Question

With regards to handling a TCP/IP connection using the TcpClient class, is there an alternative for checking whether the remote host has closed the connection other than waiting for the NetworkStream.Read method to return a 0?

Answer 1

You can use IOControlCode.KeepAliveValues on the TcpClient.Client to guarantee that a keep-alive check is made at least on the specified interval and then check the TcpClient.Client.Connected property.

An example how to use it:

struct tcp_keepalive
{
    public int OnOff;
    public int KeepAliveTime;
    public int KeepAliveInterval;

    public unsafe byte[] Buffer
    {
        get
        {
            var buf = new byte[sizeof(tcp_keepalive)];
            fixed(void* p = &this) Marshal.Copy(new IntPtr(p), buf, 0, buf.Length);
            return buf;
        }
    }
};

static void KeepAliveTest()
{
    using(var c = new TcpClient())
    {
        c.Connect("www.google.com", 80);
        var s = c.Client;
        var ka = new tcp_keepalive();
        ka.OnOff = 1; // enable
        ka.KeepAliveTime = 1000 * 60; // 60 seconds of inactivity allowed
        ka.KeepAliveInterval = 1000; // 1 second interval on keep-alive checks (default)
        s.IOControl(IOControlCode.KeepAliveValues, ka.Buffer, null);
        var ns = c.GetStream();
        Console.WriteLine("Connected to " + s.RemoteEndPoint);
        while(true)
        {
            SocketError se;
            s.Blocking = false;
            s.Receive(new byte[0], 0, 0, SocketFlags.Peek, out se);
            s.Blocking = true;
            if(!s.Connected)
            {
                // se==SocketError.ConnectionReset||SocketError.NetworkReset if the connection was closed because of a keep-alive check
                Console.WriteLine("Socket disconnected: " + se);
                break;
            }
            // do other stuff
            if(ns.DataAvailable) ns.Read(new byte[100], 0, 100);
            else Thread.Sleep(10);
        }
    }
}