BeforeDraw问题请教！

<P>private ISceneGraphEvents_BeforeDrawEventHandler pBeforeDraw;<BR>this.pBeforeDraw = new ISceneGraphEvents_BeforeDrawEventHandler(BeforeDraw);<BR><FONT color=#ff0000>(this.pScene as ISceneGraphEvents).BeforeDraw+=this.pBeforeDraw;</FONT></P>
<P><FONT color=#000000>请问我这样实现<FONT color=#000000>BeforeDraw</FONT>事件为什么会出错呢！请各位大哥帮忙解决下。</FONT></P>
<P>我想实现打开文件的时候能有一个显示打开进度的进度条。不知道怎么实现。</P>