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CA1816: Why should you call GC.SuppressFinalize when you have not provided ~class()

    Question

  • Hi,

    When I was running the FxCop on one of our assembly, I got a violation for the rule CA1816 on one of the class. This is the scenario:

    Suppose that you have a class called ManagedResourcesClass that uses only Managed resources. Lets assume that this uses a lot of managed resources and hence it is necessary to implement IDisposable.

    Now, for such a class, from my understanding of the IDisposable pattern, there's no need to write a ~ManagedResourcesClass(). So, if I have not written ~ManagedResourcesClass(), then there's no need for me to call GC.SuppressFinalize() in the ManagedResourcesClass.Dispose(), is there ?

    Also, just wanted to clarify: Since System.Object has Finalize() method on it, does it mean that all objects, by default, will be put on the finalization queue ?

    Thanks,
    -Mahesh
    Wednesday, December 10, 2008 5:16 AM

Answers

  • When you have implemented your own dispose implementation, you can speed up the garbage collection to indicate that you have already cleaned up all resources. To do this, you call the SuppressFinalize. The finalize method of your object won't be called then anymore.

    By default all objects will be placed on the finalization queue.


    Ewald
    Wednesday, December 10, 2008 7:38 AM
  • Yes I mean that the System.Object.Finalize() is called for each object. When you don't override the method, it might perform some cleanup (which you could check out, since the .net framework code is now debuggable).

    It is not wise to suppress the finalize in each constructor, otherwise the garbage collection won't free up memory. When you need explicit dispose functionality, then the best practice is to put it in the Dispose method and call the SuppressFinalize method. Otherwise do nothing and let the default .net implementation do its work.
    Ewald
    Wednesday, December 10, 2008 10:00 AM
  •  The CA rule checks whether the Dispose implementation applies to the best practice of Microsoft. When you do not want to apply to this best practice, fine: uncheck the rule. It is only an advice!

    Microsoft states that when you don't call SupressFinalize, the resources of the object are freed twice, which is a waste of time.
    Ewald
    Wednesday, December 10, 2008 12:13 PM

All replies

  • When you have implemented your own dispose implementation, you can speed up the garbage collection to indicate that you have already cleaned up all resources. To do this, you call the SuppressFinalize. The finalize method of your object won't be called then anymore.

    By default all objects will be placed on the finalization queue.


    Ewald
    Wednesday, December 10, 2008 7:38 AM
  • The finalize method of your object won't be called then anymore.

    Did you mean that System.Object.Finalize() wouldn't be called anymore ? because, the class ManagedResourcesClass does not override this function.
    Basically, Finalize()method of a ManagedResourcesClass object (which is basically the do-nothing function System.Object.Finalize()) does nothing and hence I do not care to avoid that getting called or not.

    By default all objects will be placed on the finalization queue.


    Does it mean that for every object that is created, there's an additonal overhead of going to the finalization queue and then it gets collected by the garbage collector.? In that case, shouldn't we ideally be calling GC.SuppressFinalize(this)in the constructor of most of the classes (if they do not implement IDisposable and there's no special code that needs to be executed before this gets garbage collected) to improve the performance ?
    Wednesday, December 10, 2008 9:54 AM
  • Yes I mean that the System.Object.Finalize() is called for each object. When you don't override the method, it might perform some cleanup (which you could check out, since the .net framework code is now debuggable).

    It is not wise to suppress the finalize in each constructor, otherwise the garbage collection won't free up memory. When you need explicit dispose functionality, then the best practice is to put it in the Dispose method and call the SuppressFinalize method. Otherwise do nothing and let the default .net implementation do its work.
    Ewald
    Wednesday, December 10, 2008 10:00 AM
  • I had looked up in the Reflector the implementation of System.Object in the mscorlib.dll. It does nothing. I mean, the IL code for it is:
    .method family hidebysig virtual instance void Finalize() cil managed
    {
        .custom instance void System.Runtime.ConstrainedExecution.ReliabilityContractAttribute::.ctor(valuetype System.Runtime.ConstrainedExecution.Consistency, valuetype System.Runtime.ConstrainedExecution.Cer) = { int32(3) int32(2) }
        .maxstack 8
        L_0000: ret 
    }
    
    So, basically it does nothing; neither do I want to do anything special in my class, and hence I do not override this method (which in C# is done through writing the ~ManagedResourcesClass(). method).

    Is the rule complaining that:
    The ManagedResourcesClass.Dispose() implementation could be changed to get some additional performance benefit (as compared to a an object that does not implement IDisposable) ?
    Or that,
    The ManagedResourcesClass.Dispose() implementation could be changed to avoid the additional performance hit associated with the class as it has overridden the Finalize() method (as compared to a an object that does not implement IDisposable) ?

