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C# tcp async listener gets stuck on my on_receive callback after client closes socket

I've got a listener socket that accepts, receives and sends as a TCP server typically does. I've given my accept and receive code below, it's not that different from the example on Microsoft's documentation. The main difference is that my server doesn't kill a connection after it stops receiving data (I don't know if this is a bad design or not?).
private void on_accept(IAsyncResult xResult)
{
Socket listener = null;
Socket handler = null;
TStateObject state = null;
Task<int> consumer = null;
try
{
mxResetEvent.Set();
listener = (Socket)xResult.AsyncState;
handler = listener.EndAccept(xResult);
state = new TStateObject()
{
Socket = handler
};
consumer = async_input_consumer(state);
OnConnect?.Invoke(this, handler);
handler.BeginReceive(state.Buffer, 0, TStateObject.BufferSize, 0, new AsyncCallback(on_receive), state);
}
catch (SocketException se)
{
if (se.ErrorCode == 10054)
{
on_disconnect(state);
}
}
catch (ObjectDisposedException)
{
return;
}
catch (Exception ex)
{
System.Console.WriteLine("Exception in TCPServer::AcceptCallback, exception: " + ex.Message);
}
}
private void on_receive(IAsyncResult xResult)
{
Socket handler = null;
TStateObject state = null;
try
{
state = xResult.AsyncState as TStateObject;
handler = state.Socket;
int bytesRead = handler.EndReceive(xResult);
UInt16 id = TClientRegistry.GetIdBySocket(handler);
TContext context = TClientRegistry.GetContext(id);
if (bytesRead > 0)
{
var buffer_data = new byte[bytesRead];
Array.Copy(state.Buffer, buffer_data, bytesRead);
state.BufferBlock.Post(buffer_data);
}
Array.Clear(state.Buffer, 0, state.Buffer.Length);
handler.BeginReceive(state.Buffer, 0, TStateObject.BufferSize, 0, new AsyncCallback(on_receive), state);
}
catch (SocketException se)
{
if(se.ErrorCode == 10054)
{
on_disconnect(state);
}
}
catch (ObjectDisposedException)
{
return;
}
catch (Exception ex)
{
System.Console.WriteLine("Exception in TCPServer::ReadCallback, exception: " + ex.Message);
}
}
This code is used to connect to an embedded device and works (mostly) fine. I was investigating a memory leak and trying to speed up the process a bit by replicating exactly what the device does (our connection speeds are in the realm of about 70kbps to our device, and it took an entire weekend of stress testing to get the memory leak to double the memory footprint of the server).
So I wrote a C# program to replicate the data transactions, but I've run into an issue where when I disconnect the test program, the server gets caught in a loop where it endlessly has its on_receive callback called. I was under the impression that BeginReceive wouldn't be triggered until something was received, and it seems to call on_receive, ends the receiving like an async callback should do, process the data, and then I want the connection to await more data so I call BeginReceive again.
The part of my test program where the issue occurs is in here:
private static void read_write_test()
{
mxConnection = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
mxConnection.Connect("12.12.12.18", 10);
if (mxConnection.Connected)
{
byte[] data = Encoding.ASCII.GetBytes("HANDSHAKESTRING"); //Connect string
int len = data.Length;
mxConnection.Send(data);
data = new byte[4];
len = mxConnection.Receive(data);
if (len == 0 || data[0] != '1')
{
mxConnection.Disconnect(false);
return;
}
}
//Meat of the test goes here but isn't relevant
mxConnection.Shutdown(SocketShutdown.Both);
mxConnection.Close();
}
Up until the Shutdown(SocketShutdown.Both) call, everything works as expected. When I make that call however, it seems like the server never gets notification that the client has closed the socket and gets stuck in a loop of endlessly trying to receive. I've done my homework and I think I am closing my connection properly as per this discussion. I've messed around with the disconnect section to just do mxConnection.Disconnect(false) as well, but the same thing occurs.
When the device disconnects from the server, my server catches a SocketException with error code 10054, which documentation says:
Connection reset by peer.
An existing connection was forcibly closed
by the remote host. This normally results if the peer application on
the remote host is suddenly stopped, the host is rebooted, the host or
remote network interface is disabled, or the remote host uses a hard
close (see setsockopt for more information on the SO_LINGER option on
the remote socket). This error may also result if a connection was
broken due to keep-alive activity detecting a failure while one or
more operations are in progress. Operations that were in progress fail
with WSAENETRESET. Subsequent operations fail with WSAECONNRESET.
I've used this to handle the socket being closed and has worked well for the most part. However, with my C# test program, it doesn't seem like it works the same way.
Am I missing something here? I'd appreciate any input. Thanks.

