Freezing mocks by Mark Seemann

My previous post about AutoFixture's Freeze functionality included this little piece of code that I didn't discuss:

var mapMock = new Mock<IPizzaMap>();
fixture.Register(mapMock.Object);

In case you were wondering, this is Moq interacting with AutoFixture. Here we create a new Test Double and register it with the fixture. This is very similar to AutoFixture's built-in Freeze functionality, with the difference that we register an IPizzaMap instance, which isn't the same as the Mock<IPizzaMap> instance.

It would be nice if we could simply freeze a Test Double emitted by Moq, but unfortunately we can't directly use the Freeze method, since Freeze<Mock<IPizzaMap>>() would freeze a Mock<IPizzaMap>, but not IPizzaMap itself. On the other hand, Freeze<IPizzaMap>() wouldn't work because we haven't told the fixture how to create IPizzaMap instances, but even if we had, we wouldn't have a Mock<IPizzaMap> against which we could call Verify.

On the other hand, it's trivial to write an extension method to Fixture:

public static Mock<T> FreezeMoq<T>(this Fixture fixture)
    where T : class
{
    var td = new Mock<T>();
    fixture.Register(td.Object);
    return td;
}

I chose to call the method FreezeMoq to indicate its affinity with Moq.

We can now rewrite the unit test from the previous post like this:

[TestMethod]
public void AddWillPipeMapCorrectly_FreezeMoq()
{
    // Fixture setup
    var fixture = new Fixture();
 
    var basket = fixture.Freeze<Basket>();
    var mapMock = fixture.FreezeMoq<IPizzaMap>();
 
    var pizza = fixture.CreateAnonymous<PizzaPresenter>();
 
    var sut = fixture.CreateAnonymous<BasketPresenter>();
    // Exercise system
    sut.Add(pizza);
    // Verify outcome
    mapMock.Verify(m => m.Pipe(pizza, basket.Add));
    // Teardown
}

You may think that saving only a single line of code may not be that big a deal, but if you also need to perform Setups on the Mock, or if you have several different Mocks to configure, you may appreciate the encapsulation. I know I sure do.

Comments

// does not work
var td = new Mock();
fixture.Register(td.Object);

// should work
fixture.Inject(td.Object);

fixture.Register(() => td.object);

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