It really depends. IEnumerable could have a lambda that leaks the world and makes a web call on every iteration for all the caller knows. You know iterating a

It really depends. IEnumerable could have a lambda that leaks the world and makes a web call on every iteration for all the caller knows. You know iterating a List will be fairly banal. So from an API perspective, I think returning IEnumerables can end up being too cute. The caller has to deal with this unknown thing. In general you should be clear about what you return and loose about what you accept. If your whole API is returning IEnumerables and some are expensive to iterate and others are not its actually less clear what I'm getting. And this isn't to say List is always the answer either.