Why was GetEnumerator() stored in a separate interface from IEnumerator?

Question

I was wondering why the GetEnumerator() method was factored out of IEnumerator and placed in IEnumerable. It seems to me that it would make more sense to keep all of the enumerator methods in IEnumerator.

Thanks,

Scott

Answer 1

Ask yourself "imagine if this were true".

If all the enumeration methods were on a single interface, how could two callers enumerate the same list at the same time?

There are two interfaces because one says, "You can enumerate me," while the other says, "here's an object that keeps track of a given enumeration task."

The IEnumerable interface is a factory that creates as many IEnumerator objects as you want. How and when those enumerators get used is up to the consumer.

Answer 2

IEnumerable implies that the object is a collection or source of data which can be iterated over in a linear fashion. IEnumerator is the interface for the actual implementation which performs the iteration.

Answer 3

Because "IEnumerable" says "come, enumerate me" (and then you say- how, give me the Enumerator), however "IEnumerator" says "I can enumerate your collection!" and you already have it, you don't need to get any more.