Why is IEnumerable<T> necessary when there is IEnumerator<T>?

Question

Disclaimer: I understand the difference between IEnumerable<T> and IEnumerator<T> and how to use both. This is not a duplicate of this or this .

This is more of a design question - since IEnumerator<T> already encapsulates all the necessary information ( .Current , .MoveNext() ) about something that can be enumerated, then what's the point of introducing a type ( IEnumerable<T> ) whose sole purpose is to return an instance of the former?

To be specific:

1. Why can't foreach be designed to iterate directly through an IEnumerator<T> , like so:

    // foreach (var e in anEnumerator) { //... } while (anEnumerator.MoveNext()) { doSomething(anEnumerator.Current); } 

2. Why can't Linq be built based directly off of IEnumerator<T> ?

Answer 1

The two interfaces each represent very different concepts. IEnumerable<T> is something that "allows enumeration", where IEnumerator<T> is the representation of the enumeration itself.

If you were to merge these together, it would be impossible to enumerate a single collection more than once at the same time (without some other mechanism in place). For example, two threads doing a foreach over an array would no longer work, where that is perfectly acceptable in the current design.