Why should we call SuppressFinalize when we don't have a destructor

Question

I have few Question for which I am not able to get a proper answer .

1) Why should we call SuppressFinalize in the Dispose function when we don't have a destructor .

2) Dispose and finalize are used for freeing resources before the object is garbage collected. Whether it is managed or unmanaged resource we need to free it , then why we need a condition inside the dispose function , saying pass 'true' when we call this overridden function from IDisposable:Dispose and pass false when called from a finalize.

See the below code I copied from net.

class Test : IDisposable
   {
     private bool isDisposed = false;

     ~Test()
     {
       Dispose(false);
     }

     protected void Dispose(bool disposing)
     {
       if (disposing)
       {
         // Code to dispose the managed resources of the class
       }
       // Code to dispose the un-managed resources of the class

       isDisposed = true;
     }

     public void Dispose()
     {
       Dispose(true);
       GC.SuppressFinalize(this);
     }
   }

what if I remove the boolean protected Dispose function and implement the as below.

   class Test : IDisposable
   {
     private bool isDisposed = false;

     ~Test()
     {
       Dispose();
     }


     public void Dispose()
     {
      // Code to dispose the managed resources of the class
      // Code to dispose the un-managed resources of the class
      isDisposed = true;

      // Call this since we have a destructor . what if , if we don't have one 
       GC.SuppressFinalize(this);
     }
   }       

Alforja

Answer 1

I'm going out on a limb here, but... most people don't need the full-blown dispose pattern. It's designed to be solid in the face of having direct access to unmanaged resources (usually via IntPtr) and in the face of inheritance. Most of the time, neither of these is actually required.

If you're just holding a reference to something else which implements IDisposable, you almost certainly don't need a finalizer - whatever holds the resource directly is responsible for dealing with that. You can make do with something like this:

public sealed class Foo : IDisposable
{
    private bool disposed;
    private FileStream stream;

    // Other code

    public void Dispose()
    {
        if (disposed)
        {
            return;
        }
        stream.Dispose();
        disposed = true;
    }
}

Note that this isn't thread-safe, but that probably won't be a problem.

By not having to worry about the possibility of subclasses holding resources directly, you don't need to suppress the finalizer (because there isn't one) - and you don't need to provide a way of subclasses customising the disposal either. Life is simpler without inheritance.

If you do need to allow uncontrolled inheritance (i.e. you're not willing to bet that subclasses will have very particular needs) then you need to go for the full pattern.

Note that with SafeHandle from .NET 2.0, it's even rarer that you need your own finalizer than it was in .NET 1.1.

---

To address your point about why there's a disposing flag in the first place: if you're running within a finalizer, other objects you refer to may already have been finalized. You should let them clean up themselves, and you should only clean up the resources you directly own.

Jonijonie

Hi Jon, just nitpicking, but the sentence "whatever holds the resource directly can deal with that", should probably be "will deal with that", (ie. "can" --> "will") to emphasize the point that it's not the outer class' job to handle it at all. – Horsemint

One more Question, Since System.Object is the base of all the objects ,which by default has a finalize method implemented , even though we have not provided a destructor , wont GC put this in the finalize queue ? or why do we say that if we dont provide a destrutor object wont be put in the finalize queue ? Because by virtue of inheritance protected members are like private members of derived class . – Alforja

Is there any legitimate reason for a subclass derived from a non-trivial parent class to have a cleanup finalizer if the parent does not? I can't think of any case where it would not be better for the derived class to encapsulate any unmanaged resources into their own class, with its own finalizer completely detached from the main one. Actually, even if all one wants is an "alarm bell" finalizer, it may be better to encapsulate that into its own class, rather than adding a finalizer to the derived class. – Salesmanship

@supercat: Yes, that's probably the case... although the "alarm bell" bit would require an extra field in each object which could otherwise be avoided. – Jonijonie

@JonSkeet: Creating a finalizable object is expensive, even if the finalizer ends up being suppressed. The cost of an extra field is pretty minor compared to that. Further, one could reduce the memory or execution-time costs of an "alarm bell" class below what would be required for per-instance finalizers, especially if one used factories to create them. A space-conscious simple alarm-bell factory, for example, could have each instance hold a reference to the factory and do an Interlocked.Decrement on a counter when disposed. May in some cases be time-inefficient due to... – Salesmanship

