Today I have seen a piece of code that first seemed odd to me at first glance and made me reconsider. Here is a shortened version of the code:
if(list != null){
list.Clear();
list = null;
}
My thought was, why not replace it simply by:
list = null;
I read a bit and I understand that clearing a list will remove the reference to the objects allowing the GC to do it's thing but will not "resize". The allocated memory for this list stays the same.
On the other side, setting to null would also remove the reference to the list (and thus to its items) also allowing the GC to do it's thing.
So I have been trying to figure out a reason to do it the like the first block. One scenario I thought of is if you have two references to the list. The first block would clear the items in the list so even if the second reference remains, the GC can still clear the memory allocated for the items.
Nonetheless, I feel like there's something weird about this so I would like to know if the scenario I mentioned makes sense?
Also, are there any other scenarios where we would have to Clear() a list right before setting the reference to null?
Finally, if the scenario I mentioned made sense, wouldn't it be better off to just make sure we don't hold multiple references to this list at once and how would we do that (explicitly)?
Edit: I get the difference between Clearing and Nulling the list. I'm mostly curious to know if there is something inside the GC that would make it so that there would be a reason to Clear before Nulling.
The list.Clear() is not necessary in your scenario (where the List is private and only used within the class).
A great intro level link on reachability / live objects is http://levibotelho.com/development/how-does-the-garbage-collector-work :
How does the garbage collector identify garbage?
In Microsoft’s
implementation of the .NET framework the garbage collector determines
if an object is garbage by examining the reference type variables
pointing to it. In the context of the garbage collector, reference
type variables are known as “roots”. Examples of roots include:
A reference on the stack
A reference in a static variable
A reference in another object on the managed heap that is not eligible for garbage
collection
A reference in the form of a local variable in a method
The key bit in this context is A reference in another object on the managed heap that is not eligible for garbage collection. Thus, if the List is eligible to be collected (and the objects within the list aren't referenced elsewhere) then those objects in the List are also eligible to be collected.
In other words, the GC will realise that list and its contents are unreachable in the same pass.
So, is there an instance where list.Clear() would be useful? Yes. It might be useful if you have two references to a single List (e.g. as two fields in two different objects). One of those references may wish to clear the list in a way that the other reference is also impacted - in which list.Clear() is perfect.
This answer started as a comment for Mick, who claims that:
It depends on which version of .NET you are working with. On mobile platforms like Xamarin or mono, you may find that the garbage collector needs this kind of help in order to do its work.
That statement is begging to be fact checked. So, let us see...
.NET
.NET uses a generational mark and sweep garbage collector. You can see the abstract of the algorithm in What happens during a garbage collection
. For summary, it goes over the object graph, and if it cannot reach a object, that one can be erased.
Thus, the garbage collector will correctly identify the items of the list as collectible in the same iteration, regardless of whatever or not you clear the list. There is no need to decouple the objects beforehand.
This means that clearing the list does not help the garbage collector on the regular implementation of .NET.
Note: If there were another reference to the list, then the fact that you cleared the list would be visible.
Mono and Xamarin
Mono
As it turns out, the same is true for Mono.
Xamarin.Android
Also true for Xamarin.Android.
Xamarin.iOS
However, Xamarin.iOS requires additional considerations. In particular, MonoTouch will use wrapped Objective-C objects which are beyond the garbage collector. See Avoid strong circular references under iOS Performance. These objects require different semantics.
Xamarin.iOS will minimize the use of Objetive-C objects by keeping a cache:
C# NSObjects are also created on demand when you invoke a method or a property that returns an NSObject. At this point, the runtime will look into an object cache and determine whether a given Objective-C NSObject has already been surfaced to the managed world or not. If the object has been surfaced, the existing object will be returned, otherwise a constructor that takes an IntPtr as a parameter is invoked to construct the object.
The system keeps these objects alive even there are no references from managed code:
User-subclasses of NSObjects often contain C# state so whenever the Objective-C runtime performs a "retain" operation on one of these objects, the runtime creates a GCHandle that keeps the managed object alive, even if there are no C# visible references to the object. This simplifies bookeeping a lot, since the state will be preserved automatically for you.
Emphasis mine.
Thus, under Xamarin.iOS, if there were a chance that the list might contain wrapped Objetive-C objects, this code would help the garbage collector.
