class Foo
{
int Data;
public Foo(int data)
{
Data = data;
}
public void Diminish()
{
this.Data--;
}
}
Foo Foo1 = new Foo(5);
Foo1.Diminish();
Is there Any way delete Foo1 When Data is 0 within Foo1?
(Free up the memory used by that instance)
An object never decides when to "delete" or garbage collect itself. It can't know what else has a reference to it. When there are no references left to an object then it gets garbage collected.
In the scenario described in the question - suppose you had a List<Foo>() and then one instance of Foo spontaneously decided, based on its own internal logic, that it should no longer exist? I don't know what that would even look like - all references to it suddenly become null?
If that were possible, the defects it would cause would be beyond all mortal comprehension. We have enough trouble when two threads modify the same object or property because of improper/nonexistent locking, and so from the viewpoint of one thread something has changed when it shouldn't.
But this would create similar scenarios even in single-threaded applications.
var myFoo = new Foo(1);
myFoo.Diminish();
var x = myFoo.Data; // Null reference exception! WTH - it nullified itself!
A fundamental concept of OOP is that classes don't need to know too much about the internal state of other classes. But now you'd need to "know" that a class could suddenly choose to opt-out and jump off the cliff into the GC, and constantly null-check objects that already weren't null.
It's a very good thing that this is not possible.
As some comments and at least one answer explain, a normal reference in a .NET program is exactly what keeps that object alive. It is not possible to "delete" the object explicitly in the way you seem to mean.
An object will remain alive as long as it is "reachable" (i.e. it is possible to start with a "rooted" reference and find the reference to that object). Once it is no longer reachable (which could happen even when a reference to the object exists, as long as that reference can't be reached from a rooted reference…e.g. a circular reference between two objects does not prevent garbage collection of those two objects, as long as the only reference to each is from the other).
All of this is discussed in detail in documentation references provided elsewhere.
Now, all that said, depending on what you're actually trying to do, you might find that the WeakReference<T> class is what you need. This class provides a way to reference objects without preventing them from being garbage-collected. You can combine this with reference-counting and an object repository to maintain non-weak references to keep the object alive to produce the results you want.
For example:
class RefCounted
{
private static readonly HashSet<RefCounted> _alive = new HashSet<RefCounted>();
private int _referenceCount;
public RefCounted()
{
AddRef();
}
public AddRef()
{
if (_referenceCount == 0)
{
// the collection ensures a strong reference to the object, to prevent
// the object from being garbage-collected even if the only other
// references are weak references.
_alive.Add(this);
}
_referenceCount++;
}
public Release()
{
if (--_referenceCount)
{
// no longer need to force the object to stay alive; if the only remaining
// references to the object are weak references, then the garbage
// collector may collect the object.
_alive.Remove(this);
}
}
}
Then you can use it like this:
WeakReference<RefCounted> o = new WeakReference<RefCounted>(new RefCounted());
RefCounted r;
if (o.TryGetTarget(out r))
{
// do stuff with r
// done with the object? Then call:
r.Release();
}
It is important to note that this is not precisely what you're asking for. The above still does not give you deterministic control over the actual garbage collection of the object. Furthermore, once you retrieve the target of the weak reference into e.g. a local variable, you now hold a strong reference to the object for the lifetime of that local variable (i.e. up to the last point that variable is used) and the object cannot be collected during that lifetime, even if the reference count goes to 0 (though at that point the object will of course be collected once that variable no longer exists or is reachable).
But if you really have a scenario where you need to use some type of reference counting to track and in some way control the lifetime of an object, the above is how you'd do it. You would have to store the WeakReference<T> objects rather than direct references to T, retrieving the target reference at each place in the code where you need it.
Of course, there remains the question of why do you think you need reference counting. And even if you need reference counting, why should the reference count be directly related to the lifetime of the object? Your question does not explain these aspects, and so I can't really address them directly. I will say that reference counting is not needed at all for effective management of managed objects in .NET programs. As long as you don't actively interfere with the maintenance and use of references to managed objects, the .NET garbage collection system will do a far better job of dealing with the lifetimes of objects than you or I could.
An object instance will remain "alive" as long as any piece of code references it. So, it's something you cannot really control from within a class.
