I'd like to know when i should and shouldn't be wrapping things in a USING block.
From what I understand, the compiler translates it into a try/finally, where the finally calls Dispose() on the object.
I always use a USING around database connections and file access, but its more out of habit rather than a 100% understanding. I know you should explicity (or with a using) Dispose() objects which control resources, to ensure they are released instantly rather than whenever the CLR feels like it, but thats where my understanding breaks down.
Are IDisposables not disposed of when they go out of scope?
Do I only need to use a USING when my object makes use of Dispose to tidy itself up?
Thanks
Edit: I know there are a couple of other posts on the USING keyword, but I'm more interested in answers relating the the CLR and exactly whats going on internally
Andrew
No, IDisposable items are not disposed when they go out of scope. It is for precisely this reason that we need IDisposable - for deterministic cleanup.
They will eventually get garbage collected, and if there is a finalizer it will (maybe) be called - but that could be a long time in the future (not good for connection pools etc). Garbage collection is dependent on memory pressure - if nothing wants extra memory, there is no need to run a GC cycle.
Interestingly (perhaps) there are some cases where "using" is a pain - when the offending class throws an exception on Dispose() sometimes. WCF is an offender of this. I have discussed this topic (with a simple workaround) here.
Basically - if the class implements IDisposable, and you own an instance (i.e. you created it or whatever), it is your job to ensure that it gets disposed. That might mean via "using", or it might mean passing it to another piece of code that assumes responsibility.
I've actually seen debug code of the type:
#if DEBUG
~Foo() {
// complain loudly that smoebody forgot to dispose...
}
#endif
(where the Dispose calls GC.SuppressFinalize)
"Are IDisposables not disposed of when
they go out of scope?"
No. If the IDisposable object is finalizable, which is not the same thing, then it will be finalized when it's garbage collected.
Which might be soon or might be almost never.
Jeff Richter's C#/CLR book is very good on all this stuff, and the Framework Design Guidelines book is also useful.
Do I only need to use a USING when my
object makes use of Dispose to tidy
itself up?
You can only use 'using' when the object implements IDisposable. The compiler will object if you try to do otherwise.
To add to the other answers, you should use using (or an explicit Dispose) whenever an object holds any resources other than managed memory. Examples would be things like files, sockets, database connections, or even GDI drawing handles.
The garbage collector would eventually finalise these objects, but only at some unspecified time in the future. You can't rely on it happening in a timely fashion, and you might have run out of that resource in the meantime.
Related
Do I have to use a using statement to dispose immediately even in a method? Or will the ending of the method cause an automatic Dispose of all local variables including graphics?
(I’m asking this because I’ve seen examples that have Dispose calls at the end of methods, and wanted to know if that’s really necessary.)
Thanks.
Yes, you do. Going out of scope does not do anything; it does not call Dispose(), it does not garbage-collect, it does not call a finalizer.
If the type is IDisposable, then yes. It is your job to tidy up after yourself (assuming that the object is actually "done with" at this point).
Possible side-effects of not doing this:
files get left open and cause access-exceptions (FileStream)
connections get left open and cause pool saturation (DbConnection)
unmanaged handles get saturated and cause resource starvation (any winforms/etc)
transactions get left open and cause blocking (TransactionScope / DbTransaction)
etc
basically, bad things.
Furthermore, in most cases where you see Dispose() at the bottom of the method, a using would be preferable. There are some cases where that isn't possible (fields on an object, for example), but the point remains: it sounds like those are bad examples.
You don't ever have to dispose of an object. But if you want to release unmanaged resources earlier rather than whenever the GC gets around to it (which is when it processes the fReachable queue) then yes you should since the scope of of a method does not determine when a disposable object's finalize will be called.
For more you might want to read Jeffrey Richter's Garbage Collection: Automatic Memory Management in the Microsoft .NET Framework
As delnan noted if there's a hard limit to the resources you're using you're much more likely to impacted if you let it get reclaimed by the GC rather than calling Dispose. DB connections are a good example of this kind of reasource and is certainly the reason why the NumberOfReclaimedConnections performance counter got created.
Yes, use using. It is a great statement.
Actually if you don't call Dispose (either manually or by using using), you can end up in a situation where your managed memory is stil quite empty, but unmanaged resources are depleted. Eventually it can cause a deadlock because other objects or other processes will wait for your unmanaged resources and you will wait for them. (Garbage collector won't run, because managed memory still won't be full.)
