returning IList<T> vs Array in C#?

Question

I was recently asking someone why he preferred to return a strongly-typed array over an IList. I had always thought that programming against an interface was the most flexible and best way program when faced with a project having a long life. So it struck me as odd when he replied:

We typically prefer immutable types over mutable ones. Arrays are immutable. IList is not.

I'm not entirely sure I understand this statement. Can anyone help clarify this?

Thanks for any help you guys can provide.

Answer 1

Whoever "he" is, is in 100% wrong on the topic. Arrays are a very much mutable. This is in fact one of the reasons not to return an array. There is no way to prevent a caller from changing the elements of an array to whatever they please.

The only way in which an Arrray is immutable is in it's length. Once an array is allocated, it's length cannot be changed. Even APIs such as Array.Resize don't actually resize the array, they just allocate a new one, copy the contents and return the new array (by reference in this case).

I do agree however that there are many cases in which it is better to return immutable data. The primary one is that it allows you to return a reference to an internal collection of a class without doing a complete copy and at the same time preventing the caller from messing with your internal state. Most mutable collections cannot make such guarantees.