IList and IReadOnlyList

You are out of luck here. IList<T> doesn’t implement IReadOnlyList<T>. List<T> does implement both interfaces, but I think that’s not what you want.

However, you can use LINQ:

• The Count() extension method internally checks whether the instance in fact is a collection and then uses the Count property.
• The ElementAt() extension method internally checks whether the instance in fact is a list and than uses the indexer.