See the question How does memory management works on Xamarin.IOS, Miguel de Icaza explains in his answer that the semantics are to "retain" the object when you take a reference and "release" it when the reference is null.
On the Objetive-C side, "release" does not mean to destroy the object. Objetive-C uses a reference count garbage collector. When we "retain" the object the counter is incremented and when we "release" the counter is decreased. The system destroys the object when the counter reaches zero. See: About Memory Management.
Therefore, Objetive-C is bad at handling circular references (if A references B and B references A, their reference count is not zero, even if they cannot be reached), thus, you should avoid them in Xamarin.iOS. In fact, forgetting to decouple references will lead to leaks in Xamarin.iOS... See: Xamarin iOS memory leaks everywhere.
Others
dotGNU also uses a generational mark and sweep garbage collector.
I also had a look at CrossNet (that compiles IL to C++), it appears they attempted to implement it too. I do not know how good it is.
It depends on which version of .NET you are working with. On mobile platforms like Xamarin or mono, you may find that the garbage collector needs this kind of help in order to do its work. Whereas on desktop platforms the garbage collector implementation may be more elaborate. Each implementation of the CLI out there is going to have it's own implementation of the garbage collector and it is likely to behave differently from one implementation to another.
I can remember 10 years ago working on a Windows Mobile application which had memory issues and this sort of code was the solution. This was probably due to the mobile platform requiring a garbage collector that was more frugal with processing power than the desktop.
Decoupling objects helps simplify the analysis the garbage collector needs to do and helps avoid scenarios where the garbage collector fails to recognise a large graph of objects has actually become disconnected from all the threads in your application. Which results in memory leaks.
Anyone who believes you can't have memory leaks in .NET is an inexperienced .NET developer. On desktop platforms just ensuring Dispose is called on objects which implement them may be enough, however with other implementations you may find it is not.
List.Clear() will decouple the objects in the list from the list and each other.
EDIT: So to be clear I'm not claiming that any particular implementation currently out there is susceptible to memory leaks. And again depending on when this answer is read the robustness of the garbage collector on any implementation of the CLI currently out there could have changed since the time writing this.
Essentially I'm suggesting if you know that your code needs to be cross platform and used across many implementations of the .NET framework, especially implementations of the .NET framework for mobile devices, it could be worth investing time into decoupling objects when they are no longer required. In that case I'd start off by adding decoupling to classes that already implement Dispose, and then if needed look at implementing IDisposable on classes that don't implement IDisposable and ensuring Dispose is called on those classes.
How to tell for sure if it's needed? You need to instrument and monitor the memory usage of your application on each platform it is to be deployed on. Rather than writing lots of superfluous code, I think the best approach is to wait until your monitoring tools indicate you have memory leaks.
As mentioned in the docs:
List.Clear Method (): Count is set to 0, and references to other
objects from elements of the collection are also released.
In your 1st snippet:
if(list != null){
list.Clear();
list = null;
}
If you just set the list to null, it means that you release the reference of your list to the actual object in the memory (so the list itself is remain in the memory) and waiting for the Garbage Collector comes and release its allocated memory.
But the problem is that your list may contain elements that hold a reference to another objects, for example:
list → objectA, objectB, objectC
objectB → objectB1, objectB2
So, after setting the list to null, now list has no reference and it should be collected by Garbage Collector later, but objectB1 and objectB2 has a reference from objectB (still be in the memory) and because of that, Garbage Collector need to analyse the object reference chain. To make it less confusing, this snippet use .Clear() function to remove this confusion.
Clearing the list ensures that if the list is not garbage collected for some reason, then at the very least, the elements it contained can still be disposed of.
As stated in the comments, preventing other references to the list from existing requires careful planning, and clearing the list before nulling it doesn't incur a big enough performance hit to justify trying to avoid doing so.
Related
Ok so I understand about the stack and the heap (values live on the Stack, references on the Heap).
When I declare a new instance of a Class, this lives on the heap, with a reference to this point in memory on the stack. I also know that C# does it's own Garbage Collection (ie. It determines when an instanciated class is no longer in use and reclaims the memory).
I have 2 questions:
Is my understanding of Garbage Collection correct?
Can I do my own? If so is there any real benefit to doing this myself or should I just leave it.
