What is the difference between System.GC.Collect and Dispose()?
Aren't they used for the same purpose, and when should each be used; what is the best practice?
Best practice is to never need to call GC.Collect, and to call Dispose on all IDisposable objects when you're done with them.
The purpose of GC.Collect is to to tell the Garbage collector that there are some objects in the memory that can be collected and it is the right time to collect. Though, as a rule of thumb you should leave that to GC itself. It is designed to do this job.
If there are some resources in your object that you think GC won't take care of. You should implement Dispose and take care of them yourself. You need to call Dispose Explicitly to dispose the resources you want. And if you are implementing IDisposable than you can also do that by the Using statement.
System.GC.Collect is called when all objects which are there in memory are to be collected by the Garbage Collector. Objects which are referenced in managed code are not considered for Garbage collection. This method is used to force the system to reclaim the available memory.
Dispose() is not part of GC but as better practice you can use this. It should only be used for objects which uses unmanaged resources like FileStream etc. It should release all the resources that it owns. When you know that certain resources will not be released by GC then you can use the Dispose() method.
The using statement ensures the correct use of IDisposable objects.
On a side note: The GC does not call Dispose, it calls the finalizer(which you should call from Dispose(false))
Also, to make ensure that resources are always released appropriately, a Dispose method should be callable multiple times without throwing an exception.
MSDN says:
"It is possible to force garbage collection by calling Collect, but
most of the time, this should be avoided because it may create
performance issues. "
Check this blog:-
GC.Collect() asks the system to perform a collection "now". You
shouldn't mess with this; the system usually has a much better idea
than you do of when collection is necessary.
So is the best practice:
You can use the using block or as you mentioned use the Dispose method.
Why should you use IDisposable and Dispose?
MSDN says
In many cases it is possible for objects that would otherwise always
need to be finalized to avoid that cost by implementing the
IDisposable interface. This interface provides an alternative method
for reclaiming resources whose lifetime is well known to the
programmer, and that actually happens quite a bit. Of course it's
better still if your objects simply use only memory and therefore
require no finalization or disposing at all; but if finalization is
necessary and there are many cases where explicit management of your
objects is easy and practical, then implementing the IDisposable
interface is a great way to avoid, or at least reduce, finalization
costs.
Also check this article on Improve garbage collector performance using finalize/dispose pattern
Related
I am happy using IDisposable and using blocks but often I find myself writing (new Obj()).Foo(); to avoid the extra instantiation code and naming etc. Does this instance hang in memory until the end of the function scope or is it disposed of directly after the enclosing parenthesis is exited...? Is there an alternative I am missing? An equivalent to implementing IDisposable for concrete objects that I want to use once and throw away?
IDisposable is not related whit when the memory allocated for the object is released. In both cases the memory is released when the Garbage Collector decides to release it.
IDisposable is used for objects that need to execute some action in order to release the resources it allocates outside them. For example if the objects open a Database connection, you must release it as soon as you don't need it anymore.
So IDisposable gives you one thing:
A way to release the resources as soon as you want, by calling the Dispose() method or using the using clause
So it's ok to do:
new Obj()).Foo();
And has no sense to make Obj implements IDisposable if it does not uses any resource that must be deallocated.
If an object implements IDisposable, it is imperative that you Dispose() the object in your own code, either explicitly (and within a try/finally construct, too!) or with the using syntax, and do not simply let the reference(s) fall out of scope.
EDIT: I feel like I used an awful lot of words to get around to explaining the most important aspect of all this, which is that the only reason an object should implement IDisposable is if it holds handles to other objects that are unmanaged (the CLR doesn't know about them) or scarce (due to resources/cost/licensing/etc.). So if an object implements IDisposable, you MUST call Dispose() YOURSELF, as soon as possible. The CLR will not do it for you. And if you implement IDisposable on your own classes, you need to be implementing a finalizer on them, too.
Simply letting a reference fall out of scope does not cause the GC to run immediately, and you have no guarantees whatsoever that a GC is going to occur any time soon. You can explicitly trigger a GC with code, but that is typically going to hurt overall performance more than help it.
