when to throw exceptions

have you ever heard the saying "exceptions are for exceptional circumstances"? well, i have, and until recently, i was a firm believer of such thoughts. i'd normally code for every "exceptional" condition and try to do everything i could to avoid having to throw an exception.

however, just a few days ago, i came across this statement from Krzysztof Cwalina (program manager for the CLR team at MS):

One of the biggest misconceptions about exceptions is that they are for “exceptional conditions.” The reality is that they are for communicating error conditions. From a framework design perspective, there is no such thing as an “exceptional condition”. Whether a condition is exceptional or not depends on the context of usage, --- but reusable libraries rarely know how they will be used. For example, OutOfMemoryException might be exceptional for a simple data entry application; it’s not so exceptional for applications doing their own memory management (e.g. SQL server). In other words, one man’s exceptional condition is another man’s chronic condition.

Cwalina then goes on to say that exceptions should be used for common errors such as (1) usage errors, (2) program errors, and finally (3) system errors. it seems to me this in direct opposition of the "exceptional exceptions" mantra.

to be honest, i never understood why exceptions should only be used in "exceptional" circumstances. it's hard to define "exceptional", and that's exactly the point Cwalina makes in his quote.

i realize, however, there's circumstances when it doesn't make sense to throw exceptions; for example, why throw a DivideByZeroException when you can easily check for the condition an appropriately terminate?

but for the most part, exceptions provide an objected oriented way to communicate errors to clients. i think i'm switching from the "exceptional exceptions" camp to the "let's use exceptions to communicate errors" camp. what about you?

Update: I have a new article on the cost of throwing exceptions. If you now think that throwing more exceptions is a good idea, you might want to check this article too.