I ask because I have a method in a For loop. Every time I go through a loop, I create a new instance of my Class. In my head I visualise all of these classes lying around in a heap, not doing anything but taking up memory and I want to get rid of them as quickly as I can to keep things neat and tidy!
Am I understanding this correctly or am I missing something?
Ok so I understand about the stack and the heap (values live on the Stack, references on the Heap
I don't think you understand about the stack and the heap. If values live on the stack then where does an array of integers live? Integers are values. Are you telling me that an array of integers keeps its integers on the stack? When you return an array of integers from a method, say, with ten thousand integers in it, are you telling me that those ten thousand integers are copied onto the stack?
Values live on the stack when they live on the stack, and live on the heap when they live on the heap. The idea that the type of a thing has to do with the lifetime of its storage is nonsense. Storage locations that are short lived go on the stack; storage locations that are long lived go on the heap, and that is independent of their type. A long-lived int has to go on the heap, same as a long-lived instance of a class.
When I declare a new instance of a Class, this lives on the heap, with a reference to this point in memory on the stack.
Why does the reference have to go on the stack? Again, the lifetime of the storage of the reference has nothing to do with its type. If the storage of the reference is long-lived then the reference goes on the heap.
I also know that C# does it's own Garbage Collection (ie. It determines when an instanciated class is no longer in use and reclaims the memory).
The C# language does not do so; the CLR does so.
Is my understanding of Garbage Collection correct?
You seem to believe a lot of lies about the stack and the heap, so odds are good no, it's not.
Can I do my own?
Not in C#, no.
I ask because I have a method in a For loop. Every time I go through a loop, I create a new instance of my Class. In my head I visualise all of these classes lying around in a heap, not doing anything but taking up memory and I want to get rid of them as quickly as I can to keep things neat and tidy!
The whole point of garbage collection is to free you from worrying about tidying up. That's why its called "automatic garbage collection". It tidies for you.
If you are worried that your loops are creating collection pressure, and you wish to avoid collection pressure for performance reasons then I advise that you pursue a pooling strategy. It would be wise to start with an explicit pooling strategy; that is:
while(whatever)
{
Frob f = FrobPool.FetchFromPool();
f.Blah();
FrobPool.ReturnToPool(f);
}
rather than attempting to do automatic pooling using a resurrecting finalizer. I advise against both finalizers and object resurrection in general unless you are an expert on finalization semantics.
The pool of course allocates a new Frob if there is not one in the pool. If there is one in the pool, then it hands it out and removes it from the pool until it is put back in. (If you forget to put a Frob back in the pool, the GC will get to it eventually.) By pursuing a pooling strategy you cause the GC to eventually move all the Frobs to the generation 2 heap, instead of creating lots of collection pressure in the generation 0 heap. The collection pressure then disappears because no new Frobs are allocated. If something else is producing collection pressure, the Frobs are all safely in the gen 2 heap where they are rarely visited.
This of course is the exact opposite of the strategy you described; the whole point of the pooling strategy is to cause objects to hang around forever. Objects hanging around forever is a good thing if you're going to use them.
Of course, do not make these sorts of changes before you know via profiling that you have a performance problem due to collection pressure! It is rare to have such a problem on the desktop CLR; it is rather more common on the compact CLR.
More generally, if you are the kind of person who feels uncomfortable having a memory manager clean up for you on its schedule, then C# is not the right language for you. Consider C instead.
values live on the Stack, references on the Heap
This is an implementation detail. There is nothing to stop a .NET Framework from storing both on the stack.
I also know that C# does it's own Garbage Collection
C# has nothing to do with this. This is a service provided by the CLR. VB.NET, F#, etc all still have garbage collection.
The CLR will remove an object from memory if it has no strong roots. For example, when your class instance goes out of scope in your for loop. There will be a few lying around, but they will get collected eventually, either by garbage collection or the program terminating.
Can I do my own? If so is there any real benefit to doing this myself or should I just leave it?
You can use GC.Collect to force a collection. You should not do it because it is an expensive operation. More expensive than letting a few objects occupy memory a little bit longer than they are absolutely needed. The GC is incredibly good at what it does on its own. You will also force short lived objects to promote to generations they wouldn't get normally.
