The OnDeserializingAttribute
Since I’m porting an existing project to .NET 4.0, I’ve thought about using code contracts in order to improve my code base (and yes, I can say that after some initial frustrations, code contracts is my pal now :)). And yes, it did improve…but since I’m using runtime exceptions, that also meant that I started getting unexpected runtime exceptions.
One of my DTOs had an invariant which specified that a specific object can never be null. To ensure that, I’ve introduced my own default constructor which was responsible for initializing that field into a valid (default) value. And everything run fine until I tried to recover an instance of that object through a WCF service. It was only then that I’ve recalled about an old problem that had bit me in the past: the constructor of a type isn’t called during deserialization!
Fortunately, there’s a solution for this problem: the OnDeserializingAttribute. That means that I’ll have to refactor the initialization code into a helper method so that it gets called during “normal” instantiation and during deserialization. A little more work, but at least I can still keep my invariants valid. Hurray!
Now, if I could only get NHibernate to play nicely with .NET HashSet…