Add an IIndexable<T> (accessible by index) interface by Daniel_Katz

Currently most of the common collections in BCL are implementing the IEnumerable<T> interface. This allows a great deal of flexibility.

But, IEnumerable is very costly on operations such as IEnumerable<T>.Count<T>(), IEnumerable<T>.Last<T>() and many others.

I'd like to propose to introduce a new interface that supports indexing, as I call it – IIndexable<T> which can be defined as:

public interface IIndexable<out T> : IEnumerable<T>
{
    T this[int index]
    {
        get;
    }

    int Count { get; }
}

It will improve the performance of linq extensions on all the supporting collections.
All the types that implement the ICollection<T> interface will be naturally compatible with IIndexable<T>.

The advantage of IIndexable<T> over ICollection<T> will be that IIdexable<T> is read-only as IEnumerable<T>, hence – can be covariant as IEnumerable<T> and avoid many casting issues.