In trying to understand the IDisposable i have a few questions that most answers haven't explicitly stated.
If we call dispose on an object, does that dispose the object at that time or just class members that we would like to clean up, and the whole object would be destroyed later by the GC.
After we suppress the finalizer in the dispose method, will the GC still clean up all the class members we didn't clean up in the dispose?
An object will be garbage collected once all references to it are gone, in a non-deterministic way. Garbage collection
from MSDN : Garabge collection fundamentals
Garbage collection occurs when one of the following conditions is
true:
The system has low physical memory.
The memory that is used by allocated objects on the managed heap
surpasses an acceptable threshold. This means that a threshold of
acceptable memory usage has been exceeded on the managed heap. This
threshold is continuously adjusted as the process runs.
The GC.Collect method is called. In almost all cases, you do not have
to call this method, because the garbage collector runs continuously.
This method is primarily used for unique situations and testing.
Garabge collection when to use
Understanding Object life cycle
First things first, there is a ton of duplicate questions here on this topic. If truly none other answer explicitly answers your question, here they are, but I expect this to be closed as a duplicate.
First, Dispose is just a method, an ordinary method. It does not deal with Garbage Collection at all.
So what does Dispose do? It should be used to clean up unmanaged resources, such as file handles, window handles, etc. Things that the .NET garbage collector does not know about.
So to answer question 1: "Yes", when you call Dispose on an object, it is disposed there and then. There is no mark set on the class to indicate cleanup later on.
Since Dispose does not deal with Garbage Collection, you can easily keep a reference to the object after disposing it, and it will not be garbage collected. Garbage collection only occurs when there are no more references to the object. This can happen even if you have not called Dispose on the object, but it will not happen because you call Dispose on the object.
Second question: When GC collects the object after it has been through the finalizer cycle, then GC will clean up the object + any object it refers to that no longer has any references left (besided the one from your object). This happens at some point, and not necessarily all at once either.
Suppressing the finalizer is just making sure GC does not do more work than necessary, by saying that "when you find this object and determine that it is eligible for collection, then just go ahead and collect it, I have taken care of the 'finalization' cleanup so you don't have to".
Firstly and most important you need to understand that disposing an object and garbage collecting an object are completely different things - make sure that you don't confuse the two!
If we call dispose on an object the only thing that happens is that the Dispose method gets called. It might be that the Dispose method then goes on to call GC.SuppressFinalize, but then again it might not do anything at all - its completely up to the implementation.
When we call GC.SuppressFinalize the only thing that happens is that it requests that the GC not call the objects finalizer when the object is collected (almost as if the object didn't have a finalizer in the first place).
As it happens a common pattern is for objects which have a finalizer to also implement IDisposable so that unmanaged resources can be cleaned up deterministicly - if this happens then there is no need for the GC to call the finalizer as the work that would have been done in the finalizer has already been performed when we called Dispose - calling GC.SuppressFinalize is just a friendly hint to the GC that it doesn't really need to call the finalizer after all.
1) When you call dispose on an object, all that happens is that the code in its Dispose() method is run. This code may of course call Dispose() for one or more of its fields, but it's not required to.
It may also call the base class' Dispose(true), if any (via base.Dispose()), but that is using the Dispose Idiom; Dispose(bool) is NOT part of the IDisposable interface.
2) The finalizer is suppressed only for the specific object for which SuppressFinalize() was called. It won't affect any other objects (including any held by fields of the object for which SuppressFinalize() was called).
Related
I need to optimize my application in memory usage. so I used .net performance profiler...
but some references in my application are still alive and are never collected by GC even if I force it to collect.
The reference that is alive is a "finalization handle" type.
I don't know what to do to remove this kind of reference.... please help.
This is not a memory leak, just sloppy coding on the part of the author(s) of AMProLibrary.
As you observed, the profiler is telling you that the referenced object is of type "Finalization Handle". What that means is that it is coming from the finalizer queue. The finalizer queue is what the .NET garbage collector uses to hold all objects that implement a finalizer method. The finalizer is the mechanism used to ensure that unmanaged resources are properly released during garbage collection. An object that contains unmanaged resources implements the IDisposable pattern, including the Finalize method, which is where the unmanaged resources are released. When the garbage collector processes "finalizable" objects (as indicated by the value of a bit in the object's header), it moves them into the finalizer queue. During collection, the GC iterates through the finalizer queue and calls the Finalize method on each of those objects.
What the author of the library evidently failed to do is to call GC.SuppressFinalize() from within the Dispose method. This normally removes the object from the finalizer queue by clearing the "finalizable" bit in the object's header, and indicates that the Finalize method does not need to be called.