First off, to Erics seminal post about The truth about value types
Secondly on Garbage collection, the collector knows far more about your running program than you do, don't try to second guess it unless you're in the incredibly unlikely situation that you have a memory leak.
So to your second question, no don't try to "help" the GC.
I'll find a post to this effect on the CG and update this answer.
Can I do my own? If so is there any real benefit to doing this myself or should I just leave it.
Yes you can with GC.Collect but you shouldn't. The GC is optimized for variables that are short lived, ones in a method, and variables that are long lived, ones that generally stick around for the life time of the application.
Variables that are in-between aren't as common and aren't really optimum for the GC.
By forcing a GC.Collect you're more likely to cause variables in scope to be in forced into that in-between state which is the opposite from you are trying to accomplish.
Also from the MSDN article Writing High-Performance Managed Applications : A Primer
The GC is self-tuning and will adjust itself according to applications
memory requirements. In most cases programmatically invoking a GC will
hinder that tuning. "Helping" the GC by calling GC.Collect will more
than likely not improve your applications performance
Your understanding of Garbage Collection is good enough. Essentially, an unreferenced instance is deemed as being out-of-scope and no longer needed. Having determined this, the collector will remove an unreferenced object at some future point.
There's no way to force the Garbage Collector to collect just a specific instance. You can ask it to do its normal "collect everything possible" operation GC.Collect(), but you shouldn't.; the garbage-collector is efficient and effective if you just leave it to its own devices.
In particular it excels at collecting objects which have a short lifespan, just like those that are created as temporary objects. You shouldn't have to worry about creating loads of objects in a loop, unless they have a long lifespan that prevents immediate collection.
Please see this related question with regard to the Stack and Heap.
In your specific scenario, agreed, if you new up objects in a for-loop then you're going to have sub-optimal performance. Are the objects stored (or otherwise used) within the loop, or are they discarded? If the latter, can you optimize this by newing up one object outside the loop and re-using it?
With regard to can you implement your own GC, there is no explicit delete keyword in C#, you have to leave it to the CLR. You can however give it hints such as when to collect, or what to ignore during collection, however I'd leave that unless absolutely necessary.
Best regards,
Read the following article by Microsoft to get a level of knowledge about Garbage Collection in C#. I'm sure it'll help anyone who need information regarding this matter.
Memory Management and Garbage Collection in the .NET Framework
If you are interested in performance of some areas in your code when writing C#, you can write unsafe code. You will have a plus of performance, and also, in your fixed block, the garbage collector most likely will not occur.
Garbage collection is basically reference tracking. I can't think of any good reason why you would want to change it. Are you having some sort of problem where you find that memory isn't being freed? Or maybe you are looking for the dispose pattern
Edit:
Replaced "reference counting" with "reference tracking" to not be confused with the Increment/Decrement Counter on object Reference/Dereference (eg from Python).
I thought it was pretty common to refer to the object graph generation as "Counting" like in this answer:
Why no Reference Counting + Garbage Collection in C#?
But I will not pick up the glove of (the) Eric Lippert :)
I'm using the new MemoryCache in .Net 4, with a max cache size limit in MB (I've tested it set between 10 and 200MB, on systems with between 1.75 and 8GB of memory). I don't set any time based expiration on the objects, as I'm using the cache simply as a high performance drive, and as long as there is space, I want it used. To my surprise, the cache refused to evict any objects, to the point that I would get SystemOutOfMemory exceptions.
I fired up perfmon, wired up my application to .Net CLR Memory\#Bytes In All Heaps, .Net Memory Cache 4.0, and Process\Private Bytes -- indeed, the memory consumption was out of control, and no cache trims were being registered.
Did some googling and stackoverflowing, downloaded and attached the CLRProfiler, and wham: evictions everywhere! The memory stayed within reasonable bounds based upon the memory size limit I had set. Ran it in debug mode again, no evictions. CLRProfiler again, evictions.
I finally noticed that the profiler forced the application to run without concurrent garbage collection (also see useful SO Concurrent Garbage Collection Question). I turned it off in my app.config, and, sure enough, evictions!
This seems like at best an outrageous lack of documentation to not say: this only works with non-concurrent garbage collection -- though I image since its ported from ASP.NET, they may not have had to worry about concurrent garbage collection.
