public  bool shouldStop ;
Thread com = new Thread(new ThreadStart(read));
com.IsBackground = true;
if(btnXX.Text=="xxx")
{shouldStop=false;
 comRW.Start(); 
}
else if(btnXX.Text=="yyy")
{
  while (!comRW.IsAlive)
  {
      Thread.Sleep(100);
      shouldStop = true;
      com.Join();
   }
}
private void readMessage()
{
 while(!shouldStop)
{
 //要执行的内容
}
}