string sCountry ;
        private void cbCountry_SelectionChanged(object sender, SelectionChangedEventArgs e)
            var item = cbCountry.SelectedItem as ComboBoxItem;
            if (item != null)
                sCountry = item.Uid;
                //cbCountry.Text = item.Uid;//赋值将再次触发Changed事件，并且无法赋值给Text
        private void cbCountry_DropDownClosed(object sender, EventArgs e)
            cbCountry.Text = sCountry;//只能通过DropDownClosed事件再赋值