What is the benefit to using List<T> over IEnumerable<T>?

Question

or the other way around?

I use generic lists all the time. But I hear occasionally about IEnumerables, too, and I honestly have no clue (today) what they are for and why I should use them. So, at the risk of having something on the net forever more proclaiming my ignorance, I humbly post this question.

Answer 1

Well, List<T> implements IEnumerable<T>... basically IEnumerable<T> is just a sequence of items. You can read through it, and that's all.

List<T> is a mutable collection - you can add to it, remove from it, sort it etc. It's more flexible in itself, but IEnumerable<T> allows you to use the same code to work with any implementation (arrays, linked lists, lists, iterators returned from methods using yield statements etc).

Answer 2

IEnumerable<T> is a more general interface, so you can substitute anything that implements that interface for your operation. If you have this method:

public void DoSomething(IEnumerable<T> enumerable) {
}

It will work on arrays, collections, lists, dictionaries, and anything else that implements the interface.

If you specify that the object is a List<T>, the method will only work on List<T> objects or instances that inherit from it.

The advantage of using List<T> is that lists have many more features than enumerables. When you need those features (insertion, searching, conversion, and many more), List<T> or IList<T> is more appropriate.