using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Declaring var items = GetItems(); inside the loop");

        for (int i = 0; i < 3; i++)

        {

            var inside_Items = GetItems();

            foreach (var item in inside_Items)

            {

                Console.WriteLine(item);

            }

        }

        Console.WriteLine("Declaring var items = GetItems(); outside the loop");

        var outside_Items = GetItems();

        for (int i = 0; i < 3; i++)

        {

            foreach (var item in outside_Items)

            {

                Console.WriteLine(item);

            }

        }

        Console.WriteLine("To only enter GetItems once, materialize the IEnumerable to eg a List by calling .ToList");

        var outsideOnlyOnce_Items = GetItems().ToList();

        for (int i = 0; i < 3; i++)

        {

            foreach (var item in outsideOnlyOnce_Items)

            {

                Console.WriteLine(item);

            }

        }

    }

    static IEnumerable<string> GetItems()

    {

        Console.WriteLine("Entered GetItems");

        yield return "1";

        Console.WriteLine("Exited GetItems");

    }

}