For testing purposes, you can force the finalizers to run by calling the GC.WaitForPendingFinalizers function. For example:
System.GC.Collect();
System.GC.WaitForPendingFinalizers();
System.GC.Collect();
However, you should not actually use code like this in a production application. Forcing a collection rarely makes sense. This will just prove the validity of the hypothesis stated above.
In general, you should not rely on the finalizer to release unmanaged resources. All objects that implement IDisposable should be explicitly disposed of by your code, either by manually calling the Dispose method or, preferably, by wrapping their creation in a using block that will automatically call Dispose when the block's scope is exited.
-----------Edit1----------
Sorry for my poor english, seems the original question can't make sense for many people, so I greatly simplified the question just to two sentence:
In msdn code sample of IDisposable pattern in MSDN, Putting releasing un-managed resource code in destructor was for the purpose of when user forgot to call the Dispose() to release the resource, the GC still can help to release at least un-managed resource, but, when a class used some un-managed resources, it must registered (implicitly or explicitly) current instance (or fields) to GC handle table and then would never be GCed, so how the code in destructor get executed in this situation?
-----------End Edit1-------
We know the famous IDisposable pattern which could see lots in web like: IDisposable pattern in MSDN
In samples, as I understood, the purpose of destructor works for the case: If user forgot explicitly call Dispose(), then GC will help and release un-managed resources.
But I just wondering that destructor in any case could get called by GC without user code explicitly call Dispose() first?
The guess was based on reason of Implementing IDisposable and had an explicit destructor:
This class used some un-managed resources. No matter by P/Invoke or managed API (BCL), As I know, both way need add a GC handle entry to GC handle table which avoid GCed, thus the destructor would never be called by GC.
So we should never expect luck on resources automatically release? and why put that in sample code?
There are a lot of questions here. Let's try to answer all of them.
In the MSDN code sample of the IDisposable pattern code to release unmanaged resources is put in the destructor. This ensure that when a user forgots to call Dispose() to release the resource, the GC still releases the unmanaged resources.
Correct.
But when a class uses some unmanaged resources, it must have registered the current instance to GC handle table and then would never be GCed.
I don't understand this sentence. What is the "GC handle table" that you speak of?
Can you show some sample code that demonstrates your scenario?
How does the code in destructor get executed in this situation?
Briefly the GC works like this. The GC knows the "roots"; these are the references that are definitely alive. The GC first marks all the objects as dead. Then it marks the roots as alive. Then it marks everything the roots refer to as alive, and then every that they refer to as alive, and so on. That is everything reachable from a root is alive. Then the GC kills everything that is still marked as dead.
Now, if a dead object is finalizable -- if it has a destructor that has not run yet -- then the GC does NOT kill the object. The object is put on a list of objects to be finalized, and this list is a root, so the object is alive again. Later on, another thread runs the destructors of every object on the finalization queue and removes them from the queue, and marks them as no longer requiring finalization.
The next time the GC runs, if the object is still dead then it is collected and the finalizer is not run again.
Can the destructor get called by the GC without user code explicitly call Dispose() first?
Yes. That's the whole point of the destructor.
GC uses a mechanism called finalization to handle such classes that implement IDisposable.
If you properly implement the class, then when Dispose was not called, GC will take care of the object when it is out of scope, and the finalizer will be called by GC. The finalizer is called implicitly, and at a unpredictable time.
More technical details can be found from books such as CLR via C#, or articles such as http://msdn.microsoft.com/en-us/magazine/cc163392.aspx
I'm reading "The C# Language", 4th Edition, it talks about garbage collection as below:
"BILL WAGNER: The following rule is an important difference between C# and other managed environments.
Prior to an application’s termination, destructor's for all of its objects that have not yet been garbage collected are called, unless such cleanup has been suppressed (by a call to the library method GC.SuppressFinalize, for example)."
So I have a couple of questions here:
Q1. Why .net is different from other managed environments (I suppose this is hinting Java?) here? Any particular design concern?
Q2. What will happened to objects that GC.SuppressFinalize is called?
I understand that this means GC will not call such objects' finalizer (destructor), if so, when will these objects got really destroyed, so that the allocated memory bits are returned to the heap? Otherwise there'll be Memory Leak?
What will happened to objects that GC.SuppressFinalize is called? I understand that this means GC will not call such objects' finalizer (destructor), if so, when will these objects got really destructed? otherwise there'll be Memory Leak right?
