I have a class that should delete some file when disposed or finalized. Inside finalizers I can't use other objects because they could have been garbage-collected already.
Am I missing some point regarding finalizers and strings could be used?
UPD: Something like that:
public class TempFileStream : FileStream
{
private string _filename;
public TempFileStream(string filename)
:base(filename, FileMode.Open, FileAccess.Read, FileShare.Read)
{
_filename = filename;
}
protected override void Dispose(bool disposing)
{
base.Dispose(disposing);
if (_filename == null) return;
try
{
File.Delete(_filename); // <-- oops! _filename could be gc-ed already
_filename = null;
}
catch (Exception e)
{
...
}
}
}
Yes, you can most certainly use strings from within a finalizer, and many other object types.
For the definitive source of all this, I would go pick up the book CLR via C#, 3rd edition, written by Jeffrey Richter. In chapter 21 this is all described in detail.
Anyway, here's what is really happening...
During garbage collection, any objects that have a finalizer that still wants to be called are placed on a special list, called the freachable list.
This list is considered a root, just as static variables and live local variables are. Therefore, any objects those objects refer to, and so on recursively is removed from the garbage collection cycle this time. They will survive the current garbage collection cycle as though they weren't eligible to collect to begin with.
Note that this includes strings, which was your question, but it also involves all other object types
Then, at some later point in time, the finalizer thread picks up the object from that list, and runs the finalizer on those objects, and then takes those objects off that list.
Then, the next time garbage collection runs, it finds the same objects once more, but this time the finalizer no longer wants to run, it has already been executed, and so the objects are collected as normal.
Let me illustrate with an example before I tell you what doesn't work.
Let's say you have objects A through Z, and each object references the next one, so you have object A referencing object B, B references C, C references D, and so on until Z.
Some of these objects implement finalizers, and they all implement IDisposable. Let's assume that A does not implement a finalizer but B does, and then some of the rest does as well, it's not important for this example which does beyond A and B.
Your program holds onto a reference to A, and only A.
In an ordinary, and correct, usage pattern you would dispose of A, which would dispose of B, which would dispose of C, etc. but you have a bug, so this doesn't happen. At some point, all of these objects are eligible for collection.
At this point GC will find all of these objects, but then notice that B has a finalizer, and it has not yet run. GC will therefore put B on the freachable list, and recursively take C, D, E, etc. up to Z, off of the GC list, because since B suddenly became in- eligible for collection, so does the rest. Note that some of these objects are also placed on the freachable list themselves, because they have finalizers on their own, but all the objects they refer to will survive GC.
A, however, is collected.
Let me make the above paragraph clear. At this point, A has been collected, but B, C, D, etc. up to Z are still alive as though nothing has happened. Though your code no longer has a reference to any of them, the freachable list has.
Then, the finalizer thread runs, and finalizes all of the objects in the freachable list, and takes the objects off of the list.
The next time GC is run, those objects are now collected.
So that certainly works, so what is the big bruaha about?
The problem is with the finalizer thread. This thread makes no assumptions about the order in which it should finalize those objects. It doesn't do this because in many cases it would be impossible for it to do so.
As I said above, in an ordinary world you would call dispose on A, which disposes B, which disposes C, etc. If one of these objects is a stream, the object referencing the stream might, in its call to Dispose, say "I'll just go ahead and flush my buffers before disposing the stream." This is perfectly legal and lots of existing code do this.
However, in the finalization thread, this order is no longer used, and thus if the stream was placed on the list before the objects that referenced it, the stream is finalized, and thus closed, before the object referencing it.
In other words, what you cannot do is summarized as follows:
You can not access any objects your object refer to, that has finalizers, as you have no guarantee that these objects will be in a usable state when your finalizer runs. The objects will still be there, in memory, and not collected, but they may be closed, terminated, finalized, etc. already.
So, back to your question:
Q. Can I use strings in finalizer method?
A. Yes, because strings do not implement a finalizer, and does not rely on other objects that has a finalizer, and will thus be alive and kicking at the time your finalizer runs.
