An example of using yield.

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Threading;

namespace YieldExample
{
    class Program
    {
        static void Main(string[] args)
        {
            var stopwatch = new Stopwatch();
            Console.WriteLine("With yield:");
            stopwatch.Start();
            foreach (var randomNumber in GetRandomNumbers(200))
            {
                Console.WriteLine("Random number = " +randomNumber);
                Console.WriteLine("Time so far = " +stopwatch.ElapsedMilliseconds);
            }
            stopwatch.Stop();
            var yeildLoopTotalTime = stopwatch.ElapsedMilliseconds;

            Console.WriteLine();
            Console.WriteLine("With list");
            Console.WriteLine("BTW I'm doing work.. honest");
            stopwatch.Reset();
            stopwatch.Start();
            foreach (var randomNumber in GetRandomNumberList(200))
            {
                Console.WriteLine("Random number = " + randomNumber);
                Console.WriteLine("Time so far = " + stopwatch.ElapsedMilliseconds);
            }
            stopwatch.Stop();
            var listLoopTotalTime = stopwatch.ElapsedMilliseconds;

            Console.WriteLine();
            Console.WriteLine("Total time:");
            Console.WriteLine("Yield : " + yeildLoopTotalTime);
            Console.WriteLine("List : " + listLoopTotalTime);

            Console.Read();
        }

        public static IEnumerable<int> GetRandomNumbers(int maxCount)
        {
            var rand = new Random();
            for (int i = 0; i < maxCount; i++)
            {
                Thread.Sleep(500);
                yield return rand.Next();
            }
        }

        public static IList<int> GetRandomNumberList(int maxCount)
        {
            var numberList = new List<int>();
            var rand = new Random();
            for (int i = 0; i < maxCount; i++)
            {
                Thread.Sleep(500);
                numberList.Add(rand.Next());
            }
            return numberList;
        }
    }
}