I have an object (Delegate) which needs to stay alive (not garbage collected) while another object (TargetObject) is alive. I want Delegate to be garbage collected when TargetObject is collected (or at least available for collection).
The difficulty is that I don't want to need to reference Delegate from TargetObject since I want it to work for existing objects unaware of Delegate, and I don't want to affect the lifetime of TargetObject. Is this at all possible?
Thanks.
Edit: Thanks for the responses so far. I'll try to clarify what I'm up to.
I'm trying to implement weak events but I'm not a fan of the WeakEventManager (particularly IWeakEventListener). I want to hold a weak reference to a delegate event handler (Delegate) which points to a method in object TargetObject. There needs to be a strong reference to Delegate while TargetObject is alive to keep Delegate alive, but if something with a longer lifetime references Delegate, it keeps TargetObject alive (defeating the purpose of the weak event).
It would be nice if objects subscribing to weak events didn't have to have any special implementation details such as having to hold on to a collection of delegates.
Edit Edit: Changed 'A' to 'Delegate' and 'B' to 'TargetObject'
Holy necro, but ConditionalWeakTable does just what you needed. It allows associating values with arbitrary keys, with the key value pairs as ephemerons (exactly what you were looking for, 2 years ago now.. unfortunately .NET 4 wasn't available then).
Even without ConditionalWeakTable a solution could have been a Dictionary<WeakReference, Delegate>, with periodic sweeping to remove old dead values (ie whenever the Dictionary doubles in size, remove all dead pairs). With this solution if a Delegate refers to TargetObject it'd prevent collection of the pair though - a problem ConditionalWeakTable was designed to resolve.
Just posting this in case anyone might find it useful.
This sounds like a design issue to me. If B doesn't need to know about the instance of A, why do you care about whether A is alive or not?
You can possibly do this using a container object with a weak reference to B and a strong reference to A, and a timer periodically checking whether the weak reference is still alive... but it would be a pretty grotty hack.
If you can explain why you think you need this, we may be able to suggest a better approach.
Why don't you just reference A from B?
That will keep A alive and does not require A to be aware of B...
I don't think it's good design to have an "alive" object that doesn't have any references to it.
You could always create a static List with references to objects that should stay alive, but of course you'd have to manage that yourself.
I don't see a clean solution for that, but maybe some of the super-beings on StackOverflow will come up with a solution.
Throw an event in the destructor (finalize) method of B, and write a handler for that event that kills A.
Related
I have a class (Model) an object of which serves as a blueprint for (short-lived) objects of another class (ModeledRelation). Model defines an event ModeledRelation always subscribes to in order to be notified when Model changes, so ModeledRelation objects can reflect these changes 1:1.
Now ModeledRelation objects are used somewhere else for some time and then finally go out of scope. But because the event still holds a reference to them they prevent garbage collection.
Of course I formally want the objects to be GC'ed already if the event is the only reference left. I know I can unsubscribe from the event in ModeledRelation.Dispose but that would require explicitely calling Dispose and this feels a little too crafty for a purely managed class. I also want to be safe in the case that the ModeledRelation object silently slips out of scope and not have a memory leak then.
Maybe I am thinking in a totally wrong direction and this has to be done differently.
You should look at the Weak Event Patterns :
https://learn.microsoft.com/en-us/dotnet/framework/wpf/advanced/weak-event-patterns
Upon meditating about the problem throughout the day I have recognized what is the core issue about my approach: subscribing to the event during construction of ModeledRelation. This leads to the natural conclusion that unsubscription has to be done during object destruction... which never happens because event still points to the object and GC is blocked.
Instead the right solution to the problem is to subscribe to the event when the ModeledRelation is actually used (in my case added to another object's collection). Because then the natural moment to unsubscribe is when it ceases to be used (if it is removed from the collection of the other object). If it is reused afterwards it will resubscribe, whereas if it slips out of scope it will just be GC'ed because no events point to it anymore!
I have the following code to let the GUI respond to a change in the collection.
myObservableCollection.CollectionChanged += ((sender, e) => UpdateMyUI());
First of all is this a good way to do this?
