IList and IReadOnlyList

Question

If I have a method that requires a parameter that,

• Has a Count property
• Has an integer indexer (get-only)

What should the type

Answer 1

You are out of luck here. IList doesn't implement IReadOnlyList. List 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.