An application sends a CB_RESETCONTENT message to remove all items from the list box and edit control of a combo box.
CB_RESETCONTENT wParam = 0; // not used; must be zero lParam = 0; // not used; must be zero
This message has no parameters.
This message always returns CB_OKAY.
If you create the combo box with an owner-drawn style but without the CBS_HASSTRINGS style, the owner of the combo box receives a WM_DELETEITEM message for each item in the combo box.