HashSet に対する Except と Where の性能比較

ExceptVsWhere.cs

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

namespace ExceptVsWhere
{
    class Program
    {
        static void Main(string[] args)
        {
            // 要素数を変えて試してみる
            var lengths = new int[] {10, 100, 1000, 10000, 100000, 1000000, 10000000};

#if NETCOREAPP1_1
            Console.WriteLine(".NET Core 1.1");
#elif NET461
            Console.WriteLine(".NET Framework 4.6.1");
#endif

            Console.WriteLine("Length  ,Except ,Where");

            foreach (var length in lengths)
            {
                (long except, long where) = Test(length);
                Console.WriteLine($"{length:D8},{except:D7},{where:D7}");
            }
        }

        private static (long exceptMilliSec, long whereMilliSec) Test(int length)
        {
            // 10 回やる
            const int loop = 10;

            var first = Enumerable.Range(1, length * 2);
            var second = new HashSet<int>(Enumerable.Range(1, length));

            // except
            var exceptSw = new Stopwatch();
            exceptSw.Start();
            Repeat(() => first.Except(second).ToArray(), loop);
            exceptSw.Stop();

            // where
            var whereSw = new Stopwatch();
            whereSw.Start();
            Repeat(() => first.Where(x => second.Contains(x)).ToArray(), loop);
            whereSw.Stop();

            return (exceptSw.ElapsedMilliseconds, whereSw.ElapsedMilliseconds);
        }

        private static void Repeat(Action action, int count)
        {
            for (var i = 0; i < count; i++)
            {
                action();
            }
        }
    }
}