If there is not much memory pressure, for instance, the CLR may wait a long time before running the GC. There just isn't a pressing need for it to run in a situation where there is light or zero contention for resources. If it ran too often, it would impact the performance of your applications negatively
The GC does not call Dispose(). The GC calls an object's finalizer if it has one. The finalizer is a special method using syntax similar to a C++ destructor, but which really is not a destructor in that sense.
So if an object implements IDisposable, but does not implement a finalizer, the GC will never release that object's resources. The object will cause both memory leaks and unresolvable resource contention.
Nor does calling Dispose() from your own code trigger a GC.
The finalizer is a last-ditch backup mechanism to try to ensure that a class holding scarce or expensive resources does eventually let go of them even if consumers of the class fail to explicitly release those resources.
IDisposable is really just a common convention, implemented with a handy well-known interface contract, to let callers generically request that any class holding scarce resources release those resources immediately.
The standard pattern is that the finalizer calls the Dispose() method (actually, it typically calls Dispose(false) and I'm intentionally ignoring that detail). The Dispose() method will release all the scarce resources, then it can call GC.SuppressFinalize() so that the GC can free the object's memory more quickly when it gets around to running.
The old tried and true "acquire late/release early" semantics apply here.
You should read up on how the GC uses multiple passes or "generations" to clean up objects with finalizers.
The GC can get rid of regular objects (objects where all the resources are managed by the CLR) in one pass. If there are no in-scope references to an object, the GC can basically just delete it.
But if an object has a finalizer (and GC.SuppressFinalize() has not been called), the GC uses at least 2 passes or generations to release the object. In the first generation, it marks the object for finalization. In the second generation (which happens the next time the GC runs), it actually calls the finalizer, and at that point the object is finally eligible to have its memory released.
The scarce/expensive resources we're talking about are typically things like network sockets, file handles, database or other server connections which may be limited or expensive or resource-intensive or all three, etc. This is also true of virtually all "unmanaged" (non .NET CLR) resources, including things like Win32 window handles, file handles, network sockets and so on. Such resources require deterministic cleanup. You need to let go of them as quick as you can, but the CLR is going to take its sweet time getting around to cleaning this stuff up. You have to do it yourself.
But if your class isn't using any of these types of scarce/expensive/"unmanaged" resources, then there is probably no reason to implement IDisposable on it at all.
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Proper use of the IDisposable interface
I have a class that has both managed and unmanaged resources. I am using IDisposable to release un-managed resources. Should I release managed resources in the dispose method? Or I can leave it to GC to release managed resources?
If you have a look at the following documentation you will find the line:
Free any disposable resources a type owns in its Dispose method.
So in your dispose method you should dispose of managed resources that also implement IDisposable. If an object doesn't implement this, you don't have to dispose of it.
I would suggest that classes with finalizers (the C# compiler generates finalizers for any classes with destructors) should avoid holding references to any objects that won't be used in finalization. With relatively few exceptions, classes which hold resources that are encapsulated in objects should avoid holding resources that are not so encapsulated. Instead, those resources should be encapsulated into their own objects, so they can be held along with the other resource-containing objects.
Once that has been taken care of, the proper behavior for any class which owns resources that are encapsulated in other objects is generally to call Dispose on all such resources within its own Dispose method, and not to implement a finalizer (and--for C#--not to have a destructor, which would cause the compiler to generate a finalizer). If the finalizer runs on an object which holds other finalizable objects, each of those objects will usually be in one of three states:
The finalizer on that other object will have already run, in which case it's not necessary to do anything to clean it up.
The finalizer on that other object will be scheduled to run, in which case it's probably not necessary to do anything to clean it up.
Other strong references may exist to that other object, in which case it shouldn't be cleaned up yet.
Only in the second case would there be any reason to think about doing any cleanup on the other object.
Note, btw, that the garbage collector should almost never be relied upon to do anything other than free up memory directly consumed by object instances. Deterministic disposal is almost always much better. The only times one should ever deliberately use the garbage collector to clean up resources is when one is going to be creating relatively-low-cost resources which are known to clean themselves up effectively when garbage-collected, and the instances will be widely-enough shared that finding out when the last one leaves scope would otherwise be impractical. Although there is sometimes good justification for abandoning disposable objects, abandoning disposable objects without justification is always a mistake (if it's appropriate to abandon disposable objects, it's appropriate to justify one's reasons for doing so).
