I have received an interresting post from Kent Boogaart on the CodeProject article that pointed out a major limitation of Composite Unit Testing:

I'm a little unsure how this tests all an interface's members. Take ICollection, for example. Your CollectionTest implementation would receive an instance of ICollection manufactured via an appropriate factory.

Say you wanted to test that ICollection.Count was returning the correct value. How would you test this without knowledge of the factory method called?

Thanks,
Kent

There he has a point. You cannot test ICollection.Count using Composite Unit Testing as it is now. You could test it through the IList test interface, but that's not the point.

A way of overcoming the limitation would be the ability for the user to give a "gold" instance along with the test instance. The gold instance would be mock of the interface as it "should" be and could be used to do the ICollection.Count test. For each factory method, the framework would look for it's mock counter part, if found, it is feeded to the test method. Let's illustrate that on ICollection:

The gold instance class (mock):

// the mock
public class CollectionMock : ICollection
{
    private int count=0;
    ...
    public int Count
    { 
        get{ return count;}
        set{this.count=value;}
    }
    ...
}

public class ArrayListFactory
{
    // provider for empty arraylist
    [Factory] // new attribute, tells the framework it is a factory method
    public ArrayList Empty
    {
        get
        {
            return new ArrayList();
        }
    }
    [MockFactory] // tells the framework it is a mock method
    public MockCollection EmptyMock // name is important must be "Method"+Mock
    {
        get
        {
            MockCollection col = new MockCollection();
            return col;
        }
    }
    // 1 provider + mock
    [Factory]
    public ArrayList One
    {
        get
        {
            return Empty.Add(null);
        }
    }
    [MockFactory]
    public MockCollection OneMock // name is important must be "Method"+Mock
    {
        get
        {
            MockCollection col = new MockCollection(); 
            col.Count=1;
            return col;
        }
    }
}

[TypeFixtue(typeof(ICollection)]
[ProviderFactory(typeof(ArrayListFactory),typeof(ICollection))] 
public class CollectionTest
{
    [Test]
    public void CountTest(ICollection tested, ICollection mock)
    {
        Assert.AreEqual(mock.Count,tested.Count,"ICollection.Count");
    }
}