If you have a finalizer, watch your ctor

time to read 2 min | 289 words

We recently got into a very bad state with our build server. We had a few dozens failing tests in an area of the code that didn’t change in months. That was annoying, to say the least. I tried to run this on my machine, to see what was going on, and we got an even worse state, a process crash.

The good thing was that this is managed code, so you get stack traces, and we were able to figure out that the issue is with the SqlClient assembly. It looks like a recent change meant that loading it by reflection will give you the netstadnard 2.0 release, which is basically just a filler that always throw PlatformNotSupported. That is perfectly fine, we’ll change the way we do things to load the direct reference.

What killed us was this:


This will kill the process. How is that?

Well, DbConnection inherits from Component, and Component has a finalizer. Care to guess how what will happen when that is called?


So here is what will happen, the ctor throws, so no one can have an instance of this class. But the finalizer is already tracking it, and will call the finalizer. The finalizer will call the Dispose method, and that will end up throwing, and an exception during finalization is fatal to the entire process. Bye!

The general idea is that when you are working with a throwing ctor, you either make sure to call GC.SupprressFinalize or you cleanup in such a way that you can safely finalize things.