The assumption that made you take the wrong path is the second sentence of the qustion:
Inside finalizers I can't use other objects because they could have been garbage-collected already.
The correct sentence would be:
Inside finalizer I can't use other objects that have finalizers, because they could have been finalized already.
For an example of something the finalizer would have no way of knowing the order in which to correctly finalize two objects, consider two objects that refer to each other and that both have finalizers. The finalizer thread would have to analyze the code to determine in which order they would normally be disposed, which might be a "dance" between the two objects. The finalizer thread does not do this, it just finalizes one before the other, and you have no guarantee which is first.
So, is there any time it is safe to access objects that also have a finalizer, from my own finalizer?
The only guaranteed safe scenario is when your program/class library/source code owns both objects so that you know that it is.
Before I explain this, this is not really good programming practices, so you probably shouldn't do it.
Example:
You have an object, Cache, that writes data to a file, this file is never kept open, and is thus only open when the object needs to write data to it.
You have another object, CacheManager, that uses the first one, and calls into the first object to give it data to write to the file.
CacheManager has a finalizer. The semantics here is that if the manager class is collected, but not disposed, it should delete the caches as it cannot guarantee their state.
However, the filename of the cache object is retrievable from a property of the cache object.
So the question is, do I need to make a copy of that filename into the manager object, to avoid problems during finalization?
Nope, you don't. When the manager is finalized, the cache object is still in memory, as is the filename string it refers to. What you cannot guarantee, however, is that any finalizer on the cache object hasn't already run.
However, in this case, if you know that the finalizer of the cache object either doesn't exist, or doesn't touch the file, your manager can read the filename property of the cache object, and delete the file.
However, since you now have a pretty strange dependency going on here, I would certainly advice against it.
Another point not yet mentioned is that although one might not expect that an object's finalizer would ever run while an object is in use, the finalization mechanism does not ensure that. Finalizers can be run in an arbitrary unknown threading context; as a consequence, they should either avoid using any types that aren't thread-safe, or should use locking or other means to ensure that they only use things in thread-safe fashion. Note finalizers should use Monitor.TryEnter rather than Monitor.Enter, and endeavor to act as gracefully as possible if a lock is unexpectedly held. Note that since finalizers aren't supposed to run while an object is still in use, the fact that a lock was unexpectedly held will often suggest that a finalizer was run early. Depending upon the design of the code which uses the lock, it may be possible to have the finalizer set a flag and try again to acquire the lock, and have any other code which uses the lock check after releasing it whether that flag is set and, if so, reregister the object for finalization.
Handling finalization cleanup correctly in all threading scenarios is difficult. Finalization might not seem complicated, but no convenient automated mechanisms exist by which objects can ensure that finalizers won't run while the objects in question are in use. Consequently, finalizers have a lot of subtle thread-safety issues. Code which ignores such issues will "usually" work, but may sometimes fail in difficult-to-diagnose ways.
You can call the dispose method inside your finalizer and have the file cleanup code in the Dispose method. Along with that you can also pass a boolean to your dispose method that indicates that you are invoking it from the finalizer.
For an excellent reference on the proper usage of Dispose and Fianlizers , read this Proper use of the IDisposable interface
Related
So, the default dispose pattern implementation looks like this:
class SomeClass : 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;
}
~SomeClass()
{
Dispose(false);
}
}
It's said that:
If the method call comes from a finalizer (that is, if disposing is
false), only the code that frees unmanaged resources executes. Because
the order in which the garbage collector destroys managed objects
during finalization is not defined, calling this Dispose overload with
a value of false prevents the finalizer from trying to release managed
resources that may have already been reclaimed.
The question is: why is it supposed that the objects that are referenced by the object of SomeClass may already have been freed and we shouldn't try to dispose them when the method is called from the finalizer? If those objects are still referenced by our SomeClass object they cannot be freed, isn't it true? It's said that:
Those with pending (unrun) finalizers are kept alive (for now) and are
put onto a special queue. [...] Prior to each object’s finalizer
running, it’s still very much alive — that queue acts as a root
object.
