IEnumerable as return type

Question

Is there a problem with using IEnumerable as a return type? FxCop complains about returning List (it advises returning Coll

Answer 1

I can't accept the chosen answer. There are ways of dealing with the scenario described but using a List or whatever else your using isn't one of them. The moment the IEnumerable is returned you have to assume that the caller might do a foreach. In that case it doesn't matter if the concrete type is List or spaghetti. In fact just indexing is a problem especially if items are removed.

Any returned value is a snapshot. It may be the current contents of the IEnumerable in which case if it's cached it should be a clone of the cached copy; if it's supposed to be more dynamic (like the resuts of a sql query) then use yield return; however allowing the container to mutate at will and supplying methods like Count and indexer is a recipe for disaster in a multithreaded world. I haven't even gotten into the ability of the caller to call Add or Delete on a container your code is supposed to be in control of.

Also returning a concrete type locks you into an implementation. Today internally you may be using a list. Tomorrow maybe you do become multithreaded and want to use a thread safe container or an array or a queue or the Values collection of a dictionary or the output of a Linq query. If you lock yourself into a concrete return type then you have to either change a bunch of code or do a conversions before returning.