Friday, November 21, 2008

Exception causes Xml Error in deserialization

This week I seem to run into quite a few nooks and crannies in the .NET framework that put me in front of some tricky issues. This time it had to do with deserialization, or so Visual Studio led me to believe.

Here is the situation. Last week I wrote some code that deserialized an xml file into an elaborate object model. Part of that code was to parse an xsd file and add some objects to the model from there. This all worked well for the past week, as proven by the unittests.

Yesterday I started writing some code to dynamicly load assemblies and classes based on some information from the xsd file. I didn't get to finish it, but as I started working on it this morning, all of a sudden it threw an exception on the deserialization code, stating that I had some error in my xml file. I checked my xml file, which I knew I hadn't changed, and off course it was fine.

So I figured that since the last change I made was in some code that parsed part of the xsd file, I would debug there. As it appeared, the exception occurred whenever I passed a certain XPathNavigator instance into a method of my Factory class for creating a plugin. My first tought was that this occurred because I passed the XPathNavigator to another method. I started to program around this approach, but to no avail.

I crunched my head on it for another half hour until I had to leave for a meeting, but as I came back and looked at my code I realized I actually called the initializer method of a Factory class that I use to load some pluggable classes. I started debugging this method and presto, it raised an exception, because of some small oversight. I fixed this exception and the deserializer code worked again.

Conclusion on this is that some code in the XmlSerializer or it's base classes, catches any exception thrown and re throws another exception which is not related to the actual problem. Fortunately, the inner exception does contain the actual exception, meaning I could have saved myself a lot of time and headaches, by simple looking an the inner exception.

Another thing I learned was that using a singleton pattern to create your factory class is a good thing, but you must pay some attention when debugging.

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