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.
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.
I think he maybe thought since an array's length property is immutable, then somehow arrays are more immutable than IList or maybe he used the wrong word and interchanged concrete with immutable. Who knows but it's an odd answer.
I think with returning a List there is something slightly implied that it's ok to modify it or that it may change while returning an array doesn't imply that as much.
For example, If you had an object model on top of a repository and had a method like GetCars() that returned a List and a junior programmer saw cars.Add(Car c) ... Would you think he was completely insane for thinking cars.Add(new Car()) might actually add a car into the repository? Arrays are just inherently more explicit.
I think List usage is more appropriate in properties, like Page.Controls.Add
I prefer returning arrays more often than List for several reasons.
Habit. Collections in 1.0/1.1 SUCKED
I prefer that my methods return the simplest & most lightweight object they can. If I need to make an Array a List it is trivial.
They can be used in .net 1.1 and it reduces the surface are of refactoring if I ever needed to support older versions of the runtime I can reuse at least some of my code or apply an identical object model.
I always prefer a ReadOnlyCollection. All the advantages of a List, but Read Only.
One point he could have meant is that IList includes the Insert, Remove, and Add methods, so the collection itself can be modified. T[], on the other hand, cannot have elements added to it.
I should add that FxCop recommends returning ReadOnlyCollection instead. The indexer is read-only, so you can't change the elements, and the Add and other such methods all throw NotSupportedException.
In principle he's right, but he doesn't know how to practice it properly...
We typically prefer immutable types over mutable ones.
That is correct. Immutable types are nicer to work with
Arrays are immutable. IList is not.
That is not correct. Neither of those are immutable.
If you want to return a collection that is immutable, return an IEnumerable<T> or a ReadOnlyCollection<T> (using the List<T>.AsReadOnly method). Still those doesn't protect the objects if they themselves are not immutable. Eventhough you can only read from the collections, you can still change the data in each object if they allow it.
Also, you should consider the "ownership" of the collection that you are returning. If you are creating an array for the sole purpose of returning it, then there is no reason not to give full control to it. If you on the other hand are returning a collection that is a member of the class, you should only allow as little access to it as is needed.
I don't understand it either. Arrays are very much mutable, except for their size. Individual elements can still be modified. Maybe he meant arrays are value types and Lists are reference types. I don't know.
Anyway, you should probably have a look at Eric Lippert's opinion on the subject. Could provide you with some ammo for discussion, too.
Perhaps this someone does not know what he is talking about?
The reason to use an Interface as return type over the actual type itself, is to hide the internal implementation from the caller. This allowes the implementation to change the actual type without repurcusions to the rest of the application.
It does not make any sense to copy an internally used collection into an array for external use, for no other reason than mutable or immutable.
Seperate issues are: - return a list of strongly typed data or not. IList or IList. The use of strongly typed data is always preferred. - mutable or immutable. ICollection,IList or IEnumerator Return with what you want to allow to the data. For a readonly list return only a IEnumerator. If the caller is allowed to modify the collection use ICollection or IList.
Basically he's saying "we prefer structures which can't be changed easily at run time, because we believe them to be less vulnerable to errors." Whether that's correct in this case is another question.
Perhaps he meant the size of the array as "immutable"? Basically, you declare the size once and you're stuck with it. With lists, you can always use "Add".
I suppose that if you're sure about the size of the list, perhaps array is a bit faster?
Well. IList is mutable in the sense that you can add and remove items, rather than just change the items that are already there. An array lets you mess with individual items, but a particular index will never become entirely invalid. There's any number of reasons to prefer immutability -- it makes the surface area for accidental data corruption much smaller, for example.
