http://developer.51cto.com/art/200908/147009.htm
Control Invoke and BeginInvoke
http://www.soft-bin.com/html/2010/07/09/c-control%E7%9A%84invoke%E5%92%8Cbegininvoke%E5%8F%8A%E5%85%B6%E5%AE%9E%E7%8E%B0%E6%9C%BA%E5%88%B6.html