MSDN中的示例，使用 BeginReceive 异步接收服务器响应，编译时出现找不到类型或命名空间名称“UdpState”，应该怎么修改？
//下面的代码是MSDN中的示例，使用 BeginReceive 异步接收服务器响应。为什么我编译时，告诉我
//错误1找不到类型或命名空间名称“UdpState”(是否缺少 using 指令或程序集引用?)
//应该怎么修改，谢谢！


public static bool messageReceived = false;

public static void ReceiveCallback(IAsyncResult ar)
{
UdpClient u = (UdpClient)((UdpState)(ar.AsyncState)).u;
IPEndPoint e = (IPEndPoint)((UdpState)(ar.AsyncState)).e;

Byte[] receiveBytes = u.EndReceive(ar, ref e);
string receiveString = Encoding.ASCII.GetString(receiveBytes);

Console.WriteLine("Received: {0}", receiveString);
messageReceived = true;
}

public static void ReceiveMessages()
{
// Receive a message and write it to the console.
IPEndPoint e = new IPEndPoint(IPAddress.Any, listenPort);
UdpClient u = new UdpClient(e);

UdpState s = new UdpState();
s.e = e;
s.u = u;

Console.WriteLine("listening for messages");
u.BeginReceive(new AsyncCallback(ReceiveCallback), s);

// Do some work while we wait for a message. For this example,
// we'll just sleep
while (!messageReceived)
{
Thread.Sleep(100);
}
}

[解决办法]
缺少程序集的引用 找到这个程序集的DLL 添加到引用里面
[解决办法]