In the following c# code, how do I get rid of the objects when it's no longer useful? Does it get taken care of automatically, or do I need to do something?
public void Test()
{
object MyObject = new object();
... code ...
}
Automatically. When MyObject goes out of scope (at the end of Test), it's flagged for garbage collection. At some point in the future, the .NET runtime will clean up the memory for you
Edit:
(I should point out for the sake of completeness that if you pass MyObject somewhere, it gets passed by reference, and will not be garbage collected - it's only when no more references to the object are floating around that the GC is free to collect it)
Edit: in a release build, MyObject will usually be collected as soon as it's unused (see my answer for more details --dp)
The short answer is: unless it has unmanaged resources (file handles etc) you don't need to worry.
The long answer is a bit more involved.
When .NET decides it wants to free up some memory, it runs the garbage collector. This looks for all the objects which are still in use, and marks them as such. Any local variable (in any stack frame of any thread) which may still be read counts as a root as do static variables. (In fact I believe that static variables are referenced via live Type objects, which are referenced via live AppDomain objects, but for the most part you can regard static variables as roots.)
The garbage collector looks at each object referred to by a root, and then finds more "live" references based on the instance variables within those objects. It recurses down, finding and marking more and more objects as "live". Once it's finished this process, it can then look at all the rest of the objects and free them.
That's a very broad conceptual picture - but it gets a lot more detailed when you think of the generational model of garbage collection, finalizers, concurrent collection etc. I strongly recommend that you read Jeff Richter's CLR via C# which goes into this in a lot of detail. He also has a two part article (back from 2000, but still very relevant) if you don't want to buy the book.
Of course all this doesn't mean you don't need to worry about memory usage and object lifetimes in .NET. In particular:
Creating objects pointlessly will cost performance. In particular, the garbage collector is fast but not free. Look for simple ways to reduce your memory usage as you code, but micro-optimising before you know you have a problem is also bad.
It's possible to "leak" memory by making objects reachable for longer than you intended. Two reasonably common causes of this are static variables and event subscriptions. (An event subscription makes the event handler reachable from the event publisher, but not the other way round.)
If you use more memory (in a live, reachable way) than you have available, your app will crash. There's not a lot .NET can do to prevent that!
Objects which use non-memory resources typically implement IDisposable. You should call Dispose on them to release those resources when you're finished with the object. Note that this doesn't free the object itself - only the garbage collector can do that. The using statement in C# is the most convenient way of calling Dispose reliably, even in the face of an exception.
The other answers are correct, unless your object is an instance of a class which implements the IDisposable interface, in which case you ought to (explicitly, or implicitly via a using statement) call the object's Dispose method.
In optimized code, it is possible and likely that MyObject will be collected before the end of the method. By default, the debug configuration in Visual Studio will build with the debug switch on and the optimize switch off which means that MyObject will be kept to the end of the method (so that you can look at the value while debugging). Building with optimize off (debug doesn't matter in this case) allows MyObject to be collected after it's determined to be unused. One way to force it to stay alive to the end of the method is to call GC.KeepAlive(MyObject) at the end of the method.
This will force the garbage collector to get rid of unused objects.
GC.Collect();
GC.WaitForPendingFinalizers();
If you want a specific object to be collected.
object A = new Object();
...
A = null;
GC.collect();
GC.WaitForPendingFinalizers();
It gets taken care of automatically.
Typically garbage collection can be depended on to cleanup, but if your object contains any unmanaged resources (database connections, open files etc) you will need to explicitly close those resources and/or exceute the dispose method on the object (if it implements IDisposable). This can lead to bugs so you need to be careful how you deal with these types of objects. Simply closing a file after using it is not always sufficient since an exception before the close executes will leave the file open. Either use a using block or a try-finally block.
Bottom line: "using" is your friend.
Related
Before you report me for asking a commonly-asked question, hear me out.
I am writing a program to read from a datalogger using C# and windows forms. Each of the channels can mean different things, such as lift, drag, and pressure. I plan to make these channels configurable. Here is how such a channel is initialized:
private Channel Lift_Channel = new Channel(1);
This initializes Lift_Channel as a Channel object that references channel 1 on the logger.
