I am comfortable with how to implement IDisposable..
However I'm not clear as to the exact consequences of not implementing this properly. Is it a case that unmanaged resources will never be cleared thus causing memory leaks or will the memory eventually get recycled anyway, just not in a timely manner?
It depends.
If the object in question fails to implement IDisposable, but does still implement a finalizer correctly, the resources will eventually get cleaned up.
However, if it doesn't properly implement a finalizer, AND it's wrapping an unmanaged resource, that resource will be leaked. This is more often a resource leak (ie: leaking a HANDLE stored as IntPtr) more than a traditional "memory leak" (though, of course, the object being pointed to does still use memory).
will the memory eventually get recycled anyway?
One point here... IDisposable has nothing to do with memory - it's about resources. These resources often use memory of their own, but implementing IDisposable does not cause your garbage collector to reclaim memory more quickly.
It depends entirely on what te implementation is, of course.
An incorrect dispose on a SqlTransaction could lead to excessive long blocking, for example - impacting multiple machines and users. An incorrect dispose on a SqlConnection could cause connection saturation and inability to connect. These are not just memory (etc) issues.
IIRC a missing graphic (pen/brush/whatever) dispose was responsible for a VS crash bug - GC didn't happen because there was no memory pressure, so instead it simply saturated the GDI handles and broke.
An incorrect file/stream dispose could cause exceptions due to a file bring unavailable for reading/writing - potentially losing data.
With unmanaged resources not properly handled, anything is possible.
Instead of thinking in terms of some vaguely-defined "resources", think of IDisposable as meaning "This object manipulates something outside itself (and possibly even outside the computer!) in a way that needs to be cleaned up, while the information necessary for such cleanup still exists." A finalizer which fires essentially says "Nobody else is using this stuff anymore, and I'm the only one who knows about it; I'd better clean it up because nobody else is going to."
It is dangerous to assume that finalizers will magically take care of things; while many objects can be safely abandoned, many others cannot. In some cases, it will be easy for an object to detect that it has been abandoned and clean up appropriately. In some other cases, it may be so difficult as to be impractical. Consider an object which is supposed to count how many times the word "quack" appears in a long-lived data stream. If the creator of the object has forgotten about it, and nobody is every going to query the count, the object may as well go away. Unfortunately, since the data stream holds a reference to the counting object (so as to let it know when data comes in), the counter object won't go away. If code creates a counter, then forgets about it, then creates another one, forgets about it, etc. this could become an unbounded memory leak.
A class that holds unmanaged resources should ensure that they are cleaned up during finalization. However, this only happens when the garbage collector gets around to it.
Not calling Dispose means waiting for the garbage collector to need memory before the resources are released.
The latter of the two. Just not in a timely manner.
The importance of this depends on the resources you are allocating. This is why you are more likely to see IDisposable interfaces on objects that open files and reference other, more crticital, resources.
Yes, unmanaged resources will never be released. (This assumes that the IDisposable types in question don't implement a "fallback" finaliser. If they do then the finaliser should sort things out... eventually.)
However, it's not just unmanaged memory allocations that you need to worry about; it's any unmanaged resources. For example, if you don't properly dispose of your database connections then you might find that the connection pool quickly gets exhausted.
Another potential problem with not implementing the disposable pattern properly - it is very easy to accidentally call .Dispose() more than once. If your Dispose method does not check for successive calls to it, and (eg.) attempts to free up an unmanaged resource more than once, you might end up with some unrecoverable error, or unexpected behaviour.
Related
Uses of "using" in C# has a nice explanation of the utilities of the using feature.
.Net has its garbage collector. How does it handle the lack of a dipose()?
Specifically for DB connections, statements and resultsets, is it required a using() for each of them? What happens if they are left behind with no using(), dispose() and neither close()?
Update: the context is web applications, therefore there may be thousands of simultaneous users, each with his own connection/stmt/rs and the app will never be closed.
Since using is a shorthand for calling Dispose, you could imitate it with try/finally. So the real question is what's the consequence of not calling Dispose at all.
Although C# has garbage collection which would eventually release resources most of the time, you want the release of critical resources to happen as soon as you are done with them. If you use using or the equivalent try/finally, the resources would be released quickly. If you let the garbage collector to release the resources for you, your program may be starved of resources while they are "in custody" of GC (i.e. your program is no longer using them, but GC has not released them yet). Moreover, since GC offers no hard guarantee of running finalizers, some resources may not get released explicitly until your program ends, which may cause resource starvation of other processes.