So, again, our SomeClass object is referenced by this queue (which is the same as to be referenced by a root). And other objects the SomeClass object has references to should be alive as well (as they are rooted through the SomeClass object). Then why and how they might have been freed by the time the SomeClass finalizer is called?
Konrad Kokosa has an impressive explanation on his book Pro .NET Memory Management.
(emphasis added)
During GC, at the end of Mark phase, GC checks the finalization queue to see if any of the finalizable objects are dead. If they are some, they cannot be yet delete because their finalizers will need to be executed. Hence, such object is moved to yet another queue called fReachable queue. Its name comes from the fact that it represents finalization reachable objects - the ones that are now reachable only because of finalization. If there are any such objects found, GC indicates to the dedicated finalizer thread there’s work to do.
Finalization thread is yet another thread created by the.NET runtime. It removes objects from the fReachable queue one by one and calls their finalizers. This happens after GC resumes managed threads because finalizer code may need to allocate objects. Since the only root to this object is removed from the fReachable queue, the next GC that condemns the generation this object is in will find it to be unreachable and reclaim it.
Moreover, fReachable queue is treated as a root considered during Mark phase because the finalizer thread may not be fast enough to process all objects from it between GCs. This exposes the finalizable objects more to a Mid-life crisis - they may stay in fReachable queue for a while consuming generation 2 just because of pending finalization.
I think the key here is:
fReachable queue is treated as a root considered during Mark phase because the finalizer thread may not be fast enough to process all objects from it between GCs.
Objects in .NET exist while any references exist to them. They cease to exist as soon as the last reference does so. Storage used by an object will never be reclaimed while the object exists, but there are a couple of things the GC does before it reclaims storage:
There is a special list, called the "finalizer queue", which holds references to all objects which have registered finalizers. After identifying every other reference that exists anywhere in the universe, the GC will examine all objects in the finalizer queue to see if any references have been found to them. If this process causes it to find an object which had not been discovered previously, it copies a reference to another list called the "freachable queue". Any time the freachable queue is non-empty and no finalizer is running, the system will pull a reference from that queue and call finalize upon it.
The GC will also inspect the targets of all weak references and invalidate any weak reference whose target hadn't been identified by any live strong reference.
Note that a finalize method does not "garbage-collect" an object. Instead, it prolongs the existence of the object until finalize is called upon it, for purpose of allowing it to satisfy any obligations it might have to outside entities. If at that time no reference to the object exists anywhere in the universe, the object will cease to exist.
Note that it's possible for two objects with finalizers to hold references to each other. In such situations, the order in which their finalizers run is unspecified.
I have a Wrapper<T> where T : class that wraps around my objects. I store WeakReference<Wrapper<T>> in a ConcurrentDictionary, to implement weakly-referenced thread-safe cache for immutable objects that gets automatically cleaned up when memory is required for something else. I need to call ConcurrentDictionary.TryRemove in the Wrapper destructor to free the weak references in the dictionary that no longer point to a valid object.
It is well-known that we should not use any locking inside destructors because of the risk of dead-lock. So I wonder, can I use ConcurrentDictionary.TryRemove safely in a destructor? I am afraid it might have been implemented using SpinLock or some other tool and thus still presents a risk of dead-lock when used in destructor.
You can see the implementation of the ConcurrentDictionary at this location and the TryRemove implementation uses 'lock(...)'.
What you could do inside the destructor is use the thread pool to perform the removal of the item from the dictionary. You would still need to mark the wrapper instance as no longer valid, so that if a call is made to any of its public methods between the finalizer running and the thread pool removing it, you could detect this and reject the call.
So the reason you DON'T want to use locking in a destructor is due to the fact that the destructor might be called by a FinalizerWorker in a sperate thread while stopping execution on all threads.
Thus, if one thread is in the middle of a ConcurrencyDicitonary operation when the FinalizerWorker is kicked off you might deadlock if the destructor tries to lock the ConcurrencyDictionary (this can be a very difficult to reproduce deadlock).
A spin lock wont help you because if ANY currently executing thread has the ConcurrencyDictionary locked or the Spinner variable locked it WILL NOT release it until the FinalizerWorker completes, which it wont because it will spin/lock forever.
