Why does ICollection implement both IEnumerable and IEnumerable

Question

Why does ICollection implement both IEnumerable and IEnumerable?

What is the purpose of this? How does

Answer 1

IEnumerable itself forces any implementation to also implement the non-generic IEnumerable. This is safe, for the same reasons that IEnumerable is covariant as of .NET 4... you can always convert the T to object for the non-generic form.

Basically this means that if you've got code which uses a parameter of type IEnumerable, you can still call it with something like List.

Eric Lippert wrote a blog post recently about why collections end up implementing many interfaces, and Brad Abrams wrote a blog post back in 2005 about the specific IEnumerable/IEnumerable decision.