Are .net finalizers always executed?

问题:

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.)

回答1:

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.

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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.

回答2:

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.