How to check if a TCP port is blocked using C#?

protected void Button1_Click(object sender, EventArgs  e)
{
      string host = "localhost";
      int port = 6900;
      IPAddress addr = (IPAddress)Dns.GetHostAddresses(host)[0];
      try
      { 
            TcpListener tcpList = new TcpListener(addr, port);
            tcpList.Start();
      }
      catch (SocketException sx)
      {
             // Catch exception here if port is blocked
      }
}