combobox inheritance

All-
I am attempting to create my own custom class which inherits from the
ComboBox class. The problem is that I also need to use my own custom object
collection for my custom class. I have already done this with a custom
ListBox control by overriding the protected method "CreateItemCollection()"
and returning a new instance of my custom collection class which inherits
from ListBox.ObjectCollection and that works perfectly. I was surprised that
no similar method exists for the ComboBox class. Am I overlooking something
here? Why would they omit this?

Thanks in advance,
Will