Why can't I access the class internals of List<T> when I derive from it?

Question

class NewList<T> : List<T>

Why can't I access it's internals like T[] _items, etc?

Why aren't they protected, but private?

Should I use composition for this?

Answer 1

List<T> isn't designed to be a base class. You should use one of the classes in System.Collections.ObjectModel instead.

Answer 2

They're private because the author (Microsoft) did not intend for you to access them, probably for safety reasons.

You should use composition instead of inheritance. Make your class expose IList<T> instead of List<T> directly.

List<T> should never be directly exposed in a public API (this is actually specified in the design guidelines). It should be used as an internal implementation detail only.

Answer 3

Why can't I access it's internals like T[] _items, etc?

Reflector says _items is private. The underscore suggests a private member

Why aren't they protected, but private?

In the case of _items, Microsoft provides the public ToArray method. It returns _items.

Answer 4

You don't need it. That is, what can you do with _items that you can't do with the Item : T property? They have given you a simple, functional abstraction to the underlying array with the indexer. It is very light weight -- all it does it check the bounds and increment the version of the list (on the setter) ...

var lst = new List<int>() { 1, 2, 3, 4, 5 };
lst[0].Dump(); // 1
lst.Dump(); // 1, 2, 3, 4, 5
lst[0] = 2;
lst.Dump(); // 2, 2, 3, 4, 5