Should event handlers in C# ever raise exceptions?

Question

As a general rule, are there ever any circumstances in which it's acceptable for a method responsible for listening to an event to throw an exception (or allow to be thrown) that the class raising the event will have to handle?

Given that such an exception would stop other listeners to that event from being called subsequently, it seems a bit 'antisocial' to allow this to happen, but on the other hand, if there is an exception, what should it do?

Answer 1

Throwing an exception from a event handler is in many ways similar to throwing an exception from a IDisposable.Dispose method (or a C++ destructor). Doing so creates havoc for your caller because you leave them with little option.

1. Ignore the exception and let it propagate. This breaks their contract to inform all listeners of an event. This is a very real problem if anyone above them on the stack catches the exception.
2. Catch it call the other handlers and rethrow. But what happens if one of the others throw as well?
3. Swallow the exception. This is just bad in general. Event sources should have no knowledge of their caller and hence can't know what they're swallowing.
4. Crash the process because you're toast.

Of all of these #4 is the best option. But this is rarely done and can't be counted on.

I think in your component you really only have a few options

• You are calling the code which is throwing and are in the best position to handle the exception. If it's not handleable by you then it's unreasonable to expect it to be handled by anyone else. Hence crash the process and be done with it.
• Don't call the API which throws

Answer 2

The only two types of exceptions that should come out of events are serious, potentially process-ending ones like System.OutOfMemoryException or System.DllNotFoundException, and things that are clearly programming errors, like System.StackOverflowException or System.InvalidCastException. Catching and dropping these kinds of exceptions is never a good idea -- let them float up to the top and let the developer decide what to do with them on an application level.

As for the rest... any common or garden-variety exception like System.IO.IOException should be handled inside your event, and you should have some mechanism for returning such error conditions to the caller.