In .NET, under which circumstances should I use GC.SuppressFinalize()?
What advantage(s) does using this method give me?
SuppressFinalize should only be called by a class that has a finalizer. It's informing the Garbage Collector (GC) that this object was cleaned up fully.
The recommended IDisposable pattern when you have a finalizer is:
public class MyClass : IDisposable
{
private bool disposed = false;
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
{
// called via myClass.Dispose().
// OK to use any private object references
}
// Release unmanaged resources.
// Set large fields to null.
disposed = true;
}
}
public void Dispose() // Implement IDisposable
{
Dispose(true);
GC.SuppressFinalize(this);
}
~MyClass() // the finalizer
{
Dispose(false);
}
}
Normally, the CLR keeps tabs on objects with a finalizer when they are created (making them more expensive to create). SuppressFinalize tells the GC that the object was cleaned up properly and doesn't need to go onto the finalizer queue. It looks like a C++ destructor, but doesn't act anything like one.
The SuppressFinalize optimization is not trivial, as your objects can live a long time waiting on the finalizer queue. Don't be tempted to call SuppressFinalize on other objects mind you. That's a serious defect waiting to happen.
Design guidelines inform us that a finalizer isn't necessary if your object implements IDisposable, but if you have a finalizer you should implement IDisposable to allow deterministic cleanup of your class.
Most of the time you should be able to get away with IDisposable to clean up resources. You should only need a finalizer when your object holds onto unmanaged resources and you need to guarantee those resources are cleaned up.
Note: Sometimes coders will add a finalizer to debug builds of their own IDisposable classes in order to test that code has disposed their IDisposable object properly.
public void Dispose() // Implement IDisposable
{
Dispose(true);
#if DEBUG
GC.SuppressFinalize(this);
#endif
}
#if DEBUG
~MyClass() // the finalizer
{
Dispose(false);
}
#endif
SupressFinalize tells the system that whatever work would have been done in the finalizer has already been done, so the finalizer doesn't need to be called. From the .NET docs:
Objects that implement the IDisposable
interface can call this method from
the IDisposable.Dispose method to
prevent the garbage collector from
calling Object.Finalize on an
object that does not require it.
In general, most any Dispose() method should be able to call GC.SupressFinalize(), because it should clean up everything that would be cleaned up in the finalizer.
SupressFinalize is just something that provides an optimization that allows the system to not bother queuing the object to the finalizer thread. A properly written Dispose()/finalizer should work properly with or without a call to GC.SupressFinalize().
Dispose(true);
GC.SuppressFinalize(this);
If object has finalizer, .net put a reference in finalization queue.
Since we have call Dispose(true), it clear object, so we don't need finalization queue to do this job.
So call GC.SuppressFinalize(this) remove reference in finalization queue.
If a class, or anything derived from it, might hold the last live reference to an object with a finalizer, then either GC.SuppressFinalize(this) or GC.KeepAlive(this) should be called on the object after any operation that might be adversely affected by that finalizer, thus ensuring that the finalizer won't run until after that operation is complete.
The cost of GC.KeepAlive() and GC.SuppressFinalize(this) are essentially the same in any class that doesn't have a finalizer, and classes that do have finalizers should generally call GC.SuppressFinalize(this), so using the latter function as the last step of Dispose() may not always be necessary, but it won't be wrong.
That method must be called on the Dispose method of objects that implements the IDisposable, in this way the GC wouldn't call the finalizer another time if someones calls the Dispose method.
See: GC.SuppressFinalize(Object) Method - Microsoft Docs
Related
Can't seem to find consistent answers on the Internet. It looks like CLR GC makes a "decision" on whether to call destructors during garbage collection. This would imply that sometimes destructors won't be called. The way our legacy code base is structured, "using" blocks can't always be used, and the object instances do have certain resources that need to be released after object is no longer needed. I need a guaranteed way for GC to call a destructor. Would implementing IDisposable guarantee .Dispose being called on garbage collection? What is the best practice here?
I tried using ~ClassName(){ } and destructor doesn't always get called.
Destructor
The preferable term is finalizer AFAIK.
It looks like CLR GC makes a "decision" on whether to call destructors during garbage collection
AFAIK GC does not call finalizers during collection, they are called after GC marks objects ready for finalization (this also results in extending object lifespan which can have performance implications) and places them into finalization queue. In "normal" program flow finalizers should eventually be run but there are some corner cases which can prevent that.