You'r main options here is to implement the IDisposable interface with a SuppressFinalize(this) call, since your object will suppress the Finalizer worker no deadlock can occure and ConcurrencyDictionary operations ARE SAFE !
Thus if you pre-empt Finalizer using object.Dispose() you should be safe use ConcurrencyDictionary, but otherwise DO NOT use any types of locks in your Finalizer Dispose(false) call or you will deadlock at some point.
// Design pattern for a base class.
public class Base: IDisposable
{
private bool disposed = false;
//Implement IDisposable.
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
//Disposing outside the FinalizerWorker (SAFE)
if (disposing)
m_pDictionary.TryRemove(this);
disposed = true;
}
}
// Use C# destructor syntax for finalization code.
~Base()
{
// Simply call Dispose(false).
Dispose (false);
}
The answers on here are leading you astray. Using locks in a destructor/finalizer is discouraged because it can easily lead to deadlocks, especially when implemented "manually" instead of using a concurrent collection, but sometimes it is necessary. Even on "stop the world" GC implementations finalizers run on a separate finalizer thread that chugs along concurrently with your application.
First thing is first though - it is VERY RARE that what you are suggesting is the ideal way of implementing your desired functionality, to the point where I am quite confident it isn't. To start, WeakReferences are not suitable for use in caching because they get collected far more often than just when "memory is needed". A proper cache implementation monitors memory levels with strong references and releases them as needed when memory usage is too high.
Even in implementations like a WeakValueDictionary where you don't want the collection holding onto the value if it can be collected, the implementation still doesn't receive object collection notifications. Instead you just remove the entry whenever you stumble upon one that was collected, or you scan the entire collection for dead entries every X operations, or every Y seconds, etc.
That said, if you do run into a situation where you NEED to do something when an object is collected, you can use concurrent collections just fine.
Assuming you don't do anything silly, queuing up an ID in a notification queue or removing an item from a concurrent dictionary is safe because those operations are fast and block for only a very short period of time and your application is not blocked while your finalizers run.
It's something you should avoid as much as possible, but sometimes it's the best and only way to implement something. Just make sure that the lock is fast and used as minimally as possible, and not part of any multi-level locking schemes that are easy to accidentally get wrong and deadlock.
Most resources state that the garbage collector figures that out on its own based on references and that I shouldn't mess with it.
I am wondering if I can explicitly tell the garbage collector that it may dispose an object while still keeping a reference.
What I would like to do is to tell the garbage collector that I currently don't need an object anymore (but might again) and then at a later point when (if) I need the object again I would like to check if it has been disposed already. If it has I simply recreate it, if it hasn't I'd like to "unmark" it from garbage collection until I am done with it again.
Is this possible?
I plan to implement something similar to the Lazy<T> class. Pseudocode:
obj = new DisposeIfNecessary<LargeObject>(() => InitLargeObject());
obj.DoSomething(); // Initializes obj using InitLargeObject()
obj.DisposeIfNecessary(); // This is where the magic happens
... // obj might get disposed at some point
obj.DoAnotherThing(); // Might or might not call InitLargeObject() again
obj.Dispose(); // I will not need it again
The WeakReference class does exactly what you want, using the IsAlive property to check for state before using it.
You can get a "strong" reference to it again via the Target property, which will affect the reference count and stop it from being eligible for collection.
Also note that Dispose doesn't directly relate to garbage collection, so disposing an item (depending on the Dispose implementation) might make it unusable anyway - but again, that is nothing to do with GC. On a general practice note, as mentioned by #HansPassant, calling Dispose on an item (or generally anything claiming to dispose) and then attempting to use it again afterwards is a code smell (or just plain wrong as other developers will expect Dispose to be a last-call method marking the object as unusable from then on).
The WeakReference class will not be responsible for re-creating collected objects, but in conjunction with IsAlive you can handle that logic yourself.
Also, to the point in the comments, the GC doesn't do anything clever with WeakReference in terms of deciding when to collect it in terms of trying to leave WeakReference items until last; it will collect the underlying object as it would others during a run if it is eligible - no special handling and definitely no "cache" behaviour.
