Detecting closeup event on a ComboBox

Hi! Have someone of you wondered why isn't there a CloseUp event similar to DropDown in the ComboBox? I'm trying to detect this situation unsuccesfully. I noticed, though, that Win32 has a CBN_CLOSEUP message (similar to a CBN_DROPDOWN message) for the ComboBox. But this message is sent to the parent Window and not to the ComboBox itself. I tried it by overriding both WndProc. The problem is that I am writting a general control and I cannot tap into the parent Window. Can someone help me?

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