From MSDN:
The .NET Framework's garbage collector manages the allocation and release of memory for your application. Each time you create a new object, the common language runtime allocates memory for the object from the managed heap. As long as address space is available in the managed heap, the runtime continues to allocate space for new objects. However, memory is not infinite. Eventually the garbage collector must perform a collection in order to free some memory. The garbage collector's optimizing engine determines the best time to perform a collection, based upon the allocations being made. When the garbage collector performs a collection, it checks for objects in the managed heap that are no longer being used by the application and performs the necessary operations to reclaim their memory.
So the garbage collector will take care of releasing the object when it goes out of scope unless you need to release some expensive (memory consumption) resource. You can sets the reference of the instance to null, this will terminate the object, unless there is some other reference to the object:
Foo Foo1 = new Foo(5);
Foo1.Diminish();
if(Foo1.Data == 0)
{
Foo1 = null;
}
C# is a managed language, you can't "delete" anything, once something can no longer be referenced the value clean up will be handled by the Garbage Collector.
There are edge cases where you need to handle clean up, or be more aware of how you are handling creation:
Instances of classes that implement IDisposable interface should be disposed of by calling the Dispose() method or wrapped in a using statement
Unmanaged resource wrappers, such as IntPtr may need to be cleaned up using Marshal.FreeHGlobal(IntPtr) or Marshal.FreeCoTaskMem(IntPtr) if you are required to manage them and you're making a call to unmanaged code
Static (and possibly long living) values will never be un-referenceable unless you actively set variables to null or remove items from collections
Related
I am using Actions in C# and I was wondering if I need to set the instance of Action to null once I wish the GC to collect the objects properly? Here is an example:
public class A
{
public Action a;
}
public class B
{
public string str;
}
public class C
{
public void DoSomething()
{
A aClass = new A();
B bClass = new B();
aClass.a = () => { bClass.str = "Hello"; }
}
}
Inside my Main method I have something like this:
public void Main(...)
{
C cClass = new C();
cClass.DoSomething();
Console.WriteLine("At this point I dont need object A or B anymore so I would like the GC to collect them automatically.");
Console.WriteLine("Therefore I am giving GC time by letting my app sleep");
Thread.Sleep(3000000);
Console.WriteLine("The app was propably sleeping long enough for GC to have tried collecting objects at least once but I am not sure if A and B objects have really been collected");
}
}
Please read the Console.WriteLine text it will help you understand what I am asking here.
If I apply my understanding of GC to this example the GC would never collect the objects since A cannot be destroyed because it holds instance of B. Am I right?
How can I properly collect those two objects? Do I need to set instances of Actions to null just to let GC collect objects before the end of application or is there already some kind of very smart mechanism by GC that knows how to destroy objects who have Actions such as A and B are?
EDIT: The question is about GC and collecting objects properly. Its not about calling the method collect().
There are numerous problems with this question. Rather than answer your question directly I'm going to answer the questions you should be asking.
Let's first disabuse you of your notions about the GC.
Will sleeping for a long time activate the garbage collector?
No.
What activates the garbage collector?
For testing purposes you can use GC.Collect() and GC.WaitForPendingFinalizers(). Use these only for testing purposes; it is a bad practice to use them in production code except in some very rare circumstances.
Under normal situations the things that trigger a GC are complicated; the GC is a highly tuned piece of machinery.
What are the semantics of garbage collection insofar as closed-over outer variables are concerned?
The lifetime of a closed-over outer variable of a lambda that is converted to a delegate is extended to be not shorter than the lifetime of the delegate.
Suppose I have a variable of type Action which is initialized with a lambda that is closed over an outer local variable of reference type. In order to make the object referred to by that variable eligable for collection, do I have to set the variable of type Action to null?
In the vast majority of cases, no. The garbage collector is very smart; just let it do its work and do not worry about it. Eventually the runtime will determine that the Action variable cannot be reached by any live root and will make it eligable for collection; the closed-over outer variable will then become eligible.
There may be extremely rare situations in which you want to throw away references to the Action sooner, but they are rare; the vast majority of time, just let the GC do its job without interference.
Are there situations in which outer variables can have their lifetimes extended too long?
