Is it safe to signal and immediately close a ManualResetEvent?

Question

I feel like I should know the answer to this, but I'm going to ask anyway just in case I'm making a potentially catastrophic mistake.

The following code executes as expected with no errors/exceptions:

static void Main(string[] args)
{
    ManualResetEvent flag = new ManualResetEvent(false);
    ThreadPool.QueueUserWorkItem(s =>
    {
        flag.WaitOne();
        Console.WriteLine("Work Item 1 Executed");
    });
    ThreadPool.QueueUserWorkItem(s =>
    {
        flag.WaitOne();
        Console.WriteLine("Work Item 2 Executed");
    });
    Thread.Sleep(1000);
    flag.Set();
    flag.Close();
    Console.WriteLine("Finished");
}

Of course, as is usually the case with multi-threaded code, a successful test does not prove that this is actually thread safe. The test also succeeds if I put Close before Set, even though the documentation clearly states that attempting to do anything after a Close will result in undefined behaviour.

My question is, when I invoke the ManualResetEvent.Set method, is it guaranteed to signal all waiting threads before returning control to the caller? In other words, assuming that I'm able to guarantee that there will be no further calls to WaitOne, is it safe to close the handle here, or is it possible that under some circumstances this code would prevent some waiters from getting signaled or result in an ObjectDisposedException?

The documentation only says that Set puts it in a "signaled state" - it doesn't seem to make any claims about when waiters will actually get that signal, so I'd like to be sure.

Answer 1

When you signal with a ManualResetEvent.Set you are guaranteed that all the threads that are waiting for that event (i.e. in a blocking state on flag.WaitOne) will be signaled before returning control to the caller.

Of course there are circumstances when you might set the flag and your thread doesn't see it because it's doing some work before it checks the flag (or as nobugs suggested if you're creating multiple threads):

ThreadPool.QueueUserWorkItem(s =>
{
    QueryDB();
    flag.WaitOne();
    Console.WriteLine("Work Item 1 Executed");
});

There is contention on the flag and now you can create undefined behavior when you close it. Your flag is a shared resource between your threads, you should create a countdown latch on which every thread signals upon completion. This will eliminate contention on your flag.

public class CountdownLatch
{
    private int m_remain;
    private EventWaitHandle m_event;

    public CountdownLatch(int count)
    {
        Reset(count);
    }

    public void Reset(int count)
    {
        if (count < 0)
            throw new ArgumentOutOfRangeException();
        m_remain = count;
        m_event = new ManualResetEvent(false);
        if (m_remain == 0)
        {
            m_event.Set();
        }
    }

    public void Signal()
    {
        // The last thread to signal also sets the event.
        if (Interlocked.Decrement(ref m_remain) == 0)
            m_event.Set();
    }

    public void Wait()
    {
        m_event.WaitOne();
    }
}

1. Each thread signals on the countdown latch.
2. Your main thread waits on the countdown latch.
3. The main thread cleans up after the countdown latch signals.

Ultimately the time you sleep at the end is NOT a safe way to take care of your problems, instead you should design your program so it is 100% safe in a multithreaded environment.

UPDATE: Single Producer/Multiple Consumers
The presumption here is that your producer knows how many consumers will be created, After you create all your consumers, you reset the CountdownLatch with the given number of consumers:

// In the Producer
ManualResetEvent flag = new ManualResetEvent(false);
CountdownLatch countdown = new CountdownLatch(0);
int numConsumers = 0;
while(hasMoreWork)
{
    Consumer consumer = new Consumer(coutndown, flag);
    // Create a new thread with each consumer
    numConsumers++;
}
countdown.Reset(numConsumers);
flag.Set();
countdown.Wait();// your producer waits for all of the consumers to finish
flag.Close();// cleanup

Answer 2

It is not okay. You are getting lucky here because you only started two threads. They'll immediately start running when you call Set on a dual core machine. Try this instead and watch it bomb:

    static void Main(string[] args) {
        ManualResetEvent flag = new ManualResetEvent(false);
        for (int ix = 0; ix < 10; ++ix) {
            ThreadPool.QueueUserWorkItem(s => {
                flag.WaitOne();
                Console.WriteLine("Work Item Executed");
            });
        }
        Thread.Sleep(1000);
        flag.Set();
        flag.Close();
        Console.WriteLine("Finished");
        Console.ReadLine();
    }

Your original code will fail similarly on an old machine or your current machine when it is very busy with other tasks.