ParallelExample.cs

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

class Program
{
    const int limit = 100_000_000;

    static void Main(string[] args)
    {
        var sw = System.Diagnostics.Stopwatch.StartNew();

        int sum = 0;

        for (int i = 0; i < limit; i++)
        {
            sum++;
        }

        sw.Stop();
        Console.WriteLine($"Sum: {sum}; Serial Elapsed: {sw.Elapsed.TotalMilliseconds}");     // Sum: 100000000; Serial Elapsed: 28.9205

        sw.Restart();

        sum = 0;

        Parallel.For(0, limit, (i) =>
        {
            Interlocked.Increment(ref sum);
        });

        sw.Stop();
        Console.WriteLine($"Sum: {sum}; Parallel Interlocked Elapsed: {sw.Elapsed.TotalMilliseconds}"); // Sum: 100000000; Parallel Interlocked Elapsed: 1336.2358

        sw.Restart();
        sum = 0;

        Parallel.For(0, Environment.ProcessorCount, (i) =>
        {
            var localSum = 0;
            var batchSize = Math.DivRem(limit, Environment.ProcessorCount, out int rem);

            if (i == Environment.ProcessorCount)
            {
                batchSize += rem;
            }

            for (int n = 0; n < batchSize; n++)
            {
                localSum++;
            }

            Interlocked.Add(ref sum, localSum);
        });

        sw.Stop();
        Console.WriteLine($"Sum: {sum}; Parallel Batched Interlocked Elapsed: {sw.Elapsed.TotalMilliseconds}"); // Sum: 100000000; Parallel Batched Interlocked Elapsed: 2.7273

        var localSums = new int[Environment.ProcessorCount];
        sw.Restart();

        sum = 0;

        Parallel.For(0, Environment.ProcessorCount, (i) =>
        {
            var localSum = 0;
            var batchSize = Math.DivRem(limit, Environment.ProcessorCount, out int rem);

            if (i == Environment.ProcessorCount)
            {
                batchSize += rem;
            }

            for (int n = 0; n < batchSize; n++)
            {
                localSum++;
            }

            localSums[i] = localSum;
        });

        for (int i = 0; i < localSums.Length; i++)
        {
            sum += localSums[i];
        }

        sw.Stop();
        Console.WriteLine($"Sum: {sum}; Parallel Batched Lock-Free Elapsed: {sw.Elapsed.TotalMilliseconds}"); // Sum: 100000000; Parallel Batched Lock-Free Elapsed: 2.365

        Console.ReadLine();
    }
}