UDP编程有两种技术，一种是直接使用Socket类，另一种是使用UdpClient类。UdpClient类对基础Socket进行了封装。
　　 与TCP有TcpListener类和TcpClient类不同，UDP只有UdpClient类。
　　 有两种建立远程主机的方式：1.使用远程主机名和端口号来创建；2.先创建不带参数的UdpClient实例，再调用Connect方法。
　　构造函数：UdpClient()、UdpClient(int port)、UdpClient(IPEndPoint localEp)、UdpClient(string remoteHost,int port)(There are two kinds of technologies in UDP programming. One is to use Socket class directly, the other is to use UdpClient class. The UdpClient class encapsulates the underlying Socket.
Unlike TCP, there are TcpListener classes and TcpClient classes. UDP has only UdpClient class.
There are two ways to build a remote host: 1. use a remote host name and port number to create; 2. first create a UdpClient instance without parameter, and then call the Connect method.
Constructors: UdpClient (), UdpClient (int port), UdpClient (IPEndPoint localEp), UdpClient (string remoteHost, int int).)