An application sends the CB_SETTOPINDEX message to ensure that a particular item is visible in the list box of a combo box. The system scrolls the list box contents so that either the specified item appears at the top of the list box or the maximum scroll range has been reached.
CB_SETTOPINDEX wParam = (WPARAM) index; // zero-based index of item list lParam = 0; // not used, must be zero
If the message is successful, the return value is zero.
If the message fails, the return value is CB_ERR.