CSharp - Iterators Composing Sequences

Introduction

Iterators are composable.

The following code outputs even Fibonacci numbers only:

Demo

using System;
using System.Collections.Generic;

class Test//w w  w  . java2  s  . c o m
{
     static void Main()
     {
         foreach (int fib in EvenNumbersOnly (Fibs(6))){
             Console.WriteLine (fib);
         }
     }

     static IEnumerable<int> Fibs (int fibCount)
     {
         for (int i = 0, prevFib = 1, curFib = 1; i < fibCount; i++)
         {
           yield return prevFib;
           int newFib = prevFib+curFib;
           prevFib = curFib;
           curFib = newFib;
         }
     }

     static IEnumerable<int> EvenNumbersOnly (IEnumerable<int> sequence)
     {
         foreach (int x in sequence)
           if ((x % 2) == 0)
             yield return x;
     }
}

Result