I am well aware that finalizers are typically used to control unmanaged resources. Under what circumstances may a finalizer deal with managed ones?
My understanding is that presence in the finalizer queue will prevent any object, or objects strongly referenced thereby, from being collected, but it will not (of course) protect them from finalization. In the normal course of events, once an object is finalized it will be removed from the queue and any objects it references will no longer be protected from collection on the next GC pass. By the time a finalizer is called, the finalizers may have been called for any combination of objects referred to by the object; one cannot rely upon finalizers being called in any particular sequence, but the object references one holds should still be valid.
It's pretty clear that a finalizer must never acquire locks, nor attempt to create a new object. Suppose, however, that I have an object that subscribes to some events, and another object which actually uses the events. If the latter object becomes eligible for garbage collection I want to have the former object unsubscribe from events as soon as practical. Note that the former object will never become eligible for finalization until no subscriptions for it are held by any live object.
Would it be practical to have a lock-free linked-list stack or queue of objects which needed to be unsubscribed, and have the main object's finalizer put a reference to the other object on the stack/queue? The linked-list item object would have to be allocated when the main object was created (since allocation within the finalizer would be forbidden), and it would probably be necessary to use something like a timer event to poll the queue (since the event unsubscription would have to run outside the finalizer thread, and it would probably be silly to have a thread whose sole purpose was to wait for something to appear on the finalizer queue), but if the finalizer could safely reference its pre-allocated linked-list object and the main queue object associated with its class, it could allow the events to be unsubscribed within 15 seconds or so of finalization.
Would that be a good idea? (Notes: I'm using .net 2.0; also, an attempt to add to the stack or queue might spin a few times on Threading.Interlocked.CompareExchange, but I wouldn't expect that it should ever be stuck very long).
EDIT
Certainly any code which subscribes events should implement iDisposable, but disposable things aren't always disposed properly. If there were, there wouldn't be any need for finalizers.
My scenario of concern would be something like the following: a class implementing iEnumerator(of T) hooks onto a changeNotify event of its associated class so that an enumeration can be sensibly handled if the underlying class changes (yes, I know Microsoft thinks all enumerators should simply give up, but sometimes an enumerator which can keep working will be more useful). It's quite possible that an instance of the class might be enumerated many thousands or even millions of times over the course of days or weeks, but not be updated at all during that time.
Ideally, the enumerator would never be forgotten about without being disposed, but enumerators are sometimes used in contexts where "foreach" and "using" aren't applicable (e.g. some enumerators support nested enumeration). A carefully-designed finalizer might allow a means to deal with this scenario.
Incidentally, I'd require that any enumeration which is supposed to continue through updates must use the generic IEnumerable(of T); the non-generic form, which doesn't handle iDisposable, would have to throw an exception if the collection gets modified.
Suppose, however, that I have an object that subscribes to some events, and another object which actually uses the events. If the latter object becomes eligible for garbage collection I want to have the former object unsubscribe from events as soon as practical. Note that the former object will never become eligible for finalization until no subscriptions for it are held by any live object.
If the "latter object" is the one that's using the events, and the "former" object is the one subscribing to the events, the "former" object has to have some way to pass the event info to the "latter" object - meaning it's going to have some reference in place to "latter". Chances are, this will keep the "latter" object from ever being a GC candidate.
That being said, I would recommend avoid this type of managed resource deallocation via the finalizer, unless absolutely necessary. The architecture you're describing seems very fragile, and very tricky to get right. This is probably a better candidate for IDisposable, with the finalizer being the "last ditch" cleanup effort.
Although IDisposable is typically about releasing native resources - it can be about releasing any resource, including your subscription information.
Also, I'd try to avoid having a single global collection of object references - it might make more sense to have your objects internally just use a WeakReference. As soon as the "latter" object is collected, the "former" object's WeakReference would no longer be valid. The next time an event subscription is raised, if the internal WeakReference is no longer valid, you can just unsubscribe yourself. No need for global queues, lists, etc - it should just work...
