The CBN_DROPDOWN notification message is sent when the list box of a combo box is about to be made visible. The parent window of the combo box receives this notification message through the WM_COMMAND message.
CBN_DROPDOWN
idComboBox = (int) LOWORD(wParam); // identifier of combo box
hwndComboBox = (HWND) lParam; // handle of combo box
Remarks
This notification message can occur only for a combo box with the CBS_DROPDOWN or CBS_DROPDOWNLIST style.
See Also