Socket server data receive confusion

Question

Writing a small c# application which would accept client connection and process accordingly. I'm using asynchronous tcp socket for this as I would like to allow multiple clients to send data to server simultaneously. Now I'm not sure how do I handle data received inside the callback function. Below is the code for Data receiving callback. Here each time a data is receive it opens the output file and append data to it. I want to get some file name / say some client name before saving file. How do I do that. I'm sending the client name terminated by "|" character to distinguish from other data but how do I handle and save it to a variable from data receiving callback? Any help would be highly appriciable.

public void OnDataReceived(IAsyncResult ar)
{
  StateObject state = (StateObject)ar.AsyncState;
  Socket clientSocket = state.m_currentSocket;
  BinaryWriter writer;

  string fileSavePath = m_strCurrFolder[m_currClientNumber] + "File";
  int bytesRead = clientSocket.EndReceive(ar);
  if (bytesRead > 0)
  {
    //If the file doesn't exist, create a file with the filename got from server. If the file exists, append to the file. 
    if (!File.Exists(fileSavePath))
    {
      writer = new BinaryWriter(File.Open(fileSavePath, FileMode.Create));
    }
    else
    {
      writer = new BinaryWriter(File.Open(fileSavePath, FileMode.Append));
    }

    writer.Write(state.dataBuffer, 0, bytesRead);
    writer.Flush();
    writer.Close();

    // Recursively receive the rest file. 
    try
    {
      clientSocket.BeginReceive(state.dataBuffer, 0, StateObject.BufferSize, 0, new AsyncCallback(OnDataReceived), state);
    }
    catch
    {
      if (!clientSocket.Connected)
      {
        //MessageBox.Show(Properties.Resources.DisconnectMsg);
        MessageBox.Show("Catched from OnDataReceived!");
      }
    }
  }
  else
  {
    // Signal if all the file received. 
  }
}

Answer 1

I have made a dll for TCP networking with a client and server. http://the7skulls.blogspot.com/2015/03/how-to-tcp-with-server-and-client-in-c.html