The main difference is that my server doesn't kill a connection after it stops receiving data (I don't know if this is a bad design or not?).
Of course it is.
it seems like the server never gets notification that the client has closed the socket and gets stuck in a loop of endlessly trying to receive
The server does get notification. It's just that you ignore it. The notification is that your receive operation returns 0. When that happens, you just call BeginReceive() again. Which starts a new read operation. Which…returns 0! You just keep doing that over and over again.
When a receive operation returns 0, you're supposed to complete the graceful closure (with a call to Shutdown() and Close()) that the remote endpoint started. Do not try to receive again. You'll just keep getting the same result.
I strongly recommend you do more homework. A good place to start would be the Winsock Programmer's FAQ. It is a fairly old resource and doesn't address .NET at all. But for the most part, the things that novice network programmers are getting wrong in .NET are the same things that novice Winsock programmers were getting wrong twenty years ago. The document is still just as relevant today as it was then.
By the way, your client-side code has some issues as well. First, when the Connect() method returns successfully, the socket is connected. You don't have to check the Connected property (and in fact, should never have to check that property). Second, the Disconnect() method doesn't do anything useful. It's used when you want to re-use the underlying socket handle, but you should be disposing the Socket object here. Just use Shutdown() and Close(), per the usual socket API idioms. Third, any code that receives from a TCP socket must do that in a loop, and make use of the received byte-count value to determine what data has been read and whether enough has been read to do anything useful. TCP can return any positive number of bytes on a successful read, and it's your program's job to identify the start and end of any particular blocks of data that were sent.

You missed this in the documentation for EndReceive() and Receive():
If the remote host shuts down the Socket connection with the Shutdown method, and all available data has been received, the Receive method will complete immediately and return zero bytes.
When you read zero bytes, you still start another BeginReceive(), instead of shutting down:
if (bytesRead > 0)
{
var buffer_data = new byte[bytesRead];
Array.Copy(state.Buffer, buffer_data, bytesRead);
state.BufferBlock.Post(buffer_data);
}
Array.Clear(state.Buffer, 0, state.Buffer.Length);
handler.BeginReceive(state.Buffer, 0, TStateObject.BufferSize, 0, new AsyncCallback(on_receive), state);
Since you keep calling BeginReceive on a socket that's 'shutdown', you're going to keep getting callbacks to receive zero bytes.
Compare with the example from Microsoft in the documentation for EndReceive():
public static void Read_Callback(IAsyncResult ar){
StateObject so = (StateObject) ar.AsyncState;
Socket s = so.workSocket;
int read = s.EndReceive(ar);
if (read > 0) {
so.sb.Append(Encoding.ASCII.GetString(so.buffer, 0, read));
s.BeginReceive(so.buffer, 0, StateObject.BUFFER_SIZE, 0,
new AsyncCallback(Async_Send_Receive.Read_Callback), so);
}
else{
if (so.sb.Length > 1) {
//All of the data has been read, so displays it to the console
string strContent;
strContent = so.sb.ToString();
Console.WriteLine(String.Format("Read {0} byte from socket" +
"data = {1} ", strContent.Length, strContent));
}
s.Close();
}
}

Multithreading: passing different custom control properties between threads (or using them asynchronously)

