using System;

using System.Linq;

using System.Collections.Generic;

using System.Diagnostics;

​

namespace SO_04

{

    class MyClass

    {

        public string Property { get; set; }

    }

​

    class Y

    {

        public MyClass X { get; set; }

​

        public Y(MyClass x)

        {

            this.X = x;

        }

    }

​

    public class Program

    {

        public static IEnumerable<TResult> SelectWhere<TIn, TResult>(IEnumerable<TIn> source,

            Func<TIn, bool> predicate, Func<TIn, TResult> selector)

        {

            foreach (var item in source)

            {

                if (predicate(item))

                {

                    yield return selector(item);

                }

            }

        }

​

        public static void Main()

        {           

            long value = 0;

​

            List<MyClass> someSource;

​

            var sw = Stopwatch.StartNew();

​

            for (int z = 0; z < 10; z++)

            {

                someSource = new List<MyClass>();

​

                for (int i = 0; i < 600; i++)

                {

                    someSource.Add(new MyClass() {Property = Guid.NewGuid().ToString()});

                }

​

                var xy = someSource.Where(x => x.Property.StartsWith("D")).Select(x => new Y(x));

​

                sw.Stop();

                value += sw.ElapsedTicks;

            }

​

            Console.WriteLine($"Result for LINQ query is : {value} ticks");

​

            long value2 = 0;

​

            sw.Restart();

​

            for (int z = 0; z < 10; z++)

            {

                someSource = new List<MyClass>();

​

                for (int i = 0; i < 600; i++)

                {

                    someSource.Add(new MyClass() { Property = Guid.NewGuid().ToString() });

                }

​

                var xy = SelectWhere(someSource, mc => mc.Property.StartsWith("D"), x => new Y(x));

​

                sw.Stop();

                value2 += sw.ElapsedTicks;

            }

​

            Console.WriteLine($"Result for LINQ query is : {value2} ticks");

        }

    }

}