How the CLR handles local variables with function scope in case an exception is thrown.
is it a must to use the finally block or the variable is disposed once the flow leaves the function
below is a small example
protected void FunctionX()
{
List<Employee> lstEmployees;
try
{
lstEmployees= new List<Employee>();
int s = lstEmployees[1].ID; // code intended to throw exception
}
catch (Exception ex)
{
ManageException(ex, ShowMessage); //exception is thrown here
}
finally { lstEmployees= null; } // Is the finally block required to make sure the list is cleaned
}
To answer your specific question, no, the finally block you've listed is not required.
Assigning null to a reference variable does not actually do anything, as garbage collection is non-deterministic. As a simplistic explanation, from time to time, the garbage collector will examine the objects within the heap to determine if there are any active references to them (this is called being "rooted"). If there are no active references, then these references are eligible for garbage collection.
Your assignment to null is not required, as once the function exits, the lstEmployees variable will fall out of scope and will no longer be considered an active reference to the instance that you create within your try block.
There are certain types (both within .NET and in third-party libraries) that implement the IDisposable interface and expose some deterministic cleanup procedures through the Dispose() function. When using these types, you should always call Dispose() when you're finished with the type. In cases where the lifetime of the instance shouldn't extend outside of the lifetime of the function, then you can use a using() { } block, but this is only required if the type implements IDisposable, which List<T> (as you used in your example) does not.
Don't be worried about the objects cleanup, that's why the .NET and most modern languages provide the garbage collection functionality in runtime.
If your object has a handle to unmanaged resource do that cleanup.
Some of the other answers are slightly misleading here.
In fact, the garbage collector has got (almost) nothing to do with the variable lstEmployees. But it never needs to be set to null, neither in normal code flow nor after an exception is thrown.
Setting references to null to free the object they point they point to is almost never required, especially not for local objects.
As a consequence, the garbage collector won’t care about the exception either.
On the other hand, unmanaged resources which aren’t handled by the CG do always require manual cleanup (via the Dispose method of the IDisposable interface). To make sure that such resources are returned after an exception was thrown, you indeed need the finally clause. Or, if you don’t intend to handle the exception locally, you can replace the try … finally by a using clause:
using (someUnmanagedResource) {
// … use the resource …
}
// Will implicitly call someUnmanagedResource.Dispose() *whatever happens*!
.NET languages are garbage collected, which means that objects lifetimes are kept track of, so the garbage collection will get rid of your list when it finds no more object references to it.
Not at all. When the variable is out of scope, the garbage collector will take care of it (when the GC decides it's time to collect all the garbage...)
The only thing you have to take in account is that maybe you don't want to wait for the GC to do its job, so resources help by an instance of a class are released (e.g. imagine you have locally created an instance that hols a reference to a database connection. The connection will be held until GC takes care of deleting the instance, and later on deleting the referenced connection, which may take a while).
In these cases, take a look at the IDisposable interface, so you can proactively free resources before your instances are removed by the GC.
.NET's garbage collector will handle this for you. In fact, setting "lastEmployees" to null accomplishes the same thing as just exiting the function.
Any item that is no longer referenced by the root application in one form or another will be marked for collection.
In .NET, you never need to worry about cleaning up managed resource. Hence, managed.
http://msdn.microsoft.com/en-us/library/0xy59wtx.aspx
Can I trust that an object is destroyed and its destructor is called immediately when it goes out of scope in C#?
I figure it should since many common coding practices (e.g. transaction objects) rely on this behaviour, but I'm not very used to working with garbage collection and have little insight to how such languages usually behave.
Thanks.
Nope, .Net and hence C# relies on a garbage collection memory management. So destructors (which in .Net is called finalizers) are not called until GC finds it proper to destroy the objects.
Additionally: most "regular" objects in C# don't have destructors. If you need the destructor pattern you should implement the IDisposable interface with the Dispose Pattern. On disposable objects you should also make sure that the Dispose method gets called, either with the using keyword or directly calling the method.