I'm trying to develop a project where I can have multiple clients making their own server requests for the purpose of stress testing. I'm having a lot of difficulty figuring out how I can manipulate custom control properties when I make a new thread and want that thread to do the work. I have upwards of 100 controls; so ideally 100 individual clients. The problem is my controls are part of the GUI and I don't know how to allow that thread in question get access from that relative control.
Here's what I have:
// Custom project.class to get access to a custom base of properties created within the tool itself.
List<custom_control_project.custom_control_widget> controlList = new List<custom_control_project.custom_control_widget>();
private async void btnStart_Click(object sender, EventArgs e)
{
...// control property initializations
foreach (var control in controlList)
{
if (control.Enabled)
{
Thread thread = new Thread(() => StartClient());
thread.Start();
// Loop until worker thread activates.
while (!thread.IsAlive);
... // Ideally the GUI updates would happen from these threads. Simple updates to labels based on status code responses and expected xml parameters received.
}
}
My StartClient() is largely based off Microsofts asynchronous socket client example here: http://msdn.microsoft.com/en-us/library/bew39x2a%28v=vs.110%29.aspx
I was running these clients asynchronously but the program was not my end result. I have made a few changes, including resetting the ManualResetEvents. However, when I run my application, all the controls still run one after the other, and I'd like them to be independent. Do I have the right approach by making new threads with the StartClient()?
Referencing microsofts example, the part I'm most interested in is the ReceiveCallback(IAsyncResult ar) method:
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();
}
// ManualResetEvent instances signal completion.
private ManualResetEvent connectDone =
new ManualResetEvent(false);
private ManualResetEvent sendDone =
new ManualResetEvent(false);
private ManualResetEvent receiveDone =
new ManualResetEvent(false);
// The response from the remote device.
private String response = String.Empty;
public void StartClient()
{
// Connect to a remote device.
try
{
// Establish the remote endpoint for the socket.
// The name of the
// remote device is "host.contoso.com".
//IPHostEntry ipHostInfo = Dns.Resolve("host.contoso.com");
//IPAddress ipAddress = ipHostInfo.AddressList[0];
//IPEndPoint remoteEP = new IPEndPoint(ipAddress, port);
// Create a TCP/IP socket.
Socket client = new Socket(AddressFamily.InterNetwork,
SocketType.Stream, ProtocolType.Tcp);
// Connect to the remote endpoint.
client.BeginConnect(asyncServerHolder, asyncPortHolder,
new AsyncCallback(ConnectCallback), client);
connectDone.WaitOne();
// Send test data to the remote device.
Send(client, (Upload)); //POST HTTP string + xml parameters
sendDone.WaitOne();
// Receive the response from the remote device.
Receive(client);
receiveDone.WaitOne();
// Write the response to the console.
//MessageBox.Show("Response received : " + response);
// Release the socket.
client.Shutdown(SocketShutdown.Both);
client.Close();
// Reset all manual events for next instance.
connectDone.Reset();
sendDone.Reset();
receiveDone.Reset();
}
catch (Exception)
{
//MessageBox.Show(e.ToString());
}
}
private void ConnectCallback(IAsyncResult ar)
{
try
{
// Retrieve the socket from the state object.
Socket client = (Socket)ar.AsyncState;
// Complete the connection.
client.EndConnect(ar);
//MessageBox.Show("Socket connected to " + client.RemoteEndPoint.ToString());
// Signal that the connection has been made.
connectDone.Set();
}
catch (Exception)
{
//MessageBox.Show(e.ToString());
}
}
private void Receive(Socket client)
{
try
{
// Create the state object.
StateObject state = new StateObject();
state.workSocket = client;
// Begin receiving the data from the remote device.
client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
new AsyncCallback(ReceiveCallback), state);
}
catch (Exception)
{
//MessageBox.Show(e.ToString());
}
}
private void ReceiveCallback(IAsyncResult ar)
{
try
{
// Retrieve the state object and the client socket
// from the asynchronous state object.
StateObject state = (StateObject)ar.AsyncState;
Socket client = state.workSocket;
// Read data from the remote device.
int bytesRead = client.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));
// Get the rest of the data.
client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
new AsyncCallback(ReceiveCallback), state);
}
else
{
// All the data has arrived; put it in response.
if (state.sb.Length > 1)
{
response1 = state.sb.ToString();
///
///
/// **THIS IS WHERE I WANT ALL THE PROCESSING TO BE DONE**
/// **AFTER THE RESPONSE IS COMPLETE!!**
///
///
}
// Signal that all bytes have been received.
receiveDone.Set();
}
}
catch (Exception)
{
//MessageBox.Show(e.ToString());
}
}
private void Send(Socket client, 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.
client.BeginSend(byteData, 0, byteData.Length, 0,
new AsyncCallback(SendCallback), client);
}
private void SendCallback(IAsyncResult ar)
{
try
{
// Retrieve the socket from the state object.
Socket client = (Socket)ar.AsyncState;
// Complete sending the data to the remote device.
int bytesSent = client.EndSend(ar);
//MessageBox.Show("Sent " + bytesSent + " bytes to server.");
// Signal that all bytes have been sent.
sendDone.Set();
}
catch (Exception)
{
//MessageBox.Show(e.ToString());
}
}
How am I able to get the relative control from my foreach statement passed into the method and then amend my GUI (invoking somehow?) based on the outcome? Also, is it even possible to make each client independent, and have multiple client to server to client requests simultaneously? That is my main objective.
If this is a far fetched or very discouraged way of attempting this, please say so. Without diving into too much programming jargon (for understanding purposes, I'm fairly new to programming), how would you go about doing this?
Thanks in advance!

