using System;

using System.Collections.Concurrent;

using System.Collections.Generic;

using System.Diagnostics;

using System.Linq;

using System.Threading.Tasks;

​

namespace ParallelExample

{

    class Program

    {

        static void Main()

        {

            // 2 million

            var limit = 2_000_000;

            var numbers = Enumerable.Range(0, limit).ToList();

​

            var watch = Stopwatch.StartNew();

            var primeNumbersFromForeach = GetPrimeList(numbers);

            watch.Stop();

​

            var watchForParallel = Stopwatch.StartNew();

            var primeNumbersFromParallelForeach = GetPrimeListWithParallel(numbers);

            watchForParallel.Stop();

​

            Console.WriteLine($"Classical foreach loop | Total prime numbers : {primeNumbersFromForeach.Count} | Time Taken : {watch.ElapsedMilliseconds} ms.");

            Console.WriteLine($"Parallel.ForEach loop  | Total prime numbers : {primeNumbersFromParallelForeach.Count} | Time Taken : {watchForParallel.ElapsedMilliseconds} ms.");

​

            Console.WriteLine("Press any key to exit.");

            Console.ReadLine();

        }

​

        /// <summary>

        /// GetPrimeList returns Prime numbers by using sequential ForEach

        /// </summary>

        /// <param name="inputs"></param>

        /// <returns></returns>

        private static IList<int> GetPrimeList(IList<int> numbers) => numbers.Where(IsPrime).ToList();

​

        /// <summary>

        /// GetPrimeListWithParallel returns Prime numbers by using Parallel.ForEach

        /// </summary>

        /// <param name="numbers"></param>

        /// <returns></returns>

        private static IList<int> GetPrimeListWithParallel(IList<int> numbers)

        {

            var primeNumbers = new ConcurrentBag<int>();

​

            Parallel.ForEach(numbers, number =>

            {

                if (IsPrime(number))

                {

                    primeNumbers.Add(number);

                }

            });

​

            return primeNumbers.ToList();

        }

​

        /// <summary>

        /// IsPrime returns true if number is Prime, else false.(https://en.wikipedia.org/wiki/Prime_number)

        /// </summary>

        /// <param name="number"></param>

        /// <returns></returns>

        private static bool IsPrime(int number)

        {

            if (number < 2)

            {

                return false;

            }

​

            for (var divisor = 2; divisor <= Math.Sqrt(number); divisor++)

            {

                if (number % divisor == 0)

                {

                    return false;

                }

            }

            return true;

        }

    }

}