说明：  .net库提供了一个性能监控包，我们能够用来监控系统或者任何应用程序。这个程序利用它来监控网络的发送、接收的数据包数量，从而计算出网络的速度。
(The .NET Class Library provides a set of performance counters that you can use to track the performance both of the system and of an application. This program makes use of counters in the Networking category to obtain information about data sent and received over the network, and calculates network speed.)