This is pretty much only for me to make sure, I got this right:
We have a large resource class implementing the IDisposal pattern. It should (by design) be implemented in a way, that enables it to get called more than one time (even if we try to call it exactly one time of course). We also implement a finalizer, which also calls the Dispose() method - just as backup. If called manually, Dispose() will also call GC.SuppressFinalize(this).
There are several examples of disposal patterns around. Most of them call GC.SuppressFinalize(this) at the end of the disposing code. Some claim, it would be better, to call it at the beginning of the Dispose() method, before any cleaning. Latter argue, this would make sure, the GC doesn't call the finalizer concurrently, while we are still cleaning up.
Question:
It seems, placing GC.SuppressFinalize at the beginning doesn't do any better? We still have a race condition, right? So is it true, that we rather should implement Dispose() in a thread safe way instead?
The GC only cleans up objects that are not 'reachable'.
A class in which code is executing is still 'reachable' because it's this pointer is on the stack. So while dispose is executing, the finalizer won't be called.
So it does not matter if you call SuppressFinalize at the begin or end.
As commentors below indicated, the CLR implementation does not appear to guarantee that your object does not get garbage collected/finalized while instance methods are executing. The only possible 'dependable' reference keeping the object alive is the one used to invoke the method on the object, but I don't know enough about the JIT internals to make statements about that, and it's behaviour might change.
I'm leaving the answer here for access to the discussion below.
While it is sometimes possible for an object to be finalized while a seemingly-live reference exists, that can only happen when nothing else is going to ever refer to the object. The GC.SuppressFinalize(this) actively refers to the present object 'this', thus guaranteeing that it will not be finalized until the GC.SuppressFinalize executes. Further, the fact that the object reference existed to dispose the object, and was available to the Dispose method, guarantees that the finalizer couldn't have been queued before Dispose started running unless the object was dead and a finalizer somewhere (either its own finalizer, or that of some other object) resurrected it.
Because there are some scenarios where an object could be scheduled for finalization and resurrected without ever being aware of it, it may not be a bad idea to protect a dispose and finalize against redundant operation. Microsoft's pattern is not a good one, however. Finalizable objects shouldn't hold references to any objects not needed for finalization. If an object would hold a mixture of managed and unmanaged resources, the unmanaged resources should be moved into their own classes (effectively turning them into managed resources), so then the main object would hold nothing but managed resources.
Dispose should not throw any exceptions.
I would make sure all the code in Dispose is thread safe so that if it gets called it will not do anything strange. Usually adding checks if variables are null already should do the trick.
In the microsoft examples i've only seen GC.SuppressFinalize at the end of the Dispose function.
(1) I've read a lot of questions about IDisposable where the answers recommend not using Finalize unless you really need to because of the process time involved.
What I haven't seen is how much this cost is and how often it's paid. Every millisecond? second? hour, day etc.
(2) Also, it seems to me that Finalize is handy when its not always known if an object can be disposed. For instance, the framework font class. A control can't dispose of it because it doesn't know if the font is shared. The font is usually created at design time so the user won't know to dispose it, therefore finalize kicks in to finally get rid of it when there are no references left. Is that a correct impression?
The main problem with finalize is that it blocks an object from being garbage collected. Instead, the finalizer is called, and the object collected "on the next run". Well, technically IIRC the finalizer runs a list of objects in a separate thread. Anyhow, this is not an "every ms" issue, more an "multiple GC runs needed to get rid of the objects.
Finalize is conceptually different than Dispose. Finalize can only free unmanaged resources. Dispose can free managed and unmanaged resources. You should use each as appropriate. (Note that a class with a Finalizer should always implement IDisposable).
Dispose must be called explicitly; Finalize can only be called by the GC.
Update: See my blog post on How to Implement IDisposable and Finalizers: 3 Easy Rules.
I've got a blog post about IDisposable and Finalizing - not about the performance though.
I'll answer your second question.
No, Finalize should not be used in this manner. In fact, with the exception of only a very few fringe cases, you should only override Finalize (or declare a destructor in C#) if the class directly holds unmanaged resources.