So please call Dispose as soon as you can. Once you don't need an object which has got that method defined, call it.
From what I can tell, it is an accepted rule that if you have a class A that has a member m that is IDisposable, A should implement IDisposable and it should call m.Dispose() inside of it.
I can't find a satisfying reason why this is the case.
I understand the rule that if you have unmanaged resources, you should provide a finalizer along with IDisposable so that if the user doesn't explicitly call Dispose, the finalizer will still clean up during GC.
However, with that rule in place, it seems like you shouldn't need to have the rule that this question is about. For instance...
If I have a class:
class MyImage{
private Image _img;
... }
Conventions states that I should have MyImage : IDisposable. But if Image has followed conventions and implemented a finalizer and I don't care about the timely release of resources, what's the point?
UPDATE
Found a good discussion on what I was trying to get at here.
But if Image has followed conventions and implemented a finalizer and I don't care about the timely release of resources, what's the point?
You've missed the point of Dispose entirely. It's not about your convenience. It's about the convenience of other components that might want to use those unmanaged resources. Unless you can guarantee that no other code in the system cares about the timely release of resources, and the user doesn't care about timely release of resources, you should release your resources as soon as possible. That's the polite thing to do.
In the classic Prisoner's Dilemma, a lone defector in a world of cooperators gains a huge benefit. But in your case, being a lone defector produces only the tiny benefit of you personally saving a few minutes by writing low-quality, best-practice-ignoring code. It's your users and all the programs they use that suffer, and you gain practically nothing. Your code takes advantage of the fact that other programs unlock files and release mutexes and all that stuff. Be a good citizen and do the same for them. It's not hard to do, and it makes the whole software ecosystem better.
UPDATE: Here is an example of a real-world situation that my team is dealing with right now.
We have a test utility. It has a "handle leak" in that a bunch of unmanaged resources aren't aggressively disposed; it's leaking maybe half a dozen handles per "task". It maintains a list of "tasks to do" when it discovers disabled tests, and so on. We have ten or twenty thousand tasks in this list, so we very quickly end up with so many outstanding handles -- handles that should be dead and released back into the operating system -- that soon none of the code in the system that is not related to testing can run. The test code doesn't care. It works just fine. But eventually the code being tested can't make message boxes or other UI and the entire system either hangs or crashes.
The garbage collector has no reason to know that it needs to run finalizers more aggressively to release those handles sooner; why should it? Its job is to manage memory. Your job is to manage handles, so you've got to do that job.
But if Image has followed conventions
and implemented a finalizer and I
don't care about the timely release of
resources, what's the point?
Then there isn't one, if you don't care about timely release, and you can ensure that the disposable object is written correct (in truth I never make an assumption like that, not even with MSs code. You never know when something accidentally slipped by). The point is that you should care, as you never know when it will cause a problem. Think about an open database connection. Leaving it hanging around, means that it isn't replaced in the pool. You can run out if you have several requests come in for one.
Nothing says you have to do it if you don't care. Think of it this way, it's like releasing variables in an unmanaged program. You don't have to, but it is highly advisable. If for no other reason the person inheriting from the program doesn't have to wonder why it wasn't taken care of and then try and clear it up.
Firstly, there's no guaranteeing when an object will be cleaned up by the finalizer thread - think about the case where a class has a reference to a sql connection. Unless you make sure this is disposed of promptly, you'll have a connection open for an unknown period of time - and you won't be able to reuse it.
Secondly, finalization is not a cheap process - you should be making sure that if your objects are disposed of properly you're calling GC.SuppressFinalize(this) to prevent finalization happening.
Expanding on the "not cheap" aspect, the finalizer thread is a high-priority thread. It will take resources away from your main application if you give it too much to do.
Edit: Ok, here's a blog article by Chris Brummie about Finalization, including why it is expensive. (I knew I'd read loads about this somewhere)
If you don't care about the timely release of resources, then indeed there is no point. If you can be sure that the code is only for your consumption and you've got plenty of free memory/resources why not let GC hoover it up when it chooses to. OTOH, if someone else is using your code and creating many instances of (e.g.) MyImage, it's going to be pretty difficult to control memory/resource usage unless it disposes nicely.
