socket.BeginReceiveFrom异步接收信息时，回调函数无法正常进入

Server:

using System;
using System.Net;
using System.Net.Sockets;
public class UdpServer
{
static Socket serverSock;
static AsyncCallback receiveCallback;
static byte[] buff;
static EndPoint senderIP ;

public static void Main()
{
buff = new byte[1024];
senderIP = new IPEndPoint(IPAddress.Any, 0);

receiveCallback = new AsyncCallback(OnDataReceived);

serverSock = new Socket(AddressFamily.InterNetwork,
SocketType.Dgram, ProtocolType.Udp);
serverSock.Bind(new IPEndPoint(IPAddress.Any, 8000));


serverSock.BeginReceiveFrom(buff, 0, buff.Length, SocketFlags.None,
ref senderIP, receiveCallback, serverSock);

Console.ReadLine();

}

public static void OnDataReceived(IAsyncResult ar)
{
Socket udpSocket = ar.AsyncState as Socket;
int bytesRecved = udpSocket.EndReceiveFrom(ar, ref (EndPoint)senderIP);

Console.WriteLine("{0}传来消息{1}", senderIP.ToString(), System.Text.Encoding.ASCII.GetString(buff, 0, bytesRecved));

serverSock.BeginReceiveFrom(buff, 0, buff.Length, SocketFlags.None,
ref senderIP, receiveCallback, serverSock);
}
}

Client:

using System;
using System.Net;
using System.Net.Sockets;
public class UdpClient
{
static Socket clientSock;
static byte[] buff;

public static void Main()
{
clientSock = new Socket(AddressFamily.InterNetwork,
SocketType.Dgram, ProtocolType.Udp);

string input;
while((input = Console.ReadLine()) != "exit")
{
buff = System.Text.Encoding.ASCII.GetBytes(input);
clientSock.SendTo(buff, 0, buff.Length, SocketFlags.None, new IPEndPoint(IPAddress.Parse("127.0.0.1"), 8000));
}

clientSock.Close();
}
}