One must assume you don't know whether it is (so it could be). If you want to realize it into a list, you can do that - but if you don't know anything about the

One must assume you don't know whether it is (so it could be). If you want to realize it into a list, you can do that - but if you don't know anything about the enumerable, you might get burned anyway. If I make an IEnumerable<long> which generates all even numbers, then anyone doing ToList() on it is in for a surprise.