Performance and correctness comparison between foreach and Parallel.ForEach

CompareForeachToParallelForeach.cs

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

namespace App
{
    /// <summary>
    /// This snippet shows a performance and correctness comparison between
    /// foreach and Parallel.ForEach when adding a small number of objects to a
    /// List{T}.
    /// 
    /// For a handful of items using a simple foreach loop is much faster and
    /// using Parallel.ForEach is much slower. Adding items to List{T}
    /// is also not thread-safe, so using Parallel.ForEach sometimes results in
    /// not adding all of the items you intended to add.
    /// </summary>
    public static class CompareForeachToParallelForeach
    {
        private static readonly int Runs = 1000000;

        private static readonly List<string> Parameters = new List<string>
        {
            "A",
            "B",
            "C",
            "D"
        };

        public static void DoIt()
        {
            var sw = Stopwatch.StartNew();
            ParallelForEach();
            sw.Stop();
            var millisecondsParallelForEach = sw.ElapsedMilliseconds;
            Console.WriteLine($"{millisecondsParallelForEach} ms using Parallel.ForEach");

            sw = Stopwatch.StartNew();
            ForEach();
            sw.Stop();
            var millisecondsForEach = sw.ElapsedMilliseconds;
            Console.WriteLine($"{millisecondsForEach} ms using ForEach");
            Console.WriteLine($"Parallel.ForEach takes {millisecondsParallelForEach / millisecondsForEach}x longer than ForEach");
        }

        private static void ParallelForEach()
        {
            int numTimes = 0;

            for (int i = 0; i < Runs; i++)
            {
                var request = new List<string>();
                Parallel.ForEach(Parameters, parameter =>
                {
                    request.Add(parameter);
                });
                if (request.Count != Parameters.Count)
                {
                    numTimes++;
                }
            }

            Console.WriteLine($"The number of items in the list is different {numTimes} times for {nameof(ParallelForEach)}");
        }

        private static void ForEach()
        {
            int numTimes = 0;

            for (int i = 0; i < Runs; i++)
            {
                var request = new List<string>();
                foreach (var parameter in Parameters)
                {
                    request.Add(parameter);
                }

                if (request.Count != Parameters.Count)
                {
                    numTimes++;
                }
            }

            Console.WriteLine($"The number of items in the list is different {numTimes} times for {nameof(ForEach)}");
        }
    }
}