Sockets 由于线程退出或应用程序请求，已放弃 I/O 操作。该怎么解决

Sockets 由于线程退出或应用程序请求，已放弃 I/O 操作。
代码如下。

Thread th= new Thread(new ThreadStart(InitialPPTTcp));

th.IsBackground = true;

th.Start();


       public void InitialPPTTcp()

        {

            TcpListener pptListener = new TcpListener(ip,8506);

            pptListener.Start();

            TcpClient pptClient = pptListener.AcceptTcpClient();

            pptBeginRead(pptClient);

        }


                private void pptBeginRead(TcpClient pptClient)

        {

            this.pptClient = pptClient;

            pptRemoteEndPoint = pptClient.Client.RemoteEndPoint.ToString();

            

            pptStreamToClient = pptClient.GetStream();

            pptBuffer = new byte[pptBufferSize];

            pptHandler = new ProtocolHandler();


            AsyncCallback pptCallBack = new AsyncCallback(pptOnReadComplete);

            pptStreamToClient.BeginRead(pptBuffer, 0, pptBufferSize, pptCallBack, are.WaitOne());

        }



        private void pptOnReadComplete(IAsyncResult ar)

        {

            int bytesRead = 0;

            try

            {

                lock (pptStreamToClient)

                    bytesRead = pptStreamToClient.EndRead(ar);   //这里出错了。


                string msg = Encoding.Unicode.GetString(buffer, 0, bytesRead);

                Array.Clear(pptBuffer, 0, pptBuffer.Length); //清空缓存，避免脏读。


                string[] protocolArray = pptHandler.GetProtocol(msg);

                foreach (string pro in protocolArray)

                {

                    ParameterizedThreadStart start = new ParameterizedThreadStart(pptHandleProtocol);

                    start.BeginInvoke(pro, null, null);

                }


                //无限循环

                lock (pptStreamToClient)

                {

                    AsyncCallback callBack = new AsyncCallback(pptOnReadComplete);

                    pptStreamToClient.BeginRead(pptBuffer, 0, pptBufferSize, callBack, null);

                }

            }

            catch (Exception ex)

            {

                if (pptStreamToClient != null)

                    pptStreamToClient.Dispose();

                pptClient.Close();

                frmMessageBoxDialog.Show(ex.StackTrace, "错误", MessageBoxButtonEx.OK, MessageBoxIconEx.Error);