Well, you can sort of manipulate any control from a different thread by using the BeginInvoke function on the control, passing the action you want to execute. The action will be executed on the UI thread.
But your main problem here is that you're failing to separate the concerns. You UI code can for sure make actions happen, but these actions should be distinct from any UI code. You should design your actions in such a way to make them reusable. In other words, if you decide to rewrite your UI from scratch, you should still be able to reuse your actions as-is.
To make this possible, your actions should not reference any UI, they even should not be aware of the existence of any UI. This will make your code more manageable. So extract all that stuff in a different class, and then use something like events for instance to communicate back with the UI. The UI code then would make the BeginInvoke call.

Server Async Sockets - Detect closed connection from client?

I have the following source code from ms site: http://msdn.microsoft.com/en-us/library/fx6588te.aspx
I'm trying to detect when the client has closed connection.
One way I tried was to keep sending data to client until I get exception. I created a new thread which gets executed once at onDataReceive, but I get an error "Cannot access disposed object" on:
m_workerSocket[socket_id].Send(bytes);
but if I put it in directly in onDataReceive it works fine. Why do I get this exception when trying to access socket from another thread?
Then I found the second solution:
static class SocketExtensions
{
public static bool IsConnected(this Socket socket)
{
try
{
return !(socket.Poll(1, SelectMode.SelectRead) && socket.Available == 0);
}
catch (SocketException) { return false; }
}
}
Which I wanted to run in a separate thread too, to check whenever socket gets closed, but I get the same error. I only get error if I try to do this from a thread, if I place both solutions in one of the functions, they execute fine. Any ideas how to get this running from a thread?

Try this code:
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;
if (handler != null && !handler.Poll(1, SelectMode.SelectRead) && handler.Connected && handler.Available == 0)
{
// Read data from the client socket.
int bytesRead = handler.EndReceive(ar);
if (bytesRead > 0)
{
// Parsing data received in here..
// Enabled received for next data
handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);
}
}
else
{
if (handler.Connected)
{
// Client are offline will be detected here
IPEndPoint clientIP = handler.RemoteEndPoint as IPEndPoint;
MessageBox.Show(String.Format("IP Client: {0}:{1} disconnected", clientIP.Address, clientIP.Port));
}
{
// Server are offline will be detected here
}
}
}

If only problem is to determine when client is disconnected:
I had the same problem in my socket server, and finally I decided to send disconnect "flag" from any client to server when it's disconnected.

All you have to do is handle your unload function correctly for your clients to detect a client side abnormal close. The client.connected is not very reliable on the server side but this should help.
IE...
protected override void UnloadContent()
{
// TODO: Unload any non ContentManager content here
if (client != null)
client.Close(); // I exist so close the connection...
// I would place a timout on the server side
// client.Available as well to drop inactive clients...
System.Threading.Thread.Sleep(3000); // Force the Client to stay open long enough to communicate a closure...
}

Could I have some advice on basic Asynchronous Socket Programming in C#?

