TCP C# Server Python Client cannot communicate

Question

I am writing some socket server, client application and I have a major problem. My goal is to create an async Server App in C# and a basic client APP in python. When I do follow simple examples both program work. But when I write an async Server with read and write handler messages being send from the client APP, it does not work.

Here is the example Server code that I am using.

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;

// 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 static void StartListening()
{
    // Data buffer for incoming data.
    byte[] bytes = new Byte[1024];

    // Establish the local endpoint for the socket.
    // The DNS name of the computer
    // running the listener is "host.contoso.com".
    IPHostEntry ipHostInfo = Dns.Resolve(Dns.GetHostName());
    IPAddress ipAddress = ipHostInfo.AddressList[0];
    IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 3333);
    IPEndPoint ipLocal = new IPEndPoint(IPAddress.Any, 3333);
    // 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(ipLocal);
        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);

            // Wait until a connection is made before continuing.
            allDone.WaitOne();
        }

    }
    catch (Exception e)
    {
        Console.WriteLine(e.ToString());
    }

    Console.WriteLine("\nPress ENTER to continue...");
    Console.Read();

}

public static void AcceptCallback(IAsyncResult ar)
{
    // Signal the main thread to continue.
    allDone.Set();

    // 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);
}

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);
    Console.WriteLine("\n Enters(0)");
    if (bytesRead > 0)
    {
        Console.WriteLine("\n Enters(1)");
        // 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("<EOF>") > -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);
            // Echo the data back to the client.
            Send(handler, content);
        }
        else
        {
            Console.WriteLine("\n Detect");
            // Not all data received. Get more.
            handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
            new AsyncCallback(ReadCallback), state);
        }
    }
    Console.WriteLine("\n Enters(1)");

}

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());
    }
}


public static int Main(String[] args)
{
    StartListening();
    return 0;
}
}

and here is the python code for a simple test client

import socket

HOST, PORT = "127.0.0.1", 3333
data = "data"
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

try:
    sock.connect((HOST, PORT))
    sock.sendall(data)
    print data
finally:
    sock.close()

As I have inserted some debug code on Server's ReadCallback, I see that the routine is called, but it doesn't process the received data as a message. It doesn't seem to have an endpoint.

Any ideas or solutions will be appreciated.

Answer 1

Your ReadCallback ignores the case when bytesRead == 0, which means the client stopped sending data. If your Python code is actually sending "data" without "<EOF>", then your connection is simply forgotten by the C# server.

---

Regarding your C# code, there are a few things you could improve:

• Use ManualResetEventSlim, it's faster, it doesn't use an operating system handle (unless you actually use the WaitHandle property)

• You should handle exceptions from EndAccept in AcceptCallback and EndReceive in ReadCallback, and probably improve exception handling in SendCallback

• You're creating a string from your StringBuilder in ReadCallback every time, which goes totally against the purpose of using a StringBuilder; you should parse each string you get from Encoding.ASCII.GetString to search for each character, <, E, O, F and > in succession

 

// Add to StateObject
public const string EOF = "<EOF>";
public int eofOffset = -1;
public int searchOffset = 0;

// In ReadCallback
string data = Encoding.ASCII.GetString(state.buffer, 0, bytesRead);
state.sb.Append(data);
int o = state.searchOffset + state.eofOffset + 1;
while (o < state.sb.Length)
{
    if (state.sb[o] != StateObject.EOF[state.eofOffset + 1])
    {
        state.eofOffset = -1;
        state.searchOffset++;
        o = state.searchOffset;
    }
    else
    {
        state.eofOffset++;
        if (state.eofOffset == StateObject.EOF.Length)
        {
            break;
        }
        o++;
    }
}

// Replace this:
//content = state.sb.ToString();
//if (content.IndexOf("<EOF>") > -1)
// with this:
if (state.eofOffset == StateObject.EOF.Length)
{
    // Here is a good place to turn the StringBuilder into a string
    // Perhaps truncate data to send back up to state.searchOffset
    // ...
}

Answer 2

Not an expert for C#, but I think You have problem with Python program.

try:
    sock.connect((HOST, PORT))
    sock.sendall("data")
    print data
finally:
    sock.close()

You're trying to print data, which is not defined. Therefore, except part would be executed (if it existed). Then, program is finished with socket being closed.