However, the constructor method Channel(int) is the only method I can use to set the channel. If I want Lift_Channel to point to channel 2, I would like to do something like this.
delete Lift_Channel;
Lift_Channel = new Channel(2);
I feel like if I can't delete Lift_Channel then recreate it, I will have memory leak errors because the Channel(1) data will be floating around.
Any ideas?
Nope, nothing to delete. After you re-assign it, Lift_Channel will no longer point to the old memory address, and the memory will be cleaned up.
In C++, any object allocated on the heap must be deleted or it will leak memory. In a typical .NET managed app that only uses managed code, developers don't have to worry about deleting an object when they are done using it. This is because .NET has a very nice and efficient garbage collector. It keeps track of all references to objects and when the last reference to an object is no longer valid, it then deletes the object on its own. The garbage collector typically runs in about the same time as a memory page fault, so it is pretty efficient.
There are times when you might need to implement some code to handle cleaning up an object if it is using unmanaged code. .NET provides an interface for this called IDisposable. The main idea behind this is for the code that instantiates the object to call the Dispose method to perform cleanup. If someone forgets to do this, then the garbage collector ends up calling your finalizer method as a fail safe. The finalizer needs to implement a call to the Dispose method. An example of this is shown in the link above.
The garbage collector (GC) runs automatically in the background and stops all active managed threads before it does any clean up work. Part of this clean up work involves compaction of memory and it can move memory around. If you have calls into unmanaged code that use pointers or references to managed data, then that data should be pinned in memory so that the GC won't move it. This can be done with the Fixed statement for use within a given method or the GCHandle.Alloc method for longer periods of time that can be used outside of the method it was declared in. The GCHandle Free method should be called to release the pin. Pinning memory requires that the code be marked as unsafe.
.NET provides a class called GC that can be used to help control the garbage collector. For example, if you have objects that are using a large amount of memory and wish to wait for the memory to be released before proceeding, then the following methods should be called to accomplish this after setting them to null:
GC.Collect();
GC.WaitForPendingFinalizers();
.Net garbage collects allocated items that cannot be reached any more. collection. The garbage collector runs periodically and will reclaim memory for objects that are not reachable by anything else.
By assigning via a new new, the memory formerly associated with a variable (before the reassignment) will not longer be reachable (assuming that variable was the only thing referring to it). So the old object becomes unreachable, then the garbage collector will pick it up the next time it runs.
Hence, there is no delete operator.
You can assign null to a variable, to make it unreachable or
you can assign a new object reference to a variable.
Finally, you can use the IDisposable interface to ensure disposal at the end of a block of code.
All of those actions achieve the effect of a delete. (Eventually, if the item is unreachable, it will be "deleted when the garbage collector runs).
I've been researching the IDispose interface and garbage collection however I'm struggling to understand what I need to do (if anything) to ensure my objects are 'taken care of' appropriately in certain instances.
I know that I should use using (i.e. the syntactic sugar for Dispose()) to make sure I am releasing unmanaged resources, however when I create a temporary object I am not sure if I need to do something to actively release it or if the garbage collector will take care of it the same way it would a temporary variable.
For example:-
public void LoadSerializedClass(string filePath)
{
Binary formatter = new BinaryFormatter();
if (File.Exists(filePath))
{
using (Stream input = File.OpenRead(filePath))
{
MySerializedClass mySerializedClass
= (MySerializedClass)formatter.Deserialize(input);
/* ... use the deserialized class ... */
/* ... the deserialized class is no longer needed... */
}
}
}
I use using to make sure the unmanaged resource is disposed of properly.
(At the moment) I do not do anything to dispose of the (intended to be) temporary instance of my MySerializedClass - I (perhaps very mistakenly) thought it would behave as a local temporary variable would, and I would not need to manage it. Am I incorrect? I got the impression from the research I did that I would not have to do anything, and the garbage collector will automatically pick it up; however a colleague gave the impression otherwise and I am obviously struggling to understand.
