Moq在构造函数中使用延迟加载

Question

I ran into a strange problem today - I was writing a unit test around a SIM object. It asserts that when the SIM object is updated and the PIN attempts remaining has changed, it will call a particular method. The test looks like this:

[Test]
public void TestUpdateSimInfoWithPinAttemptsChangedCallsOnPinAttemptsRemaining()
{
    var info = new SimPinInfo {PinAttemptsRemaining = 10};

    var sim = new Mock<Sim>(info);

    info.PinAttemptsRemaining = 2;

    sim.Object.UpdateSimInfo(info);

    sim.Verify(s => s.FireOnPinAttemptsRemaining(), Times.Once());
}

So the mock SIM object is created with 10 PIN attempts remaining. The SimPinInfo object then has the PinAttemptsRemaining value reduced to 2 before being passed to the UpdateSimInfo() method.

SIM constructor (trimmed for clarity):

internal Sim(SimPinInfo info) : this()
{
     _pinAttemptsRemaining = info.PinAttemptsRemaining;
     _pukAttemptsRemaining = info.PukAttemptsRemaining;
     ......
}

And the UpdateSimInfo() method (trimmed):

internal void UpdateSimInfo(SimPinInfo info)
{
    lock(_locker)
    {
        if (_pinAttemptsRemaining != info.PinAttemptsRemaining)
        {
            Log("PinAttemptsRemaining changed");
            _pinAttemptsRemaining = info.PinAttemptsRemaining;
            FireOnPinAttemptsRemaining();
        }
        .....
    }
}

A pretty simple test - what should happen is the above if statement will be true (pin attempts remaining has changed) so the OnPinAttemptsRemaining event will be fired. However, the test failed (although not all the time - it passed when I slowly stepped through the code!). What was happening was the if statement was false - both _pinAttemptsRemaining and info.PinAttemptsRemaining were 2. It appears that the SIM mock is not actually created when expected - when the info.PinAttemptsRemaining was 10.

To demonstrated this, I added a comment:

var sim = new Mock<Sim>(info);
info.PinAttemptsRemaining = 2;
Console.WriteLine("SIM's pin attempts = " + sim.Object.PinAttemptsRemaining);

I also put a breakpoint in the SIM object's constructor. The breakpoint was hit when stepping over the Console.WriteLine line, not the new Mock... line. So the object is not being created until needed.

I believe this is called lazy-loading or lazy-evaluation.

There were various workaround to this behaviour - I ended up creating a new SimPinInfo object to pass to UpdateSimInfo() .

Has anyone come across this behaviour before? I couldn't find any references to it.

Answer 1

From what I've read you're trying to test a Mock.

sim.Object.UpdateSimInfo(info);

Mocks are meant to substitute dependencies, you don't use them to substitute the code you mean to test. Without seeing the whole code, I'd guess that you don't even need a Mock to test this behaviour. Assuming that FireOnPinAttemptsRemaining raises an event I'd recommend something like this for a test method:

[Test]
public void TestUpdateSimInfoWithPinAttemptsChangedCallsOnPinAttemptsRemaining()
{
    int eventFiredCount = 0;
    var info = new SimPinInfo {PinAttemptsRemaining = 10};
    var sim = Sim(info);
    sim.OnPinAttemptsRemaining += (sender, e) => { eventFiredCount++; };

    info.PinAttemptsRemaining = 2;
    sim.UpdateSimInfo(info);

    Assert.AreEqual(1, eventFiredCount);
}

I'm not 100% on the event handler since I don't know the Fire method is raising an event or not, or the event name, but it should give you an idea.