...cache contention, but would still be cheaper than adding a finalizer to an object. Actually, one might be able to reduce cache contention by having the factory hold an array of references to single-element arrays and randomly select one to pass to each created instance, thus dividing the contention among a plurality of resources. – Salesmanship

I'd go further than "most people don't need the full-blown dispose pattern": Nobody needs it, because having both managed and unmanaged resources that need clean-up held directly as fields is an anti-pattern, and while the "dispose pattern" is a smart way of dealing with a case where one has that, that just makes it a smart way of dealing with the fact that you've done something dumb. The only time anyone should use it is when its forced upon them by inheriting from something that does. – Tennessee

@JonHanna: Were I in charge of coding standards, I'd probably require that no object to which public references might exist should implement Finalize; instead, resources requiring finalizer cleanup should be encapsulated in privately-held objects whose purpose is simply to encapsulate those resources. The encapsulating object may also optionally hold a reference to an object holding a "long" WeakReference back to the main object (to ensure that the weak reference remains valid as long as the main object exists, a static reference should also exist to the weakreference-holding object). – Salesmanship

@JonHanna: When the finalizer runs, it would then be able to check whether the WeakReference has been invalidated yet (if the finalizable object holds no strong reference to the main object, neither the finalizer nor anything else should prevent the main object from ceasing to exist). If the WeakReference hasn't been invalidated yet, the finalizer should regard its invocation as a false alarm and re-register itself for finalization. Doing that would guard against the possibility of evil outside code using resurrection to trick an object's finalizer into running while the object is in use. – Salesmanship

Legitimate question (not sarcasm): How is this not thread safe? I'm trying to learn about properly implementing disposable pattern and I came across this answer. So I'd love to learn why. – Slide

@void.pointer: Suppose two threads call Dispose at the same time. The disposed field is false, so they both proceed, and they both call stream.Dispose(), which is presumably not ideal. – Jonijonie

I understand the threading issue in that context. I guess I'm asking: Under what normal circumstance is the dispose method called simultaneously from two threads? – Slide

Why make the argument for thread safety of it's not intentional that it is called in a threaded context? – Slide

@void.pointer: It can still be important, depending on exactly what you're doing in the Dispose method. I didn't "make the argument for thread safety" - I just made it clear that this isn't thread-safe... and explicitly noted that it's probably not important. – Jonijonie

Isn't that a tautology though? – Slide

@void.pointer: No, I don't see why. All I've done is alerted readers to something they would need to address if they needed it to be thread-safe. How is that a tautology? It really feels like you're making a lot of a single line of the answer... – Jonijonie

It's confusing to bring up thread safety issues when 1) you yourself admitted it is "probably not important" and 2) there's no example of a real world use case where multiple threads dispose the same object. There's also a possible #3 that brings up the question of: Would this level in the code even be the appropriate place to solve a threading issue? Probably not. Why are two threads disposing a single object? That's the bug; not the dispose pattern itself. Overall I just think your concern is not pragmatic, and only adds confusion to an already confusing topic. – Slide

Also: Yes it's a single line in your answer, but threading is a serious and sophisticated topic and when I personally see it mentioned, I start to wonder what the details are, how it is important, what considerations need to be made, etc. So I think it would do less harm to mention it, given what we've discussed so far. Others might see it as a red flag like I have. – Slide

@void.pointer: If it's important to your situation, then you should think about it carefully. That's precisely why it's worth mentioning that this isn't thread-safe, but that it's not important for most situations. I'm keeping it in. (Note that in over 10 years, you're the first person to mention it.) – Jonijonie

"If it's important to your situation": For someone learning about dispose pattern, how would I know? This is exactly my point. You can amend almost every answer on SO with "But if you have threads...", but most folks don't do that. First because threading wasn't directly mentioned in the question. Second because it adds exponential complexity to the answer. I stand by my opinion that you should have not mentioned it, in order to keep things less confusing and more directly in line with what the OP asked. – Slide

@void.pointer: I think we'll have to agree to disagree. I don't think we're going to achieve anything by continuing comments here. – Jonijonie

Answer 2

Here are the main facts

1) Object.Finalize is what your class overrides when it has a Finalizer. the ~TypeName() destructor method is just shorthand for 'override Finalize()' etc

2) You call GC.SuppressFinalize if you are disposing of resources in your Dispose method before finalization (i.e. when coming out of a using block etc). If you do not have a Finalizer, then you do not need to do this. If you have a Finalizer, this ensures that the object is taken off of the Finalization queue (so we dont dispose of stuff twice as the Finalizer usually calls the Dispose method as well)

3) You implement a Finalizer as a 'fail safe' mechanism. Finalizers are guaranteed to run (as long as the CLR isnt aborted), so they allow you to make sure code gets cleaned up in the event that the Dispose method was not called (maybe the programmer forgot to create the instance within a 'using' block etc.