Many classes require that Dispose be called to ensure correctness. If some C# code uses an iterator with a "finally" block, for example, the code in that block will not run if an enumerator is created with that iterator and not disposed. While there a few cases where it would be impractical to ensure objects were cleaned up without finalizers, for the most part code which relies upon finalizers for correct operation or to avoid memory leaks is bad code.
If your code acquires ownership of an IDisposable object, then unless either the object's cleass is sealed or your code creates the object by calling a constructor (as opposed to a factory method) you have no way of knowing what the real type of the object is, and whether it can be safely abandoned. Microsoft may have originally intended that it should be safe to abandon any type of object, but that is unrealistic, and the belief that it should be safe to abandon any type of object is unhelpful. If an object subscribes to events, allowing for safe abandonment will require either adding a level of weak indirection to all events, or a level of (non-weak) indirection to all other accesses. In many cases, it's better to require that a caller Dispose an object correctly than to add significant overhead and complexity to allow for abandonment.
Note also, btw, that even when objects try to accommodate abandonment it can still be very expensive. Create a Microsoft.VisualBasic.Collection (or whatever it's called), add a few objects, and create and Dispose a million enumerators. No problem--executes very quickly. Now create and abandon a million enumeartors. Major snooze fest unless you force a GC every few thousand enumerators. The Collection object is written to allow for abandonment, but that doesn't mean it doesn't have a major cost.
If an object you're using implements IDisposable, it's telling you it has something important to do when you're finished with it. That important thing may be to release unmanaged resources, or unhook from events so that it doesn't handle events after you think you're done with it, etc, etc. By not calling the Dispose, you're saying that you know better about how that object operates than the original author. In some tiny edge cases, this may actually be true, if you authored the IDisposable class yourself, or you know of a bug or performance problem related to calling Dispose. In general, it's very unlikely that ignoring a class requesting you to dispose it when you're done is a good idea.
Talking about finalizers - as has been pointed out, they have a cost, which can be avoided by Disposing the object (if it uses SuppressFinalize). Not just the cost of running the finalizer itself, and not just the cost of having to wait till that finalizer is done before the GC can collect the object. An object with a finalizer survives the collection in which it is identified as being unused and needing finalization. So it will be promoted (if it's not already in gen 2). This has several knock on effects:
The next higher generation will be collected less frequently, so after the finalizer runs, you may be waiting a long time before the GC comes around to that generation and sweeps your object away. So it can take a lot longer to free memory.
This adds unnecessary pressure to the collection the object is promoted to. If it's promoted from gen 0 to gen 1, then now gen 1 will fill up earlier than it needs to.
This can lead to more frequent garbage collections at higher generations, which is another performance hit.
If the object's finalizer isn't completed by the time the GC comes around to the higher generation, the object can be promoted again. Hence in a bad case you can cause an object to be promoted from gen 0 to gen 2 without good reason.
Obviously if you're only doing this on one object it's not likely to cost you anything noticeable. If you're doing it as general practice because you find calling Dispose on objects you're using tiresome, then it can lead to all of the problems above.
Dispose is like a lock on a front door. It's probably there for a reason, and if you're leaving the building, you should probably lock the door. If it wasn't a good idea to lock it, there wouldn't be a lock.
Even if you don't care in this particular case, you should still follow the standard because you will care in some cases. It's much easier to set a standard and follow it always based on specific guidelines than have a standard that you sometimes disregard. This is especially true as your team grows and your product ages.
Scenario
Lets say that I've decided I really need to call GC.Collect();
What do I need to do to ensure that an object is prepared to be garbage collected appropriately before I actually call this method?
Would assigning null to all properties of the object be enough? Or just assigning null to the object itself?
If you really need to know why.....
I have a distributed application in WCF that sends a DataContract over the wire every few seconds, containing 8 Dictionaries as DataMembers.
This is a lot of data and when it comes into the Client-side interface, a whole new DataContract object is created and the memory usage of the application is growing so big that I'm getting OutOfMemory Exceptions.
Thanks
EDIT
Thanks for all the comments and answers, it seems that everyone shares the same opinion.
What I cannot understand is how I can possibly dispose correctly because the connection is open constantly.
Once I've copied the data over from the incoming object, I don't need the object anymore, so would simply implementing IDisposable on that DataContract object be enough?