I need a guaranteed way for GC to call a destructor.
There is no such way. Finalizers are non-deterministic and should be used as "last resort" measure.
Would implementing IDisposable guarantee .Dispose being called on garbage collection?
No, IDisposable has nothing to do with GC, IDisposable will be called only "manually" (i.e. either directly or via using), that's it.
What is the best practice here?
Implement IDisposable and invoke it (manually or via using).
Useful resources:
Already referenced docs
Possibly a bit dated but wonderful When everything you know is wrong, part one and part 2 by Eric Lippert
What is the difference between using IDisposable vs a destructor in C#?
GC.WaitForPendingFinalizers
The finalizer (destructor) is unreliable.
Ideally you should implement the IDisposable interface and use using blocks whenever possible. This article from microsoft has info about this
For your case a combination of IDisposable and finalizers may be the best case. There is a pattern from microsoft that you can also generate automatically in Visual Studio.
From microsoft:
using System;
class BaseClassWithFinalizer : IDisposable
{
// To detect redundant calls
private bool _disposedValue;
~BaseClassWithFinalizer() => Dispose(false);
// Public implementation of Dispose pattern callable by consumers.
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
// Protected implementation of Dispose pattern.
protected virtual void Dispose(bool disposing)
{
if (!_disposedValue)
{
if (disposing)
{
// TODO: dispose managed state (managed objects)
}
// TODO: free unmanaged resources (unmanaged objects) and override finalizer
// TODO: set large fields to null
_disposedValue = true;
}
}
}
If Finalizer (destructor) is implemented in a class and GC.SupressFinalizer() is called from the overriden Dispose() method will the Garbage Collector still take care of any managed resources the instances of that class might have?
What confuses me is Microsofts documentation on this matter. For example the implemenation of IDisposable pattern in the virtual Dispose method takes care of the managed resources as well as the unmanaged. Why is that being done if the GC is taking care of the managed resources by default?
If I define a class like this:
public class Car
{
string plateNum;
}
and use this type as a filed in a class that also deals with unmanaged resources, according to the Microsofts documentation, the proper way to handle the disposal of the objects would be to call Dispose on the Car as well. For one to do so Car has to implement the IDisposable interface. Car is only dealing with the managed resources, there is no reason for doing so in the Car class, I have no idea what Dispose() would even do there, maybe give null to the plateNum? Also why would anyone want to implement IDisposable on the class that deals with the managed resources only?
Having that in mind, why is there a section in the code in the virtual Dispose() method (in the example in the MS documentation) in which managed resources are disposed?
protected virtual void Dispose(bool disposing)
{
// Check to see if Dispose has already been called.
if(!this.disposed)
{
// If disposing equals true, dispose all managed
// and unmanaged resources.
if(disposing)
{
// Dispose managed resources.
component.Dispose();
}
// Call the appropriate methods to clean up
// unmanaged resources here.
// If disposing is false,
// only the following code is executed.
CloseHandle(handle);
handle = IntPtr.Zero;
// Note disposing has been done.
disposed = true;
}
}
The only reason that I can think of is that GC.SupressFinalize(Object) is telling to the GC that it doesn't need to take care of anything related to the object argument that is given. But this shouldn't be the case because implementing the finalizer should only mean that the object that implements it should be put on the Finalizer queue only after the object has been dealt with by the GC because the Finalizer method, that is explicitly implemented by the user, should be called.
Also if a finalizer is defined does that change the way the GC collects the managed resources that the instance contains or does it just mean that additional code contained in the finalizer will be executed?
There are many Q+A about this already on SO so I'll give you a very practical answer: You won't ever need to write a finalizer.
Summary: In the rare case that you have an unmanaged resource, use the SafeHandle class to make it a managed resource.
When you inspect the full pattern carefully you can see that without unmanaged resources the destructor (aka the Finalizer, ~MyClass() {}) code path does exactly nothing.
And actually having that Finalizer without SuppressFinalize() is very expensive, it will delay the cleanup of your object to the next GC. Promoting its data to gen 1.
The remaining reason for the full pattern with virtual void Dispose(bool) is inheritance. A resource holding class should almost never need that. So make it sealed and all you need (want) is:
public sealed MyClass : IDisposable
{
private SomeResource _myResource;
public void Dispose()
{
_myResource?.Dispose();
}
}
And when you need inheritance then this is the official pattern.