Thank you!
You only need to Dispose of objects that implement IDisposable. Your variable is fine. Even with objects which implement IDisposable the GC will get rid of them at some point if you do not dispose them. Problem is that the unmanaged resources would not be released in the meantime and you can starve the operating system of that resource or cause other problems. For example if you open a file and don't close it the OS will think the file is in use and won't let the user delete it. The GC will eventually collect the managed object and release the file but that may take quite a long time if your program is not under memory pressure.
For more information on how the GC works you may want to check some article about it. Basically if the GC decides that it needs to free some memory it starts with all the variables that are in scope + all static fields and goes through all the objects they reference and then recursively all the way down. When it is done marking it just deletes the unmarked objects. This is called Mark and Sweep (there are different strategies for GC this is just one of them). The GC does not look for unused objects it looks for used objects and deletes the rest.
As for IDisposable a properly implemented IDisposable will have a Finalizer (syntax is like C++ destructor ~). If the GC sees an object with a Finalizer it makes a note to call it before deleting an object. Normally the finalizer will do what the Dispose method would and release the unmanaged object. However note that the GC is only concerned with memory used by the program so it does not care about file handles or network connections. Maybe the OS needs these resources but from the point of view of the GC everything is fine since there is enough memory and it does not feel like calling these finalizers. This is why we explicitly need to release (dispose) the unmanaged resources.
I'm using C# but probably the same in VB.NET. I C++ I would just set a break-point on an objects destructor to know when/if it was deleted/free'd. I understand that in winforms the base class calls SupressFinalize so that form destructors are never called, so I guess I can't do it that way. Is there another method to know if an object has been garbage collected? It seems like a catch-22 because if there was you would probably need a reference to check, but by holding that reference the garbage collected will not crush it.
I've read this What strategies and tools are useful for finding memory leaks in .NET?, and I understand there are tools and/or frameworks out there to handle this "big picture" and I'm sure in a few weeks I'll be trying out several of these methods. For now I just have a really strong feeling I might have a leak related to forms not being deleted, so just want to check this one thing (and I want to know just for the sake of knowing).
I know I can watch Dispose, but I'm pretty sure Dispose can be called but still end up with the form object still being around. To test that theory I created a known-issue where I registered for a callback event in my form, then closed the form without unregistering it. Sure enough, Dispose was called (and "disposing" was true), but later when the event was fired, it still hit my break-point inside the form that was supposedly disposed already.
There are really two issues here:
As for your original question, you can use a WeakReference to monitor an object's existence without affecting its lifetime.
Your underlying question suggests that you have a misunderstanding of what garbage collection is, and how it works. The point of garbage collection is that you should never care if an object has been collected or not. Instead, you should focus on the references to the object, and if they have been reassigned or made inaccessible from rooted references. Don't worry about the instance, worry about the references to it.
The entire concept of a managed language is that you don't need to care when an object is actually garbage collected. Lots and lots of time and effort go into the GC to make sure that it doesn't collect objects that it shouldn't, that it doesn't leave objects that it should collect (when it decides to do a pass over the generation that it's in) and that all of this is done reasonably efficiently. This means that if an object consumes a large amount of managed resources and also implements IDisposable (say, a DataTable or DataSet) is disposed it is still consuming a lat of memory, and disposing of it doesn't make it get garbage collected any quicker (although you should still dispose of it to ensure any managed resources go away).
The GC is built to work best when you leave it alone and let it do it's job rather than interfering with it by trying to, for example, manually cause collections to take place. This is occasionally useful for debugging purposes or learning about your program/the language, but it virtually never belongs in a production application.
Disposing has nothing to do with garbage collection or collecting an object. Disposing is the mechanism in place for dealing with a managed object that is holding onto an unmangaed resource (or another object that is holding onto an unmangaed resource). Disposing of the object is telling it to clear up that unmanaged resource, but it has nothing to do with the garbage collector freeing the managed resources of that object. The destructor is there so that you don't free the managed resources before freeing the unmanaged resources, but it's perfectly acceptable (and in fact should always happen) that the unmanaged resources are cleaned up (via dispose) before the managed resource is freed.