Second: what's the code to unsubscribe from this event? Is it the same but with -= (and then the complete anonymous method again)?
First of all... yes its a good way of doing it, it's clean, small form and easy to read & understand... the caveat of course is "Unless you later want to unsubscribe".
I believe Jon Skeet pointed out before that
"the specification explicitly doesn't guarantee the behaviour either way when it comes to equivalence of delegates created with anonymous methods."
So if you need to unsubscribe from the event at a later time, you'd be best to actually create a delegate instance so you can hang onto the reference for later.
var myDelegate = delegate(sender, e){UpdateMyUI()};
myObservableCollection.CollectionChanged += myDelegate;
myObservableCollection.CollectionChanged -= myDelegate;
If you need to unsubscribe from an event, you need an instanced reference. Unfortunately, that means you can't use that particular syntax.
It's an ok way to go, unless myObservableCollection is going to live longer than 'this', in which case you could end up with a memory leak, as the delegate which is created behind the scenes will conserve a reference to your 'this', which will keep it alive. If you are repeatedly creating and 'destroying' whatever is listening to the event, you will find that they aren't being collected by the garbage collector.
If this is a problem, you can go the route suggested in the answers, conserving a reference to the handler, which you must create first.
Another solution is to use weak references to create an event handler which will allow the subscriber to be collected if there are not other references. I've explored this solution in this question and answer.
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.
Say I want to make an attached property that attaches a list of object references to a view instance (a DependencyObject/FrameworkElement)...
When does it release all of those references? Will it call Dispose on attached property values if they implement it?
Its perhaps easier to think that "references" never actually get "released". They simply fall in to disuse and can no longer be found by following any chain of reference from the global space or any thread stack. They are then considered garbage and at some point the collector will come and collect the memory they occupy. If they have finalisers it will not collect the memory immediately but place the object on a queue that will call the finaliser first.
A FrameworkElement should not, for all sorts of reasons, call Dispose on any reference held in its value dictionary when it is being unloaded. Primarly because it can't know that its responsible for doing that.
Imagine a scenario in which code elsewhere created a disposable object and also attached it to a framework element. What would happen if that code later attempted to use the object only to find that the framework element had taken it upon itself to dispose it?
It is the responsibility of the code that creates a disposable object to ensure that its disposed at the appropriate point.
Think of dependency properties as key-value pairs in DependencyObjects. When you assign a value to an attached dependency property (or a regular dependency property), this value is put into the dictionary. That means items in the list will be collected by garbage collector when the view itself is garbage collected. Dispose behavior is usual too.
I have the following code to let the GUI respond to a change in the collection.
myObservableCollection.CollectionChanged += ((sender, e) => UpdateMyUI());
First of all is this a good way to do this?
Second: what's the code to unsubscribe from this event? Is it the same but with -= (and then the complete anonymous method again)?
First of all... yes its a good way of doing it, it's clean, small form and easy to read & understand... the caveat of course is "Unless you later want to unsubscribe".
I believe Jon Skeet pointed out before that
"the specification explicitly doesn't guarantee the behaviour either way when it comes to equivalence of delegates created with anonymous methods."
So if you need to unsubscribe from the event at a later time, you'd be best to actually create a delegate instance so you can hang onto the reference for later.
var myDelegate = delegate(sender, e){UpdateMyUI()};
myObservableCollection.CollectionChanged += myDelegate;
myObservableCollection.CollectionChanged -= myDelegate;
If you need to unsubscribe from an event, you need an instanced reference. Unfortunately, that means you can't use that particular syntax.
It's an ok way to go, unless myObservableCollection is going to live longer than 'this', in which case you could end up with a memory leak, as the delegate which is created behind the scenes will conserve a reference to your 'this', which will keep it alive. If you are repeatedly creating and 'destroying' whatever is listening to the event, you will find that they aren't being collected by the garbage collector.
If this is a problem, you can go the route suggested in the answers, conserving a reference to the handler, which you must create first.
Another solution is to use weak references to create an event handler which will allow the subscriber to be collected if there are not other references. I've explored this solution in this question and answer.