I have a c# class. Whenever this class is not in use anymore I want to do some things. For example log the current state and so on.
I want to be sure that this method is run everytime when the class is not used anymore.
I don't want just use a simple method because I can't be sure that every user is calling it.
I have no resources (like file handles) to clear up.
Is the best way to use a destructor?
"not in use" is when (for example):
a user uses my class in a form and the form is closed
the class is used in an application and this application is shut down
It depends. C# .NET utilizes a garbage collector that implicitly cleans up objects for you. Normally, you cannot control the clean up of objects - the garbage collector does that. You can implement a destructor in your class if you desire, but you may get a performance hit. MSDN has this to say on destructors:
In general, C# does not require as much memory management as is needed
when you develop with a language that does not target a runtime with
garbage collection. This is because the .NET Framework garbage
collector implicitly manages the allocation and release of memory for
your objects. However, when your application encapsulates unmanaged
resources such as windows, files, and network connections, you should
use destructors to free those resources. When the object is eligible
for destruction, the garbage collector runs the Finalize method of the
object.
and finally on performance:
When a class contains a destructor, an entry is created in the
Finalize queue. When the destructor is called, the garbage collector
is invoked to process the queue. If the destructor is empty, this just
causes a needless loss of performance.
There are other ways to manage resources besides a destructor:
Cleaning Up Unmanaged Resources
Implementing a Dispose Method
using Statement (C# Reference)
No that would not be the best way, a destructor is costly.
The best way would be to add a Close() or maybe the Dispose() (IDiposable interface) method.
But you need to define very carefully what "not in use anymore" means, and if you want the extra trouble to manage and track that.
You can use a destructor to automate it, but it would be better to make that conditional (Debug config only). Also consider that the destuctor implements "non deterministic" finalization.
If you want something to run when it's done, you should implement IDisposable.
Related
I was working on serializing and deserializing a class object using XML when I came across this blog post that shows how to do it on Windows Phone 7 using the isolated storage area. Windows Phone 7 is the platform I am developing for:
In this example, the only object he explicitly calls Dispose() on is the TextReader object. I looked up the TextReader object on MSDN and found that the documentation said this:
Releases the unmanaged resources used by the TextReader and optionally releases the managed resources.
So I assume the reason he does this is to release immediately the unmanaged resources acquired by the TextReader object. It would not have occurred to me to do this if it weren't for his blog post. Obviously I don't want to start calling Dispose() on every object in sight, so what is a good rule of thumb for at least investigating when a particular object should have Dispose() called on it or not? Are there some guidelines for this or a list somewhere, at least of the popular .NET objects that require this special handling?
Obviously I don't want to start calling Dispose() on every object in
Wrong.
In general, any object that implements IDisposable should be disposed as soon as you're finished with it, typically using the using statement.
Most objects that do not have unmanaged resources do not implement IDisposable (and do not have Dispose() methods), so you have nothing to worry about.
The only exceptions are base classes that implement IDisposable in case some derived implementations have something to dispose (eg, IEnumerator, Component, or TextReader).
However, it is not always obvious which concrete implementations need to be disposed (and it may change at any time), so you should always dispose them anyway.
Obviously I don't want to start calling Dispose() on every object in sight, so what is a good rule of thumb for at least investigating when a particular object should have Dispose() called on it or not?
This is not a problem. The compiler won't let you call Dispose() on an object that doesn't implement it.
And you should be calling Dispose() for every object that does implement it (which it will do via the IDisposable interface). That is the guideline you should be following. In fact, that's what it means when an object implements IDisposable: that it has unmanaged resources that need to be released.
It becomes much less of a chore if you'll simply wrap the creation and use of the objects in a using statement, e.g.:
using (DisposableObject obj = new DisposableObject(...))
{
obj.DoWork();
} // obj.Dispose() is automatically called here, even if an exception is thrown
Actually you do have to dispose of objects which implement IDisposable.
The standard way of doing that as opposed to directly calling the Dispose() is:
using(AnyIDisposable obj = ...)
{
// work with obj here
}
//The Dispose() method is already called here
Please correct me if i'm wrong.
As far a i read/understood, all clases of the .NET Framework are managed (to the view of the programmer, although underderlaying they might use unmanaged code), so theoretically you dont need to call Dispose() or using, because the gc will take care. But sometimes it's very recommended to use them, see IDisposable Interface and
Which managed classes in .NET Framework allocate (or use) unmanaged memory? and http://blogs.msdn.com/b/kimhamil/archive/2008/11/05/when-to-call-dispose.aspx
EDIT: (you are right noob) For clarification i'll add Nayan answer from IDisposable Interface
It recommended to call dispose or using, when:
1.You class has many objects and there are of lots of cross references. Even though its all managed, GC may not be able to reclaim
the memory due to alive references. You get a chance (other than
writing a finalizer) to untangle the references and break up the links
the way you attached them. Hence, you are helping the GC to reclaim
the memory.
2.You have some streams open which are alive till the object of the class dies. Even though such implementations of files/network etc are
managed, they go deep down to handles in Win32 mode. Hence, you get a
chance to write a Dispose method where you can close the streams. The
same is true for GDI objects, and some more.