4) Finalizers are expensive as Types that have finalizers cant be garbage collected in a Generation-0 collection (the most efficient), and are promoted to Generation-1 with a reference to them on the F-Reachable queue, so that they represent a GC root. it's not until the GC performs a Generation-1 collection that the finalizer gets called, and the resources are released - so implement finalizers only when very important - and make sure that objects that require Finalization are as small as possible - because all objects that can be reached by your finalizable object will be promoted to Generation-1 also.

Oilla

Answer 3

Keep the first version, it is safer and is the correct implementation of the dispose pattern.

1. Calling SuppressFinalize tells the GC that you have done all the destruction/disposing yourself (of resources held by your class) and that it does not need to call the destructor.

2. You need the test in case the code using your class has already called dispose and you shouldn't tell the GC to dispose again.

See this MSDN document (Dispose methods should call SuppressFinalize).

Havildar

Yes I do understand the that SupressFinalze will prevent the GC from calling Finalize . But my doubt is when I am not having a destuctor why we need to call SupressFinalze . Because Finalize will be ony called for those objets in the finalzer queue and my object dont have destructor so any way GC is not going to call . 2) My second pint is , why dispose pattens insist about an overloaded dispose funtion which takes a boolean . which will control the release of managed or unmanaged resources . when the object is going to be disposed , why we need to treat resource seperatly , lets free all. – Alforja

That rule is only relevant when you need a finalizer or you need to allow subclasses to have a finalizer. In many situations this isn't the case. – Jonijonie

@somaraj: The point is that your class may not have a finalizer, but a subclass might. – Jonijonie

@JonSkeet: Why would you want to supress a subclass' finalizer? base.Dispose() is called from the subclass Dispose() so there should already be a GC.SupressFinalize there. – Stool

@adrianm: You would want to suppress finalization if you've been disposed, on the reasonable assumption that all the clean-up required will have taken place. GC.SuppressFinalize() is normally in the parameterless Dispose method, rather than the one with the "disposing" parameter which is virtual - in the standard (long) pattern. – Jonijonie

@adrianm: Ideally, SupressFinalize() should only be called after all derived classes have finished their cleanup logic. The only thing that knows that is the non-virtual wrapper. – Salesmanship

Answer 4

You should always call SuppressFinalize() because you might have (or have in the future) a derived class that implements a Finalizer - in which case you need it.

Let's say you have a base class that doesn't have a Finalizer - and you decided not to call SuppressFinalize(). Then 3 months later you add a derived class that adds a Finalizer. It is likely that you will forget to go up to the base class and add a call to SuppressFinalize(). There is no harm in calling it if there is no finalizer.

My suggested IDisposable pattern is posted here: How to properly implement the Dispose Pattern

Underlay

Answer 5

1. Answer for the first question

Basically, you don't have to call SuppressFinalize method if your class doesn't have a finalize method (Destructor). I believe people call SupressFinalize even when there is no finalize method because of lack of knowledge.

2. Answer for the second question

Purpose of the Finalize method is to free un-managed resources. The most important thing to understand is that, Finalize method is called when the object is in the finalization queue. Garbage collector collects all the objects that can be destroy. Garbage Collector adds objects those have got finalization to the finalization queue before destroy. There is another .net background process to call the finalize method for the objects those are in the finalization queue. By the time that background process execute the finalize method, that particular object's other managed reference may have been destroyed. Because there is no specific order when it comes to the finalization execution. So, the Dispose Pattern wants to make sure that finalize method do not try to access managed objects. That's why managed objects are going in side "if (disposing)" clause which is unreachable for the finalize method.

Transferor

There is an analyzer rule which obliges you to call SuppressFinalize: learn.microsoft.com/en-us/visualstudio/code-quality/… – Astound