Why does List implement IReadOnlyList in .NET 4.5?

Question

Why does List implement IReadOnlyList in .NET 4.5?

List isn\'t read only...

Answer 1

Because List implements all of the necessary methods/properties/etc. (and then some) of IReadOnlyList. An interface is a contract that says "I can do at least these things."

The documentation for IReadOnlyList says it represents a read-only collection of elements.

That's right. There are no mutator methods in that interface. That's what read-only means, right? IReadOnlyList is used in the "typical" (contract) way, not as a marker.