An application sends a CB_DELETESTRING message to delete a string in the list box of a combo box.
CB_DELETESTRING wParam = (WPARAM) index; // item to delete lParam = 0; // not used; must be zero
The return value is a count of the strings remaining in the list. If the index parameter specifies an index greater than the number of items in the list, the return value is CB_ERR.
If you create the combo box with an owner-drawn style but without the CBS_HASSTRINGS style, Windows sends a WM_DELETEITEM message to the owner of the combo box so the application can free any additional data associated with the item.