multithreading - C# Form closing all threads join -

i have simple piece of code:

private volaile bool working; private volatile list<thread> threads = new list<thread>(); private volatile form face;  public void start(int n)  {     working = true;     (int = 0; <= n; i++)      {         thread worker = new thread(() =>         {             while(working)              {                 // work                  }         });         threads.add(worker);         worker.start();     } }  public void stop()  {     if(working)     {         working = false;          logger.info("waiting threads join");         foreach (thread worker in threads)         {             worker.join();         }         logger.info("threads joined");     } }  private void face_closing(object sender, system.componentmodel.canceleventargs e) {     face.invoke(new action(() => {stop();}));     system.environment.exit(0); } 

face form creates on programm start , have controls, when use start() , stop() methods, works fine (all threads join normally).

but when press "x" form button, programm stacks on "waiting threads join". why? missing?

thread sample = new thread(new threadstart(thread1));         sample.isbackground= true; 

try set threads background threads.