using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    // see: https://stackoverflow.com/a/30887187/1016343

    public static void Main()

    {

        Console.WriteLine("===== Example 1 =====");

        // 1. Using IEnumerator allows to get item by item

        var i=Iterator.IterateOne(x => true); // iterate endless

​

        Console.WriteLine("===== Example 1a =====");

​

        // 1.a) get item by item

        i.MoveNext(); Console.WriteLine(i.Current);

        i.MoveNext(); Console.WriteLine(i.Current);

​

        Console.WriteLine("===== Example 1b =====");

​

        // 1.b) loop until 100

        int j; while (i.MoveNext() && (j=i.Current)<=100) { Console.WriteLine(j); }

        // -----------------------

        Console.WriteLine("===== Example 2 =====");

​

        // 2. Using IEnumerable makes looping and LINQ easier   

        var k=Iterator.IterateAll(x => x<100); // limit iterator to 100

​

        Console.WriteLine("===== Example 2a =====");

​

        // 2.a) Use a foreach loop

        foreach(var x in k){ Console.WriteLine(x); } // loop

​

        Console.WriteLine("===== Example 2b =====");

​

        // 2.b) LINQ: take 101..200 of endless iteration

        var lst=Iterator.IterateAll(x=>true).Skip(100).Take(100).ToList(); // LINQ: take items

        foreach(var x in lst){ Console.WriteLine(x); } // output list

    }

    // 2 iterators, one as IEnumerator, one as IEnumerable

    public class Iterator

    {

        public static IEnumerator<int> IterateOne(Func<int, bool> condition)

        {

            for(var i=1; condition(i); i++) { yield return i; }     

        }

        public static IEnumerable<int> IterateAll(Func<int, bool> condition)

        {

            for(var i=1; condition(i); i++) { yield return i; }     

        }

    }

}