Why does this string extension method not throw an exception?

Question

I\'ve got a C# string extension method that should return an IEnumerable of all the indexes of a substring within a string. It works perfectly for it

Answer 1

Enumerators, as the others have said, aren't evaluated until the time they start getting enumerated (i.e. the IEnumerable.GetNext method is called). Thus this

List indexes = "a.b.c.d.e".AllIndexesOf(null).ToList();

doesn't get evaluated until you start enumerating, i.e.

foreach(int index in indexes)
{
    // ArgumentNullException
}