Cannot apply indexing with [] to an expression of type 'System.Collections.Generic.IEnumerable<>

Question

Is there any specific reason why indexing is not allowed in IEnumerable.

I found a workaround for the problem I had, but just curious to know why it does not allow indexing.

Thanks,

Answer 1

Because it's not.

Indexing is covered by IList. IEnumerable means "I have some of the powers of IList, but not all of them."

Some collections (like a linked list), cannot be indexed in a practical way. But they can be accessed item-by-item. IEnumerable is intended for collections like that. Note that a collection can implement both IList & IEnumerable (and many others). You generally only find IEnumerable as a function parameter, meaning the function can accept any kind of collection, because all it needs is the simplest access mode.

Answer 2

The IEnumerable<T> interface does not include an indexer, you're probably confusing it with IList<T>

If the object really is an IList<T> (e.g. List<T> or an array T[]), try making the reference to it of type IList<T> too.

Otherwise, you can use myEnumerable.ElementAt(index) which uses the Enumerable.ElementAt extension method. This should work for all IEnumerable<T>s . Note that unless the (run-time) object implements IList<T>, this will cause all of the first index + 1 items to be enumerated, with all but the last being discarded.

EDIT: As an explanation, IEnumerable<T> is simply an interface that represents "that which exposes an enumerator." A concrete implementation may well be some sort of in-memory list that does allow fast-access by index, or it may not. For instance, it could be a collection that cannot efficiently satisfy such a query, such as a linked-list (as mentioned by James Curran). It may even be no sort of in-memory data-structure at all, such as an iterator, where items are generated ('yielded') on demand, or by an enumerator that fetches the items from some remote data-source. Because IEnumerable<T> must support all these cases, indexers are excluded from its definition.