We all know the System.IDisposable pattern. It's been described a zillion time, also here on StackOverflow:
link: Dispose() for cleaning up managed resources?
The Disposable patterns advises that I should only dispose managed resources if my object is being disposed, not during finalize
You can see that this happens because the following code is advised:
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
}
I know that my class should implement System.IDisposable whenever my class has a (private) member that implements System.IDisposable. The Dispose(bool) should call the Dispose() of the private member if the boolean disposing is true.
Why would it be a problem if the Dispose would be called during a finalize? So why would the following Dispose be a problem if it is called during finalize?
protected void Dispose(bool disposing)
{
if (myDisposableObject != null)
{
myDisposableObject.Dispose();
myDisposableObject = null;
}
}
When an object's finalizer runs, one of the following will be true of almost any IDisposable object it might hold:
It held the only reference to that other object, and that other object's finalizer has already run, so there's no need to do anything with it.
It held the only reference to that other object, and that other object's finalizer is scheduled to run, even though it hasn't yet, and there's no need to do anything with it.
Other objects are still using the IDisposable object, in which case the finalizer must not call Dispose.
The other object's Dispose method cannot be safely run from a finalizer threading context (or, more generally, any threading context other than the one where the object was created), in which case the finalizer must not call Dispose.
Even in the cases where none of the above would apply, code which knows that will probably know a lot about its cleanup requirements beyond the fact that it implements IDisposable, and should often use a more detailed cleanup protocol.
Generally speaking, you should get rid of resources ASAP. If you don't need a resource why would you keep it around to waste memory?
Also, in order for Dispose() to be called during finalization, you would have to create a finalizer for your object, that is - a destructor in C#. However, the exact time of invoking an object's finalizer is non-deterministic, meaning that your managed objects might not be around / accessible safely anymore at that moment. Even the thread that executes your finalizer is non-deterministic which could also result in problems that are difficult to foresee.
For these reasons finalizers should be created to release unmanaged resources only.
Very few programmers understand 'fully' how finalization works. For instance, it is the garbage collector that recognizes that your type has a finalizer during object instantiation and puts your instance in a special internal data structure called finalizequeue. Indeed when you debug your application with sos.dll (windbg) you can use the command !finalizeQueue to display the objects that have finalizers and are yet to be finalized at some later point in time.
I have a COM object implemented in C# (see below). It owns some IDisposable resource. Where should I dispose that resource?
Looks like finalizer is never called and I cannot overload IUnknown::Release method.
[ComVisible(true)]
[Guid("1992EC2F-087A-4264-B5B2-5E2E757F1A75")]
public class ComServer
{
IDisposable disposableResource; //where to dispose IDisposable resource?
public ComServer()
{
disposableResource = File.Open(#"c:\somefile.txt", FileMode.OpenOrCreate);
Console.WriteLine("ComServer.ComServer");
}
~ComServer() //finalizer is never called!
{
disposableResource.Dispose();
Console.WriteLine("ComServer.~ComServer");
}
}
Edited: the COM server is used in a native third party application, it's impossible to call Dispose on client side or make any changes there.
There is no guarantee when of if the finalize will be called. Your ComServer should implement the IDiposable interface itself and release it's disposable members within it.
From MSDN (if you override Object.Finalize)
If you are defining a base class that uses unmanaged resources and
that either has, or is likely to have, subclasses that should be
disposed, you should implement the IDisposable.Dispose method and
provide a second overload of Dispose, as discussed in the next
section.
class BaseClass : IDisposable
{
// Flag: Has Dispose already been called?
bool disposed = false;
// Public implementation of Dispose pattern callable by consumers.
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
// Protected implementation of Dispose pattern.
protected virtual void Dispose(bool disposing)
{
if (disposed)
return;
if (disposing) {
// Free any other managed objects here.
//
}
// Free any unmanaged objects here.
//
disposed = true;
}
~BaseClass()
{
Dispose(false);
}
}
Then call Dispose explicitly if you need it disposed or use using where ever possible.
using(ComServer com = new ComServer()) {
// operate on the ComServer
} // Dispose will be called automatically when the leaving the using block
Finalizer will never be called if your object is not marked for garbage collection. Go and look for the reason.
