Question

This is a continuation of the this question. I am new to network programming, so I am just writing small sample stuff to gain understanding, but somewhat struggling with explaining results.

It seems setting NetworkStream.ReceiveTimeout is not working correctly when client that was supposed to be sending data simply closes before sending all the expected data.

Here is the sample code:

public static void Main(string[] args)
{
    TcpListener listener = new TcpListener(IPAddress.Any, 10001);
    listener.Start();

    ThreadPool.QueueUserWorkItem(WriterThread);

    using (TcpClient client = listener.AcceptTcpClient())
    using (NetworkStream stream = client.GetStream())
    {
        client.ReceiveTimeout = (int)new TimeSpan(0, 0, 2).TotalMilliseconds;
        stream.ReadTimeout = (int)new TimeSpan(0, 0, 2).TotalMilliseconds;
        ReceiveMessage(stream, 1024);
    }

    listener.Stop();

    Console.WriteLine("Done.");
    Console.ReadKey(true);
}


private static void WriterThread(object state)
{
    using (TcpClient client = new TcpClient())
    {
        client.Connect(new IPEndPoint(IPAddress.Loopback, 10001));
        using (NetworkStream stream = client.GetStream())
        {
            byte[] bytes = Encoding.ASCII.GetBytes("obviously less than 1024 bytes");
            stream.Write(bytes, 0, bytes.Length);

            Thread.Sleep(10000); // comment out 
        }
    }
}


private static byte[] ReceiveMessage(Stream stream, int length)
{
    byte[] buffer = new byte[length];
    int bufferFill = 0;

    while (true)
    {
        bufferFill += stream.Read(buffer, bufferFill, buffer.Length - bufferFill);
        if (buffer.Length == bufferFill)
            return buffer;

        Thread.Sleep(100);
    }
}

This version works correctly triggering exception on the stream.Read() call. However If I comment out Thread.Sleep(10000), the client closes connection, but listener fails to recognize it. Main thread gets stuck inside the while(true) loop. The stream.Read() keeps returning zero, but no exception thrown.

Is this normal? If so how am I expected to handle abnormal client disconnections?

Answer 1

Yes, this sounds normal. There is no receive- or read timeout because the client has disconnected. This means that no more data is available for reading and the stream will return 0 immediately just as documented.

I would modify your ReceiveMessage method to something like the following:

private static byte[] ReceiveMessage(Stream stream, int length)
{
   byte[] buffer = new byte[length];
   int bufferFill = 0;

   while (true)
   {
      int bytesRead = stream.Read(buffer, bufferFill, buffer.Length - bufferFill);
      if (bytesRead == 0)
         throw new Exception("No more data available.");
      bufferFill += bytesRead;
      if (buffer.Length == bufferFill)
         return buffer;

      Thread.Sleep(100);
   }
}

Clearly if the stream.Read() call returns 0 before we have received all the expected bytes there must have been some form of disconnection or similar. Either way we will never get any more data from the stream.

Edit: The Stream class has no notion of a “message”. The Read method blocks until more data becomes available if none is already in the buffer. It will however return 0 when no more data can be received, which in this case means the connection is closed.