Home/ Questions/Q 186957
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-11T15:44:19+00:00

Suppose that ThreadA and ThreadB both call WaitOne() in that order on the same

  • 0

Suppose that ThreadA and ThreadB both call WaitOne() in that order on the same AutoResetEvent. When the event is set, why does ThreadB get released instead of ThreadA?

I ran a test to find out what happens when you set an AutoResetEvent on which mutiple threads are waiting:

    private static void Test()     {         // two threads - waiting for the same autoreset event         // start it unset i.e. closed i.e. anything calling WaitOne() will block         AutoResetEvent autoEvent = new AutoResetEvent(false);          Thread thread1 = new Thread(new ThreadStart(WriteSomeMessageToTheConsole));         thread1.Start();  // this will now block until we set the event          Thread thread2 = new Thread(new ThreadStart(WriteSomeOtherMessageToTheConsole));         thread2.Start();  // this will now also block until we set the event          // simulate some other stuff         Console.WriteLine("Doing stuff...");         Thread.Sleep(5000);         Console.WriteLine("Stuff done.");          // set the event - I thought this would mean both waiting threads are allowed to continue         // BUT thread2 runs and thread1 stays blocked indefinitely         // So I guess I was wrong and that Set only releases one thread in WaitOne()?         // And why thread2 first?         autoEvent1.Set();     }

The code is of course not useful, it’s just a simple example.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. IIRC, which thread is released by an auto-reset event is unspecified. As everyone else mentioned, you want a manual reset event if you want to broadcast a condition. If you want to release an exact number (say exactly 3 of n), then you probably want to use a semaphore.

    If you really want to dig into why the order might be different than you would expect, take a look at ‘Windows Internals’ or anything that Mark Russinovich has written. Chances are that he explains the wait order on executive resources somewhere.

    • 0