Should I return an IEnumerable or IList? [closed]

Question

I wish to return an ordered list of items from a method. Should my return type be IEnumerable or IList?

Answer 1

There is a hierarchy here:

interface IList<T> : ICollection<T> { }  interface ICollection<T> : IEnumerable<T> { } 

You want to aim for the least possible coupling, so return an IEnumerable<T> if that is enough. It probably will be.

Return an IList<T> if the situation requires that the caller gets a List that it can use to Add/Insert/Remove. But even then it might be better if the caller created his own List from the IEnumerable collection.

Answer 2

It depends what you want to do with the result. If you need to get the count of items or get random access to individual items, go with an IList.

If callers just want to iterate through the items then go with IEnumerable - but you should document whether or not the returned value will be evaluated lazily or not - many IEnumerable instances these days represent queries that will be executed when the collection is enumerated. To be on the safe side, if what you are returning won't be evaluated on demand, I'd go with IList.