>>I want to combine a combo with a treeview, so when the arrow is clicked, I am going to drop down the tree instead.
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>So does your ComboBox *ever* act like a ComboBox? Or are you simply using it to get a treeview to display? Are you trying to mimic the behavior of an old application that you're converting? I'm sorry, I just can't see why you want this kind of behavior from a ComboBox ... it is certainly not what most users would expect to see happen when they click on a Combo's down-arrow.

One example I can think of right off is Windows Explorer. It has a combo/treeview type control at the top called the Address Bar. Another great example is a drop down calendar control.