Yes. Consider:
void M()
{
Expensive e = new Expensive();
Cheap c = new Cheap();
Q.longLived = ()=>c; // static field
Q.shortLived = ()=>e; // static field
}
When M() is executed a closure is created for both delegates. Suppose that shortLived is going to be set to null soon, and longLived is set to null far in the future. Unfortunately both local variables have their lifetimes extended to the lifetime of the object referred to by longLived, even though only c is still reachable. The expensive resource e is not released until the reference in longLived is dead.
Numerous programming languages have this problem; some implementations of JavaScript, Visual Basic, and C# all have this problem. There is some talk of fixing it in the Roslyn release of C# / VB but I do not know if that will come to fruition.
In that case the solution is to avoid the situation in the first place; don't make two lambdas that share a closure if one of the delegates will live much longer than the other.
Under what circumstances does a local that is not a closed-over outer variable become eligable for collection?
The moment that the runtime can prove that a local cannot be read from again, the thing it references becomes eligable for collection (assuming the local is the only root of course.) In your example program there is no requirement that the references in aClass and bClass remain alive until the end of the method. In fact, there are rare but possible circumstances in which the GC can be deallocating an object on one thread while it is still in its constructor on another thread! The GC can be very aggressive about determining what is dead, so be careful.
How do I keep something alive in the face of an aggressive GC?
GC.KeepAlive() of course.
I am using Actions in C# and I was wondering if I need to set the instance of Action to null once I wish the GC to collect the objects properly?
Not necessarily. As long as no object is reachable which references the delegate, the delegate will be eligible for GC.
That being said, in your example, aClass and bClass are still valid variables, and refer to reachable objects. This means that aClass.a is still reachable, and not eligible for GC, so it will not be collected.
If you wanted these to be garbage collected, you would need to explicitly set the object reference (aClass) to null so that the A instance, and it's contained delegate, were no longer reachable objects, and then you'd have to explicitly call GC.Collect to trigger the GC, since nothing will cause the GC to trigger in your code.
I have a class that contains a pointer to an object inheriting off IDisposable
class Foo
{
private Bar bar;
private test()
{
bar = CreateBarFactory();
}
};
bar never is used beyond when it is set, in test();
I'm wondering if the GC is permitted to dispose of bar even if the Foo instance is still around?
if so is there a way I can prevent this? bar is a pointer to an object that contains a ref count, if it's Disposed() then the ref is released causing some functionality to be disabled
If the instance of foo is still around (i.e. someone is holding a reference to it.. keeping it alive), bar will not be collected. It will be treated as 'reachable' by the GC i.e. in use.
Members of reachable objects are also considered to be reachable.
Please don't mix up terminology. C# does have pointers, but that's not what you have there. You have a reference. Importantly, references do affect the garbage collector, whereas pointers, in and of themselves, do not.(*)
So long as the instance of Foo is reachable, it's reference to bar will keep that object reachable too. If Foo and Bar have finalizers, then, in the finalizer of Foo is the one circumstance where you shouldn't assume anything about bar - it may have already been finalized itself.
Even though, as you've indicated, no other method will be called that accesses bar, the GC and JIT don't perform this sort of analysis. For object references, the entire object is considered to be reachable, and any references it contains are followed and the objects located similarly are marked as reachable. This has to be done so that Reflection based access to an object will never obtain an invalid reference. This is also why, if your object has allocated a large helper object that you will no longer use, it can be (in this limited circumstance) useful to set the reference to null.
The only lifetime analysis that does consider what future code will run is the code within a single method body, with respect to the local variables. A local reference variable within a method is not sufficient to keep an object alive if no further references to that variable will occur. This is worked out collaboratively between the JIT and GC.
(*) people sometimes think that pointers keep objects alive. This is not strictly true. The act of pinning, which can yield a pointer, will keep an object alive, but there's nothing stopping you from keeping a pointer beyond the time that the object was pinned. Of course, de-referencing the pointer at any such time would be unsafe...
And, another terminology issue:
I'm wondering if the GC is permitted to dispose of bar
The GC doesn't dispose of anything. Disposal occurs when an object implements IDisposable and the user of the object calls Dispose on it (either directly or via e.g. a using block).
