I am testing a method that is populating a custom collection.
My custom collection class has a method that requires some advanced stuff that is not important for my test, so I tried to mock the collection object to setup a mock for that method. The custom collection is input parameter to the method I am testing, so I send in the mocked object as input.

This seems to work up to the point where elements are added to the collection. The Add method runs, but the collection is not updated and Count is still 0.

Why is not the elements added to the mocked collection object?