The issue you described is one of ownership. The owner of an IDisposable class is responsible for its lifetime and the decision of when to call Dispose. Other parts of code are free to use that class, but since they cannot claim ownership they should not participate in the lifetime management of that class.
Unfortunately I am not very familiar with the Font class nor how it might relate to the specific scenario that was the impetus for your question, but I can make a general statement that might apply to you. If your code did not create the instance (via the constructor) directly then your code should not be considered the owner. In that case you can assume the responsibility for disposal is left to something else.
Finalize is extremely useful as a double check. If a crash or someone's bad code doesn't dispose your object before it goes out of scope, guarantee that its resources will be released in the finalizer.
You can do some fancy footwork in your disposer though by calling GC.SuppressFinalize(this) which will allow you to write a method which will work in both situations and give you a guarantee that the code will work nicely.
You could even fire an MDA if you were writing a framework to remind people that they should dispose your object.
The penalty of the finalizer is basically that you end up pushing your object into the level 2 queue which takes longer to run. If you are consistently using objects and they are finalizing this could result in a level 2 collection running more often than neccessary just to run your finalizer threads.
What are the methods to destroy Excel COM Interop objects in C#, besides these:
object_instance = null;
System.GC.collect();
&
System.Runtime.InteropServices.Marshal.ReleaseComObject(object);
Please suggest only inbuilt techniques, not other tools like bigcannon, etc.
The kicker is that if you haven't dropped all references to the object, even GC.Collect won't destroy it.
The rule in C# (or .NET generally) is that you can't destory an object. Dispose() won't do it. A finalizer won't do it (rule number 2, don't use a finalizer unless you know why you need it, and don't ever call it directly).
For .NET, these are the things you need to do:
If an object holds references to unmanaged objects, implement IDispose pattern. And implement it fully; it's more than just providing a Dispose() method.
The only reason to have a finalizer is to release unmanaged objects, and only in the event that your object hasn't been properly disposed. Once your dispose method is called, it should do the cleanup, then "kill" the finalizer by calling GC.SuppressFinalize.
If you are using an object that implements a Dispose() method, call Dispose() when you're done with that object. Best practices is to allocate that object with a using block. Exit from the using block will call Dispose() automatically.
When you are done with an object, drop all references to the object, including event handlers, delegates, etc.
Beyond that, trust the garbage collector to do its job. Don't mess with the garbage collector; you're only likely to make things worse, unless you really, really, really know what you're doing and why.
The garbage collector is the only mechanism that can destroy a managed object, but you usually don't invoke it explicitly. You just let it do its thing.
Just like you never take your own trash to the depot, you just leave it sitting on the corner. It's always the garbage man's responsibility.
You can release references to things and clean them up with IDisposable, finalizers and destructors but not destroy them.
By using System.GC you can ask the garbage man to do things early - request a custom run just for yourself - but this usually screws up his schedule and he has a lot more trash to deal with than just yours so it's not recommended.
For the most part, you have to remove all references to an object. Only then will the garbage collector see it and destroy it.
Be aware of this:
object_instance = null;
All this does is kill the object_instance reference. This works if it's the only reference. If there are other references, then it won't be collected.
var skywalker = new Person();
var object_instance = skywalker;
...
object_instance = null;
//It's still alive and won't be collected because skywalker lives...
If your object allocates significant resources, it's best to implement IDisposable and explicitly call Dispose. If you can't call Dispose for some reason and your class reserves unmanaged memory, you can use GC.AddMemoryPressure (with a matching GC.RemoveMemoryPressure in the finalizer) to tell the GC your class is heavier than it looks, prioritizing it for earlier cleanup.
If you really need to explicitly free the memory occupied by an object, and you are sure that you will choose the moment to exterminate it better than GC, then the only reasonable option is to allocate it on stack and let it die when you return from the method where it was allocated.
If you want control so that you can manage resources of the object, then implement the IDisposable interface.
http://msdn.microsoft.com/en-us/library/system.idisposable.aspx
If resources are not the reason why you want to explicitly control object destruction, I can't fathom why you want that level of control... Do you use object pooling?