CBN_DROPDOWN
idComboBox = LOWORD(wParam);
hwndComboBox = (HWND)lParam;
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.
idComboBox
Value of low word of wParam. Specifies the combo-box ID.
hwndComboBox
Value of lParam. Specifies the combo-box window handle.
This message does not occur if the combo box does not contain a drop-down list box.
WM_COMMAND