Question

There is a sample on MSDN for .NET ThreadPool. If I run this code, the output is completely erratic, sometimes, I get a completely empty output on the Console.

If I add a Thread.Sleep() call, even just for a few ms, the output is fine.

AFAIK Console.WriteLine() is thread-safe, so the output should always be there. But its not, at least not on my i7 2600 x64 compiled version. Obviously, if I add a breakpoint everything is fine, but it drives me mad.

I added a ConcurrentBag to make sure stuff is there, but even printing the elements of that is empty. Again, if I add a breakpoint everything is fine.

{
    public class TaskInfo
    {
        public string m_text;

            public int m_value;
        public ConcurrentBag<int> m_bag;
    public TaskInfo(string text, int value, ConcurrentBag<int> bag)
    {
        m_text = text;
        m_value = value;
        m_bag = bag;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Program p = new Program();
        p.Run();
    }

    void Run()
    {
        ConcurrentBag<int> concurrentBag = new ConcurrentBag<int>();
        for (int i = 0; i < 10; i++)
        {
            TaskInfo ti = new TaskInfo("Hello Thread", i, concurrentBag);
            bool b = ThreadPool.QueueUserWorkItem(new WaitCallback(MyThreadFunction), ti);
            if (!b)
            {
                Console.WriteLine("Damn!");
            }
            //Thread.Sleep(5);
        }

        for (int j = 0; j < concurrentBag.Count; j++)
        {
            Console.WriteLine("This is in the bag: {0}", concurrentBag.ElementAt(j));
        }
    }

    static void MyThreadFunction(object stateInfo)
    {
        TaskInfo ti = (TaskInfo)stateInfo;
        ti.m_bag.Add(ti.m_value);
        Console.WriteLine(ti.m_text + ti.m_value.ToString());
    }
}

}

Answer 1

I can very well understand that the output can be empty.

You push 10 jobs on a queue and then immediately start consuming the results. Not all jobs will have finished, and it could be none has even started yet.

And when running in the debugger, the program will terminate before you can see the WrtieLine()s from the MyThreadFunction.