So has anyone else seen this? I'd love to get some other experiences out there, and maybe some more educated insights.
Update 1
I've reproduced the issue within a single method: it seems that the cache must be written to in parallel for the cache evictions not to fire (in concurrent garbage collection mode). If there is some interest, I'll upload the test code to a public repo. I'm definitely getting toward the deep end of the the CLR/GC/MemoryCache pool, and I think I forgot my floaties...
Update 2
I published test code on CodePlex to reproduce the issue. Also, possibly of interest, the original production code runs in Azure, as a Worker Role. Interesting, changing the GC concurrency setting in the role's app.config has no effect. Possibly Azure overrides GC settings much like ASP.NET? Further, running the test code under WPF vs a Console application will produce slightly different eviction results.
You can "force" a garbage collection right after the problematic method and see if the problem reproduces executing:
System.Threading.Thread.Sleep(200);
GC.Collect();
GC.WaitForPendingFinalizers();
right at the end of the method (make sure that you free any handles to reference objects and null them out). If this prevents memory leakage, and then yes, there may be a runtime bug.
Stop-the-world garbage collection is based on determining whether a strong live reference to an object exists at the moment the world is stopped. Concurrent garbage collection usually determines whether a strong live reference to an object has existed since some particular time in the past. My conjecture would be that many strong references to objects held in WeakReferences are being individually created and discarded. If a stop-the-world garbage collector fires between the time a particular object is created and the time it's discarded, that particular object will be kept alive, but previously-discarded objects will not. By contrast, a concurrent garbage collector may not detect that all strong references an object have been discarded until a certain amount of time goes by without any strong references to that object being created.
I've sometimes wished that .net would offer something between a strong reference and a weak one, which would prevent an object from being wiped from memory, but would not protect it from being finalized or having weak WeakReferences to it invalidated. Such references would slightly complicate the GC process, requiring every object to have separate flags indicating whether strong and quasi-weak references exist to it, and whether it has been scanned for both strong and quasi-weak references, but such a feature could be helpful in many "weak event" scenarios.
I found this entry while searching for a similiar topic and I'm focusing on your Out of Memory exception.
If you put an object in the cache then it still may be referencing other objects and therefore these objects would not be garbage collected -- hence the out of memory exception and probably a CPU being pegged out due to Gen 2 garbage collection.
Are you putting "used" objects on the cache or clones of "used" objects on the cache? If you put a clone on the cache then the "used" object that possible references other objects could be garbage collected.
If you shut off your caching mechanism does your program still run out of memory? If it doesn't run out of memory then that would prove that the objects you would otherwise be putting on the cache are still holding references to other objects hindering garbage collection.
Forcing garbage collection is not a best practice and shouldn't have to be done. In this scenario forcing a garbage collection wouldn't dispose of referenced objects anyway.
MemoryCache definately has some issues. It ate 160Mb of memory on my asp.net server, just changed to simple list and added some extra logic to get the same functionality.
This question already has answers here:
Understanding garbage collection in .NET
(2 answers)
Closed 7 years ago.
I'm learning C# coming from python and wish to know how the C# garbage collector works - I found that I understood a lot more about python once I figured out what it was doing behind the scenes, and wish to avoid making the sort of noob errors I made at first when learning python.
I've not been able to find any good clear explanations of when an item is garbage collected and am left with questions such as
"What happens to an object when its last reference passes out of scope?" Does that object get garbage collected or is it still there when you pass back into the scope in which it was defined?
"At what point is the number of refernces decremented?" Leading me to wonder whether it even uses reference counting or some other technique...
Answers to these, or even better a clear consise overview of what's actually going on will win cookies (or upvotes), and even better if your answer compares it to the python way of doing things. I'm not interested in which is better, just the details. Also answers on my original post on programmers.stackexchange would be much appreciated...
The dotnet GC engine is a mark-and-sweep engine rather than a reference-counter engine like you're used to in python. The system doesn't maintain a count of references to a variable, but rather runs a "collection" when it needs to reclaim RAM, marking all of the currently-reachable pointers, and removing all the pointers that aren't reachable (and therefore are out of scope).
You can find out more about how it works here:
http://msdn.microsoft.com/en-us/library/ee787088.aspx
The system finds "reachable" objects by starting at specific "root" locations, like global objects and objects on the stack, and traces all objects referenced by those, and all the objects referenced by those, etc., until it's built a complete tree. This is faster than it sounds.
