Clean way of adding elements to IEnumerable

Is there a good way of adding an element to an IEnumerable without having to create a copy of the collection as such?:

var CatA = new Cat();
var CatB = new Cat();
IEnumerable<Cat> cats = new[]{ CatA };
var catList = cats.ToList();
catList.Add(CatB));
cats = catList;

What I want to do is something like this:

var CatA = new Cat();
var CatB = new Cat();
IEnumerable<Cat> cats = new[]{ CatA };
cats.ToList().Add(CatB));

But ToList() makes a copy of the IEnumerable cats, and CatB is placed into the copy, instead of the IEnumerable cats.

Use the Concat Linq method

Hah, speaking of Cats. Thanks. I'll try it out

Why are you not just starting with a List?

And to be clear, Concat isn't going to do what you want either

Concat produces a new enumerable, which returns the contents of the first enumerable, then the contents of the added elements

You cannot add elements to an IEnumerable

yeah, but note that it does not copy the collection

it only makes a new iterator object

The purpose of IEnumerable is just to assure iterability, if you want to be able to add thing then IList

Wait, array also implements IList, donnit?

iirc Array is IList but throws exceptions upon adding so am unsure

Yes