Never ever catch System.Exception

…except when you should. :)

Patrick Cauldwell wrote a great post on how to handle exceptions and I agree with everything he says except for this:

“If you find yourself arguing for catching Exception, you probably have a design problem.”

In most cases he is completely right. In every web app I write I never catch System.Exception and I have no reason to.  If I’m code reviewing and I see somebody catch System.Exception I flag it as something that needs to be changed.  But in the last app I wrote I needed to catch System.Exception and there is only one good reason why: Microsoft gave me no choice.

Yep.  I have no other option.  Here’s the background: I wrote a nice little server application that receives multiple kinds of data.  Xml files, CSV, structured text documents, etc.  Each piece of data is labeled with what kind data it is and as the data comes in I store it and it’s label to disk.  I have client software that has plugins that are specifically written for each type of data.  When the clients start up they read in from a config file what types of data they support and register themselves with the server and start polling.  The server then responds with a payload that the client supports and the client loads up the plugin that handles that data type and pass the payload along to it.  The data is processed and then the client polls for more and the process repeats until there is nothing left to process.

So here is where the big problem is that makes me catch System.Exception: When the plugin crashes, the client needs to keep on chugging.  It tells the server there is a problem, marks the data as problematic, notifies the appropriate people and then it keeps on keeping on.  I know that catching System.Exception is bad.  What if I’m out of memory or the application domain is being torn down for some reason?  What if it really is one of those unrecoverable exceptions? But you never can tell the difference between the unrecoverable and recoverable exceptions since they all inherit from System.Exception. The other part of this problem is that I have no control over what exceptions the plugin can throw and I need to handle all of them.

Microsoft tried to have different base classes (System.ApplicationException) for different types of exceptions. That didn’t work to well and since System.ApplicationException has only 10 exceptions in the entire framework that inherit from it.

I would love to be able to just catch System.ApplicationException and know that anything else that should crash my app will crash my app but since I can’t I have to go on catching System.Exception.