Best practice: How to expose a read-only ICollection

Question

I have an ICollection called foos in my class which I want to expose as read-only (see this question). I see that the interface defines a

Answer 1

I typically return an IEnumerable.

Once you make a collection readonly (so methods like Add, Remove and Clear no longer work) there's not much left that a collection supports that an enumerable doesn't - just Count and Contains, I believe.

If consumers of your class really need to treat elements like they're in a collection, it's easy enough to pass an IEnumerable to List's constructor.