using System;
using System.Collections;

namespace YieldDemo
{
    
    class Program
    {
        public static IEnumerable Power(int num, int exponent)
        {
            int counter = 0;
            int result = 1;
            while (counter++ < exponent)
            {
                //if (counter == 4) yield break;
                if (counter == 4) yield return null;
                result = result * num;
                yield return result;
            }
        }
        public static IEnumerator Power2(int num, int exponent)
        {
            int counter = 0;
            int result = 1;
            while (counter++ < exponent)
            {
                //if (counter == 4) yield break;
                if (counter == 4) yield return null;
                result = result * num;
                yield return result;
            }
        }
        static void Main(string[] args)
        {
            foreach (var item in Power(2,8))
            {
                Console.WriteLine(item);
            }

            Console.WriteLine("---");
            var e = Power2(2, 8);
            while (e.MoveNext())
            {
                var i = e.Current;
                Console.WriteLine(i);
            }

            Console.WriteLine("main done");
            Console.ReadKey();
        }
    }
}

C# yield return; yield break;