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

/// <summary>
/// TcpClientWithTimeout is used to open a TcpClient connection, with a
/// user definable connection timeout in milliseconds (1000=1second)
/// Use it like this:
/// TcpClient connection = new TcpClientWithTimeout('127.0.0.1',80,1000).Connect();
/// </summary>
public class TcpClientWithTimeout
{
  protected string _hostname;
  protected int _port;
  protected int _timeout_milliseconds;
  protected TcpClient connection;
  protected bool connected;
  protected Exception exception;

  public TcpClientWithTimeout(string hostname,int port,int timeout_milliseconds)
  {
    _hostname = hostname;
    _port = port;
    _timeout_milliseconds = timeout_milliseconds;
  }
  public TcpClient Connect()
  {
    // kick off the thread that tries to connect
    connected = false;
    exception = null;
    Thread thread = new Thread(new ThreadStart(BeginConnect));
    thread.IsBackground = true; // So that a failed connection attempt
    // wont prevent the process from terminating while it does the long timeout
    thread.Start();

    // wait for either the timeout or the thread to finish
    thread.Join(_timeout_milliseconds);

    if (connected == true)
    {
      // it succeeded, so return the connection
      thread.Abort();
      return connection;
    }
    if (exception != null)
    {
      // it crashed, so return the exception to the caller
      thread.Abort();
      throw exception;
    }
    else
    {
      // if it gets here, it timed out, so abort the thread and throw an exception
      thread.Abort();
      string message = string.Format("TcpClient connection to {0}:{1} timed out",
        _hostname, _port);
      throw new TimeoutException(message);
    }
  }
  protected void BeginConnect()
  {
    try
    {
      connection = new TcpClient(_hostname, _port);
      // record that it succeeded, for the main thread to return to the caller
      connected = true;
    }
    catch (Exception ex)
    {
      // record the exception for the main thread to re-throw back to the calling code
      exception = ex;
    }
  }
}

And here's a little example of how to use this to open a connection, send 10 bytes, and receive 10 bytes:

// connect with a 5 second timeout on the connection
TcpClient connection = new TcpClientWithTimeout("www.google.com", 80, 5000).Connect();
NetworkStream stream = connection.GetStream();

// Send 10 bytes
byte[] to_send = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0xa};
stream.Write(to_send, 0, to_send.Length); 

// Receive 10 bytes
byte[] readbuf = new byte[10]; // you must allocate space first
stream.ReadTimeout = 10000; // 10 second timeout on the read
stream.Read(readbuf, 0, 10); // read

// Disconnect nicely
stream.Close(); // workaround for a .net bug: http://support.microsoft.com/kb/821625
connection.Close();