The CBN_DROPDOWN notification message is sent when the list box of a combo box is about to be dropped down (made visible). The parent window of the combo box receives this notification message through a WM_COMMAND message.
wParam
Specifies the identifier of the combo box.
lParam
Specifies the handle of the combo box in the low-order word, and specifies the CBN_DROPDOWN notification message in the high-order word.
This notification message can occur only for a combo box with the CBS_DROPDOWN or CBS_DROPDOWNLIST style.