using System;

using System.Collections;

using System.Collections.Generic;

using System.Threading;

using System.Diagnostics;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var stopwatch = new Stopwatch();

        stopwatch.Start();

        var people = GetPeople();

        int underageCount = people.Where(person => person.Age < 18).Count();

        int allCount = people.Count();

        stopwatch.Stop();

        Console.WriteLine($"There are {underageCount} underage people out of {allCount}. This took {stopwatch.ElapsedMilliseconds}ms");

        //Repeat the same, but this time "freeze" the list

        stopwatch.Reset();

        stopwatch.Start();

        people = GetPeople().ToList(); //Freeze the list

        underageCount = people.Where(person => person.Age < 18).Count();

        allCount = people.Count();

        stopwatch.Stop();

        Console.WriteLine($"There are {underageCount} underage people out of {allCount}. Took only {stopwatch.ElapsedMilliseconds}ms");

    }

    private static IEnumerable<Person> GetPeople()

    {

        Thread.Sleep(100); //Simulo un costo prestazionale nel produrre le Person

        yield return new Person { FullName = "Shepherd", Age = 24 };

        Thread.Sleep(100);

        yield return new Person { FullName = "Samsara", Age = 32 };

        Thread.Sleep(100);

        yield return new Person { FullName = "Talos", Age = 11 };

    }

    private class Person {

        public string FullName { get; set; }

        public int Age { get; set; }

    }

}