I've developing (or trying to, anyway) a program that uses Asynchronous Socket to, supposedly, pass strings to and fro the server and client, at any time.
This program requires no more than one client be connected to a server. I tried Socket Programming, but I found out it blocks the program until either one receives something.
Since I have only a basic understanding of Asynchronous socket programming, I just went for the simplest one I could find, or at least, the simplest one I could understand.
Here's my code for the Server:
public Socket g_server_conn;
public byte[] g_bmsg;
public bool check = false;
private void net_As_Accept(IAsyncResult iar)
{
Socket server_conn = (Socket)iar.AsyncState;
g_server_conn = server_conn.EndAccept(iar);
g_bmsg = new byte[1024];
check = true;
g_server_conn.BeginReceive(g_bmsg, 0, g_bmsg.Length, SocketFlags.None, new AsyncCallback(net_As_Receive), g_server_conn);
}
private void net_As_Send(IAsyncResult iar)
{
Socket server_conn = (Socket)iar.AsyncState;
server_conn.EndSend(iar);
}
private void net_As_Receive(IAsyncResult iar)
{
try
{
Socket server_conn = (Socket)iar.AsyncState;
server_conn.EndReceive(iar);
if (g_bmsg.Length != 0)
{
net_Data_Receive(Encoding.ASCII.GetString(g_bmsg, 0, g_bmsg.Length));
check = false;
}
}
catch (Exception ex)
{
MessageBox.Show(ex.ToString(), "GG");
}
}
public void net_Data_Send(string msg2snd) // Function for sending through socket
{
MessageBox.Show(msg2snd);
byte[] byData = System.Text.Encoding.ASCII.GetBytes(msg2snd);
g_server_conn.BeginSend(byData, 0, byData.Length, SocketFlags.None, new AsyncCallback(net_As_Send), g_server_conn);
g_server_conn.BeginReceive(g_bmsg, 0, g_bmsg.Length, SocketFlags.None, new AsyncCallback(net_As_Receive), g_server_conn);
}
private void net_Data_Receive(string txt)
{
if (lblBuffer.InvokeRequired)
lblBuffer.Invoke(new MethodInvoker(delegate { net_Data_Receive(txt); }));
else
lblBuffer.Text = txt;
if (txt.StartsWith("&"))
{
// Do something
}
}
And here's my code for the Client:
private void net_As_Connect(IAsyncResult iar)
{
try
{
Socket client_conn = (Socket)iar.AsyncState;
client_conn.EndConnect(iar);
g_bmsg = new byte[1024];
check = true;
string toSendData = "&" + net_Name;
net_Data_Send(toSendData);
g_client_conn.BeginReceive(g_bmsg, 0, g_bmsg.Length, SocketFlags.None, new AsyncCallback(net_As_Receive), g_client_conn);
}
catch (Exception ex)
{
MessageBox.Show(ex.ToString(), "GG");
}
}
private void net_As_Send(IAsyncResult iar)
{
Socket client_conn = (Socket)iar.AsyncState;
client_conn.EndSend(iar);
}
private void net_As_Receive(IAsyncResult iar)
{
if (g_bmsg.Length != 0)
{
net_Data_Receive(Encoding.ASCII.GetString(g_bmsg, 0, g_bmsg.Length));
check = false;
}
}
public void net_Data_Send(string msg2snd)
{
byte[] byData = System.Text.Encoding.ASCII.GetBytes(msg2snd);
g_client_conn.BeginSend(byData, 0, byData.Length, SocketFlags.None, new AsyncCallback(net_As_Send), g_client_conn);
g_client_conn.BeginReceive(g_bmsg, 0, g_bmsg.Length, SocketFlags.None, new AsyncCallback(net_As_Receive), g_client_conn);
}
private void net_Data_Receive(string txt)
{
if (lblBuffer.InvokeRequired)
lblBuffer.Invoke(new MethodInvoker(delegate { net_Data_Receive(txt); }));
else
lblBuffer.Text = txt;
if (txt.StartsWith("&"))
{
// Do Something
}
else if (txt.StartsWith("$"))
{
// Do something Else
}
}
Now, the Client can connect to the Server fine. The Client can even send in a string containing the user's name to the Server, which will then be displayed on the Server. The Server then sends out the name of its user to the Client, which the client receives and displays. Whatever is sent is stored in a Label (lblBuffer)
But afterwards, say I have the following code:
private void btnSendData_Click(object sender, EventArgs e)
{
string posMov = "Stuff to send";
net_Data_Send(posMov);
}
The Client receives nothing. Putting a Message Box in net_Data_Send(msg2snd) function reveals that the server does in fact send out the message. In fact, putting in the Message Box in that function makes it work (the Client receives it), for reasons I don't know. Since I haven't tried sending a message from the Client to the Server (other than the name when the Client Connects), I assume the Client will have the same problem sending to the Server.
Also, when it does send the second message (by putting a Message Box in the net_Data_Send function), only parts of the Label (lblBuffer) are overwritten. So if I my name is "Anon E. Moose", and the Server sends that when the Client connects, and I try to send out, say, "0.0" (via button press) the Label on the Client would then read "0.0n E. Moose".
What did I do wrong? Can I have some help on this, please?
Perhaps I have a problem with net_Data_Receive and net_Data_Send?

