title: Why i'm getting exception: A blocking operation was interrupted by a call to WSACancelBlockingCall?
body: <p>I have this method:</p>
  
  <pre><code>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&lt;string&gt; ntpServersList = new List&lt;string&gt;();
              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 &lt; ntpServersList.Count; i++)
              {
                  addresses = Dns.GetHostEntry(ntpServersList[i]).AddressList;
                  if (addresses.Length &gt; 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(()=&gt;
                  {
                     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();
          }
  </code></pre>
  
  <p>In the top of Form1 i did:</p>
  
  <pre><code>AutoResetEvent flag;
  </code></pre>
  
  <p>In the constructor:</p>
  
  <pre><code>flag = new AutoResetEvent(false);
  </code></pre>
  
  <p>Then in the method above the GetNetworkTime() the part i changed was:</p>
  
  <pre><code>Thread th = new Thread(()=&gt;
                  {
                     socket.Receive(ntpData);
                     flag.Set();
                  });
                  th.IsBackground = true;
                  th.Start();
  
                  //Block the current thread for 5 seconds
                  flag.WaitOne(5000, false);
  </code></pre>
  
  <p>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:</p>
  
  <pre><code>socket.Receive(ntpData);
  </code></pre>
  
  <p>So i added this flag and thread code.
  And now when running my program i'm getting this exception on the line:</p>
  
  <pre><code>socket.Receive(ntpData);
  </code></pre>
  
  <p>SocketException
  A blocking operation was interrupted by a call to WSACancelBlockingCall</p>
  
  <pre><code>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.&lt;&gt;c__DisplayClass1.&lt;GetNetworkTime&gt;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: 
  </code></pre>