title: Cannot close Asynchronous C# Server Socket connection
body: <p>I am writing code allowing a Android client to connect to a C# server socket. The client and server are working correctly but I am unable to shutdown or disconnect the socket.</p>
  
  <p>The server is started by an on click event:</p>
  
  <pre><code>private void btnStartServer_Click(object sender, EventArgs e)
  {
      AsynchronousSocketListener Async = new AsynchronousSocketListener();
      receiveThread = new Thread(new ThreadStart(Async.StartListening));
      receiveThread.Start();
  
      btnStartServer.Enabled = false;
      btnStopServer.Enabled = true;
      MessageBox.Show("Server Started");
  }
  </code></pre>
  
  <p>Then the bulk of the server code:</p>
  
  <pre><code>// State object for reading client data asynchronously
  public class StateObject
  {
      // Client  socket.
      public Socket workSocket = null;
      // Size of receive buffer.
      public const int BufferSize = 1024;
      // Receive buffer.
      public byte[] buffer = new byte[BufferSize];
      // Received data string.
      public StringBuilder sb = new StringBuilder();  
  }
  
  public class AsynchronousSocketListener
  {
      // Thread signal.
      public static ManualResetEvent allDone = new ManualResetEvent(false);
  
      public AsynchronousSocketListener()
      {
      }
  
      public void StartListening()
      {
          // Data buffer for incoming data.
          byte[] bytes = new Byte[1024];
  
          // Establish the local endpoint for the socket.
          IPHostEntry ipHostInfo = Dns.Resolve(Dns.GetHostName());
          IPAddress ipAddress = ipHostInfo.AddressList[0];
          IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 3000);
          System.Diagnostics.Debug.WriteLine(ipAddress);
  
          // Create a TCP/IP socket.
          Socket listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp );
  
          // Bind the socket to the local endpoint and listen for incoming connections.
          try
          {
              listener.Bind(localEndPoint);
              listener.Listen(100);
  
              while (true)
              {
                  // Set the event to nonsignaled state.
                  allDone.Reset();
  
                  // Start an asynchronous socket to listen for connections.
                  Console.WriteLine("Waiting for a connection...");
                  listener.BeginAccept(new AsyncCallback(AcceptCallback), listener );
  
                  Singleton s = Singleton.Instance;
  
                  if (s.getIsEnded() == false)
                  {
                      // Wait until a connection is made before continuing.
                      allDone.WaitOne();
                  }
                  else
                  {
                      listener.Shutdown(SocketShutdown.Both);
                      listener.Disconnect(true);
                      break;
                  }
              }
          }
          catch (Exception e)
          {
              Console.WriteLine(e.ToString());
          }
      }
  
      public static void AcceptCallback(IAsyncResult ar)
      {
          // Get the socket that handles the client request.
          Socket listener = (Socket) ar.AsyncState;
          Socket handler = listener.EndAccept(ar);
  
          // Create the state object.
          StateObject state = new StateObject();
          state.workSocket = handler;
          handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);
  
          // Signal the main thread to continue.
          allDone.Set();
      }
  
      public static void ReadCallback(IAsyncResult ar)
      {
          String content = String.Empty;
  
          // Retrieve the state object and the handler socket
          // from the asynchronous state object.
          StateObject state = (StateObject) ar.AsyncState;
          Socket handler = state.workSocket;
  
          // Read data from the client socket. 
          int bytesRead = handler.EndReceive(ar);
  
          if (bytesRead &gt; 0)
          {
              // There  might be more data, so store the data received so far.
              state.sb.Append(Encoding.ASCII.GetString(state.buffer,0,bytesRead));
  
              // Check for end-of-file tag. If it is not there, read 
              // more data.
              content = state.sb.ToString();
              if (content.IndexOf("&lt;EOF&gt;") &gt; -1)
              {
                  // All the data has been read from the 
                  // client. Display it on the console.
                  Console.WriteLine("Read {0} bytes from socket. \n Data : {1}", content.Length, content );
                  if (content.Equals("end&lt;EOF&gt;"))
                  {
                      Console.WriteLine("Should end");
                      Singleton s = Singleton.Instance;
                      s.setIsEnded(true);
                  }
                  // Echo the data back to the client.
                  Send(handler, content);
              }
              else
              {
                  // Not all data received. Get more.
                  handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
                  new AsyncCallback(ReadCallback), state);
              }
          }
      }
  
      private static void Send(Socket handler, String data)
      {
          // Convert the string data to byte data using ASCII encoding.
          byte[] byteData = Encoding.ASCII.GetBytes(data);
  
          // Begin sending the data to the remote device.
          handler.BeginSend(byteData, 0, byteData.Length, 0, new AsyncCallback(SendCallback), handler);
      }
  
      private static void SendCallback(IAsyncResult ar)
      {
          try
          {
              // Retrieve the socket from the state object.
              Socket handler = (Socket) ar.AsyncState;
  
              // Complete sending the data to the remote device.
              int bytesSent = handler.EndSend(ar);
              Console.WriteLine("Sent {0} bytes to client.", bytesSent);
  
              handler.Shutdown(SocketShutdown.Both);
              handler.Close();
          } 
          catch (Exception e)
          {
              Console.WriteLine(e.ToString());
          }
      }
  }
  </code></pre>
  
  <p>Using a singleton I can keep a unique variable to check if the server should be running or not. This is checked in the <code>StartListening()</code> method above:</p>
  
  <pre><code>public class Singleton
  {
      private static Singleton instance;
      private Boolean isEnded = false;
  
      private Singleton() { }
  
      public static Singleton Instance
      {
          get
          {
              if (instance == null)
              {
                  instance = new Singleton();
              }
              return instance;
          }
      }
  
      public void setIsEnded(Boolean setter)
      {
          isEnded = setter;
      }
  
      public Boolean getIsEnded()
      {
          return isEnded;
      }
  }
  </code></pre>
  
  <p>Finally  have attempted to stop the server by sending it a message with the String <code>"end&lt;EOF&gt;"</code>. The servers logic at <code>ReadCallback()</code> will notify the singleton to set <code>isEnded = true</code>. This is not a great solution but it was the only way that I could get half working at the time of writing. The Logic for disconnecting the socket is in <code>StartListening()</code>. Ideally it would disconnect so that the socket can be started again.</p>
  
  <p>This error occurs when I try to disconnect and then start the socket again:</p>
  
  <pre><code>A first chance exception of type 'System.Net.Sockets.SocketException' occurred in System.dll
  System.Net.Sockets.SocketException (0x80004005): Only one usage of each socket address (protocol/network address/port) is normally permitted
      at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress)
      at System.Net.Sockets.Socket.Bind(EndPoint localEP)
      at StartServer.AsynchronousSocketListener.StartListening() in c:\Users\Conor\Desktop\StartServer\StartServer\StartServer.cs:line 89
  </code></pre>
  
  <p>If I stop the server and then try send a string from the android client, the message is received on the server and then I get the following message on the server console:</p>
  
  <pre><code>System.Net.Sockets.SocketException (0x80004005): A request to send or receive data was disallowed because the socket is not connected and (when sending on a datagram socket using a sendto call) no address was supplied
      at System.Net.Sockets.Socket.Shutdown(SocketShutdown how)
      at StartServer.AsynchronousSocketListener.StartListening()
  </code></pre>