Use yield to return a few items from an iterator.

gistfile1.cs

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

namespace YieldExample
{
    class Program
    {
        static void Main(string[] args)
        {
            
            var stopwatch = new Stopwatch();
            stopwatch.Start();
            
            var yieldedRandoms = GetRandoms().Take(10);
            
            stopwatch.Stop();
            var yeildLoopTotalTime = stopwatch.ElapsedMilliseconds;

            Console.WriteLine();

            stopwatch.Reset();
            stopwatch.Start();

            var listedRandoms = GetRandomsList().Take(10);

            stopwatch.Stop();
            var listLoopTotalTime = stopwatch.ElapsedMilliseconds;

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

            Console.Read();
        }

        public static IEnumerable<int> GetRandoms()
        {
            var rand = new Random();
            for (int i = 0; i < 5000000; i++)
            {
                yield return rand.Next();
            }
        }

        public static IList<int> GetRandomsList()
        {
            var numberList = new List<int>();
            var rand = new Random();
            for (int i = 0; i < 5000000; i++)
            {
                numberList.Add(rand.Next());
            }
            return numberList;
        }
    }
}