The following works as expected when building in Debug – the execution is done after three seconds. But, for some reason, it fails when in Release.

class Program
{
    static void Main(string[] args)
    {
        var w = new Worker();
        while (!w.IsDone);
        Console.WriteLine("The work is done.");
    }
}

class Worker
{
    public bool IsDone;

    public Worker()
    {
        var thread = new Thread(Job);
        thread.Start();
    }

    private void Job()
    {
        Thread.Sleep(3000);
        IsDone = true;
    }
}

Could you explain what is happening? How could you fix it?

Tomorrow, I will share the answer.

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3 Comments
  1. This was a nice lesson. I’m still quite new to multithreading, so I had to learn what was going on. Ended up diving in C# documentation to know more. Below is the answer, so if you haven’t done it, SPOILER ALERT!

    This happens because the compiler assumes your application is single-threaded, and optimizes it accordingly. Therefore, when Worker is created, it only sees the need to check for the variable’s value one single time, since it isn’t being changed on that thread. Simply adding the keyword “volatile” on the variable’s declaration will tell the compiler to keep watching for changes.

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