You don't know when .net's garbage collector is called and run, so it's allowing you to do it yourself when you don't need it. So, when your code gets out of using() it's dispose object used in using() instead of waiting for GC to run on its own schedule.
If you don't use with DB connection, then GC will dispose it on its own way based on criteria of algorithm it's implementing. It might get too late(in terms of computer clock) to sweep it.
Garbage collector is a background thread which doesn't run every millisecond. It has specific schedule and its own algorithm which tends it to work on a specific time. E.g., some GC algorithms check for objects having no references then they sweep those objects when GC runs.
Specifically for DB connections, statements and resultsets, is it
required a using() for each of them? What happens if they are left
behind with no using(), dispose() and neither close()?
Actually the worst consequence of a memory leak is holding some memory until you restart the PC. However in this case probably the worst consequence is leaking memory until you restart the application.
If memory growth increases up to where GC cannot clean any longer, in fact if Gen 2 of Small Object Heap is overflow (Large object heap also can overflow), it will throw out of memory exception and close the application.
.Net has its garbage collector. How does it handle the lack of a
dipose()?
All the standard database connection related classes have implement Dispose and Finalize methods properly. Generally there are unmanaged resources in those classes. Unmanaged resources are the resouces (eg: file handlers, database connection handlers and etc) which could cause worse memory leaks that may hold memory until you restart the PC. However, that's where GC's finalization comes handy. If you don't call the Dispose for such Disposable object, garbage collector will execute the Finalize method(if there is a ~destructor) and clear unmanaged resources.
That's the reason why it is required to implement IDispose Pattern properly Dispose and Finalization as required. Finalization is required only if it has Unamanged resources.
The most likely consequence of failing to promptly Dispose database objects is that the program will ask a database server to open a database connections on its behalf with a promise that it will tell the server when they are no longer needed (i.e. close them), but may leave the connections open for quite awhile after they're no longer needed. Such behavior may increase the number of connections the database server will need to keep open simultaneously. Depending upon the server, there may be no consequences, or the extra connections may impair performance, or they may cause some connection requests to get needlessly denied.
Although .NET will try to ensure that database servers will get notified when database objects are abandoned, even if Dispose is not called, the code which uses database objects will generally know when it will no longer need them, long before .NET can determine that they're abandoned. Note also that while some .NET database-related libraries may keep connections open for a little while after a Dispose (so that if code needs the database again it can resume using the earlier connection) such libraries may use timers to limit how long connections are maintained in expectation of further use, rather than depend upon the garbage-collector (which might go a very long time without noticing that an object has been abandoned).
I came across the following quote "Desctructors are not guaranteed to be called." and this scares me a bit.
It goes on to say that even a try finally block can be interrupted, causing memory leaks.
It does give a solution by either placing your code in CER (constrained execution region) or derive from the CriticalFinalizerObject.
My question is
What are the tradoffs by using CriticalFinalizerObject, if any?
Are their any cases were you found deriving from CriticalFinalizerObject was really usefull?
Should I only worry about using this when I start running into Memory leaks?
Why are you relying on desctructors ? most of the time you don't have any need of them.
Perhaps have a look at IDisposeable and the Dispose Pattern.
Here some links that helpes me to understand this subject
-> Everybody thinks about garbage collection the wrong way
-> How To implement dispose Pattern
-> Implementing Finalize and Dispose to Clean Up Unmanaged Resources
Regarding question #3: memory leaks would typically be caused by:
Unmanaged resources not being freed. For those, using IDisposable (with a fallback call to Dispose() in the finalizer) is the best approach.
References to managed objects that are maintained because other objects still point to them, even though they are supposed to be removed. IHMO, that's more a problem of code quality than a low-level issue with garbage collection.
Unless you run into actual memory leaks, you should not even worry about it, and not try to force any behavior.
I would suggest using the IDisposable interface for all resources that need to be destroyed, and use them in a using block.
Typically the differences between a normal finalizers and critical finalizers only become important on AppDomain unload. Since most unmanaged resources automatically go away on process unload you usually need only to worry about critical finalization if you want to unload AppDomains cleanly while keeping the process running.
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.
I'm busy getting confused about non-deterministic destruction. In an answer to another question I got the advice that destructors/finalisers (which I assume are the same thing in c#, i.e. the function called ~classname()) are expensive and not required. But looking at this example a destructor is used and from the comment sounds like it may be vital. Anyone got some advice about how this all fits together and should I remove the destructor from my code?
Thanks again.
