CBN_SELCHANGE
idComboBox = LOWORD(wParam);
hwndComboBox = (HWND)lParam;
The CBN_SELCHANGE notification message is sent when the selection in the list box of a combo box is about to be changed as a result of the user either clicking in the list box or changing the selection by using the arrow keys. The parent window of the combo box receives this 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.
WM_COMMAND