CODEWITHSUNDEEP
.netgarbage-collectionfinalizer
mafu
mafu

Reputation: 32650

Are .net finalizers always executed?

Are finalizers guaranteed to be executed in .NET at some point (spare power outages and the like)? I know how GC works and that it is nondeterministic when exactly they'll run.

(The search did not display good answers, so I'm adding this question in high expectation of a merge with the not-so-easy-to-discover actual answers. Apart from that, I already know the answer and am going to add it after a few days in case nobody mentioned it.)

Upvotes: 19

Views: 2515

Answers (2)

OregonGhost
OregonGhost

Reputation: 23759

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.

Upvotes: 26

Oded
Oded

Reputation: 499042

If a finalizer throws an exception, other finalizers will not execute.

You can also suppress finalizers if you call SuppressFinalizer on the object.

From MSDN (Object.Finalize):

The Finalize method might not run to completion or might not run at all in the following exceptional circumstances:

  • Another finalizer blocks indefinitely (goes into an infinite loop, tries to obtain a lock it can never obtain and so on). Because the runtime attempts to run finalizers to completion, other finalizers might not be called if a finalizer blocks indefinitely.
  • The process terminates without giving the runtime a chance to clean up. In this case, the runtime's first notification of process termination is a DLL_PROCESS_DETACH notification.

Upvotes: 6

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