udp 接收数据

Question

为什么在最后一句出现错误 ？
System.Net.Sockets.UdpClient receiver = new System.Net.Sockets.UdpClient();
Byte[] receiveBytes;
System.Threading.Thread.Sleep(2000);
System.Net.IPEndPoint  ep =new System.Net.IPEndPoint(System.Net.IPAddress.Any,0);
receiveBytes = receiver.Receive(ref ep);

Answer 1

MSDN上是这样写的 
// This constructor arbitrarily assigns the local port number.
UdpClient udpClient = new UdpClient();
    try{
         udpClient.Connect("www.contoso.com", 11000);         // Sends a message to the host to which you have connected.
         Byte[] sendBytes = Encoding.ASCII.GetBytes("Is anybody there?");
      
         udpClient.Send(sendBytes, sendBytes.Length);         // Sends a message to a different host using optional hostname and port parameters.
         UdpClient udpClientB = new UdpClient();
         udpClientB.Send(sendBytes, sendBytes.Length, "AlternateHostMachineName", 11000);         //IPEndPoint object will allow us to read datagrams sent from any source.
         IPEndPoint RemoteIpEndPoint = new IPEndPoint(IPAddress.Any, 0);         // Blocks until a message returns on this socket from a remote host.
         Byte[] receiveBytes = udpClient.Receive(ref RemoteIpEndPoint); 
         string returnData = Encoding.ASCII.GetString(receiveBytes);
   
         // Uses the IPEndPoint object to determine which of these two hosts responded.
         Console.WriteLine("This is the message you received " +
                                      returnData.ToString());
         Console.WriteLine("This message was sent from " +
                                     RemoteIpEndPoint.Address.ToString() +
                                     " on their port number " +
                                     RemoteIpEndPoint.Port.ToString());          udpClient.Close();
          udpClientB.Close();
          
          }  
       catch (Exception e ) {
                  Console.WriteLine(e.ToString());
        }
Byte[] receiveBytes = udpClient.Receive(ref RemoteIpEndPoint); 
人家是udpClient，不是udpClientB

Answer 2

udpClient.Connect("www.sina.com", 11000);
在我的代码前加上一句就不报错了
为什么呢？