At some indeterminate point in time after the last reference to an object disappears, the object will be collected.
The second part of your first question doesn't make sense.
If you can get back into the scope in which an object was defined (eg, a lambda expression), there is obviously still a reference.
The GC does not use reference counting at all.
Rather, it uses a mark-and-sweep algorithm.
Garbage collection is not triggered by references going out of scope. Garbage collection is usually triggered when allocating storage for new objects - specifically when generation zero's budget is exhausted. I.e. there may be a significant delay between when objects are eligible for garbage collection and when they are actually collected. As others have already pointed out, the CLR doesn't use reference counting. Instead it employs a mark and sweep approach.
A good source of information on all the details about how garbage collection works is Jeffrey Ricther's book CLR via C#. The book goes into great detail about how the heap is partitioned and how garbage collection works. Highly recommended if you're interested in .NET implementation details.
When an object no longer has a strong reference to it, nothing immediately happens. Each object is assigned a generation, 0, 1, or 2. Initially, all objects are placed in the 0 generation. Using the Mark-and-Sweep algorithm, the garbage collector will periodically check a generation. If the instance still has any strong references, it is promoted to a higher generation up to 2 (Which are usually checked less frequently). The theory behind this is that most objects are generally short lived, so if an object has lived long enough to make it to Generation 1, it doesn't need to be checked as often.
There is no reference counting in the .NET GC. Rather it uses mark and sweep.
I recently came across a piece of Java code with WeakReferences - I had never seen them deployed although I'd come across them when they were introduced. Is this something that should be routinely used or only when one runs into memory problems? If the latter, can they be easily retrofitted or does the code need serious refactoring? Can the average Java (or C#) programmer generally ignore them?
EDIT Can any damage be done by over-enthusiastic use of WRs?
Weak references are all about garbage collection. A standard object will not "disappear" until all references to it are severed, this means all the references your various objects have to it have to be removed before garbage collection will consider it garbage.
With a weak reference just because your object is referenced by other objects doesn't necessarily mean it's not garbage. It can still get picked up by GC and get removed from memory.
An example: If I have a bunch of Foo objects in my application I might want to use a Set to keep a central record of all the Foo's I have around. But, when other parts of my application remove a Foo object by deleting all references to it, I don't want the remaining reference my Set holds to that object to keep it from being garbage collected! Really I just want it to disappear from my set. This is where you'd use something like a Weak Set (Java has a WeakHashMap) instead, which uses weak references to its members instead of "strong" references.
If your objects aren't being garbage collected when you want them to then you've made an error in your book keeping, something's still holding a reference that you forgot to remove. Using weak references can ease the pain of such book keeping, since you don't have to worry about them keeping an object "alive" and un-garbage-collected, but you don't have to use them.
You use them whenever you want to have a reference to an object without keeping the object alive yourself. This is true for many caching-like features, but also play an important role in event handling, where a subscriber shall not be kept alive by being subscribed to an event.
A small example: A timer event which refreshes some data. Any number of objects can subscribe on the timer in order to get notified, but the bare fact that they subscribed on the timer should not keep them alive. So the timer should have weak references to the objects.
Can any damage be done by
over-enthusiastic use of WRs?
Yes it can.
One concern is that weak references make your code more complicated and potentially error prone. Any code that uses a weak reference needs to deal with the possibility that the reference has been broken each time it uses it. If you over-use weak references you end up writing lots of extra code. (You can mitigate this by hiding each weak reference behind a method that takes care of the checking, and re-creates the discarded object on demand. But this may not necessarily be as simple as that; e.g. if the re-creation process involves network access, you need to cope with the possibility of re-creation failure.)
A second concern is that there are runtime overheads with using weak references. The obvious costs are those of creating weak references and calling get on them. A less obvious cost is that significant extra work needs to be done each time the GC runs.
A final concern is that if you use a weak references for something that your application is highly likely to need in the future, you may incur the cost of repeatedly recreating it. If this cost is high (in terms of CPU time, IO bandwidth, network traffic, whatever) your application may perform badly as a result. You may be better off giving the JVM more memory and not using weak references at all.
