IEnumerable<T> and IEnumerator<T> Recap
To set the scene, we need to refresh our knowledge of the
IEnumerable<T> interface and its little brother,
IEnumerator<T>. The LINQ to Objects standard query operators are implemented as extension methods on the former interface, meaning that any query expressions can target any enumerable sequence. Because collection types implement this interface, this makes LINQ to Objects available for all of those (including arrays).
IEnumerable<T> interface basically provides the capability to request an enumerator, which acts like a cursor over a sequence of objects of type
T. Multiple enumerators can be active simultaneously, each with its own cursor into the underlying sequence: