gistfile1.cs

namespace ConsoleTest
{
    using System;
    using System.Collections;
    using System.Collections.Generic;

    internal class Program
    {
        #region Methods: private

        static void Main(string[] args)
        {
            var p = new Producer();
            foreach(var s in p.RepeatForever())
            {
                Console.Out.WriteLine(s);
                Console.ReadKey();
            }
        }

        #endregion
    }

    internal class Producer : IEnumerable<string>
    {
        public IEnumerator<string> GetEnumerator()
        {
            Console.Out.WriteLine("Enumerator called - I may have side effets");
            yield return "Foo";
            yield return "Bar";
            yield return "Baz";
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }

    public static class IEnumerableExtensions
    {
        public static IEnumerable<T> RepeatForever<T>(this IEnumerable<T> self)
        {
            var list = new List<T>();
            foreach(var elem in self)
            {
                list.Add(elem);
                yield return elem;
            }
            for(int i = 0;; i = (i + 1) % list.Count)
                yield return list[i];
        }
    }
}