ComboBox的SelectionChanged获取不到当前文本的问题

private void cmb_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            string str = "";
            foreach (ComboBoxItem item in e.AddedItems)
            {
                str = item.Content.ToString();
            }

            if ((str != null) && (str.Length != 0))
            {
                MessageBox.Show(str);
            }
        }

用 (ComboBox)sender 获得Text属性只能获得SelectionChanged前的字符串，要获得改变后的要利用参数：

SelectionChangedEventArgs