I think you need to call BeginReceive on your client again, it looks like you are only calling it once, so after it has received the server name, it isn't listening for any more data from the server
private void net_As_Receive(IAsyncResult iar)
{
var bytesRead = g_client_conn.EndReceive(iar);
if (bytesRead != 0)
{
net_Data_Receive(Encoding.ASCII.GetString(g_bmsg, 0, bytesRead));
check = false;
}
g_client_conn.BeginReceive(g_bmsg, 0, g_bmsg.Length, SocketFlags.None, new AsyncCallback(net_As_Receive), g_client_conn);
}
also, as I mentioned in my comment, use the bytesRead value to work out how much of the buffer you need to use.
You will need to work out if the data you have received from the socket is the full amount, or if you need to read more data to make up the current message from the other side.

BeginReceive doesn't just call its callback whenever a new packet (string in your case arrives). In fact. BeginReceive or any raw socket method works in a stream based fasion, not packet based. See http://msdn.microsoft.com/en-us/library/bew39x2a.aspx for an example.
What you need to do, is in your 'net_As_Receive' callback method (naming is terrible imo), you need to make a call first to socket.EndRecieve(IAsyncResult), which in turn returns the total bytes currently available. After that, you have to make a decision whether to receive more data or not.
For example:
private StringBuilder packetBuilder;
{
if (packetBuilder == null)
packetBuilder = new StringBuilder();
// finalyze the receive
int length = g_server_conn.EndReceive(iar);
if (length != 0)
{
// get the total bytes received. Note that the length property is of that of the number of bytes received rather than that of the buffer
packetBuilder.Append(Encoding.ASCII.GetString(g_bmsg, 0, length));
net_Data_Receive(packetBuilder.ToString());
check = false;
}
// receive the next part
g_server_conn.BeginReceive(g_bmsg, 0, g_bmsg.Length, SocketFlags.None, new AsyncCallback(net_As_Receive), g_server_conn);
}
Note that this example doesnt care about packages. It will work if your lucky but there is a good change either a part of a string will be shown or 2 different strings will be combined. A good implementation will look for a string end and only show that part while buffering the rest untill a new string end is found. You can also use a StreamReader for making your life much easier

How to connect to modems with tcp clients, in multiple port or other way?

I have around 5000 modem (thin clients), and I want to communicate with them, one of a my method is like this : string GetModemData(modemID), now I have an open port in server that listens to modem and I'm using socket programming to send data to modems (calling related function), but when i want send data to multiple modem in a same time and get response from them, I don't know what should i do? I can send data to one modem and waiting for its response and then send another data to other modems (sequential), but the problem is client should be wait long time to get answer(may be some different client want to get some information from modems so they all will be wait into the Q or something like this), I think one way to solving this problem is to use multiple port and listen for each modem to related port, but it takes too many ports and also may be memory usage going up and exceed my available memory space, so some lost may be occurred (is this true?). what should to do ? I'd thinking about Parallelism, but i think its not related i should to wait for one port, because i don't know should to pass current received data to which client. I'm using asp.net.
currently I'm doing like this:
private void StartListener()
{
ModemTcpListener = new TcpListener(ModemPort);
//ClientTcpListener = new TcpListener(ClientPort);
ModemTcpListener.Start();
ModemTcpListener.BeginAcceptTcpClient(new AsyncCallback(DoAcceptModemCallback), ModemTcpListener);
}
and in return
private void DoReadModemCallback(IAsyncResult ar)
{
try
{
bool bRet = ar.AsyncWaitHandle.WaitOne(420000);
Modem modem = ar.AsyncState as Modem;
if (!bRet || modem == null)
{
return;
}
}
catch{}
// now send data to which client?????? if i'm going to use async????
}
and :
private void DoAcceptModemCallback(IAsyncResult ar)
{
try
{
ModemTcpListener.BeginAcceptTcpClient(new AsyncCallback(DoAcceptModemCallback), ModemTcpListener);
TcpClient tcpClient = ModemTcpListener.EndAcceptTcpClient(ar);
Modem modem= new Modem(tcpClient, "");
tcpClient.GetStream().BeginRead(modem.Buffer, 0, tcpClient.ReceiveBufferSize, new AsyncCallback(DoReadModemCallback), modem);
ModemTcpListener.BeginAcceptTcpClient(new AsyncCallback(DoAcceptModemCallback), ModemTcpListener);
Log.Write("a Modem connect ...");
}
catch (Exception ex)
{
}
}

