How to affect definition of COMBOBOX window class

Question

I have a subclass of CComboBox that behaves differently in two different contexts - both in native C++. In the working (test) context the top-level application was created using VS2008. In the non-working case the application is greatly more complex and was probably originally created by an earlier VS version.

In the failure case the control won't display a dropdown box, but responds to up/down arrow key controls. Also in the failure case it always returns zero when queried using GetMinVisible and always returns failure from SetMinVisibleItems.

It appears that the problem is rooted in the definition of the COMBOBOX window class. I have dumped the data returned from ::GetClassInfoEx in both cases. Aside from the address of the wndProc, there are two differences. First, the working case has style CS_GLOBALCLASS set while the failure case doesn't. Also the working case specifies 4 bytes of extra window storage (cbWndExtra) while the failure case specifies 8 bytes.

So my questions are: 1) What configuration properties might affect the definition of the COMBOBOX window class?, and 2) How can I determine the correct wndProc to use if I make my own window class?

EDIT: The behavior of the dropdown box changes with the specification of the character set property in the General property tab. It works as expected if character set is unicode and fails when it's not set. The combo box subclass is in an ActiveX control, but the setting at the application level seems to affect the window class, which winds up messing up the combo box in ActiveX control. Interestingly there are other combo boxes, albeit in dialogs, elsewhere in the application that seem to behave just fine.

Answer 1

SetMinVisibleItems uses CB_SETMINVISIBLE and it requires comctl32 v6, this is your clue that this is a visual styles issue.

The older common controls version of the combobox actually uses the controls height as the size of the drop-down list so if you adjust that you might be able to fix it without adding a manifest.