public delegate void XmlNodeEventHandler( object sender, XmlNodeEventArgs e);

This declared delegate type is used to receive event notifications from the XmlSerializer instance when an unknown node is encountered. The UnknownNode( ) event is fired before the UnknownAttribute( ) or UnknownElement( ) event. If no XmlNodeEventHandler is assigned to the XmlSerializer.UnknownNode event, any unknown nodes will be handled by the appropriate XmlAttributeEventHandler or XmlElementEventHandler, if one is assigned.