[ryanmolden]

There is also some trickiness around when things get executed. If you return an IEnumerable<T> that is really a LINQ query, and the caller doesn't immediately enumerate it, they run the risk of getting exceptions when they do enumerate if you mutate the collection the query runs over. You can of course force its evaluation, but that also loses some of the benefits of the laziness. Bring in multi-threading and even if you properly synchronize collection access (and that is hard to do without custom collection types, since LINQ is like the honey badger w.r.t. synchronized access) you can still see the collection modified exceptions talked about above.