Heres an example keeping track of all your clients. I've compacted it for readability. You should really split it up into multiple classes.
I'm using Pool (which I just created and commited) and SimpleServer. Both classes are part of a library that I'm currently building (but far from done).
Don't be afraid of having 5000 sockets open, they do not consume much resources when you are using asynchronous operations.
public class SuperServer
{
private List<ClientContext> _clients = new List<ClientContext>();
private SimpleServer _server;
private Pool<byte[]> _bufferPool;
public SuperServer()
{
// Create a buffer pool to be able to reuse buffers
// since your clients will most likely connect and disconnect
// often.
//
// The pool takes a anonymous function which should return a new buffer.
_bufferPool = new Pool<byte[]>(() => new byte[65535]);
}
public void Start(IPEndPoint listenAddress)
{
_server = new SimpleServer(listenAddress, OnAcceptedSocket);
// Allow five connections to be queued (to be accepted)
_server.Start(5);
}
// you should handle exceptions for the BeginSend
// and remove the client accordingly.
public void SendToAll(byte[] info)
{
lock (_clients)
{
foreach (var client in _clients)
client.Socket.BeginSend(info, 0, info.Length, SocketFlags.None, null, null);
}
}
// Server have accepted a new client.
private void OnAcceptedSocket(Socket socket)
{
var context = new ClientContext();
context.Inbuffer = _bufferPool.Dequeue();
context.Socket = socket;
lock (_clients)
_clients.Add(context);
// this method will eat very few resources and
// there should be no problem having 5000 waiting sockets.
context.Socket.BeginReceive(context.Inbuffer, 0, context.Inbuffer.Length, SocketFlags.None, OnRead,
context);
}
//Woho! You have received data from one of the clients.
private void OnRead(IAsyncResult ar)
{
var context = (ClientContext) ar.AsyncState;
try
{
var bytesRead = context.Socket.EndReceive(ar);
if (bytesRead == 0)
{
HandleClientDisconnection(context);
return;
}
// process context.Inbuffer here.
}
catch (Exception err)
{
//log exception here.
HandleClientDisconnection(context);
return;
}
// use a new try/catch to make sure that we start
// read again event if processing of last bytes failed.
try
{
context.Socket.BeginReceive(context.Inbuffer, 0, context.Inbuffer.Length, SocketFlags.None, OnRead,
context);
}
catch (Exception err)
{
//log exception here.
HandleClientDisconnection(context);
}
}
// A client have disconnected.
private void HandleClientDisconnection(ClientContext context)
{
_bufferPool.Enqueue(context.Inbuffer);
try
{
context.Socket.Close();
lock (_clients)
_clients.Remove(context);
}
catch(Exception err)
{
//log exception
}
}
// One of your modems
// add your own state info.
private class ClientContext
{
public byte[] Inbuffer;
public Socket Socket;
}
}
Used classes:
Pool: http://fadd.codeplex.com/SourceControl/changeset/view/58858#1054902
SimpleServer: http://fadd.codeplex.com/SourceControl/changeset/view/58859#1054893

You need to use the asynchronous tcp/ip methods. This article shows how:
http://www.codeproject.com/KB/IP/asyncsockets.aspx
The critical piece is the BeginReceive() and related callback functions. Any more q's, please leave comments to this answer ;) BEST OF LUCK!

You need multi threading, whenever a client establishes a connection to the server start a new thread for it and start communication send/receive.
Here are some articles explaining multithreading in c#,
c-sharpcorner
codeproject
And here's a sample server application with multithreading,
http://www.dotnetspider.com/resources/2829-A-multi-readed-server-C-which-finds-prime-num.aspx