using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Iterating without extension");

        var myList = new List<int>();

        myList.AddRange(MyInts());

        foreach(var item in myList)

            Console.WriteLine("Iterating {0}", item);

​

        Console.WriteLine("\nIterating With extension");

        var otherList = new List<int>();

        foreach (int i in otherList.AddRangeLazily(MyInts()))

        {

            if (i > 1)

                break;

            Console.WriteLine(i);

        }

    }

    public static IEnumerable<int> MyInts()

    {  

        for (int i = 0; i < 5; i++)

        {

            Console.WriteLine(i);

            yield return i;

        }

    }

​

}

​

public static class Extension

{

    public static IEnumerable<T> AddRangeLazily<T>(this ICollection<T> col, IEnumerable<T> values)

    {

        foreach (T i in values)

        {

            yield return i;

            col.Add(i);

        }

    }

}