CB_RESETCONTENT
wParam = 0; /* not used, must be zero */
lParam = 0L; /* not used, must be zero */An application sends a CB_RESETCONTENT message to remove all items from the list box and edit control of a combo box.
This message has no parameters.
The return value is always CB_OKAY.
If the combo box was created 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.
This example removes all items from the list box and edit control of a combo box:
SendDlgItemMessage(hdlg, ID_MYCOMBOBOX, CB_RESETCONTENT, 0, 0L);