A few weeks ago, I hit a problem that an application that I wrote suddenly started randomly dropping out with an unhandled exception. I can’t remember all the specifics anymore, but I do remember that the event that was written to the logs (that can be viewed in Event Viewer in Administrative Tools in Control Panel): -
EventType : clr20r3
P9 : mxyabj2rsfg4uknkgmspj2kfpmzxhcc5
The most confusing thing about this was that I have a system built in to the application to catch unhandled exceptions, and this had been working fine in the past for a few years. I couldn’t understand why it wasn’t catching this exception.
What was even more confusing was that it was randomly affecting several machines in different sets of circumstances. I eventually managed to find something that would reliably reproduce the problem, but when I tried it on my development PC in the debugger, it worked fine (I still haven’t worked out why this is).
I initially thought that maybe it was because I had just upgraded to Visual Studio 2010 and tried to back-convert my application back to Visual Studio 2008, but the problem continued. I also tried using remote debugging to track down the problem, which also caused a lot of problems as the application still seemed to be dropping out even with the remote debugger running!
I eventually tracked down the problem using a Try, Catch clause around code which I knew would reproduce the problem. The exception was about a missing resource file. I hunted around and eventually found that the resx file for the error handling dialogue window seemed to have been excluded from the project, presumably due to some bug while it was being upgrade to Visual Studio 2010.
After I put the resx file back, I was able to see that the application was throwing an exception. My code for dealing with the unhandled exception had caught it and been throwing the other exception, which didn’t have anything to catch it, so the Common Language Runtime (or Windows?) killed it.
Edit: The actual exception type that was being thrown was System.Resources.MissingManifestResourceException