Off course, this does not mean you should avoid using weak references entirely. Just that you need to think carefully. And probably you should first run a memory profiler on your application to figure out where your memory usage problems stem from.
A good question to ask when considering use of a WeakReference is how one would feel if the weak reference were invalidated the instant no strong references existed to the object. If that would make the WeakReference less useful, then a WeakReference is probably not the best thing to use. If that would be an improvement over the non-deterministic invalidation that comes from garbage-collection, then a WeakReference is probably the right choice.
I need to dispose of an object so it can release everything it owns, but it doesn't implement the IDisposable so I can't use it in a using block. How can I make the garbage collector collect it?
You can force a collection with GC.Collect(). Be very careful using this, since a full collection can take some time. The best-practice is to just let the GC determine when the best time to collect is.
Does the object contain unmanaged resources but does not implement IDisposable? If so, it's a bug.
If it doesn't, it shouldn't matter if it gets released right away, the garbage collector should do the right thing.
If it "owns" anything other than memory, you need to fix the object to use IDisposable. If it's not an object you control this is something worth picking a different vendor over, because it speaks to the core of how well your vendor really understands .Net.
If it does just own memory, even a lot of it, all you have to do is make sure the object goes out of scope. Don't call GC.Collect() — it's one of those things that if you have to ask, you shouldn't do it.
You can't perform garbage collection on a single object. You could request a garbage collection by calling GC.Collect() but this will effect all objects subject to cleanup. It is also highly discouraged as it can have a negative effect on the performance of later collections.
Also, calling Dispose on an object does not clean up it's memory. It only allows the object to remove references to unmanaged resources. For example, calling Dispose on a StreamWriter closes the stream and releases the Windows file handle. The memory for the object on the managed heap does not get reclaimed until a subsequent garbage collection.
Chris Sells also discussed this on .NET Rocks. I think it was during his first appearance but the subject might have been revisited in later interviews.
http://www.dotnetrocks.com/default.aspx?showNum=10
This article by Francesco Balena is also a good reference:
When and How to Use Dispose and Finalize in C#
http://www.devx.com/dotnet/Article/33167/0/page/1
Garbage collection in .NET is non deterministic, meaning you can't really control when it happens. You can suggest, but that doesn't mean it will listen.
Tells us a little bit more about the object and why you want to do this. We can make some suggestions based off of that. Code always helps. And depending on the object, there might be a Close method or something similar. Maybe the useage is to call that. If there is no Close or Dispose type of method, you probably don't want to rely on that object, as you will probably get memory leaks if in fact it does contain resourses which will need to be released.
If the object goes out of scope and it have no external references it will be collected rather fast (likely on the next collection).
BEWARE: of f ra gm enta tion in many cases, GC.Collect() or some IDisposal is not very helpful, especially for large objects (LOH is for objects ~80kb+, performs no compaction and is subject to high levels of fragmentation for many common use cases) which will then lead to out of memory (OOM) issues even with potentially hundreds of MB free. As time marches on, things get bigger, though perhaps not this size (80 something kb) for LOH relegated objects, high degrees of parallelism exasperates this issue due simply due to more objects in less time (and likely varying in size) being instantiated/released.
Array’s are the usual suspects for this problem (it’s also often hard to identify due to non-specific exceptions and assertions from the runtime, something like “high % of large object heap fragmentation” would be swell), the prognosis for code suffering from this problem is to implement an aggressive re-use strategy.
A class in Systems.Collections.Concurrent.ObjectPool from the parallel extensions beta1 samples helps (unfortunately there is not a simple ubiquitous pattern which I have seen, like maybe some attached property/extension methods?), it is simple enough to drop in or re-implement for most projects, you assign a generator Func<> and use Get/Put helper methods to re-use your previous object’s and forgo usual garbage collection. It is usually sufficient to focus on array’s and not the individual array elements.
It would be nice if .NET 4 updated all of the .ToArray() methods everywhere to include .ToArray(T target).
Getting the hang of using SOS/windbg (.loadby sos mscoreei for CLRv4) to analyze this class of issue can help. Thinking about it, the current garbage collection system is more like garbage re-cycling (using the same physical memory again), ObjectPool is analogous to garbage re-using. If anybody remembers the 3 R’s, reducing your memory use is a good idea too, for performance sakes ;)