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 to combo box
This notification message can occur only for a combo box with the CBS_DROPDOWN or CBS_DROPDOWNLIST style.
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Requires version 1.0 or later.
Header: Declared in winuser.h.
Combo Boxes Overview, Combo Box Messages, CBN_CLOSEUP, WM_COMMAND