3.You are writing a class which uses unmanaged resources, and you want to ship your assembly to third parties. You better use disposable
pattern to make sure you are able to free the handles to avoid the
leakage.
4.Your class implements lots of event handlers and hooks them up to events. The objects of classes which
expose the events, like Form etc., will not be freed up by GC since
the implementations local to your class (maybe) are still hooked into
those events. You can unhook those event handlers in Dispose; again
helping GC.
I am a little confused about memory clean up in an ASP.NET application. I had defined several destructors--I know this isn't the new .NET way of doing things, but I am a creature of habit and I always did it this way in c++-- that were working wonderfully in just about every scenario. However, I have noticed that they are sometimes not called in my ASP.NET applications.
I am thinking about implementing IDisposable, but I am under the impression that IDisposable is for other users of your code, and I am not sure that ASP.NET would call Dispose when it is finished with the object. Could someone clarify on this?
What is the best, and by best I mean that it will always work-- way to clean up my unmanaged memory?
Edit
This seems to indicate that if the class containing potential unmanaged memory is a member of an encapsulating class, then the destructor is the best strategy. This certainly makes sense to me since I could hardly put a try or a using around a class member. Even then however, that brings me back to my question, it sometimes never gets called in my ASP.NET app.
All classes which handle unmanaged resources should implement the IDisposable interface.
For a little more info, there are two issues with the garbage collector. First, you have no idea when it's going to run. Second, it has zero knowledge of unmanaged resources.. That's why they are called unmanaged.
Therefore it's up to the calling code to properly dispose of objects that utilize unmanaged resources. The best way to do this is to implement the above interface and either wrap the object in a using ( ) { } statement or, at the least, a try .. finally. I generally prefer the using statement.
Also, by implementing IDisposable you are signaling to other developers that this class deals with unmanaged resources so they can take the appropriate steps to ensure things are called correctly.
When working with managed resources, you don't need to implement IDisposable or a destructor. All you have to do for "cleanup" is set all top-level ("rooted") references to null (statics are normally considered to be top-level), and the garbage collector will take care of the rest.
Destructors as such are primarily useful with unmanaged resources in cases where callers either forget to call Dispose, or where such a call isn't possible. However, the runtime doesn't guarantee that destructors will ever be called; only that they will be called before the memory associated with the object is finally freed. You don't have to implement IDisposable; it's just a convention. It's perfectly reasonable to have a Close() or Cleanup() method that releases unmanaged resources.
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
What's the purpose of implementing the IDisposable interface? I've seen some classes implementing it and I don't understand why.
If your class creates unmanaged resources, then you can implement IDisposable so that these resources will be cleaned up properly when the object is disposed of. You override Dispose and release them there.
When your classes makes use of some system resource, it's the class' responsibility to make sure the resource is freed too. By .Net design you're supposed to do that in the Dispose method of the class. The IDisposable interface marks that your class needs to free resource when it's no longer in use, and the Dispose method is made available so that users of your class can call it to free the consumed resources.
The IDisposable method is also essential if you want auto clean-up to work properly and want to use the using() statement.
As well as freeing unmanaged resources, objects can usefully perform some operation the moment they go out of scope. A useful example might be an timer object: such objects could print out the time elapsed since their construction in the Dispose() method. These objects could then be used to log the approximate time taken for some set of operations:
using(Timer tmr=new Timer("blah"))
{
// do whatever
}
This can be done manually, of course, but my feeling is that one should take advantage wherever possible of the compiler's ability to generate the right code automatically.
It all has to do with the garbage collection mechanism. Chris Sells describes garbage collection, finalizers, and the reason for the Dispose pattern (and the IDisposable interface) episode 10 of .NET Rocks! (starting about 34 minutes in).
Many objects manipulate other entities in ways that will cause problems if not cleaned up. These other entities may be almost anything, and they may be almost anywhere. As an example, a Socket object may ask another machine to open up a TCP connection. That other machine might not be capable of handling very many connections at once; indeed, it could be a web-equipped appliance that can only handle one connection at a time. If a program were to open a socket and simply forget about it, no other computer would be able to connect to the appliance unless or until the socket got closed (perhaps the appliance might close the socket itself after a few minutes of inactivity, but it would be useless until then).
If an object implements IDisposable, that means it has the knowledge and impetus required to perform necessary cleanup actions, and such actions need to be performed before such knowledge and impetus is lost. Calling IDisposable.Dispose will ensure that all such cleanup actions get carried out, whereupon the object may be safely abandoned.
Microsoft allows for objects to request protection from abandonment by registering a method called Finalize. If an object does so, the Finalize method will be called if the system detects that the object has been abandoned. Neither the object, nor any objects to which it holds direct or indirect references, will be erased from memory until the Finalize method has been given a chance to run. This provides something of a "backstop" in case an object is abandoned without being first Disposed. There are many traps, however, with objects that implement Finalize, since there's no guarantee as to when it will be called. Not only might an object be abandoned a long time before Finalize gets called, but if one isn't careful the system may actually call Finalize on an object while part of it is still in use. Dangerous stuff. It's far better to use Dispose properly.
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