You have a misunderstanding of what finalization is for. Finalization is for cleaning up resources that are not managed memory.
Suppose you have an object of reference type that contains an integer field. That integer field just happens to be a handle to a file that was obtained by calling into unmanaged code to open the file.
Since some other program might want to access that file, it is polite to close the file as soon as possible. But the .NET runtime has no idea that this integer has any special meaning to the operating system. It's just an integer.
The way you solve this problem typically is you mark the object as implementing IDisposable, and then you call "Dispose" on the object as soon as you're done with it. Your implementation of "Dispose" then closes the file.
Note that there is nothing special going on here. It is just a convention that a method that cleans up an unmanaged resource is called "Dispose" and an object that needs to be disposed implements IDisposable. The garbage collection knows absolutely nothing about this.
So now the problem arises: what if someone forgets to call Dispose? Does the file stay open forever? (Clearly the file will be closed when the process ends, but what if the process runs for a long time?)
To solve this problem, you use a finalizer. How does that work?
When an object is about to be garbage collected, the garbage collector checks it to see if it has a finalizer. If it does, then instead of garbage collecting it, it puts it on the finalizer queue. At some unspecified point in the future, a thread runs that examines the queue and calls a special "Finalize" method on every object. After that, the object is removed from the finalization queue and marked as "hey, I've already been finalized". The object is now once again eligable for collection, and so the garbage collector eventually runs and collects the object without putting it on the finalization queue.
Clearly "Finalize" and "Dispose" frequently need to do the same thing.
But now another problem arises. Suppose you dispose an object. Now it does not need to be finalized. Finalization is expensive; it keeps a dead object alive for much longer than it needs to be. Therefore, traditionally when one disposes an object, the implementation of Dispose not only closes the unmanaged resource, it also marks the object as "this object has already been finalized, don't finalize it again". That way it tricks the garbage collector into not putting the object on the finalization queue.
So let's answer your specific questions:
What will happened to objects that GC.SuppressFinalize is called?
When the object is dead the garbage collector will simply reclaim the memory of the object without putting the object on the finalizer queue.
I understand that this means GC will not call such objects' finalizer
The GC never calls a finalizer. The finalizer thread is the only thing that calls finalizers.
when will these objects got really destructed?
It is not clear what you mean by "destructed". If you mean "when will the finalizers run?" the answer is "never" because you said to suppress finalization. If you mean "when will the memory in the managed heap be reclaimed?", the answer is "as soon as the object is identified as dead by the garbage collector". That will happen sooner than normal because the object will not be kept alive by the finalizer queue.
Q1: I suspect it's because it cares more about achieving great performance, as opposed to Java which has made quite a few sacrifices for simplicity.
Q2: Since finalizers aren't even guaranteed to be called in the first place (even if SuppressFinalize didn't exist), this should be only used for performance reasons, when you've disposed the resources already. Otherwise you should use IDisposable to dispose of resources.
Finalization != Destruction
Destructors (in the C++ sense) don't exist in .NET -- because every object, in a sense, has a "destructor", called the garbage collector. :)
What C# calls "destructors" are in fact finalizers.
Finalizers are for disposing things other than the memory allocated by the object, such as file handles, etc. Not every object, therefore, had a finalizer. The memory is always freed by the GC, so you don't get a memory leak that way.
I only know the second answer: SuppressFinalize is called when the object has already been destructed, i.e., by IDisposable.Dispose. So it shouldn't be destructed again.
This is because finalizers are a resource cleanup that occurs at a non-deterministic time. IDisposable was added to allow resource cleanup that occurs at a specific, predictable time.
EDIT: At process termination, memory leaks are immaterial. When a process terminates, its heaps are collected by Windows, so it doesn't matter if an object's memory is returned to the heap or not.
I try to answer Q2:
If your class has a finalizer (shown by the ~ClassName), then it will not be directly collected by the garbage collection, instead it will be put on the finalizer queue which will be called later. Now when finalizer is finally called it usually frees any unmanaged resources, the class created. If for any reason the user already did it, usually by calling dispose, the finalizer don't need to clean the unmanaged memory, thus calling SuppressFinalizer is necessary. Otherwise it would try to free the resources again.
So the only memory leak you will get is by any resources that you requested on creation of your class. The finalizer is just to free everything that is not already managed by the framework.
There is because once call GC.Collect()
The object with finalize will move to next generation at least once.
We can call GC.SuppressFinalize, and GC will know this object is de-reference, we can remove this object and compact heap then.
