PrimesPerfTest

using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp39 {
    class Program {
        static void Main(string[] args) {
            int total = 0;
            var sw = Stopwatch.StartNew();

            Parallel.For(1, 80_000_001,
                () => 0,
                (i, loopstate, local) => {
                    if (Utils.IsPrime(i)) {
                        local++;
                    }
                    return local;
                },
                local => Interlocked.Add(ref total, local)
            );

            sw.Stop();
            Console.WriteLine($"{sw.Elapsed} {total}");
        }
    }

    internal static class Utils {

        public static int CountPrimes(int from, int to) {
            int count = 0;
            for (int i = from; i <= to; i++) {
                if (IsPrime(i)) count++;
            }
            return count;
        }

        public static bool IsPrime(this int value) {
            if (value > 1) {
                //if ((value % 2) == 0) return value == 2;
                if ((value & 1) == 0) return value == 2;
                int sqrt = (int)Math.Sqrt(value);
                for (int t = 3; t <= sqrt; t = t + 2) {
                    if (value % t == 0) {
                        return false;
                    }
                }
                return true;
            }
            return false;
        }
    }

}