wpf passwordbox控件 光标移到最后

 /// <summary>
 /// 设置光标位置
 /// </summary>
 /// <param name="passwordBox"></param>
 /// <param name="start">光标开始位置</param>
 /// <param name="length">选中长度</param>
 private void SetSelection(PasswordBox passwordBox, int start, int length)
 {
     passwordBox.GetType()
                .GetMethod("Select", BindingFlags.Instance | BindingFlags.NonPublic)
                .Invoke(passwordBox, new object[] { start, length });
 } 

以上方法中 start传光标位置，length传0，即可控制