Combo Box Differences

A combo box in a version 4.0 application passes the control color messages (WM_CTLCOLOR*) from its child components (edit control and list box) to the parent window of the combo box. In a version 3.x application, a combo box passes those messages to the DefWindowProc function.

A combo box in a version 4.0 application uses the WM_CAPTURECHANGED message to hide its drop-down list box if it is open. Windows 95/98 sends the message when another window takes the mouse capture, which typically happens when the user clicks another window. In a version 3.x application, a combo box does not use WM_CAPTURECHANGED to hide the drop-down list.

A combo box in a version 3.x application uses the WM_CTLCOLORLISTBOX message to retrieve the text and background colors. In a version 4.0 application, a combo box uses the WM_CTLCOLOREDIT or WM_CTLCOLORSTATIC message instead. The combo box uses WM_CTLCOLORSTATIC if it is disabled or contains a read-only selection field (in an edit control); otherwise, it uses WM_CTLCOLOREDIT.

In a version 3.x application, the background of the static text area in read-only combo boxes is filled with the system highlight color (COLOR_HILIGHT). In a version 4.0 application, Windows 95/98 fills the background of the static text area only for a combo box that is not owner drawn.

In a version 4.0 application, Windows 95/98 adds the ODS_COMBOBOXEDIT value to the itemState member of the DRAWITEMSTRUCT structure when Windows 95/98 sends the WM_DRAWITEM message to the parent window of an owner-drawn combo box to draw an item in the selection field. The ODS_COMBOBOXEDIT value tells the parent window that the drawing takes place in the selection field of the combo box rather than in the list box.