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
Related
This is a follow up question to this question:
Finalize/Dispose pattern in C#
So I understand that if I'm creating a class that uses unmanaged resources, I should dispose them. The answer in the linked question says that the finalizer disposes of the unmanaged resources. However, the Dispose(Boolean) method is also disposing of unmanaged resources:
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
// get rid of managed resources
}
// get rid of unmanaged resources
}
So what is the difference between the disposal of the finalizer and the disposal of the dispose method?
The only reason you would use it (and its extremely controversial).
A finalizer allows the clearing an object before it will be deleted by a garbage collector. (That's to say, The GC is responsible for calling it, and clearing the object from memory) If the developer forgot to call Dispose() method of an object, then it will be possible to free the unmanaged resources and thus, avoid the leak.
There are many reasons not to, and many ways to get it wrong. In short there is seldom a reason why you need to do this, or want to do it
In addition to given answer: finalizer is is called by garbage collector when it runs.
So you can't rely on time of releasing unmanaged resources in finalizer! Because it is unknown.
Also, finalizer runs on another thread, so when garbage collection finishes, finalization may be still running! So there has to by another garbage collection, to completely get rid of an object.
So, first garbage collection call finalezrs, but object doesn't get collected (and also objects, that the object holds references to), it will be collected on the second garbage collection.
An object with finalizer goes through two phases of GC: First time, the finalizer is run and second time, the object is actually collected and the memory is freed. Apart from increasing the GC pressure and delaying the memory release back to pool, finalizers also have the feature of working with objects whose fields may not be in valid state. Also, throwing an exception on finalizer thread tears down the whole application instantly without any friendly information on what just happened.
This is why the Dispose pattern implementation always features the call to GC.SuppressFinalize which causes the finalizer not to run in case the object was already disposed and the GC can directly free the memory on first run.
Generally, using finalizers can be very complex and tricky if your application should survive critical exceptions such as out of memory or thread abort and subsequent AppDomain unloads - this is the case for applications such as SQL Server or IIS.
Long story short: Do not use finalizers unless you absolutely have to, and if you have to (e.g. using unmanaged resources), there's quite a bit of research awaiting you.
You can find more reading on this topic in the following blog posts:
Eric Lippert - When everything you know is wrong
Joe Duffy - Never write a finalizer again
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.
I was posed this question, and while I believe I know the answer, I believe I should be 100% certain else I start spreading misinformation!
I have this object:
public class MyObj(){
private SqlConnection conn;
public MyObj(string connString){
var stream = File.Open(#"c:\\file.txt");
conn = new SqlConnection(connString);
}
}
When this object is created, a file handle is associated with the file, and a connection to a database is created. Neither has Dispose called on it as they should.
When this object goes out of scope and is eventually garbage collected, are these two resources then released, or do they remain in memory?
When this object goes out of scope and is eventually garbage collected, are these two resources then released, or do they remain in memory?
Usually some type - often somewhat hidden from the public API - has a handle on a native resource, possibly via SafeHandle, and that's what ends up releasing the resource. It's possible to write code which doesn't do this, of course, in which case the native resources would only be released on process exit through the normal OS clean-up, but I'd expect any Microsoft-provided APIs to do the right thing.
Of course, you should still dispose of resources explicitly :)
Any object that directly uses an unmanaged resource like a file handle or a database connection should always implement a finalizer that releases the unmanaged resource. When the object is garbage collected the finalizer is executed and the unmanaged resource is freed. So to answer your question:
When this object goes out of scope and is eventually garbage collected, are these two resources then released, or do they remain in memory?
Yes, the unmanaged resources will eventually be freed by the garbage collector that calls the finalizer of the object.
As you are aware of, leaving it to the garbage collector to clean up unmanaged resources is normally a bad thing. If you open and read a file you would prefer that the file is closed and available to other processes when the read has completed instead at some random future time when the garbage collector decides to release the now unused file object.
.NET provides the IDisposable interface to enable deterministic release of unmanaged resources. When you dispose a FileStream object the underlying unmanaged file handle is released and the file is closed.
An important part of implementing the IDisposable interface is that if the unmanaged resource is released via a call to Dispose then the object no longer needs to be finalized. That is why you see a call to GC.SuppressFinalize(this) when an object implements IDisposable and has a finalizer. Avoiding finalization is a good thing to reduce the amount of resources that the garbage collector has to use. Also, finalizers run within tight constraints established by the garbage collector so they are best avoided.
Note that most of the time your objects will not have a finalizer because you do not use any unmanaged resource. Instead you will get access to unmanaged resources using a managed objects like SafeHandle. In that case the object does not need a finalizer but should implement IDisposable and forward calls to Dispose to any aggregated IDisposable objects.
Short answer: They are automatic released.
Just one time, years ago, I got a inexplicable memory leak due to a object not being released.
At the time I need to put some explicit call to the garbage collector to fix the problem.
Never found the real cause, maybe a framework bug, maybe something to do with a OS resource but I can say 99.99% of time you can be free of worries about it in C#.
-----------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
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).