I'm going to call the objects "publisher" and "subscriber" and restate my understanding of the problem:
In C#, the publisher will (effectively) hold references to the subscribers, preventing subscribers from being garbage collected. What can I do so that the subscriber objects can be garbage collected without explicitly managing the subscriptions?
First, I would recommend doing everything I could to avoid this situation in the first place. Now, I'm going to move on and assume you have, considering you're posting the question anyway =)
Next, I would recommend hooking the add and remove accessors of the publisher's event(s) and using a collection of WeakReferences. You can then automatically unhook those subscriptions whenever the event is invoked. Here's an extremely rough, untested example:
private List<WeakReference> _eventRefs = new List<WeakReference>();
public event EventHandler SomeEvent
{
add
{
_eventRefs.Add(new WeakReference(value));
}
remove
{
for (int i = 0; i < _eventRefs; i++)
{
var wRef = _eventRefs[i];
if (!wRef.IsAlive)
{
_eventRefs.RemoveAt(i);
i--;
continue;
}
var handler = wRef.Target as EventHandler;
if (object.ReferenceEquals(handler, value))
{
_eventRefs.RemoveAt(i);
i--;
continue;
}
}
}
}
Let me make sure I understand -- are you worried about leaks from event subscribers that remain subscribed to a collected event publisher?
If that's the case, then I don't think you have to worry about it.
Here's what I mean assuming that the "former" object is the event subscriber and the "latter" object is the event publisher (raises the event):
The only reason that the subscriber (former) is "subscribed" is because you created a delegate object and passed that delegate to the publisher ("latter").
If you look at the delegate members, it has a reference to the subscriber object and to the method on the subscriber that will be executed. So there is a reference chain that looks like this: publisher --> delegate --> subscriber (publisher references delegate, which references subscriber). It's a 1-way chain -- the subscriber does not hold a reference to delegate.
So, the only root that keeps the delegate around is on the publisher ("latter"). When latter becomes eligible for GC, so does the delegate. Unless there is some special action you want for your subscribers to take when they unsubscribe, they will effectively become unsubscribed when the delegate gets collected -- there is no leak).
Edit
Based on supercat's comments, it sounds like the problem is that the publisher is keeping the subscriber alive.
If that's the problem, then finalizers won't help you. Reason: Your publisher has a real, bonafide reference to your subscriber (via the delegate), and the publisher is rooted (otherise it would be eligible for GC), so your subscribers are rooted, and will not be eligible for finalization or GC.
If you are having trouble with publisher keeping subscriber alive, I would suggest that you search for weak-ref events. Here are a couple links to get you started: http://www.codeproject.com/KB/cs/WeakEvents.aspx http://www.codeproject.com/KB/architecture/observable_property_patte.aspx.
I had to deal with this once as well. Most of the effective patterns involve changing the publisher so that it holds a weak-ref to the delegate. Then you have a new problem -- the delegate isn't rooted, and you hvae to keep it alive somehow. The articles above probably do something like that. Some techniques use reflection.
I used a technique once that did not rely upon reflection. It required that you be able to make changes to the code in both the publisher and the subscriber, though. If you would like to see a sample of that solution, let me know.
Let's try this again. Can you add your event handlers to your publisher like this:
var pub = new Publisher();
var sub = new Subscriber();
var ref = new WeakReference(sub);
EventHandler handler = null; // gotta do this for self-referencing anonymous delegate
handler = (o,e) =>
{
if(!ref.IsAlive)
{
pub.SomeEvent -= handler; // note the self-reference here, see comment above
return;
}
((Subscriber)ref.Target).DoHandleEvent();
};
pub.SomeEvent += handler;
This way, your delegate doesn't keep a direct reference to the subscriber, and automatically unhooks itself whenever the subscriber gets collected. You could implement this as a private static member of the Subscriber class (for the purposes of encapsulation), just make sure it's static to prevent inadvertently holding onto a direct reference to the "this" object.
