Question

How can a thread blocking method like WaitOne method exposed by AutoResetEvent not take up resources (CPU etc.)?

I would imagine that such a method would simply have a while loop like:

public void WaitOne()
{
    while(IsSet == false)
    {
        // some code to make the thread sleep    
    }

    // finally call delegate
}

But that’s clearly wrong, since it will make the CPU spin. So what’s the secret behind all this black magic?

Answer 1

The method is implemented in the kernel. For each thread that isn’t ready to run, Windows keeps a list of all the waitable objects (events, etc.) that the thread is waiting on. When a waitable object is signalled, Windows checks if it can wake up any of the waiting threads. No polling required.

This channel9 talk has a lot of information about how it works:
http://channel9.msdn.com/shows/Going+Deep/Arun-Kishan-Farewell-to-the-Windows-Kernel-Dispatcher-Lock/