The GC doesn't call Dispose. It may call a finalizer, if one is defined on an object (and so my warning above would apply) and it then collects the object.
To work properly with this class, you should inherit it from IDisposable iterface and realize method Dispose() that will call bar.Dispose(). It will garantee that this expansive resource will be freed exactly when it can be freed, not when GC decide to Collect.
I'm reading through a C# topic on Dispose() and ~finalize and when to use which. The author argues that you should not use references within your ~finalize because it's possible the object you're referencing may already be collected. The example specifically stated is, ".. you have two objects that have references to each other. If object #1 is collected first, then object #2's reference to it is pointing to an object that's no longer there."
In what scenarios would an instance of an object be in a state where it has a reference in memory to an object that is being GC'd? My guess is there are at least two different scenarios, one where the object reference is pointing at an object and one where the object reference is pointing at another object reference (eg. when it was passed by ref in a method).
You can have objects that reference each other, and the entire set can be eligible for GC.
Here is a simple code sample:
class Test {
public Test Other { get; set;}
static void Main()
{
Test one = new Test();
Test two = new Test { Other = one; }
one.Other = two;
one = null;
two = null
// Both one and two still reference each other, but are now eligible for GC
}
}
Normally the GC will only reclaim memory for objects that don't have any reference pointing to them. However, objects with finalizers are treated differently.
Here's what MSDN says about it:
Reclaiming the memory used by objects with Finalize methods requires
at least two garbage collections. When the garbage collector performs
a collection, it reclaims the memory for inaccessible objects without
finalizers. At this time, it cannot collect the inaccessible objects
that do have finalizers. Instead, it removes the entries for these
objects from the finalization queue and places them in a list of
objects marked as ready for finalization. [...]
The garbage collector calls the Finalize methods for the objects in this list and then removes the entries from the list. A future garbage collection will determine that the finalized objects are truly garbage because they are no longer pointed to by entries in the list of objects marked as ready for finalization.
So there is no guarantee that other objects referenced in a finalizer will still be usable when the Finalize method gets executed by the GC, since they may already have been finalized during an earlier garbage collection while the object itself was waiting to be finalized.
In short, objects that are not reachable from a GC root (static field, method parameter, local variable, enregistered variable) by following the chain of references are eligible to garbage collection. So it is fully possible that, say, object A refers to B that refers to C that refers to D, but suddenly A nulls out its reference to B, in which case B, C and D all can be collected.
Unfortunately, there is a lot of sloppy use of terminology surrounding garbage collection, which causes much confusion. A "disposer" or "finalizer" does not actually destroy an object, but rather serves to delay the destruction of an object which would otherwise be eligible for destruction, until after it has had a chance to put its affairs in order (i.e. generally by letting other things know that their services are no longer required).
It's simplest to think of the "stop the world" garbage collector as performing the following steps, in order:
Untag all items which are new enough that they might be considered "garbage".
Visit every garbage-collection root (i.e. thing which is inherently "live"), and if it hasn't been copied yet, copy it to a new heap, update the reference to point to the new object, and and visit all items to which it holds references (which will copy them if they haven't been copied). If one visits an item in the old heap that had been copied, just update the reference one used to visit it.
Examine every item that has registered for finalization. If it hasn't yet been copied, unregister it for finalization, but append a reference to it on a list of objects which need to be finalized as quickly as possible.
Items on the immediate-finalization list are considered "live", but since they haven't yet been copied yet, visit every item on that list and, if not yet copied, copy it to the new heap and visit all items to which it holds references.
Abandon the old heap, since nobody will hold references to anything on it anymore.
It's interesting to note that while some other garbage-collection systems work by using double-indirected pointers for references, the .net garbage collector (at least the normal "stop the world" one) uses direct pointers. This increases somewhat the amount of work the collector has to do, but it improves the efficiency of code that manipulates objects. Since most programs spend more of their time manipulating objects than they spend collecting garbage, this is a net win.
" ... then object #2's reference to it is pointing to an object that's no longer there."
That will never happen. Whenever your code has access to a reference then the object being referred to still exists. This is called memory safety and this still holds when an object is being finalized in the background. A reference never points to a collected instance.