My original problem is here - Distributed OutOfMemory Exceptions
As long as nothing else can see the object it is already eligible for collection; nothing more is required. The key point here is to ensure that nothing else is watching it (or at least, nothing with a longer lifetime):
is it in a field somewhere?
is it in a variable in a method that is incomplete? (an infinite loop or iterator block, perhaps)
is it in a collection somewhere?
has it subscribed to some event?
it is captured in a closure (lambda / anon-method) that is still alive?
I genuinely doubt that GC.Collect() is the answer here; if it was eligible it would have already been collected. If it isn't elgible, calling GC.Collect() certainly won't help and quite possibly will make things worse (by tying up CPU when nothing useful can be collected).
You don't generally need to do anything.
If the object is no longer referenced then it's a candidate for collection. (And, conversely, if the object is still referenced then it's not a candidate for collection, however you "prepare" it.)
You need to clean up any unmanaged resources like database connections etc.
Typically by implementing IDisposable and call Dispose.
If you have a finalizer you should call GC.SuppressFinilize.
The rest is cleaned up by the garbage collector.
Edit:
And, oh, naturally you need to release all references to your object.
But, and here is this big but. Unless you have a very very special case you don't need to call GC.Collect. You probably forgets to release some resources or references, and GC.Collect won't help you with that. Make sure you call Dispose on everything Disposable (preferably with the using-pattern).
You should probably pick up a memory profiler like Ants memory profiler and look where all your memory has gone.
If you have no more direct reference to an object, and you're running out of memory, GC should do this automatically. Do make sure you call .Dispose() on your datacontext.
Calling GC.Collect will hardly ever prevent you from getting OutOfMemory exceptions, because .NET will call GC.Collect itself when it is unable to create a new object due to OOM. There is only one scenario where I can think of and that is when you have unreferenced objects that are registered in the finalizable queue. When these objects reference many other objects it can cause a OOM. The solution to this problem is actually not to call GC.Collect but to ensure that those objects are disposed correctly (and implement the dispose pattern correctly when you created those objects).
Using GC.Collect in general
Since you are trying to get rid of a very large collection, it's totally valid to use GC.Collect(). From the Microsoft docs:
... since your application knows more about its behavior than the runtime does, you could help matters by explicitly forcing some collections. For example, it might make sense for your application to force a full collection of all generations after the user saves his data file.
"Preparing" your objects
From the excellent Performance Considerations for Run-Time Technologies in the .NET Framework (from MSDN):
If you keep a pointer to a resource around, the GC has no way of knowing if you intend to use it in the future. What this means is that all of the rules you've used in native code for explicitly freeing objects still apply, but most of the time the GC will handle everything for you.
So, to ensure it's ready for GC, Make sure that you have no references to the objects you wish to collect (e.g. in collections, events, etc...). Setting the variable to null will mean it's ready for collection before the variable goes out of scope.
Also any object which implements IDisposable should have it's Dispose() method called to clean up unmanaged resources.
Before you use GC.Collect
Since it looks like your application is a server, using the Server GC may resolve your issue. It will likely run more often and be more performant in a multi-processor scenario.
The server GC is designed for maximum throughput, and scales with very high performance.
See the Choosing Which Garbage Collector to Use within Performance Considerations for Run-Time Technologies in the .NET Framework (from MSDN):
I get the need to clean up resources during the teardown of an object, but I have always found the differences between Dispose, Finalize, and the destructor methods a bit confusing.
I found this great article that concisely describes the distinctions between them, that I am going to have to save for future reference:
"Difference between Destructor, Dispose and Finalize methods" - Sanjay Saini
http://sanjaysainitech.blogspot.com/2007/06/difference-between-destructor-dispose.html
The fundamental question I am trying to ask here is this.
If a language offers destructors (for example C# [refuted]) what
value do Dispose and
Finalize add to the equation?
Am I just a curmudgeon that is used to the old school way of doing everything in the destructor, or is there something I am missing that is only possible by breaking the tear-down of an object into three parts?
UPDATE:
As noted in some of the replies, C# does not actually have destructors. The question may be moot at this point in recognition of that. When I read in the above referenced article that C# actually had a separate deconstructor (an error apparently), it threw me for a loop and I started wondering what the point of Dispose and Finalize would be if you had a final destructor to wrap up everything. I suppose that in a GC langauge like C# the concept of a single destructor to provide the denemount for an object doesn't make much sense.
