关于MapControl和Map添加事件监听无效的问题

用9.3做项目的时候，发现Map对象IActiveViewEvents接口中的事件时而有效，时而无效，异常神奇，仔细研究，终于发现原因。

在对地图事件中监听中，一般用到两个接口，MapControl的IMapControlEvents2和Map的IActiveViewEvents接口，这两个接口中有相同的方法，我以为使用方法差不多，后来发现不然。IMapControlEvents2是地图控件的事件接口，只要地图控件不变 ，监听一直有效，而IActiveViewEvents是地图对象的接口，当MapControl中的地图对象更换后，对之前添加的地图事件监听就不再有效了。Engine中自带的一个例子估计误导了很多人，如下。在这个例子中，如果按这样的顺序操作：运行，点击Step1按扭，添加数据（注意是添加，不是打开文档），进行其它地图操作，那么所添加的事件监听全部有效，但是，如果点击Step1按扭后，不是添加数据，而是打开地图文档，那么Step1步骤中添加的事件监听都不再有效，原因正如前面所说的，地图对象被更换掉了，而IActiveViewEvents接口中的事件是与地图对象相关联的。理解了这个之后，就明白如果要在地图更换后仍对地图对象监听，那么就要重新添加监听。

private void btnStep1_Click(object sender, EventArgs e)

{

m_map = (axMapControl1.Object as IMapControl4).Map; // Explict Cast

TextBox3.Clear();

//Create an instance of the delegate, add it to AfterDraw event

m_ActiveViewEventsAfterDraw = new ESRI.ArcGIS.Carto.IActiveViewEvents_AfterDrawEventHandler(OnActiveViewEventsAfterDraw);

((ESRI.ArcGIS.Carto.IActiveViewEvents_Event)(m_map)).AfterDraw += m_ActiveViewEventsAfterDraw;

//Create an instance of the delegate, add it to AfterItemDraw event

m_ActiveViewEventsAfterItemDraw = new ESRI.ArcGIS.Carto.IActiveViewEvents_AfterItemDrawEventHandler(OnActiveViewEventsItemDraw);

((ESRI.ArcGIS.Carto.IActiveViewEvents_Event)(m_map)).AfterItemDraw += m_ActiveViewEventsAfterItemDraw;

//Create an instance of the delegate, add it to ContentsChanged event

m_ActiveViewEventsContentsChanged = new ESRI.ArcGIS.Carto.IActiveViewEvents_ContentsChangedEventHandler(OnActiveViewEventsContentsChanged);

((ESRI.ArcGIS.Carto.IActiveViewEvents_Event)(m_map)).ContentsChanged += m_ActiveViewEventsContentsChanged;

//Create an instance of the delegate, add it to ContentsCleared event

m_ActiveViewEventsContentsCleared = new ESRI.ArcGIS.Carto.IActiveViewEvents_ContentsClearedEventHandler(OnActiveViewEventsContentsCleared);

((ESRI.ArcGIS.Carto.IActiveViewEvents_Event)(m_map)).ContentsCleared += m_ActiveViewEventsContentsCleared;

........................................

}