But an existing, non collected object could already have been Finalized (Disposed). That is probably what your warning refers to.
class Foo
{
private StreamWriter logFile = ...
private StringBuilder sb = new StringBuilder("xx");
~Foo()
{
if (sb.ToString() == "xx") // this will always be safe and just work
{
// the next line might work or
// it might fail with "logFile already Disposed"
logFile.Writeline("Goodbye");
}
}
}
There is a term called resurrection in .NET.
In short, resurrection may happen when your object is in finalization queue but when the finalizer (~ClassName() method) is called, it moves the object back to the game. For example:
public class SomeClass
{
public static SomeClass m_instance;
...
~SomeClass()
{
m_instance = this;
}
}
You can read more on this here: Object Resurrection using GC.ReRegisterForFinalize. But I would really recommend the book CLR via C# by Jeffrey Richter as it explained this subject in depth in one of the chapters.
Does setting the object to null make it MARKED for GC?
EDIT: A class has got multiple static methods. when using these methods in your program, what is the best way to ensure the objects are marked for GC after a certain point?
Methods aren't garbage collected at all - so it's not really clear what your question means.
Likewise you never set an object to null. You can make the value of a variable null, but that doesn't nothing to any object that the variable previously referred to. It just means that next time the garbage collector looks for live object, that variable won't contribute any object to the set of objects which must be kept alive at the end of the GC.
I suggest you read Jeffrey Richter's article on garbage collection for a bit more background, then ask any further specific questions when you've got to grips with the basics.
If you are asking about what happens to objects referenced by variables in static methods, then those objects become elligible for garbage collection when they are no longer in scope.
If you are talking about objects referenced by static fields then these will, in simple terms, not be collected until they the references are set to null.
The following example may illustrate this better:
class Example
{
private static object field1 = new object();
public static void SomeMethod()
{
object variable1 = new object();
// ...
}
public static void Deref()
{
field1 = null;
}
}
The object referenced by field1 will be created when the class is loaded and will remain rooted even as objects of class Example are created and destroyed. The only way to have that object garbage collected would be to call the Deref() method which will dereference it by setting the reference to null. (Actually, it is possible to unload classes by unloading an app domain but this is somewhat more advanced and not something you are likely to come across that often.)
Conversely, the static method SomeMethod() creates an object and references it by the variable variable1. This object is elligible for garbage collection as soon as it goes out of scope (at the end of the method). Pratically, it could be collected earlier if the remainder of the method does not reference it.
objects are not marked for GC, They are marked (by the existence of a variable that references or points to them) to NOT be Garbage collected. When every variable or reference to an object in all running threads, and in all global static variables, and in all cpu registers, has been deleted, gone out of scope or been set to null, then the object is no longer accessible, and the GC will collect it.
Think of static methods as class methods. They are available whether an object exists or not. They have no affect on garbage collection.
I am fascinated by the way the CLR and GC works (I'm working on expanding my knowledge on this by reading CLR via C#, Jon Skeet's books/posts, and more).
Anyway, what is the difference between saying:
MyClass myclass = new MyClass();
myclass = null;
Or, by making MyClass implement IDisposable and a destructor and calling Dispose()?
Also, if I have a code block with a using statement (eg below), if I step through the code and exit the using block, is the object disposed of then or when a garbage collection occurs? What would happen if I call Dispose() in the using block anyay?
using (MyDisposableObj mydispobj = new MyDisposableObj())
{
}
Stream classes (eg BinaryWriter) have a Finalize method? Why would I want to use that?
It's important to separate disposal from garbage collection. They are completely separate things, with one point in common which I'll come to in a minute.
Dispose, garbage collection and finalization
When you write a using statement, it's simply syntactic sugar for a try/finally block so that Dispose is called even if the code in the body of the using statement throws an exception. It doesn't mean that the object is garbage collected at the end of the block.
Disposal is about unmanaged resources (non-memory resources). These could be UI handles, network connections, file handles etc. These are limited resources, so you generally want to release them as soon as you can. You should implement IDisposable whenever your type "owns" an unmanaged resource, either directly (usually via an IntPtr) or indirectly (e.g. via a Stream, a SqlConnection etc).
