即使要用ping,也应该用using块包起来
其实压根就用不着去轮询,操作系统设计有相应的事件通知,.Net已经将其封装为委托
using System.Net.NetworkInformation;
NetworkChange.NetworkAvailabilityChanged += new NetworkAvailabilityChangedEventHandler(NetworkChange_NetworkAvailabilityChanged);
http://www.cnblogs.com/94cool/archive/2009/08/14/1546286.html