The CBN_SETFOCUS notification message is sent when a combo box receives the keyboard focus. The parent window of the combo box receives this notification message through the WM_COMMAND message.
CBN_SETFOCUS
idComboBox = (int) LOWORD(wParam); // identifier of combo box
hwndComboBox = (HWND) lParam; // handle to combo box
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_KILLFOCUS, WM_COMMAND