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.