Sorry for the downvotes on some of you guys, but a couple people didn't read the question carefully and thought I was asking about the difference between Dispose and Finalize, which really wasn't the point.
The author of that blog post is a bit confused...
In C#, there is no such thing as a "destructor". Only Finalizers and IDisposable.
The ~ClassName() method is not called a "destructor". It is called a finalizer.
Dispose exists to release resources from code, where the finalizer exists to be called from the GC. Very often, the finalizer calls the Dispose() method, but the "Dispose Pattern" sets you up to only handle unmanaged resources from the finalizer.
You see, when the finalizer gets called, you are on a different thread, and any managed object you have is not necessarily valid. Because of this, if you call Dispose() from the finalizer, you should really be calling Dispose(false) which tells the "Dispose Pattern" to only dispose unmanaged resources.
Further, the "Dispose Pattern" suggests that when Dispose(true) is called, you should suppress the finalizer on that object.
Only managed objects can be finalized automatically. If you want to offer implicit disposal of unmanaged objects, the Finalizer can be used. If you want to offer explicit control of disposal to a caller of your object, you can allow them to call Dispose.
I like this article.
It's hard answering this since you already link to articles that explains the difference.
But let me try anyway.
With garbage collection you have non-deterministic memory management, and since GC will run your finalizer, you have guaranteed, but non-deterministic resource management.
This is good, in the sense that you know things will be cleaned up.
However, it is bad, since you don't know when it will happen.
For instance, if you open a file and lock it while you have it open, not knowing when the file will become available to open again later on is bad, but the guarantee that at some point it will be closed is good.
Dispose and IDisposable serves that bad part by adding deterministic resource management. You choose when to dispose of your resources, closing the file, the network connection, the database connection, whatever. It's your choice, and you will typically dispose of the resource when you no longer need it. Thus, when you no longer need the file, you dispose of the object keeping it open. The object will stay in memory (non-deterministic memory management) for a while longer, but the file will be closed when you say so, ready to be opened again right away.
Thus you gain deterministic handling of resources, and couple that with non-deterministic memory management and you got the best of two worlds.
What looks like a destructor in C# (~Foo()) is really just an different way of spelling "Finalize".
Perhaps in hindsight the choice of syntax wasn't the best because ~Foo() in C# is rather different from ~Foo() in C++.
Sorry to answer my own question, but I think I have assembled from a variety of sources, including some of the answers a concise answer to my question addressing the differences in object lifecycles when transitioning to a GC platform.
Non GC Object Lifecycle:
Initialization: Happens in the constructor which is called when the first reference is added to object.
Cleanup: Happens in destructor which is called when last variable referencing object goes out of scope or is explicitly set to nothing/null/another object.
GC Object Lifecycle:
Initialization: Happens in the constructor which is called when the first reference is added to object.
Clean-up (first pass): Happens in the Dispose Method, which must be explicitly called by the client or implicitly called at the end of a Using statement. This clean-up is specifcially to free up resources that must be released immediately and not wait on the GC.
Clean-up (Final Pass): Happens in Finalize Method which is called on the GC's timetable, which can be delayed. If the finalize method is implemented, it is called unless the GC.SuppressFinalize has been called previously on the object (normally from within the dispose method) to flag it as already cleaned up.
Did I get this right? Feel free to edit if I missed something here.
c# does not offer destructors, so your question is somewhat moot?
I'm not sure on the reasoning between Destructors and Finalizers, but Dispose exists so that you can free the resources before the Garbage Collector gets to the object (which may be never!)
The problem with waiting for the finalizer to run to release resources, is that it does not run in a predictable deterministic manner. The finalizer is called when the GC is ready to destroy the object, and this can occur at anytime in your application.
This is why you have dispose. It allows you to deterministicly release resources at a known point in time, ensuring that resources will be freed up. Otherwise you have no way of controlling when the finalizer will run.
Finalize cannot access managed objects, so managed object should be handled in Dispose. The difference between those two is that you should use Dispose for managed and Finalize for unmanaged. Your Destructor should be calling your Dispose so you've properly cleared memory. Take a look at the following articles:
Code Project - IDisposable: What Your Mother Never Told You About Resource Deallocation
Gil Fink - Memory Leak And The IDisposable Pattern
When would I implement IDispose on a class as opposed to a destructor? I read this article, but I'm still missing the point.
