C# – Why i’m getting exception: A blocking operation was interrupted by a call to WSACancelBlockingCall

c++multithreading

I have this method:

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();
        }

In the top of Form1 i did:

AutoResetEvent flag;

In the constructor:

flag = new AutoResetEvent(false);

Then in the method above the GetNetworkTime() the part i changed was:

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);

Before using the flag and the Thread the program was hang/freeze and i did DEBUG > Break All (Pause)
And it stopped on the line:

socket.Receive(ntpData);

So i added this flag and thread code.
And now when running my program i'm getting this exception on the line:

socket.Receive(ntpData);

SocketException
A blocking operation was interrupted by a call to WSACancelBlockingCall

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: 

Best Solution

After the waiting you close the socket. This cancels the synchronous read call, which is a good thing. That also means that you should probably handle this exception. Be sure to match on the ErrorCode=10004 property. You don't want to swallow other errors.

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