Usually, this implement with dispose pattern
This is pretty much only for me to make sure, I got this right:
We have a large resource class implementing the IDisposal pattern. It should (by design) be implemented in a way, that enables it to get called more than one time (even if we try to call it exactly one time of course). We also implement a finalizer, which also calls the Dispose() method - just as backup. If called manually, Dispose() will also call GC.SuppressFinalize(this).
There are several examples of disposal patterns around. Most of them call GC.SuppressFinalize(this) at the end of the disposing code. Some claim, it would be better, to call it at the beginning of the Dispose() method, before any cleaning. Latter argue, this would make sure, the GC doesn't call the finalizer concurrently, while we are still cleaning up.
Question:
It seems, placing GC.SuppressFinalize at the beginning doesn't do any better? We still have a race condition, right? So is it true, that we rather should implement Dispose() in a thread safe way instead?
The GC only cleans up objects that are not 'reachable'.
A class in which code is executing is still 'reachable' because it's this pointer is on the stack. So while dispose is executing, the finalizer won't be called.
So it does not matter if you call SuppressFinalize at the begin or end.
As commentors below indicated, the CLR implementation does not appear to guarantee that your object does not get garbage collected/finalized while instance methods are executing. The only possible 'dependable' reference keeping the object alive is the one used to invoke the method on the object, but I don't know enough about the JIT internals to make statements about that, and it's behaviour might change.
I'm leaving the answer here for access to the discussion below.
While it is sometimes possible for an object to be finalized while a seemingly-live reference exists, that can only happen when nothing else is going to ever refer to the object. The GC.SuppressFinalize(this) actively refers to the present object 'this', thus guaranteeing that it will not be finalized until the GC.SuppressFinalize executes. Further, the fact that the object reference existed to dispose the object, and was available to the Dispose method, guarantees that the finalizer couldn't have been queued before Dispose started running unless the object was dead and a finalizer somewhere (either its own finalizer, or that of some other object) resurrected it.
Because there are some scenarios where an object could be scheduled for finalization and resurrected without ever being aware of it, it may not be a bad idea to protect a dispose and finalize against redundant operation. Microsoft's pattern is not a good one, however. Finalizable objects shouldn't hold references to any objects not needed for finalization. If an object would hold a mixture of managed and unmanaged resources, the unmanaged resources should be moved into their own classes (effectively turning them into managed resources), so then the main object would hold nothing but managed resources.
Dispose should not throw any exceptions.
I would make sure all the code in Dispose is thread safe so that if it gets called it will not do anything strange. Usually adding checks if variables are null already should do the trick.
In the microsoft examples i've only seen GC.SuppressFinalize at the end of the Dispose function.
Three questions:
What kind of variables should be disposed manually in .NET/Java? I know that SqlConnection should always be either disposed manually or used in a using{} block. Is it right? What are the other kind of variables that should be disposed?
I read somewhere that unmanaged code must be disposed manually. Is that right? What exactly is unmanaged code and how do I know if a variable is managed or unmanaged?
Finally, how do I dispose variables? I know that the Dispose() method does not really dispose a variable. So what does Dispose() do? Should I set them to null()? What is the logic by which the garbage collector works?
This answer deals only with the .NET part of your question
What kind of variables should be disposed manually in .NET/Java? I know
that SqlConnection should always be
either disposed manually or used in a
using{} block. Is it right? What are
the other kind of variables that
should be disposed?
In .NET, all objects that implement IDisposable should be disposed explicitly (or used in a using block).
I read somewhere that unmanaged code must be disposed manually. Is
that right? What exactly is unmanaged
code and how do I know if a variable
is managed or unmanaged?
You probably mean unmanaged resources, as code can't be disposed... All classes that use unmanaged resources (memory allocated not on the managed heap, win32 handles...) should implement IDisposable, and should be disposed explictly, since they are not managed by the garbage collector.
Finally, how do I dispose variables? I know that the Dispose()
method does not really dispose a
variable. So what does Dispose() do?
Should I set them to null()? What is
the logic by which the garbage
collector works?
I'm not sure I understand your question... you don't dispose a variable, it is managed by the garbage collector. All managed memory is automatically released when it's not used anymore (i.e. it is not accessible by code because there are no references left to it). The IDisposable.Dispose method is only intended for resources that are not managed by the GC.
EDIT: as a side note, I would like to add that IDisposable is primarily intended to clean-up unmanaged resources, but is also frequently used to perform other cleanup actions and guarantee state or data integrity. For instance, IDbTransaction implements IDisposable in order to rollback the transaction if an exception occurs before the transaction was committed :
using (var trx = connection.BeginTransaction())
{
// Do some work in the transaction
...
