为什么我会收到异常：A blocking operation was interrupted by a call to WSACancelBlockingCall？

Question

我有这个方法：

public DateTime GetNetworkTime()
        {
            DateTime networkDateTime = DateTime.Now;
            try
            {
            IPAddress[] addresses = null;
            //default Windows time server
            const string ntpServer = "time.windows.com";
            const string ntpServer1 = "time.nist.gov";
            const string ntpServer2 = "time-nw.nist.gov";
            const string ntpServer3 = "time-a.nist.gov";
            const string ntpServer4 = "time-b.nist.gov";
            List<string> ntpServersList = new List<string>();
            ntpServersList.Add(ntpServer);
            ntpServersList.Add(ntpServer1);
            ntpServersList.Add(ntpServer2);
            ntpServersList.Add(ntpServer3);
            ntpServersList.Add(ntpServer4);

            // NTP message size - 16 bytes of the digest (RFC 2030)
            var ntpData = new byte[48];

            //Setting the Leap Indicator, Version Number and Mode values
            ntpData[0] = 0x1B; //LI = 0 (no warning), VN = 3 (IPv4 only), Mode = 3 (Client Mode)

            for (int i = 0; i < ntpServersList.Count; i++)
            {
                addresses = Dns.GetHostEntry(ntpServersList[i]).AddressList;
                if (addresses.Length > 0)
                {
                    break;
                }
            }


            //The UDP port number assigned to NTP is 123
            var ipEndPoint = new IPEndPoint(addresses[0], 123);
            //NTP uses UDP
            var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);

            socket.Connect(ipEndPoint);
            socket.Send(ntpData);
                Thread th = new Thread(()=>
                {
                   socket.Receive(ntpData);
                   flag.Set();
                });
                th.IsBackground = true;
                th.Start();

                //Block the current thread for 5 seconds
                flag.WaitOne(5000, false);
            socket.Close();

            //Offset to get to the "Transmit Timestamp" field (time at which the reply 
            //departed the server for the client, in 64-bit timestamp format."
            const byte serverReplyTime = 40;

            //Get the seconds part
            ulong intPart = BitConverter.ToUInt32(ntpData, serverReplyTime);

            //Get the seconds fraction
            ulong fractPart = BitConverter.ToUInt32(ntpData, serverReplyTime + 4);

            //Convert From big-endian to little-endian
            intPart = SwapEndianness(intPart);
            fractPart = SwapEndianness(fractPart);

            var milliseconds = (intPart * 1000) + ((fractPart * 1000) / 0x100000000L);

            //**UTC** time
            networkDateTime = (new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)).AddMilliseconds((long)milliseconds);
            }
            catch(Exception err)
            {
                MessageBox.Show("error" + err.ToString());
            }
            return networkDateTime.ToLocalTime();
        }

在Form1的顶部，我进行了以下操作：

AutoResetEvent flag;

在构造函数中：

flag = new AutoResetEvent(false);

在上述的GetNetworkTime()方法中，我修改的部分是：

Thread th = new Thread(()=>
                {
                   socket.Receive(ntpData);
                   flag.Set();
                });
                th.IsBackground = true;
                th.Start();

                //Block the current thread for 5 seconds
                flag.WaitOne(5000, false);

在使用标识和线程之前，程序一直卡住/冻结，我进行了调试 > 暂停操作，它停在了以下代码行：

socket.Receive(ntpData);

我添加了这个标志和线程代码。现在当运行我的程序时，我在这一行遇到了异常：

socket.Receive(ntpData);

套接字异常 阻塞操作被WSACancelBlockingCall调用中断。

System.Net.Sockets.SocketException was unhandled
  HResult=-2147467259
  Message=A blocking operation was interrupted by a call to WSACancelBlockingCall
  Source=System
  ErrorCode=10004
  NativeErrorCode=10004
  StackTrace:
       at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
       at System.Net.Sockets.Socket.Receive(Byte[] buffer)
       at TestDateTime.Form1.<>c__DisplayClass1.<GetNetworkTime>b__0() in d:\C-Sharp\TestDateTime\TestDateTime\TestDateTime\Form1.cs:line 125
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

- user2760148

---

当套接字正在尝试“接收”时，您正在强制关闭它。 结果令人惊讶吗？ - Jon

Answer 1

等待后，您会关闭套接字。这会取消同步读取调用，这是一件好事。这也意味着您应该处理此异常。请确保匹配ErrorCode=10004属性。不要吞咽其他错误。

- usr