First Chance .NET Exception Handling

I saw this article posted on Reddit’s programming feed*, which talks about a Visual Studio debugging technique for getting first crack at exceptions, before any upstream handlers run. The Debug->Exceptions dialog can be used to set the debugger to break before any exception handlers fire for a particular exception. This is useful not only for debugging code that interfaces with third-party libraries (where it is often unclear why an exception might be thrown), but also your own code. Why?

Imagine that you have some code that runs in an interactive session, but which has a high-level catch block to catch and report errors. This catch block may attempt to continue execution when an error occurs – for example, writing one file in a batch might fail, but the code should continue writing other files. Unfortunately, the exception log produced by this might not provide sufficient information to debug the issue. For example, many of the I/O exception types fail to include information about which file or directory was being modified when the exception was thrown!

In tracking down cases like these, it’s often easier to set the debugger to catch that particular exception at the first chance it gets, and then examine the state of the calling function where the exception was thrown. Having both a call stack and the specific line of code where the exception was thrown often allows you to see the problem immediately, without further investigation.

Another illuminating debugging exercise is to turn on first chance exception handling for all exceptions, and then see where they are thrown. Code that masks exceptions (by using an untyped, empty catch block) is particularly nasty, as it can leave the program in an ill-defined state, but without any feedback to indicate that something failed. A different problem results from the frequent use of exceptions: they have a significant negative performance impact. Turning on first chance exception handling makes it trivial to find these cases, if you hadn’t already noticed the debug window spam that frequent exceptions tend to create.



* There’s a lot of stuff that gets posted there that gets my (figurative) blood pressure up, so we’ll see how long I stick with it. But I figured it would be a good exercise to read it regularly and write about things I see there.

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