Why should you be returning an IEnumerable

I’ve seen in many places where a method returns a List<T> (or IList<T>) when it appears that it may not actually really be required, or even desirable when all things are considered.

A List is mutable, you can change the state of the List. You can add things to the List, you can remove things from the List, you can change the items the List contains. This means that everything that has a reference to the List instantly sees the changes, either because an element has changed or elements have been added or removed. If you are working in a multi-threaded environment, which will be increasingly common as time goes on, you will get issues with thread safety if the List is used inside other threads and one or more threads starts changing the List.

Return values should, unless you have a specific use case in mind already, be returning an IEnumerable<T> which is not mutable. If the underlying type is still a List (or Array or any of a myriad of other types that implement IEnumerable<T>) you can still cast it. Also, some LINQ expressions will self optimise if the underlying type is one which better supports what LINQ is doing. (Remember that LINQ expressions always take an IEnumerable<T> or IQueryable<T> anyway so you can do what you like regardless of what the underlying type is).

If you ensure that your return values are IEnumerable<T> to begin with yet further down the line you realise you need to return an Array or List<T> from the method it is easy to start doing that. This is because everything accepting the return value from the method will still be expecting an IEnumerable<T> which List<T> and Array implement. If, however, you started with a List<T> and move to returning an IEnumerable<T> then because so much code will have the expectation of a List<T> without actually needing it you will have a lot of refactoring to do just to update the reference types.

Have I convinced you yet? If not, think about this. How often are you inserting items into a collection of objects after the initial creation routine? How often do you remove items from a collection after the initial creation routine? How often do you need to access a specific item by index within a collection after the initial creation routine? My guess is almost never. There are some occasions, but not actually that many.

It took me a while to get my head around always using an IEnumerable<T>, until I realised that I almost never require to do the things in the above paragraph. I almost always just need to loop over a collection of objects, or filter a collection of objects to produce a smaller set. Both of those things can be done with just an IEnumerable<T> and LINQ.

But, what if I need a count of the objects in the List<T>, that would be inefficient with an IEnumerable<T> and LINQ? Well, do you really need a count? Oftentimes I just need to know if there are any objects at all in the collection, I don’t care how many object there actually are, in which case the LINQ extension method Any() can be used. If you do need a count LINQ is clever enough to work out that the underlying type may expose a Count property and it calls that (anything that implements ICollection<T> such as arrays, lists, dictionaries, sets, and so on) so it is not iterating over all the objects counting them up each time.

Remember, there is nothing necessarily wrong with putting a ToArray() to ToList() before returning as a reference to an IEnumerable<T> something to which a LINQ expression has been applied. That removes the issues that deferred execution can bring (e.g. unexpected surprises when it suddenly evaluates during the first iteration but breaks in the process) or repeatedly applying the filter in the Where() method or the transformation in the Select() method.

Just because an object is of a specific type, doesn’t mean you have to return that specific type.

For example, consider the services you actually need on the collection that you are returning, remembering how much LINQ gives you. The following diagram shows what each of the interfaces expect to be implemented what a number of the common collection types implement themselves.

Incidentally, the reason some of the interfaces on the Array class are in a different colour is that these interfaces are added by the runtime. So if you have a string[] it will expose IEnumerable<string>.

I’d suggest that as a general rule IEnumerable<T> should be the return type when you have anything that implements it as the return type from the method, unless something from an ICollection<T> or IList<T> (or any other type of collection) as absolutely desperately in needed and not just because some existing code expects, say, an IList<T> (even although it is using no more services from it that it would had it been an IEnumerable<T>).

The mutability that implementations of ICollection<T> and IList<T> give will prove problematic in the long term. If you have a large team with members that don’t fully understand what is going on (and this is quite common given the general level developer documentation) they are likely to change the contents of the collection without understanding its implications. In some situations this may fine, in others it may be disastrous.

Finally, if you absolutely do need to return a more specific collection type then instead of returning a reference to the concrete class, return a reference to the lowest interface that you need. For example, if you have a List<T> and you need to add further items to it, but not at specific locations in the list, then ICollection<T> will be the most suitable return type.


  1. Marc Gravell says:


    – the caller doesn’t have a guaranteed way to check things like the Length/Count cheaply, or use a get-indexer – with a list they know that is cheap
    – it is ambiguous with IEnumerable<T> whether it is buffered / non-buffered, and (separately) immediate / deferred execution
    – if you mix LINQ in with that, you get into a testability pickle, since composition is different between providers (obviously you’d need to call .AsQueryable() to get back into composition mode, so this is a corner-case, bordering on self-inflicted)
    – many IEnumerable implementations are non-repeatable; again, the result is unclear whether it is a: possible to enumerate it twice, or b: cheap to enumerate it twice (does it repeat the query? is it the same buffered data?)

    in many cases (and especially in things like MVC where you want to give the view the *constructed* data, not a deferred hook to a query that hasn’t been executed yet)

    It would be nice if there was a BCL IReadOnlyList (insert better name here), with IEnumerable, Length and this[int index] {get;} (which means it would also be covariant; win:win) – but there isn’t sadly. In the absence of such, IMO the simplest approach is usually to return List; as long as this is an *isolated* list (i.e. we only return it to this caller), then if they want to call .Add() on it, I’m honestly not too bothered – they’ve only messed their local copy, and I don’t care about that ;p It does, however, make the API really obvious, i.e.

    – this is not deferred / streaming: you have the data *now*
    – (or maybe a Task<List> if you want a “future” ?)
    – the caller can query length etc

    All of this is just thoughts; my main point: it isn’t entirely a no-brainer, IMO. There are pluses and minuses on both sides here.

  2. Colin Mackay says:

    If the IEnumerable<T> reference can be cast to ICollection or ICollection<T> then LINQ will cast it and use the Count property on that interface. Obviously, if your IEnumerable<T> can’t be cast then you are no better off than before. I agree, you don’t have a guarantee whether it will be an O(1) or O(n) operation tho’.

    If I am performing a LINQ statement in a method and returning the result, I pretty much always put a .ToArray() at the end to force the execution so that I don’t have nasty surprises later on. So, for my code I don’t have the issue of deferred execution or whether it enumeration is repeatable. It means that enumeration is always cheap, repeatable and returns the same items.

    I like your idea of a IReadOnlyList although I think perhaps something between IEnumerable<T> and ICollection that exposes just Count and Contains() would also be useful. This also gets around the issue of certain methods in ICollection that throw a NotSupportedException when implemented on arrays.

    You are right that it is not entirely a no-brainer. Maybe I was a little strong in my assertions to use IEnumerable<T> because I’ve hit so many issues where mutable things (to be more general about it) have been returned and been changed in ways that have caused issues (and increasingly so as more multi-threaded code gets created). However, if a situation calls for a specific type of collection then by all means return it from a method. I frequently do but it tends to be in the initial construction phase (as I mentioned above) of the List and once it is populated I never really want to change it.

    But, if a developer is not sure which to use I’d start with IEnumerable<T> as that is easier to refactor to ICollection<T>, IList<T> or even the concrete types later on.

    In the end, this is just my opinion and it appears to work well. 🙂

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