Program.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace MultiThread_25
{
    class Program
    {
        private const int MaxCount = 100_000_000;
        private static IEnumerable<int> Collection => Enumerable.Repeat(1, MaxCount);

        static async Task Main(string[] args)
        {
            GetSum("Обычное", (() => Collection.Sum(x => x)));

            var portions = PreparePortions();
            await GetSum("Параллельное Task,", async () =>
            {

                var threads = new List<Task<int>>();

                for (var i = 0; i < portions.Count; i++)
                {
                    int index = i;

                    var thread = new Task<int>(() =>
                    {
                        return portions[index].Sum(x => x);
                    });

                    threads.Add(thread);
                    thread.Start();
                }
                
                return (await Task.WhenAll(threads)).Sum();

            });
            
            GetSum("Параллельное с помощью LINQ", (() => Collection.AsParallel().Sum()));
            
            Console.ReadKey();
        }

        private static IReadOnlyList<IEnumerable<int>> PreparePortions()
        {
            var portions = new List<IEnumerable<int>>();
            const int partLength = MaxCount / 4;

            for (int i = 0; i < 4; i++)
            {
                portions.Add(Collection.Skip(i * partLength).Take(partLength));
            }

            return portions;
        }

        private static void GetSum(string descr, Func<int> f)
        {
            var stopwatch = new Stopwatch();

            stopwatch.Start();
            var result = f();
            stopwatch.Stop();

            Console.WriteLine($"{descr}: сумма = {result} to {stopwatch.ElapsedMilliseconds} мс.");
        }

        private static async Task GetSum(string descr, Func<Task<int>> f)
        {
            var stopwatch = new Stopwatch();

            stopwatch.Start();
            var result = await f();
            stopwatch.Stop();

            Console.WriteLine($"{descr}: сумма = {result} to {stopwatch.ElapsedMilliseconds} мс.");
        }
    }
}