Related
I have a class holding a WeakReference pointing to an object implmenting IDisposable. After the object is disposed there is a period of time before it is garbage collected. During that time the WeakReference can still be used. This can result in unexpected behavior because we are now making calls against an object that has been disposed.
Does anyone have a suggested approach for dealing with this, checking the weak reference if the target has been disposed, etc.?
Background:
We have a WinForms application with a controller holding data. Multiple UI controls may be presenting the data at any given time. A form adds and removes the controls (and calls Dispose when it removes them) but is ignorant about what the controls are doing or what data they need. Previously the controls would subscribe to events from the controller to receive notifications when the data was updated. This results in memory leaks. To address this the controller now keeps weak references to the controls, and notifies the ones that are still alive.
Since IDisposable and WeakReference have mutual exclusive semantic you need to choose single one. Combining them is dangerous since after a disposal an object most likely will have inconsistent state which isn't suitable for a reusing. IDisposable implies that you'd like to control the life-cycle of your objects and aren't going to take back. It's like to put something into a dumpster and to say: "I'm done with this thing, now I don't care what happens with that". The WeakReference semantic is like to put something near a dumpster and say: "I'm gonna get back in 30 minutes or so, if noone puts this into this dumpster I'll take it back, but if it's inside I don't care".
Weak references imply that objects referenced by them don't have a long life that's they're good for quick memory reclaim but at the same time they give some chance to be reused so in your case you can rely exclusively on WeakReference<T>:
MyType instance;
if (weakRef.TryGetTarget(out instance))
{
// resurrected, still can use it
}
else
{
// object is collected, the new one should be created
}
Just in case anyone else stumbles across this question they way I resolved it was to make the referenced objects have well defined behavior after being disposed (per the comment by Alexei Levenkov). The easiest way to do this was to ignore function calls after the object had been disposed.
public void OnDataChanged(object model)
{
if (IsDisposed)
{
return;
}
...
// method implementation
}
This was only necessary in the single function being called from the weak reference.
Multi-threading was not an issue as these are UI objects and Invoke and BeginInvoke were already required to make calls modifying them.
Background:
I've a got a main object that live on my app's UI thread. This long-lived parent / container object contains a sub-object as a private member that is internally multi-threaded.
Based on some event published from the child / contained object I'd like the parent object to simply Dispose() the child and recreate it.
The contained object owns some unmanaged resources and well as some large managed memory buffers.
I could create an event handler on the top level object that does this, but this means that the object about to be Disposed will be in the call stack of the method call to about to replace it. (!)
This is because the event handler delegate will be called by one of the child object's state handling functions on it's own thread.
This seems...wrong. No?
In particular, the the child's FireAnEvent() method will resume execution after the delegate calls are processed, except that now execution will resume in the context of an already "disposed" object.
Intuitively, I can't see this leading to good things.
Question:
Is there an established C# pattern to destroy a contained object as a result of it's own event?
Or, is there GC magic that makes such a simple event handler good enough somehow?
Or, am I missing some key bit of understanding?
Calling IDisposable.Dispose() doesn't signal anything special to the .NET framework. The only thing you need to do is remove any references to the object you wish to remove. Once this is done and the object is out of the call stack, it will become a candidate for garbage collection.
Note that your object will not necessarily be garbage collected immediately, or even the next time the GC runs; it is merely an assumed eventuality.
The only purpose of IDisposable is to provide a standard means for requesting that an object clean itself up and release resources. You can hold a reference to a "disposed" object for as long as you like, which will prevent the GC from collecting the object. IDisposable.Dispose() is just another method; technically, you can make it do anything you want.
This question has a very nicely detailed answer that may help you understand IDisposable a bit more: Proper use of the IDisposable interface
as mentioned, an IDisposable object is nothing magical. It just lets you use the using shorthand, which is just a shorthand for:
try { // code in the using block }
catch{}
finally{
disposableObject.Dispose()
}
Have you considered incorporating a third type of object into the mix? It is ill advised that contained object be conscious of their container.
