Thread is not terminated when the main app closed

public void threadHandler()
        {
            Socket handlersocket = (Socket)alSockets[alSockets.Count - 1];
            NetworkStream networkStream = new NetworkStream(handlersocket);
            int thisRead = 0;
            int blockSize = 1024;
            byte[] dataByte = new byte[blockSize];
            lock (this)
            {
                // Only one process can access
                // the same file at any given time
                Stream fileStream = File.OpenWrite(@"c:\upload");
                while (true)
                {
                    thisRead = networkStream.Read(dataByte, 0, blockSize);
          
                    fileStream.Write(dataByte, 0, thisRead);
                    if (thisRead == 0) break;
                }
                fileStream.Close();
                networkStream.Close();
            }
            lBConnections.Items.Add("File Recieved");
            handlersocket = null;
        }
        public void ThreadListener()
        {
            
                TcpListener tcpListener = new TcpListener(8880);
                tcpListener.Start();
                while (true)
                {
                    Socket handlerSocket = tcpListener.AcceptSocket();
                    if (handlerSocket.Connected)
                    {

                        lBConnections.Items.Add(handlerSocket.RemoteEndPoint.ToString() + "..Connected.");

                        lock (this)
                        {
                            alSockets.Add(handlerSocket);
                        }
                        ThreadStart thdstHandler = new ThreadStart(threadHandler);
                     Thread    thdHandler = new Thread(thdstHandler);
                        
                        thdHandler.Start();
                     
                    }

                }                                       
        }

        private void ReceiveFile_Load(object sender, EventArgs e)
        {
            this.CenterToScreen();
            IPHostEntry IPHost = Dns.GetHostByName(Dns.GetHostName());
            lBConnections.Text = "Host IP address is " +
            IPHost.AddressList[0].ToString();
            alSockets = new ArrayList();

          Thread   thdListener = new Thread(new ThreadStart(ThreadListener));
            thdListener.Start();                       
        }