Know when ThreadPool is empty

Question

I can;t seem to find an easy way to determine when a ThreadPool has finished with all the queued tasks. I found some answers here but none of which can help me.

For sake of simplicity let's say:

for (int i=0;i<10;i++)
{
   threadpool.queueuserworkitem(Go);

}
void Go()
{
   Console.WriteLine("Hello");
}

So how would I go about sending a final "All done" console.writeline after all the 10 background threads have finished?

Thanks

Answer 1

It is impossible and dangerous to rely on thread pool emptiness. What you can do is to count your active tasks yourself. A low level approach with monitor:

   class Program
    {
        static object _ActiveWorkersLock = new object();
        static int _CountOfActiveWorkers;

        static void Go(object state)
        {
            try
            {
                Console.WriteLine("Hello");
            }
            finally
            {
                lock (_ActiveWorkersLock)
                {
                    --_CountOfActiveWorkers;
                    Monitor.PulseAll(_ActiveWorkersLock);
                }
            }
        }

        static void Main(string[] args)
        {
            for (int i = 0; i < 10; i++)
            {
                lock (_ActiveWorkersLock)
                    ++_CountOfActiveWorkers;
                ThreadPool.QueueUserWorkItem(Go);
            }

            lock (_ActiveWorkersLock)
            {
                while (_CountOfActiveWorkers > 0)
                    Monitor.Wait(_ActiveWorkersLock);
            }

            Console.WriteLine("All done");
        }
    }