// Commit
trx.Commit();
} // the transaction is implicitly rolled back when Dispose is called
In Java, you "close" rather than "dispose".
JDBC Connections, unless you got them from a pool.
JDBC ResultSets, depending on the JDBC connector.
InputStreams, OutputStreams, Readers and Writers (with the exception of those that are byte array and string backed).
Some third-party Java libraries or frameworks have classes that need to be manually disposed / closed / destroyed.
.NET
All objects that implement the IDisposable interface are asking that their Dispose methods are called when they are no longer required. The using block is just C# sugar for a try-finally block that disposes the IDiposable in the finally block, as long as it is not null. Consequently, disposal occurs even if an exception is thrown in the try block.
Unmanaged Code.
The Dispose method does whatever code the developer who wrote the Dispose method put in it! Typically this involves releasing 'unmanaged' resources, which are not managed by the runtime, such as database connections, window handles and file handles. Note that the GC manages the heap, so setting a reference to null just before the object it references becomes unreachable doesn't help all that much.
This is a good read.
This was mostly covered by Thomas, but to expand on the third point:
Finally, how do I dispose variables? I know that the Dispose()
method does not really dispose a
variable. So what does Dispose() do?
Should I set them to null()? What is
the logic by which the garbage
collector works?
They key is that the Dispose() tells the object to free any unmanaged resources that it currently holds, it does not free the object itself.
The garbage collector knows how to free the object, but for IDisposable objects, only the object itself knows how to dispose of it's private resources, so you have to make sure that Dispose() gets called before the garbage collector frees the object.
For Java:
SWT is a framework where we have to dispose resources (like images), because the framework uses native libs and system handles which have to released. SWT classes are documented and tell, when dispose is required.
Following up on other .NET solutions...
If your object owns unmanaged resources, it's not enough to simply clean them up on Dispose() calls, because you have no guarantee that Dispose will get called. The full solution is:
Add a finalizer for the class.
Add a public bool property called Disposed, which indicates that the object has been disposed.
Add a protected Dispose(bool) method. The public Dispose() method has no parameters.
When Dispose() is called, if the Disposed property is false, it will call Dispose(true), and call GC.SuppressFinalize() to disable the finalizer. Ths is essential to keep .NET's garbage collector happy. Classes with unspressed finalizers go to the end of the line for garbage collector cleanup, so they become almost a memory leak.
The public Dispose() method should then set the Disposed property to true.
The finalizer calls Dispose(false).
The Dispose(bool) will always clean up its own managed resources, but it will clean up unmanaged resources only when called as Dispose(true). Note that all resource cleanup is in the protected Dispose(bool) method, not in the public Dispose() method.
If the class is not sealed, then the protected Dispose(bool) method should be a virtual method. Subclasses that need to do cleanup can override Dispose(bool) and followht e same logic as above. In addition, they should call base.Dispose(bool) with the parameter they were given.
If .net, if you can use an object in a "using" statement, that means the object implements iDisposable and you should when practical call it's disposal method when you're done with it. Note that the disposal method may not always be called "Dispose", but you can always call it by casting the object to iDisposable and then calling Dispose on that.
Note that some types of "iDisposable" objects don't handle any unmanaged resources, but do subscribe to events. If such an object isn't properly disposed, it might not get garbage-collected unless or until all of the objects for which it holds subscribed events are themselves garbage-collected. In some cases, that may never happen until an application exits.
For example, the enumerator returned by an iEnumerable collection might subscribe to that object's "collection changed" event, unsubscribing when its dispose method is called. If the dispose method is never called, the enumerator would remain around as long as the collection did. If the collection itself stayed around a long time and was enumerated frequently, this could create a huge memory leak.
I'm in agreement with the above, I'll just add to the subsequent question about setting a variable to null.
You don't need to do this with variables used in a method (they will fall out of scope so it's only if they have state within them that must be signalled as needing clean-up, through IDisposable.Disopse that you have to worry abou this).
It's rarely useful for instance or static members, as memory isn't as precious a resource as people often think (it is a precious resource, but most attempts to deal with it in a few lines of code is like turning off your taps while you've a burst main.
It is though useful if you have a static or instance member of a class and (A) it is a large object (B) the "owning" object is likely to stay in memory for a long time and (C) you know you won't need that value gain, to set that member to null.
In practice this isn't a very common combination. If in doubt, just leave alone.
Now, do read the other answers here, as what they say about the Dispose() method is much more important.