How Can Servers Detect Client Disconnections on Sockets?

Server-side socket client disconnect monitoring

In distributed applications, timely detection of client disconnection is critical to maintaining system integrity. However, unlike the client being able to notify the server using events or callbacks, the server faces the challenge of proactively identifying client termination.

Detection method

The following methods have been proven to be ineffective at detecting client disconnects on server sockets:

• handler.Available: This method returns the number of bytes that can be read, but this is not an indication of a disconnection.
• handler.Send: If the client has disconnected, sending data will fail, but this is a destructive test and will cause an error during the reconnection attempt.
• handler.Receive: Likewise, if the client is not connected, trying to receive data will result in an exception.

Polling using extension methods

Since there is no dedicated event for socket disconnection, periodic polling is recommended. To simplify this process, you can create an extension method:

<code class="language-csharp">static class SocketExtensions
{
  public static bool IsConnected(this Socket socket)
  {
    try
    {
      return !(socket.Poll(1, SelectMode.SelectRead) && socket.Available == 0);
    }
    catch (SocketException) { return false; }
  }
}</code>

This method utilizes the Poll method to check data availability. If the poll is successful but Available returns 0, this indicates that the connection may have been disconnected, which can be confirmed by capturing SocketException during the send operation.

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