LazyFibonacci.cs

private static IEnumerable<int> ZipWith(Func<int, int, int> combinePair,
                                        IEnumerable<int> listA,
                                        IEnumerable<int> listB)
{
    return listA.Zip(listB, combinePair);
}

private static IEnumerable<int> Tail(IEnumerable<int> list)
{
    return list.Skip(1);
}

private static int Plus(int x, int y)
{
    return x + y;
}

//fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
private static IEnumerable<int> Fibs()
{
    yield return 0;
    yield return 1;

    foreach (int sum in ZipWith(Plus, Fibs(), Tail(Fibs())))
        yield return sum;
}
        
public static void Main()
{
    foreach (int f in Fibs().TakeWhile(x => x < 100))
        Console.WriteLine(f);

    Console.ReadLine();
}