    If it is the second case, then I'm afraid that the rule should also see if the Finalize() method is overridden or not before raising the violation.
    Wednesday, December 10, 2008 10:17 AM
  •  The CA rule checks whether the Dispose implementation applies to the best practice of Microsoft. When you do not want to apply to this best practice, fine: uncheck the rule. It is only an advice!

    Microsoft states that when you don't call SupressFinalize, the resources of the object are freed twice, which is a waste of time.
    Ewald
    Wednesday, December 10, 2008 12:13 PM
  • Hello Ewald,

    I have same question :

    Each object is going to be added in Finalize queue and F-reachable queue before collection even if user has not overridden Finalize method?

    If this is the case then is it not unnecessary burden? Can Such objects not be garbage collected in first Collect cycle directly without calling their Finalize method?

    I read an article about garbage collection from following link :

    http://msdn.microsoft.com/hi-in/magazine/bb985010(en-us).aspx

    From this article I thought that if you do not override Finalize method in your class then object of your class will not be finalizable. And it will be Garbage collected in one Collect cycle. But then question comes whether microsoft considers only overridden Finalize methods during Garbage Collection?

    Thanks and Regards,
    Vishal Shah
    Monday, July 27, 2009 6:46 AM
  • Hello,

    I also had question that ---- By default all objects will be placed on the finalization queue?

     I posted it. Here is the link for it.

    http://social.msdn.microsoft.com/Forums/en-US/clr/thread/7bbde004-3c68-469c-a245-7e9184fdf464

    That says that,

    ==================

    Reference of the object will be added to Finalize queue only when class of that newly created object has overridden Finalize method. In short CLR will identify whether this class has overriden the Finalize or not and if class has overridden then and only then CLR will add refernce of the newly created object to Finalize queue. If class has not overridden Finalize then class will have imlpementation of System.Object class's Finalize method and in that case object of that class will not be considered as Finalizable and CLR will not add reference for such objects in Finalize queue.

    Please correct me if I am wrong.

    Thanks and Regards,
    Vishal
    ===========================

    Please forgive me If you feel that I should not post to this thread. And also correct this if any thing is not proper.

    Thanks and Regards,
    Vishal
    Tuesday, July 28, 2009 6:03 AM
  • You are correct that only objects with a declared finalizer in a System.Object subclass will be subject to finalization.  However, this does not necessarily mean that calling GC.SuppressFinalize is an "unnecessary burden" even if there is no finalizer implementation in an object's inheritance chain.

    The main reason for this is that adding a finalizer is not a breaking change.  If your class B inherits from a class A in another assembly, the author of class A can add a finalizer, recompile and redistribute his assembly without requiring your assembly to be recompiled.  When you originally compiled the assembly containing class B, it didn't have an implemented finalizer, but that's no guarantee that it won't have one at run-time if any of its superclasses above System.Object reside in another assembly.

    Because of this scenario, I think the rule makes sense as-is.  If you write a lot of IDispose implementations for classes that do not have an inheritance hierarchy that reaches outside their own assembly until they reach System.Object, perhaps it might make sense for you to substitute the Microsoft-provided rule with one that handles this special case.  However, I would not recommend suppressing violations of this rule since you could hide problems that might emerge later as the code evolves.
    Thursday, July 30, 2009 6:29 PM
  • Hello Nicole,

    You talked about a Nice scenario. That is quite possible. Thanks for sharing this one.

    Regards,
    Vishal
    Friday, July 31, 2009 4:52 AM
  • Umm... I'm still not buying this rationale. Nicole has a good scenario but did class A already implement IDisposable to begin with? If so...

    Then class B should've been calling base.Dispose (if it wasn't, that was a bug already). When the finalizer was added to class A, surely the author must've added the GC.SuppressFinalize(this) call to A.Dispose. So it's all good and B didn't have to do it.

    If not...

    Then as a finalizer is added to class A, a Dispose should to be added to it as well that calls GC.SuppressFinalize(this). Regardless of whether that is done or not, B isn't calling base.Dispose() because it didn't exist so we already have a problem. In fact, adding GC.SuppressFinalize(this) is B.Dispose() makes things worse because when B is disposed, it would take the object out of the finalization queue and A.Finalize() will not run... ever! The unmanaged resources held on to A will be leaked.


    Wednesday, May 16, 2012 4:21 PM
  • When you have implemented your own dispose implementation, you can speed up the garbage collection to indicate that you have already cleaned up all resources. To do this, you call the SuppressFinalize. The finalize method of your object won't be called then anymore.

    By default all objects will be placed on the finalization queue.


    Ewald

    "By default all objects will be placed on the finalization queue". I think this is a misleading and terribly wrong statement. Please find another discussion about this here http://social.msdn.microsoft.com/Forums/en-US/clr/thread/7bbde004-3c68-469c-a245-7e9184fdf464
    Wednesday, July 18, 2012 3:35 PM