Now, it is still possible, given all of this, for a program to have a memory leak, but there are a few questions you really need to be asking yourself first. How large is the leak? Is it a one off, continuous over time, every time we do function X, etc? What would it take for the program to consume so much memory as to be disruptive (either run out of memory, cause other programs to run out of memory, etc) and is that likely to occur with common or legitimate uses of the program? Usually you don't need to start asking these types of questions until you start getting out of memory exceptions, or notice that you start running out of physical memory for a program that oughtn't to. If you notice these problems then you can start looking around for objects that implement IDisposable that aren't being disposed, to see if you're holding onto references to very large objects (or collections of objects) that you no longer need, etc.
Sorry for the wall of text.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Setting Objects to Null/Nothing after use in .NET
Do you need to dispose of objects and set them to null?
For large, or high traffic website:
First question:
Will set Object=null (not Disposable ) release memory?
Or is other way to release memory?
Second question:
Is explicitly release memory as above necessary in normal code?
No, it won't do it immediately, and no, it's usually unnecessary if you're writing your code the right way.
If you have a resource that implements IDisposable, call Dispose() on it, or even better, put it in a using(...) block - it's much faster and will release the proper resources. If you have several large objects in the same scope that aren't COM objects or don't implement some form of disposal mechanism, doing:
someObject = null;
GC.Collect();
might help, but you're probably better off restructuring your code so you don't end up in that situation.
If you're doing this before objects go out of scope, it's completely superfluous and makes things worse. More so if you set things to null in your finalizer. For example, never do this:
public void aFunction() {
SomeThing anObject = new SomeThing();
// ...
anObject = null;
}
nor this:
public ~MyClass() {
this.Something = null; // WRONG!
this.SomethingElse.Dispose(); // DANGEROUS!
this.SomeObject.Notify("I got finalized!"); // ALSO DANGEROUS!
}
And, for the sake of completeness, you do this:
Marshal.ReleaseComObject(someObj);
to release a COM object.
Setting a reference to null won't delete the memory used by the object but it will eventually get garbage-collected.
Suppose you have two objects A and B, with A referencing B.
If there are no longer any references to A, then A and B will both be garbage-collected. There's no point in clearing A's reference to B.
However, if A is a long-lived object and B is no longer required, it can be worth clearing the reference to B so that it will be garbage-collected.
In the context of asp.net, you'll mostly be dealing with short-lived objects so this is typically a non-issue.
Setting an object reference to null won't release memory, however it will allow the garbage collector to release the memory at some later time.
As long as you hold a reference to an object, it won't be garbage collected, but in normal ASP.NET code you'll seldom have to think about it since most objects are only referenced for the duration of the request. When the request is garbage collected, most objects created during it will be automatically garbage collected too.
The major exception to this "soft rule" is static or shared resources where leaving references may cause them to live for many requests and build up over time. In other words, avoid static variables and you'll mostly be fine by default.
No, it isn't. If the object isn't disposeable, the garbage collector will take care of it just fine. Also, you aren't releasing memory, you are modifying a variable.
You don't "release" memory in a garbage-collected environment. Start by reading this.
I havent worked for a very heavy traffic websites or web apps. But i have my business 2 businness portal and Practice Management System running on the web where i am getting around 1000-3000 hits daily with concurrent users. For that i am using static objects throughout the layers. Keeping public static variable in mind. It gives me few benefits ,
I need lesser objects to creates, as a single static object gets shared
Cleaner code for later maintenance.
static ctors calls, as they are called once.
Object cleanup , I left on GC, as GC checks for any orphan objects and automatically free ups its space.
AFAIK, for your questions,
setting null will resets the pointer instead freeing up the memory.
It is not mandatory that you release memory in normal code, but its a good practice to remove all un-necessary objects from the memory so as free up the memory and let your app work appropriately without any issue.
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):