As to Dispose, only you can decide when to dispose your objects. It depends on the architecture, lifecycle of your objects. Generally speaking, you should call Dispose() when the object is no longer needed.
The best thing you can do is to implement IDisposable on your ComServer and invoke Dispose where your objects are no longer needed, or you can just wrap them in using constructions.
Tseng provided an example of standard implementation of IDisposable pattern, though consider that a good program does not need finalizers. Generally, it's a bad practice to leave your objects with intention to rely upon the mechanism of finalizing.
If you want to test your finalizer, than you can create your object, do not dispose it, call GC.Collect(). This call will force the garbage collection and if your object is eligible for garbage collection your finalizer will be invoked. But just remember never to use GC.Collect() as part of your production code.
I have a question about how to use Dispose() and destructors. Reading some articles and the MSDN documentation, this seems to be the recommended way of implementing Dispose() and destructors.
But I have two questions about this implementation, that you can read below:
class Testing : IDisposable
{
bool _disposed = false;
protected virtual void Dispose(bool disposing)
{
if (!_disposed) // only dispose once!
{
if (disposing)
{
// Not in destructor, OK to reference other objects
}
// perform cleanup for this object
}
_disposed = true;
}
public void Dispose()
{
Dispose(true);
// tell the GC not to finalize
GC.SuppressFinalize(this);
}
~Testing()
{
Dispose(false);
}
}
GC.SupressFinalize(this) on Dispose()
When the programmer uses using or calls Dispose() explicity, our class is calling to GC.SupressFinalize(this). My question here is:
What this exactly means? Will the object be collected but without calling the destructor?. I guess that the anwswer is yes since destructors are converted by the framework to a Finalize() call, but I'm not sure.
Finalizing without a Dispose() call
Suppose that the GC is going to clean our object but the programmer did not call Dispose()
Why don't we dispose resource at this point? In other words, why can't we free resources on destructor?
What code must be executed in the if inside, and what outside?
if (!_disposed) // only dispose once!
{
if (disposing)
{
//What should I do here and why?
}
// And what here and why?
}
Thanks in advance
1. What does SuppressFinalize do?
It unregisters the object from the finalizer list, ie when the GC later collects the object it will ignore the presence of the destructor. This is a big gain in performance since the destructor would otherwise require the object's collection, and that of everything it references, to be delayed.
2. Why don't we dispose [managed] resource at this point? In other words, why can't we free [managed] resources on destructor?
You could, but it is certain to be pointless: the object you're in has become unreachable so all those owned managed resources are unreachable too. They will be Finalized and collected by the GC in the same run and calling Dispose() on them is unnecessary but not totally without risk or cost.
2a What code must be executed in the if inside, and what outside?
Inside the if(disposing), call _myField.Dispose()
In other words, Dispose of managed resources (objects with a Dispose)
Outside, call code to cleanup (close) unmanaged resources, like Win32API.Close(_myHandle).
Note that when you don't have unmanaged resources, as will usually be the case (look up SafeHandle), you don't need a destructor and therefore no SuppressFinalize.
And that makes that the complete (official) implementation of this pattern is only needed because of the possibility that Test is inherited from.
Note that the Dispose(bool) is protected. When you declare your class Testing to be sealed, it is totally safe and conforming to omit the ~Testing().
First part:
When GC.SupressFinalize(this) is called, GC is informed that the object has already freed its resources and it can be garbage-collected as any other object. And yes, finalization and "destructors" are the same thing in .NET.
Second part:
Finalization is done by a separate thread and we have no control on time and order of finalization, so we don't know whether any other objects are still available or are already finalized. For this reason, you can't reference other object outside the disposing block.
The vast majority of the time when an object which owns IDisposable resources is finalized, at least one of the following statements will apply to each of those resources:
It has already been finalized, in which case no cleanup is necessary.
Its finalizer hasn't yet run, but is scheduled to do so, in which case no cleanup is necessary.
It can only be cleaned up within a particular thread (which isn't the finalizer thread), in which case the finalizer thread must not try to clean it up.
It may still be in use by someone else, in which case the finalizer thread must not try to clean it up.
In some of the rare circumstances where none of the above apply, cleanup within the finalizer might be appropriate, but one shouldn't even consider it unless one has first examined the above four possibilities.