Lately, I’ve been considering the merits and demerits of returning IEnumerable<T>.

On the plus side, it is about as minimal as an interface gets, so it leaves you as method author more flexibility than committing to a heavier alternative like IList<T> or (heaven forbid) an array.

However, as I outlined in the last post, an IEnumerable<T> return entices callers to violate the Liskov Substitution Principle. It’s too easy for them to use LINQ extension methods like Last() and Count(), whose semantics IEnumerable<T> does not promise.

What’s needed is a better way to lock down a returned collection without making such temptations so prominent. (I am reminded of Barney Fife learning this lesson the hard way.)

(Part 1 of the video is available here.)

Enter IReadOnlyCollection<T>, new in .NET 4.5. It adds just one property to IEnumerable<T>: the Count property. By promising a count, you assure your callers that your IEnumerable<T> really does have a terminus. They can then use LINQ extension methods like Last() with a clear conscience.