Exposing types as a less capable type doesn’t prevent use as their real type

A great example of this is when internal lists are exposed as IEnumerable properties, e.g.:

private readonly List<string> internalStrings = new List<string>();
public IEnumerable<string> AllStrings { get { return internalStrings; } }

You’d likely think nobody can modify internal strings.

Wrong!
It's just too easy:

((List<string>)x.AllStrings).Add("Hello");

Even AsEnumerable won’t help as that’s a LINQ method that does nothing in this case   :-(

You can use AsReadOnly which creates a wrapper over the list that throws when you try and set anything however this provides a good pattern for doing similar things with your own classes should you need to expose a subset of internal structures if unavoidable.