Are .net finalizers always executed?

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Finalizers may actually be never executed, as Raymond Chen explains. Kind of funny that this question is asked during his annual CLR week, just two days after he explained it 🙂

For the lazy ones, the (or rather, one) conclusion is:

A correctly-written program cannot assume that finalizers will ever run.

If you are wondering whether you can rely on finalizers, this is already everything you have to know: Don’t rely on finalizers.

As Raymond Chen also states in the linked article:

Finalizers are a safety net, not a primary means for resource reclamation.

If you’re looking for how to release resources, have a look at the Disposable pattern.

A finalizer may not run, for example, if:

  • Another finalizer throws an exception.
  • Another finalizer takes more than 2 seconds.
  • All finalizers together take more than 40 seconds.
  • An AppDomain crashes or is unloaded (though you can circumvent this with a critical finalizer (CriticalFinalizerObject, SafeHandle or something like that)
  • No garbage collection occurs
  • The process crashes

(Note: The time values may have changed over time, but were certainly true some time ago.)

I guess there are a lot more things that can cause finalizers to never run. The bottom line is, other than the quote from Mr. Chen, that finalizers are a safety net that decrease the impact of bugs, because for example resources are released sometime, which is better than never, if you forget to do it explicity.

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