We Keep Coding

How does one identify and differentiate clients connected in a client-server socket application?

In the scenario where there is a SINGLE asynchronous socket server, and n-number of clients connected, on the server side how can I differentiate the clients? I need to know which client the data is coming from. Note - It may not be possible to append an identifier to requests.
Clients are created at runtime:
public void CreateSockets()
{
Parallel.For(1, Convert.ToInt32(numericUpDown1.Value) + 1, i =>
{
new Thread(() => Connect(i)).Start();
});
}
Abbreviated client creation loop:
public void Connect(int i)
{
ClientSocket[i] = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
try
{
ClientSocket[i].Connect(ip, port);
}
catch (SocketException sex)
{
Console.WriteLine(sex);
}
RequestLoop();
}
The sockets are being created, connect, and are sending data to the server. The server is directly from the MS site examples of Asynchronous Socket Server.
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
{
// 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();
// Client socket.
public Socket workSocket = null;
}
public class AsynchronousSocketListener
{
// Thread signal.
public static ManualResetEvent allDone = new ManualResetEvent(false);
public AsynchronousSocketListener()
{
}
public static void StartListening()
{
// Establish the local endpoint for the socket.
// The DNS name of the computer
// running the listener is "host.contoso.com".
IPHostEntry ipHostInfo = Dns.GetHostEntry(Dns.GetHostName());
IPAddress ipAddress = ipHostInfo.AddressList[0];
IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 11000);
// Create a TCP/IP socket.
Socket listener = new Socket(ipAddress.AddressFamily,
SocketType.Stream, ProtocolType.Tcp );
// Bind the socket to the local endpoint and listen for incoming connections.
try {
listener.Bind(localEndPoint);
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);
if (bytesRead > 0) {
// 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 {
// Not all data received. Get more.
handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
new AsyncCallback(ReadCallback), state);
}
}
}
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;
}
}

The combination of srcip:srcport:dstip:dstport is a unique value.
Use that Value to identify your client. When a client connects, have the client send an identifier. Maintain state in your server mapping that combination to your client.
You will never have two identical values of this on any socket on your network. (Except in the case of private networks with identical ip addresses).

Having issues sending an array of bytes using asynchronous socket in C#?

I 've following one of the .NET tutorials by Microsoft on how to implement an Asynchronous Server Socket. My goal is to implement a new function called writeToClient(byte[] input) inside Microsoft given code.
This function will be executed by another function located in a different application and upon being called, it will received an array of byte as an input and forward it to another application via socket running on a Raspberry Pi using Send() ideally.
The main issue I am facing so far is that Send(Socket handler, string data) function requires two arguments. To this end, I haven't been able to find a way I could combine these two applications into one and achieve my goal.
Any thoughts?
Thanks,
using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
// State object for reading client data asynchronously
namespace UDCManager
{
public class StateObject
{
// 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();
// Client socket.
public Socket workSocket = null;
}
public class AsynchronousServerSocket
{
// Thread signal.
public static ManualResetEvent allDone = new ManualResetEvent(false);
public static void StartListening()
{
IPAddress IP = IPAddress.Any;
IPEndPoint EP = new IPEndPoint(IP, 24000);
Socket listner = new Socket(IP.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
// Bind the socket to the local endpoint and listen for incoming connections.
try
{
listner.Bind(EP);
listner.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...");
listner.BeginAccept(
new AsyncCallback(AcceptCallback),
listner);
// 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);
if (bytesRead > 0)
{
// 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
{
// Not all data received. Get more.
handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
new AsyncCallback(ReadCallback), state);
}
}
}
public 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);
}
**public static void writeToClient(byte[] input)
{
}**
public 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());
}
}
}
}

If I understood the question correctly, you'd want something like this
// Expose a socket as a private variable, for example
private static Socket _socket;
public static void StartListening()
{
IPAddress IP = IPAddress.Any;
IPEndPoint EP = new IPEndPoint(IP, 24000);
_socket = new Socket(IP.AddressFamily, SocketType.Stream, ProtocolType.Tcp); // <<<< HERE
// ...
}
// then you could use it in the write method
public static void writeToClient(byte[] input)
{
// validate input for null, and size
// validate that _socket has been created (optionally)
_socket.BeginSend(input, 0, input.Length, 0,
new AsyncCallback(SendCallback), handler)
}
It may be simpler if you could start with synchronous code, and after that is working, convert it to new style async code using async/await

