Why doesn't IReadOnlyCollection<T> include method for CopyTo(T[] array, int index)?

[aeroelec]

I'm curious why IReadOnlyCollection<T> did not include CopyTo method similar to ICollection<T>. To me, this significantly reduces the usefulness of IReadOnlyCollection<T> versus IEnumerable<T>, being that the former still requires the use of the latter to extract data from the underlying object. With the ability to add default implementations, this functionality could easily be added without affecting existing codebase:

void CopyTo(T[] array, int index) => foreach (T item in this) array[index++] = item;

Thus enabling optimization of classes that consume IReadOnlyCollection<T> for enumeration. For example, I created a generic undo/redo stack implementation that is an IReadOnlyCollection<T>. (See UndoRedoStack.) When initializing one stack from another, the only optimization I can achieve is to use IReadOnlyCollection<T>'s Count property to initialize the array size and then enumerate through the collection, unlike with an ICollection<T> where CopyTo implements a far more efficient copy method.

[huoyaoyuan]

Adding default interface implementation to the core interfaces is never an easy work. Previous attempts have caused some breakings in the whole toolchain.