How do I detect when a telnet client is improperly closed to a tcp server?

问题

I am experimenting in creating a small IRC server to learn some new programming concepts (and others I haven't used in forever). The first step is to get a basic client connecting via TCP to send plaintext commands to the server.

To listen for connection I have the following code:

public NetworkClient(Server server, TcpClient socket, int id)
{
    _socket = socket;
    _id = id;
    _server = server;
}

private async void ListenForClients()
{
    int numClients = 0;
    while (IsRunning)
    {
        var tcpClient = await _listener.AcceptTcpClientAsync();
        var netClient = new NetworkClient(this, tcpClient, numClients);
        netClient.Start();
        Console.WriteLine("Client Connected");

        numClients++;
    }
}

Then in my NetworkClient class my Start() method looks like:

public async void Start()
{
    using (var reader = new StreamReader(_socket.GetStream()))
    {
        while (_server.IsRunning)
        {
            var line = await reader.ReadLineAsync();
            Console.WriteLine("Client {0} wrote: {1}", _id, line);
        }
    }
}

This works well while a telnet client is connected, however once I close my telnet client reader.ReadLineAsync(); constantly returns null. I would add a check to see if line == null but I'm not sure that is the correct way to detect if a client has disconnected.

To make matters worse, _socket.Connected is constantly returning true all while nulls are getting "received" by reader.ReadLineAsync().

What is the proper way to detect when tcp clients have been disconnected?

回答1:

A read on a TCP/IP socket will return 0 bytes when the connection has been gracefully closed. This situation causes ReadLineAsync to return null. So, yes, you should check for null and treat it as a graceful socket closure.

Sockets may be closed other ways, too; any socket operation may throw an exception if the socket is abortively closed. If the exception happens at an acceptable part of the protocol (where a close is not considered an error), then you should treat that exception as though it were a graceful closure as well.

Oh, and TcpClient.Connected (like Socket.Connected) is practically useless; it only tells you whether the socket was connected, not whether it is connected. Just pretend that property doesn't exist.

Finally, a couple of notes:

Avoid async void. If your methods return Task, then you have a "handle" to see when they complete (and whether they raised exceptions). My recent MSDN article explains why async void is not recommended.
It's best to periodically send data over the connection to determine if it is still viable. I wrote a TCP/IP .NET sockets FAQ that covers this in more detail.

来源：https://stackoverflow.com/questions/16138302/how-do-i-detect-when-a-telnet-client-is-improperly-closed-to-a-tcp-server

标签

tcplistener