using System;

using System.Collections;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var stateless = new Stateless();

        var getNumbers = stateless.GetNumbers();        

        Console.WriteLine("Printing the state-less IEnumerable once:");

        Print(getNumbers);

        Console.WriteLine("Printing the state-less IEnumerable twice:");

        Print(getNumbers);

        getNumbers = stateless.GetNumbersYield();

        Console.WriteLine("Printing the state-less yielded IEnumerable once:");

        Print(getNumbers);

        Console.WriteLine("Printing the state-less yielded IEnumerable twice:");

        Print(getNumbers);

        var statefull = new Statefull();

        getNumbers = statefull.GetNumbers().ToIEnumerable();

        Console.WriteLine("Printing the state-full IEnumerable once:");

        Print(getNumbers);

        Console.WriteLine("Printing the state-full IEnumerable twice:");

        Print(getNumbers);

    }

    private static void Print(IEnumerable<int> toPrint){

        bool printed = false;

        foreach(int i in toPrint){

            Console.WriteLine(i);

            printed = true;

        }

        if(!printed){

            Console.WriteLine("No Elements");

        }

    }

}

​

public class Stateless

{

    private List<int> integers = new List<int> {1, 2, 3, 4};

    // Only encapsulation.

    public IEnumerable<int> GetNumbers(){

        return this.integers;

    }

    // Although yielding, still state-less IEnumerator.

    public IEnumerable<int> GetNumbersYield(){

        foreach(int i in integers){

            yield return i;

        }

    }

}

​

public class Statefull : IEnumerable<int>

{

        IEnumerator IEnumerable.GetEnumerator()

        {

            return this.GetNumbers();

        }

​

        IEnumerator<int> IEnumerable<int>.GetEnumerator()

        {

            return this.GetNumbers();

        }

        // Yielding the state.

        public IEnumerator<int> GetNumbers(){

            for(int i = 1; i<5; i++){

                yield return i;

            }

        }

}

​

 public static class IterationHelper

    {

        public static IEnumerable<T> ToIEnumerable<T>(this IEnumerator<T> enumerator)

        {

            while (enumerator.MoveNext())

            {

                yield return enumerator.Current;

            }

        }

    }