Hi Jeff,

The drop down part of a ComboBox control is a separate window, which
enables the drop down part to open up over other controls.

To implement a custom drop down window, we could use a WinForm, whose
FormBorderStyle property is set to None. But this solution is not very
good, because the form would take the focus and inactivate the main window
form. The functionality of the dropdown would be difficult to manage too.

A better solution is to derive a class from NativeWindow and create a popup
window, similar to a tooltip window, that can be shown using the
SW_SHOWNOACTIVATE setting and ShowWindow function via interoop.  A class
created in this way would mimic the behaviour of the base Win32 controls
and it would be easier to create the desired internal functionality such as
item selection and so-on.

You may visit the following link for more information on how to do it using
NativeWindow.

http://vbaccelerator.com/home/NET/Code/Controls/Popup_Windows/Popup_Windows/
article.asp

I searched the web site ' www.codeproject.com' and also found a smarter
solution of using ToolStripDropDown to implement the drop down window. You
may visit the following link for more information on how to do this.

http://www.codeproject.com/useritems/simplepopup.asp

Hope this helps.
If you have any question, please feel free to let me know.

Sincerely,
Linda Liu
Microsoft Online Community Support

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