You should only include a finalizer if you absolutely have to run some cleanup at some point, whether it's performed explicitly or not. In such cases you should always have an explicit way of performing clean-up in a timely manner anyway, and that should suppress finalization anyway, so that "good" clients don't see any performance penalty.
You would normally only need a finalizer if you have a direct handle to unmanaged resources - if you only have a reference to another class which has a handle on the resource (e.g. FileStream) then you should leave it to the other class to have a finalizer.
With the advent of SafeHandle in .NET 2.0, situations where it's worth writing your own finalizer are very rare indeed.
The performance penalty of finalizers is that they make your objects live for longer than they need to: in the first GC cycle where they're otherwise considered eligible for collection, they get put on the finalizer queue - and bumped up to the next generation just like any other object which survives a GC cycle. The finalizer will then run in another thread (at some point) and only then will they be eligible to really be collected. So instead of (say) getting collected in the first gen1 collection, they live on past that until the next gen2 collection, which may be considerably later.
Usually, implementing a destructor is useful in on case: when it is not guaranteed, that client code will close all the resources(file streams, db connections and so on) properly. So, if client code will fail to do so, you will have code, which will close it, which is better than just left the resources open.
You only need the full Disposable pattern when you are dealing with an unmanaged resource directly. And then it is up to your calling code to make sure the destructor is (almost) never used.
When dealing with managed resources (= indirect ownership of unmanaged resources), the destructor is useless:
class FileWrapper
{
private FileStream fs; // managed resource
~FileWrapper()
{
if (fs != null)
fs.Dispose(); // fs is already on the GC finalizer queue
}
}
Whenever a FileWrapper object is being collected by the GC, it is certain that the fs object is in the same batch. So the call to fs.Dispose() is useless, only testing the correct (allowing multiple calls) behaviour of the FileStream.Dispose().
The only useful destructor here is the one in FileStream.
i have a stupid question, but i want to hear the community here.
So here is my code:
using (FtpWebResponse response = (FtpWebResponse)request.GetResponse())
{
return true;
}
My question, is it any different than:
(FtpWebResponse)request.GetResponse();
return true;
Which one is better in general? which one in terms of GC and why?
The first is better, but not in terms of GC. WebResponse implements IDisposable because it has resources that need to be released such as network connections, memory streams, etc. IDisposable is in place because you, as the consumer, know when you are done with the object and you call Dispose to notify the class you're done with it, and it is free to clean up after itself.
The using pattern really just calls Dispose under the hood. E.g. your first example is actually compiled to be this:
FtpWebResponse response = (FtpWebResponse)request.GetResponse()
try
{
}
finally
{
((IDisposable)response).Dispose();
}
return true;
This is separate from the garbage collector. A class could also clean up resources in the finalizer, which gets called from GC, but you should not rely on that to happen for two reasons:
It could be a very long time before GC finally comes around, which means those resources are sitting there being consumed for no good reason.
It might not actually clean up resources in the finalizer anyway
The first one is better.
The difference is that the using statement will call dispose on the object wrapped in it. It will correctly dispose of held resources.
From MSDN, the using statement:
Provides a convenient syntax that ensures the correct use of IDisposable objects.
I appreciate that you are not interested in the response. However, the response object has been created, initialised and returned to your code whether you wanted it or not. If you don't dispose it, then it will hang around consuming resources until the GC eventually gets round to finalizing it.
I agree with what others said, but if you have such hate for the using() syntax, you could do:
((IDisposable)request.GetResponse()).Dispose();
return true;
The GC doesn't care about anything other than memory, so if you've got plenty of memory and are not consuming much of it, it might be a very long time until it does a GC. During that entire time the unmanaged resources, such as network connections, file handles, database connections, graphics handles etc., remain tied up by the objects that are waiting to be GC'd. This can cause you to run out of these resources and the GC will be oblivious because it doesn't monitor unmanaged resources.
So if you put your code in a loop and kept calling it, without calling Dispose, you might find it quickly degrades in performance(processes fighting for scarce resources) or gives you an exception due to a lack of the unmanaged resources. It depends on the context of how it is being called and how you are creating other related objects. Rather than analyze each situation, it is always safest to call Dispose as soon as you are done with the instance.
It is true that eventually Dispose will be called by the GC on an object that has gone out of scope/is no longer referenced, but this is indeterministic. Being indeterministic, you could see different results in how soon resources are free'd when testing vs production and lead to random Exceptions for failure to allocate resources when they run out. If you could choose between consistent deterministic behavior of your application and inderterministic, you'd probably want deterministic, unless maybe you are trying to confuse your Testers.