Inheritance: IExternalEventHandler
 public ExternalEventExampleDialog(ExternalEvent exEvent, ExternalEventExample handler)
 {
  InitializeComponent();
  m_ExEvent = exEvent;
  m_Handler = handler;
 }
  protected override void OnFormClosed(FormClosedEventArgs e)
  {
   // 保存的实例需要释放
   m_ExEvent.Dispose();
   m_ExEvent = null;
   m_Handler = null;

   base.OnFormClosed(e);
  }
  // 外部命令调用此方法
  public void ShowForm(UIApplication uiapp)
  {
   // 如果没有创建对话框,创建并显示它
   if (m_MyForm == null || m_MyForm.IsDisposed)
   {
    // 新建一个外部事件响应实例
    ExternalEventExample handler = new ExternalEventExample();

    // 新建一个外部事件实例
    ExternalEvent exEvent = ExternalEvent.Create(handler);

    // 把上面两个实例传给对话框.
    m_MyForm = new ExternalEventExampleDialog(exEvent, handler);
    m_MyForm.Show();
   }
  }