My assumption is that if I implement IDispose on an object, I can explicitly 'destruct' it as opposed to waiting for the garbage collector to do it. Is this correct?
Does that mean I should always explicitly call Dispose on an object? What are some common examples of this?
A finalizer (aka destructor) is part of garbage collection (GC) - it is indeterminate when (or even if) this happens, as GC mainly happens as a result of memory pressure (i.e. need more space). Finalizers are usually only used for cleaning up unmanaged resources, since managed resources will have their own collection/disposal.
Hence IDisposable is used to deterministically clean up objects, i.e. now. It doesn't collect the object's memory (that still belongs to GC) - but is used for example to close files, database connections, etc.
There are lots of previous topics on this:
deterministic finalization
disposing objects
using block
resources
Finally, note that it is not uncommon for an IDisposable object to also have a finalizer; in this case, Dispose() usually calls GC.SuppressFinalize(this), meaning that GC doesn't run the finalizer - it simply throws the memory away (much cheaper). The finalizer still runs if you forget to Dispose() the object.
The role of the Finalize() method is to ensure that a .NET object can clean up unmanaged resources when garbage collected. However, objects such as database connections or file handlers should be released as soon as possible, instead on relying on garbage collection. For that you should implement IDisposable interface, and release your resources in the Dispose() method.
The only thing that should be in a C# destructor is this line:
Dispose(False);
That's it. Nothing else should ever be in that method.
There is a very good description on MSDN:
The primary use of this interface is
to release unmanaged resources.
The garbage collector automatically
releases the memory allocated to a
managed object when that object is no
longer used. However, it is not
possible to predict when garbage
collection will occur. Furthermore,
the garbage collector has no
knowledge of unmanaged resources
such as window handles, or open
files and streams.
Use the Dispose method of this
interface to explicitly release
unmanaged resources in conjunction
with the garbage collector. The
consumer of an object can call this method when the object is no
longer needed.
Your question regarding whether or not you should always call Dispose is usually a heated debate. See this blog for an interesting perspective from respected individuals in the .NET community.
Personally, I think Jeffrey Richter's position that calling Dispose is not mandatory is incredibly weak. He gives two examples to justify his opinion.
In the first example he says calling Dispose on Windows Forms controls is tedious and unnecessary in mainstream scenarios. However, he fails to mention that Dispose actually is called automatically by control containers in those mainstream scenarios.
In the second example he states that a developer may incorrectly assume that the instance from IAsyncResult.WaitHandle should be aggressively disposed without realizing that the property lazily initializes the wait handle resulting in an unnecessary performance penalty. But, the problem with this example is that the IAsyncResult itself does not adhere to Microsoft's own published guidelines for dealing with IDisposable objects. That is if a class holds a reference to an IDisposable type then the class itself should implement IDisposable. If IAsyncResult followed that rule then its own Dispose method could make the decision regarding which of its constituent members needs disposing.
So unless someone has a more compelling argument I am going to stay in the "always call Dispose" camp with the understanding that there are going to be some fringe cases that arise mostly out of poor design choices.
It's pretty simple really. I know it's been answered but I'll try again but will try to keep it as simple as possible.
A destructor should generally never be used. It is only run .net wants it to run. It will only run after a garbage collectoin cycle. It may never actually be run during the lifecycle of your application. For this reason, you should not ever put any code in a destructor that 'must' be run. You also can't rely on any existing objects within the class to exist when it runs (they may have already been cleaned up as the order in which destructors run in is not garanteed).
IDisposible should be used whenever you have an object that creates resources that need cleaning up (ie, file and graphics handles). In fact, many argue that anything you put in a destructor should be putin IDisposable due to the reasons listed above.
Most classes will call dispose when the finalizer is executed but this is simply there as a safe guard and should never be relied upon. You should explicitly dispose anything that implements IDisposable when you're done with it. If you do implement IDisposable, you should call dispose in finalizer. See http://msdn.microsoft.com/en-us/library/system.idisposable.aspx for an example.
Here is another fine article which clears up some of the mist surrounding IDisposable, the GC and dispose.
Chris Lyons WebLog Demystifying Dispose