Listbox selectedindex always -1

Dear all,
             I populate a list box and want to swap the item when they click a up button. But the
selectedindex of the list box always return -1 no matter which item was selected. The following
is my code. Please help.
protected void LoadSubMenu()
        {
            string strSQL = "";
            DBConnection myDB = new DBConnection();
            strSQL = "select * from menu where parent_id is not null";
            myDB.ReadData(IntranetDSN,strSQL);
            ListBox1.DataTextField = "menu_name&qu ...