c# - What is the basic concept behind WaitHandle?

Question

What is the basic concept behind WaitHandle in C# .net threading? Whats is its use? When to use it? What is the use of WaitAll and WaitAny methods inside it?

Answer 1

Whenever you would want to control the execution of multiple threads in your app. Though this does not only mean that only one thread increments the counter; but let the threads start/stop or pause upon an event.

See WaitHandles - Auto/ManualResetEvent and Mutex

--EDIT--

WaitHandles are the mechanism that you "use" to control the threads' execution. Its not about handles not being accessible within a thread; its about using them within the thread.

This may be a fat example, but please bear with me; think about, a lady gives five different toned whistles to five girls, and tells them to blow the whistle whenever something would happen; the process is for each girl to blow a whistle, and the lady would know who blew the whistle.

Now, its not about sharing-the-whistles with each other, its about, probably for the lady, to use them to "control" the execution or the process how girls would blow the whistle.

Therefore, technically the process would be to:

1. Create a wait event(ManualResetEvent object)
2. Register the events, WaitHandle.WaitAny(events);
3. After you are done performing operation in your thread, the .Set(), which would tell the WaitHandle that 'I am done!'.

For instance, consider the example from the link provided. I have added the steps for you to understand the logic. These aren't the hardcoded steps, but just so you may understand.

class Test
{
    static void Main()
    {
    //STEP 1: Create a wait handle
        ManualResetEvent[] events = new ManualResetEvent[10];//Create a wait handle
        for (int i=0; i < events.Length; i++)
        {
            events[i] = new ManualResetEvent(false);
            Runner r = new Runner(events[i], i); 
            new Thread(new ThreadStart(r.Run)).Start();
        }

    //STEP 2: Register for the events to wait for
        int index = WaitHandle.WaitAny(events); //wait here for any event and print following line.

        Console.WriteLine ("***** The winner is {0} *****", 
                           index);

        WaitHandle.WaitAll(events); //Wait for all of the threads to finish, that is, to call their cooresponding `.Set()` method.

        Console.WriteLine ("All finished!");
    }
}


class Runner
{
    static readonly object rngLock = new object();
    static Random rng = new Random();

    ManualResetEvent ev;
    int id;

    internal Runner (ManualResetEvent ev, int id)
    {
        this.ev = ev;//Wait handle associated to each object, thread in this case.
        this.id = id;
    }

    internal void Run()
    {
    //STEP 3: Do some work
        for (int i=0; i < 10; i++)
        {
            int sleepTime;
            // Not sure about the thread safety of Random...
            lock (rngLock)
            {
                sleepTime = rng.Next(2000);
            }
            Thread.Sleep(sleepTime);
            Console.WriteLine ("Runner {0} at stage {1}",
                               id, i);
        }

    //STEP 4: Im done!
        ev.Set();
    }
}