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 the WM_COMMAND message.
CBN_SELCHANGE idComboBox = (int) LOWORD(wParam); // identifier of combo box hwndComboBox = (HWND) lParam; // handle of combo box