To further (hopefully) clarify: deterministic disposal is useful in .Net e.g. when you need to explicitly free resources that is not managed by the .Net runtime. Examples of such resources are file handles, database connections, etc. It is usually important that these resources be freed as soon as they no longer are needed. Thus we cannot afford to wait for the GC to free them.
In order to get deterministic disposal (similar to the scope behavior of C++) in the non-deterministic world of the .Net GC, the .Net classes rely on the IDisposable interface. Borrowing from the Dispose Pattern, here are some examples:
First, instantiating a disposable resource and then letting the object go out of scope, will leave it up to the GC to dispose the object:
1. {
2. var dr = new DisposableResource();
3. }
To fix this we can explicitly dispose the object:
1. {
2. var dr = new DisposableResource();
3.
4. ...
5.
6. dr.Dispose();
7. }
But what if something goes wrong between line 2 and 6? Dispose will not be called. To further ensure that Dispose will finally be called regardless of any exceptions we can do the following:
1. var dr = new DisposableResource();
2. try
3. {
4. ...
5. }
6. finally
7. {
8. dr.Dispose();
9. }
Since this pattern is often needed, C# includes the using keyword to simplify things. The following example is equivalent to the above:
1. using (var dr = new DisposableResource())
2. {
3. ...
4. }
No. An object doesn't actually go "out of scope," the reference to it (i.e. the variable you use to access it) does.
Once there are no more references to a given object, that object becomes eligible for garbage collection (GC) should the need arise. Whenever the GC decides it needs to reclaim the space your no-longer-referenced object, that's when the objects finalizer will be called.
If your object is a resource (e.g. a file handle, database connection), it should implement the IDisposable interface (which obligates the object to implement a Dispose() method to clean up any open connections, etc). The best practice for you in this case would be to create the object as part of a using block, so that when this block is completed, your application will automatically call the objects Dispose() method, which will take care of closing your file/db connection/whatever.
e.g.
using (var conn = new DbConnection())
{
// do stuff with conn
} // conn.Dispose() is automatically called here.
The using block is just some syntactic sugar which wraps your interactions with the conn object in a try block, along with a finally block which only calls conn.Dispose()
There is no such thing als a C++-like destructor in C#. (There is a different concept of destructor in C#, also called a finalizer, which uses the same syntax as C++ destructors, but they are unrelated to destroying objects. They're intended to provide a cleanup mechanism for unmanaged resources.)
The garbage collector will cleanup objects sometime after they are no longer referenced. Not immediately, and there is no way to guarantee this either.
Luckily there is also no real reason why you would want to guarantee this. If you need the memory, then the GC will reclaim it then. If you don't, why care if there's still some garbage object around? It's not a memory leak: the GC can still find it and clean it up any time.
No, this isn't guaranteed. Similar to languages such as Java, in C# the garbage collector runs when it's needed (i. e. when the heap is getting too full). However, when your objects implement IDisposable, i. e. they have a Dispose() method and it has to be called, then you can take advantage of the using keyword:
using (var foo = new DisposableObject()) {
// do something with that
}
That way Dispose() will be called immediately when leaving that using block.
Note: IDisposable is found in many types, most notably GDI+ but also database connections, transactions, etc. so it may really be the right pattern here.
Note 2: Behind the scenes above block will get translated into a try/finally block:
var foo = new DisposableObject();
try
{
// do something with that
}
finally
{
foo.Dispose();
}
But that translation is done by the compiler and very handy for not forgetting to call Dispose().
I don't think you should rely on garbage collectors in this way. Even if you deduct how they operate it might very well be that in the next release they've reimplemented it.
In any case, objects are not garbage collected the moment you unreference them. Typically they are collected until some threshold is reached and then they are released.
Especially in java programs this is very noticeable when you look at the memory consumption on the task manager. It grows and grows and all of a sudden every minute it drops again.
No. If you refer to CLI specification (p. 8.9.6.7 about Finalizers) http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-335.pdf you can find the following
the CLI should ensure that finalizers are called soon after the instance becomes
inaccessible. While relying on memory pressure to
trigger finalization is acceptable, implementers should consider the use of additional
metrics
but it must not.