Garbage collection itself is only about memory - with one little twist. The garbage collector is able to find objects which can no longer be referenced, and free them. It doesn't look for garbage all the time though - only when it detects that it needs to (e.g. if one "generation" of the heap runs out of memory).
The twist is finalization. The garbage collector keeps a list of objects which are no longer reachable, but which have a finalizer (written as ~Foo() in C#, somewhat confusingly - they're nothing like C++ destructors). It runs the finalizers on these objects, just in case they need to do extra cleanup before their memory is freed.
Finalizers are almost always used to clean up resources in the case where the user of the type has forgotten to dispose of it in an orderly manner. So if you open a FileStream but forget to call Dispose or Close, the finalizer will eventually release the underlying file handle for you. In a well-written program, finalizers should almost never fire in my opinion.
Setting a variable to null
One small point on setting a variable to null - this is almost never required for the sake of garbage collection. You might sometimes want to do it if it's a member variable, although in my experience it's rare for "part" of an object to no longer be needed. When it's a local variable, the JIT is usually smart enough (in release mode) to know when you're not going to use a reference again. For example:
StringBuilder sb = new StringBuilder();
sb.Append("Foo");
string x = sb.ToString();
// The string and StringBuilder are already eligible
// for garbage collection here!
int y = 10;
DoSomething(y);
// These aren't helping at all!
x = null;
sb = null;
// Assume that x and sb aren't used here
The one time where it may be worth setting a local variable to null is when you're in a loop, and some branches of the loop need to use the variable but you know you've reached a point at which you don't. For example:
SomeObject foo = new SomeObject();
for (int i=0; i < 100000; i++)
{
if (i == 5)
{
foo.DoSomething();
// We're not going to need it again, but the JIT
// wouldn't spot that
foo = null;
}
else
{
// Some other code
}
}
Implementing IDisposable/finalizers
So, should your own types implement finalizers? Almost certainly not. If you only indirectly hold unmanaged resources (e.g. you've got a FileStream as a member variable) then adding your own finalizer won't help: the stream will almost certainly be eligible for garbage collection when your object is, so you can just rely on FileStream having a finalizer (if necessary - it may refer to something else, etc). If you want to hold an unmanaged resource "nearly" directly, SafeHandle is your friend - it takes a bit of time to get going with, but it means you'll almost never need to write a finalizer again. You should usually only need a finalizer if you have a really direct handle on a resource (an IntPtr) and you should look to move to SafeHandle as soon as you can. (There are two links there - read both, ideally.)
Joe Duffy has a very long set of guidelines around finalizers and IDisposable (co-written with lots of smart folk) which are worth reading. It's worth being aware that if you seal your classes, it makes life a lot easier: the pattern of overriding Dispose to call a new virtual Dispose(bool) method etc is only relevant when your class is designed for inheritance.
This has been a bit of a ramble, but please ask for clarification where you'd like some :)
When you dispose an object, the resources are freed. When you assign null to a variable, you're just changing a reference.
myclass = null;
After you execute this, the object myclass was referring to still exists, and will continue to until the GC gets around to cleaning it up. If Dispose is explicitly called, or it's in a using block, any resources will be freed as soon as possible.
The two operations don't have much to do with each others.
When you set a reference to null, it simply does that. It doesn't in itself affect the class that was referenced at all.
Your variable simply no longer points to the object it used to, but the object itself is unchanged.
When you call Dispose(), it's a method call on the object itself. Whatever the Dispose method does, is now done on the object. But this doesn't affect your reference to the object.
The only area of overlap is that when there are no more references to an object, it will eventually get garbage collected. And if the class implements the IDisposable interface, then Dispose() will be called on the object before it gets garbage collected.
But that won't happen immediately after you set your reference to null, for two reasons.
First, other references may exist, so it won't get garbage collected at all yet, and second, even if that was the last reference, so it is now ready to be garbage collected, nothing will happen until the garbage collector decides to delete the object.
Calling Dispose() on an object doesn't "kill" the object in any way. It is commonly used to clean up so that the object can be safely deleted afterwards, but ultimately, there is nothing magical about Dispose, it's just a class method.