Question

Can anyone tell me when I should use either.

For example, I think I should use an IList when I want to access the .Count of the collection or an individual item, correct?

Thank you.

Answer 1

Generally speaking, you should try and use the least specific type that suits your purpose. IEnumerable is less specific than IList (IList implements IEnumerable) so unless you want something specific from IList (such as Count as you suggest, or perhaps Add, Delete, etc), I’d use IEnumerable.

One benefit of remaining with IEnumerable is that you can write iterator methods to return this type (look up “yield return” and iterator methods if you are not familiar with them). This allows you to write very memory efficient “pipelines” for your loops.