Roughly this would be your workflow:
contained object decides it should be restarted.
contained object frees resources.
contained object writes to a queue on a third object (not the containing object).
containing object accesses the queue when you feel you should create new contained objects and reinstantiates the objects. Alternatively adding to the queue raises an event to the container to empty it.
The third object might seem pointless but it would make your life a lot easier if you ever decided to refactor.
Objects which subscribe to events for the purpose of notification should be prepared to receive notifications at any time, even after they have been disposed. The purpose of a notification is to tell an object to do whatever it needs to do in response to something that has happened. If an object can't do anything useful in response to a notification, it should simply not do anything.
Further, the purpose of Dispose isn't to "destroy" an object, nor the resources it contains, but rather to release it from any obligations it may have had to outside objects, and allow it to release any outside entities from any obligations they might have had toward it. In many cases, an object will be useless once it releasing the services of outside entities that were committed to it, and thus objects which have been disposed cannot be expected to be useful; if, as is likely, a method which is called after Disposed cannot satisfy its duties because necessary outside entities have been released, it should throw ObjectDisposedException rather than failing some other way.
Putting these observations together, while many methods on a disposed object should throw ObjectDisposedException, notification event handler methods should not, since they're instructing the object to "do whatever you need to do to meet your obligations, given that something has happened". If an object has been disposed, it has no obligations. Thus, being disposed doesn't prevent an object from satisfying an event handler contract; instead, it allows the object to meet the contract by silently ("successfully") doing nothing.
I have a class which has a static List<T> field inside which I hold all my objects; each object represents a process that is running and its properties.
After the process finishes its job, the relevant object is removed from the List<T>, then my UI is updated.
After the object is removed from my list, should I change this object to null to free resources?
Once you remove the object from the list, the object still exists but if no reference is being made to that object, the garbage collector will clean it up and you won't have to worry
C# is garbage collected so either way the memory will be taken care of.
The moment you remove that object from list,the object would go out of scope and it would be eligible for GC..
In terms of Events:
Even if that object has some events to which other objects subscribe,it would still be elgible for GC..
So, its a one way relation
Broadcaster -> Subscribers
If Broadcaster goes out of scope it would be eligible for GC even if it has subscribers..
But Broadcaster would prevent GC of Subscribers
Have a look at Do event handlers stop garbage collection from occuring?
In short, no.
Once a reference falls out-of-scope, the Garbage Collector will get rid of the object/references to it. This can also be handled with the using (if the object implements IDisposable) keyword as well. Basically, don't set it to null and let the clean-up mechanisms within C# handle the work for you.
Recently I have come across an increasing number of people who have code similar to the following:
private AsynchronousReader r;
public SynchronousReader()
{
r = new AsynchronousReader();
// My practice is to put this here
// and then never remove it and never add it again
// thus cleaning up the code and preventing constant add/remove.
//r.ReadCompleted += this.ReadCompletedCallback;
}
private ReadCompletedCallback()
{
// Remove the callback to "clean things up"...
r.ReadCompleted -= this.ReadCompletedCallback;
// Do other things
}
public Read()
{
r.ReadCompleted += this.ReadCompletedCallback;
// This call completes asynchronously and later invokes the above event
r.ReadAsync();
r.WaitForCompletion();
}
Folks say that this practice is better than the one I indicated above and have given several reasons specific to Silverlight. They state it prevents memory leaks, threading issues, and even that it is the normal practice.
I have not done much Silverlight, but it seems silly to do this still.
Are there any specific reasons one would use this method instead of just rigging up the callback in the constructor once and for the lifetime of the object?
This is as simple as I could make my example. Ignore the fact that it's a sort of wrapper that turns an asynchronous object into a synchronous one. I'm only curious about the way events are added and removed.