Trying to Connect RPI to Unity via TCP

I am working on a small project and I am at the point where I want to connect my Raspberry Pi to Unity (running on my PC). I am able to make a connection running Visual Studio, but when I run the same code in Unity, no connections happen.
C# Code
using System;
using System.Globalization;
using System.IO;
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.GetHostEntry(Dns.GetHostName());
//IPAddress ipAddress = Array.Find(ipHostInfo.AddressList, x => x.AddressFamily == AddressFamily.InterNetwork);
IPAddress ipAddress = IPAddress.Any;
IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 6670);
Console.WriteLine($"Connected on {localEndPoint}");
// Create a TCP/IP socket.
Socket listener = new Socket(ipAddress.AddressFamily,
SocketType.Stream, ProtocolType.Tcp );
// Bind the socket to the local endpoint and listen for incoming connections.
try {
listener.Bind(localEndPoint);
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 );
Console.WriteLine("Accepted");
// Wait until a connection is made before continuing.
allDone.WaitOne();
Console.WriteLine("After wait");
}
} 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);
if (bytesRead > 0) {
// There might be more data, so store the data received so far.
var stringRead = Encoding.ASCII.GetString(
state.buffer, 0, bytesRead);
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 {
string timestamp = DateTime.UtcNow.ToString("yyyy-MM-dd HH:mm:ss.fff",
CultureInfo.InvariantCulture);
Console.WriteLine(timestamp + ": " + stringRead);
// Not all data received. Get more.
handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
new AsyncCallback(ReadCallback), state);
//Test Code
Send(handler, "boogieboogie");
}
}
}
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) {
Thread t = new Thread(StartListening);
t.Start();
//FingerMovement.Service();
return 0;
}
}
The Python code is just doing a socket connect to the IP and port.
So to restate the question: Why does it not connect to the RPI when I run this in Unity? No connections occur, however, if I run the same exact code through VS, it connects just fine.
I allowed Unity in my firewall, tried running it in Admin, still nothing.

It's a bit of a long shot but have you enabled 'run in the background' checkbox in the player settings? I've had unity miss a whole lot of system events / transmissions while I was switching windows when this was off. I think all the child processes freeze when Unity looses focus by default, and I think that inclueds the async quasi-processes, or at the very least it surely includes your main thread which communicates with you

Why C# socket server can't find the eof from python client?

update
no matter \r\n or '\n' neither of them works.
if I change the indexof("<eof>")> -1 to index("<eof>") == -1, it worked. And I Google it and find that the eof always be at the -1 but I don't know why, the msdn can't be wrong.
The code is simple I just copy them from
python tutorial
C# MSDN
If I use the server code in the first link in Python, the python can print out the msg sent from client. But if I use the C# server, the indexof("<EOF>") always be -1, which means the msg sent from client never be printed by the C# server, why?
Server code
C#
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, 11000);
// 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(localEndPoint);
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);
if (bytesRead > 0)
{
// 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();
int res = 0;
res = content.IndexOf("<EOF>");
if (res > -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
{
// Not all data received. Get more.
handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
new AsyncCallback(ReadCallback), state);
}
}
}
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;
}
}
client code
python
#!/usr/bin/python3 # This is client.py file
import socket
# create a socket object
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# get local machine name
host = socket.gethostname()
port = 11000
# connection to hostname on the port.
s.connect((host, port))
msg='Hi Server!'+ "\r\n"
s.send(msg.encode('ascii'))
# Receive no more than 1024 bytes
msg = s.recv(1024)
s.close()
print (msg.decode('ascii'))

This can be due to different line endings: \n and \r\n.
Check your apps are sending and awaiting the same.

TCP C# Server Python Client cannot communicate

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.

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
// ...
}

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.