Careful with those enumerables

Ever since .Net 2.0 introduced the yield keyword for creating enumerators, and even more after the introduction of LINQ in C# 3.0, I've seen more and more APIs return IEnumerable<T> instead of IList<T> or ICollection<T> or their older cousins the ArrayList and the array object.

That makes sense most of the time, especially for collections that aren't meant to be modified, and choosing between those different return types is not what I'm about to discuss here. You can find endless articles and threads about that.

I'm a big sinner when it comes to IEnumerable. Need to remember that lazy evaluation can lead to unexpected side-effects.