears you're doing some sort of integration test with multiple layers of mocks (using a mock IUhh to get a value from a concrete FruitWrapper which has a mock IFruit ). Try to break it down to using the class under test with mock dependencies.

A good idea is to use the AAA (Arrange, Act, Assert) pattern. For example:

[Test]
public void Test_Apple_Pen()
{
    //Arrange
    var fruitMock = new Mock<IFruit>(); 
    fruitMock.Setup(x => x.GetFruit()).Returns("apple");

    var fw = new FruitWrapper(fruitMock.Object);

    //Act
    var result = fw.GetFruit();

    //Assert
    Assert.AreEqual("apple", result);
}