using System;

using System.Collections.Generic;

using System.Diagnostics;

using System.Linq;

​

namespace ConsoleApplication1

{

    public class Program

    {

        public static void Main(string[] args)

        {

            var test = new Test();

            test.someSetRoutineSlower(Test.MakeSet(1));

            test.someSetRoutineFaster(Test.MakeSet(1));

            var exampleSet = Test.MakeSet(100000);

            var t1 = Stopwatch.StartNew();

            test.someSetRoutineSlower(exampleSet);

            t1.Stop();

            var t2 = Stopwatch.StartNew();

            test.someSetRoutineFaster(exampleSet);

            t2.Stop();

            Console.WriteLine("Slow: {0,10}", t1.ElapsedTicks);

            Console.WriteLine("Fast: {0,10}", t2.ElapsedTicks);

            Console.ReadKey();

        }

    }

​

    class IExampleAble { }

    class Test

    {

        public static HashSet<IExampleAble> MakeSet(int count)

        {

            return new HashSet<IExampleAble>(Enumerable.Range(0, count).Select(_ => new IExampleAble()));

        }

​

        HashSet<IExampleAble> alreadyProcessed = MakeSet(10000000);

        public void someSetRoutineSlower(HashSet<IExampleAble> exampleSet)

        {

​

            foreach (var item in exampleSet)

            {

                if (!alreadyProcessed.Contains(item))

                {

                    // do Stuff

                }

            }

        }

​

        public void someSetRoutineFaster(HashSet<IExampleAble> exampleSet)

        {

            exampleSet.ExceptWith(alreadyProcessed);//doesnt this have to check each item of it's collection against the other one, thus actually looping twice?

            foreach (var item in exampleSet)

            {

                // do Stuff

            }

        }

    }

}

​