ManualResetEventSlim.Set() 并不总是解锁任务内的等待

Question

I am trying to use the ManualResetEventSlim class to communicate between a few parallel Tasks.

Here is a simplified version of the code:

class Program
{
    private static void Main(string[] args)
    {
        Stopwatch stopwatch = Stopwatch.StartNew();

        ManualResetEventSlim eventSlim = new ManualResetEventSlim(false);

        Task.Run(() =>
        {
            Task.Run(() =>
                            {
                                Thread.Sleep(1000);

                                eventSlim.Set();
                            });

            eventSlim.Wait();

            Console.WriteLine($"Hello from the Task! {stopwatch.Elapsed}");
        });

        eventSlim.Wait();

        Console.WriteLine($"Hello from the Main thread! {stopwatch.Elapsed}");

        stopwatch.Stop();

        Console.ReadLine();
    }
}

Most of the times the code runs fine and outputs:

Hello from main thread! 00:00:01.1017591
Hello from task! 00:00:01.1017630

However once in every five or six times I run the code I only get this output:

Hello from main thread! 00:00:01.1017591

And the code after the Wait inside the Task never gets called.

I'm using .NET Core 2.0.2 on Windows Server 2012 R2 with Visual Studio 15.4.1 .

Can anyone reproduce this behaviour?

Can anyone confirm if my code is right or if there is any problems with it please?

Update After @ArhiChief suggested to test the results in the Release configuration, I figured that the problem only appears when I am using the IDE to debug my code.

When I build and run the code in the command line in either Debug/Release configuration, there seem to be no problems.

I tried to close/reopen the IDE and clean the project and do a fresh rebuild, and now the IDE seems to be working fine too.

Results: I haven't faced the issue so far after restarting the IDE, cleaning the project and doing a fresh rebuild of the project. I'm suspecting a minor bug in the IDE. However I can't report it because it has disappeared now.

I'm leaving this question open in case someone else encounters this issue and would want to track and report the bug.

Answer 1

I'm also have an idea what your first task may be disposed and collected. Consider this code

static void Main(string[] args)
{
Stopwatch stopwatch = Stopwatch.StartNew();

ManualResetEventSlim eventSlim = new ManualResetEventSlim(false);

Task.Run(() =>
{
    Task.Run(() =>
    {
        Thread.Sleep(1000);

        eventSlim.Set();
    });

    eventSlim.Wait();

    Console.WriteLine($"Hello from the Task! {stopwatch.Elapsed}");
});

eventSlim.Wait();

Console.WriteLine($"Hello from the Main thread! {stopwatch.Elapsed}");

stopwatch.Stop();

//Console.ReadLine();
}

I get message Hello from the Main thread! 00:00:01.0111623 Hello from the Main thread! 00:00:01.0111623 . I also read that

A single call to Set() signals the event, and any waiting Tasks are released. New calls to Wait() don't block until Reset() method called.

But lets return to our code. If you rewrite it like this

static void Main(string[] args)
{
Stopwatch stopwatch = Stopwatch.StartNew();

ManualResetEventSlim eventSlim = new ManualResetEventSlim(false);

var t = Task.Run(() =>
{
    Task.Run(() =>
    {
        Thread.Sleep(1000);

        eventSlim.Set();
    });

    eventSlim.Wait();

    Console.WriteLine($"Hello from the Task! {stopwatch.Elapsed}");
});

eventSlim.Wait();

Console.WriteLine($"Hello from the Main thread! {stopwatch.Elapsed}");

stopwatch.Stop();

t.Wait();
//Console.ReadLine();
}

you will find that everything works as you expected.