Question

I have an application in which the user will choose to do a number of tasks along with the maximum number of threads. Each task should run on a separate thread. Here is what I am looking for:

If the user specified “n less than t” where n is the maximum number of threads and t is the number of tasks. The program should run “n” threads and after they finish, the program should be notified by some way and repeat the loop untill all tasks are done.

My Question is:
How to know that all running threads has finished their job so that I can repeat the loop.

Answer 1

I recommend using the ThreadPool for your task. Its algorithm will generally be more efficient than something you can roll by hand.

Now the fun part is getting notified when all of your threads complete. Unless you have really specific needs which make this solution unsuitable, it should be easy enough to implement with the CountdownEvent class, which is a special kind of waithandle that waits until its been signaled n times. Here’s an example:

using System;
using System.Linq;
using System.Threading;
using System.Diagnostics;

namespace CSharpSandbox
{
    class Program
    {
        static void SomeTask(int sleepInterval, CountdownEvent countDown)
        {
            try
            {
                // pretend this did something more profound
                Thread.Sleep(sleepInterval);
            }
            finally
            {
                // need to signal in a finally block, otherwise an exception may occur and prevent
                // this from being signaled
                countDown.Signal();
            }
        }

        static CountdownEvent StartTasks(int count)
        {
            Random rnd = new Random();

            CountdownEvent countDown = new CountdownEvent(count);

            for (int i = 0; i < count; i++)
            {
                ThreadPool.QueueUserWorkItem(_ => SomeTask(rnd.Next(100), countDown));
            }

            return countDown;
        }

        public static void Main(string[] args)
        {
            Console.WriteLine("Starting. . .");
            var stopWatch = Stopwatch.StartNew();
            using(CountdownEvent countdownEvent = StartTasks(100))
            {
                countdownEvent.Wait();
                // waits until the countdownEvent is signalled 100 times
            }
            stopWatch.Stop();
            Console.WriteLine("Done! Elapsed time: {0} milliseconds", stopWatch.Elapsed.TotalMilliseconds);
        }
    }
}