GC.Collect() and Finalize

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Ok, it’s known that GC implicitly calls Finalize methods on objects when it identifies that object as garbage.

No no no. That is not known because in order to be knowledge a statement must be true. That statement is false. The garbage collector does not run finalizers as it traces, whether it runs itself or whether you call Collect. The finalizer thread runs finalizers after the tracing collector has found the garbage and that happens asynchronously with respect to a call to Collect. (If it happens at all, which it might not, as another answer points out.) That is, you cannot rely on the finalizer thread executing before control returns from Collect.

Here’s an oversimplified sketch of how it works:

  • When a collection happens the garbage collector tracing thread traces the roots — the objects known to be alive, and every object they refer to, and so on — to determine the dead objects.
  • “Dead” objects that have pending finalizers are moved onto the finalizer queue. The finalizer queue is a root. Therefore those “dead” objects are actually still alive.
  • The finalizer thread, which is typically a different thread than the GC tracing thread, eventually runs and empties out the finalizer queue. Those objects then become truly dead, and are collected in the next collection on the tracing thread. (Of course, since they just survived the first collection, they might be in a higher generation.)

As I said, that’s oversimplified; the exact details of how the finalizer queue works are a bit more complicated than that. But it gets enough of the idea across. The practical upshot here is that you cannot assume that calling Collect also runs finalizers, because it doesn’t. Let me repeat that one more time: the tracing portion of the garbage collector does not run finalizers, and Collect only runs the tracing part of the collection mechanism.

Call the aptly named WaitForPendingFinalizers after calling Collect if you want to guarantee that all finalizers have run. That will pause the current thread until the finalizer thread gets around to emptying the queue. And if you want to ensure that those finalized objects have their memory reclaimed then you’re going to have to call Collect a second time.

And of course, it goes without saying that you should only be doing this for debugging and testing purposes. Never do this nonsense in production code without a really, really good reason.

More Related Contents:

  1. Why should we call SuppressFinalize when we don’t have a destructor
  2. Proper use of the IDisposable interface
  3. Understanding garbage collection in .NET
  4. Use of Finalize/Dispose method in C#
  5. Do event handlers stop garbage collection from occurring?
  6. When is it acceptable to call GC.Collect?
  7. Do you need to dispose of objects and set them to null?
  8. Best Practice for Forcing Garbage Collection in C#
  9. Implementing IDisposable correctly
  10. Memory Leak in C#
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  12. .Net and Bitmap not automatically disposed by GC when there is no memory left
  13. Does garbage collector call Dispose()? [duplicate]
  14. GC.Collect()
  15. Monitoring Garbage Collector in C#
  16. How can I write a unit test to determine whether an object can be garbage collected?
  17. Is correct to use GC.Collect(); GC.WaitForPendingFinalizers();?
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  19. Timer, event and garbage collection : am I missing something?
  20. GC.Collect() not collecting immediately?
  21. When should I use GC.SuppressFinalize()?
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  23. Guidelines For Dispose() and Ninject
  24. Does WeakReference make a good cache?
  25. How does garbage collection and scoping work in C#? [duplicate]
  26. Garbage Collection should have removed object but WeakReference.IsAlive still returning true
  27. How do I get .NET to garbage collect aggressively?
  28. What’s the purpose of GC.SuppressFinalize(this) in Dispose() method?
  29. Should we use “workstation” garbage collection or “server” garbage collection?
  30. Garbage Collection not happening even when needed

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