In the case you mention it would make sense to hook it up once, but potentially the objects (parent and/or child) may not get garbage collected as the event handlers still reference them.
According to Marc Gavel here
i.e. if we have:
publisher.SomeEvent += target.SomeHandler;
then "publisher" will keep "target" alive, but "target" will not keep
"publisher" alive.
A more important point to bear in mind might be the lifespan of the child object. If it is the same as the parent, then one-off subscription in the constructor makes more sense. If it is dynamic you will likely want to remove the handlers as I have seen them leak (resulting in multiple callbacks).
Note: If the constructor-only method turns out to leak objects, you can always put an unsubscribe in the Dispose() I guess, but I can't say I have ever seen that.
It sounds like you have two issues:
You're attempting to reuse an object that really should only be used once.
That object needs to get properly cleaned up.
You should really either only use an instance of the SynchronousReader object only once (thus avoiding the two async calls racing with one failing to finish like you mentioned elsewhere) or you should implement IDisposable in order to unsubscribe from the event and prevent the memory leak.
A third solution might be possible: keep the single instance of SynchronousReader, but each call to SynchronousReader.Read would create a new instance of AsynchronousReader (rather than storing it as a private field within the instance). Then you could keep most of the code above which you don't like, but which properly handles event subscriptions.
If yes, is there a way to unwire it globally for all wired events
Edit: Say for example. I have objects, each tagged with an event like orm.NatureChanged += Nature_Changed; I tag these events when i create each orm instance. If I didn't unwire by means like orm.NatureChanged -= Nature_Changed; will it cause memory leak?
Regardless of what you're asking, the technical answer to your question is "no". Technically, unless you discover a bug in the CLR, there are no true "memory leaks" with managed objects (that's a big part of what makes them a Good Thing). To answer what I think you're actually asking, though, it sounds like you're asking one of two things:
Is there something that needs to be done with events that do not have any delegates attached to them?
Can an event prevent objects from getting cleaned up by the garbage collector?
The answer to the first question is simply "no". Move along, nothing to see here.
The answer to the second has been discussed at length here on SO and other areas of the web. The short version is that an attached event handler means that the GC will consider the target instance as "reachable" by the event-firing instance. This can cause objects to remain in memory longer than expected, since this reachability is somewhat transparent to the user (developer) because of the way delegates are constructed.
In other words, say I have two objects: Producer and Consumer. Producer fires an event that Consumer...consumes.
public class Producer
{
public event EventHandler OmgIDidSomething;
}
public class Consumer
{
public void AttachTo(Producer producer)
{
producer.OmgIDidSomething += new EventHandler(producer_OmgIDidSomething);
}
private void producer_OmgIDidSomething(object sender, EventArgs e)
{
// ...
}
}
In this example, any instance of Consumer where AttachTo is called will remain reachable as far as the GC is concerned until the instance of Producer that it attached to is eligible for collection, because the delegate behind the implementation of the OmgIDidSomething event has a reference to the instance of Consumer that it corresponds to.
No, because when you unwire an event, the delegate (it's an object) which was wired to the event is no longer rooted, and will be collected when the GC sees fit to do so. This is assuming of course the event delegate isn't attached to multiple handlers, in which case it won't be collected until it is unwired from all events.
If you meant whether events that don't get unwired can cause a memory leak, the answer is that it can if the actual lifetime of the object holding the event delegate is much longer than the useful lifetime of the object to which the delegate refers. For example, if a collection's enumerator hooked a CollectionChanged event, and someone were to get enumerators without ever disposing them, then every time the collection was enumerated (without disposing the enumerator) a new enumerator object would be created that would stay in memory as long as the underlying collection.
It's not a memory leak, it just doesn't hook any handlers to that event if they aren't wired, automatically or otherwise. So the event fires to nobody, gets cleaned up, and life goes on.
This conversation talks on this subject: How do events cause memory leaks in C# and how do Weak References help mitigate that?
See some background information here: What does AutoEventWireUp page property mean?