A Loop That Never Stops

Everything so far has been a cost. Here is the payoff: because an iterator only computes what is asked for, it can describe a sequence that has no end.

static IEnumerable<int> Naturals()
{
    for (int i = 1; ; i++)
    {
        yield return i;
    }
}
   

That for loop has no exit condition. Written anywhere else it would hang the program. Here it is harmless, because it only advances when someone calls MoveNext.

foreach (var n in Naturals().Take(5))
{
    Console.WriteLine(n);
}
   

1
2
3
4
5

Take(5) stops asking after the fifth value, so the loop stops running. The sixth number is never computed — it does not exist anywhere.

Fibonacci, without deciding how many

The usual version of this function takes a count parameter, because it has to know how big an array to fill. An iterator does not have to know.

static IEnumerable<long> Fibonacci()
{
    long a = 0;
    long b = 1;

    while (true)
    {
        yield return a;

        long next = a + b;
        a = b;
        b = next;
    }
}