Yeah - this is much easier in C#, where all of the built in collection types support IEnumerable, and if you were creating a new collection type then not supporting IEnumerable would be a bit silly.

(And yes, I know that C# isn't suitable for all sorts of things.)

Does even the ordinary array type support IEnumerable too? (I presume that C# still has ordinary array types, and doesn't actually require you to instantiate a dynamically allocated collection object every time you want to store n things in a row.)

Yup. Array implements IEnumerable and IEnumerable<>. I think it does some jiggery-pokery under the covers to do so, but I've never dug into how they actually work. I would have suspected they were significantly slower than pure C++, but it seems that I am incorrect:
http://www.mightyware.com/cpp_vs_csharp_arrays.bhs