I suppose this is a very silly question but I've been looking around and couldn't find answers on the following questions. Really appreciate answers shedding light on this.
1) What happens to the previous object if instantiating a new one within the same method. Example:
DataTable dataTable = new DataTable();
dataTable = new DataTable(); // Will the previously created object be destroyed and memory freed?
2) Same question as (1) but on static variable. Example:
static private DataView dataView;
private void RefreshGridView()
{
dataView = new DataView(GetDataTable()); // Will the previously created objects be destroyed and memory freed?
BindGridView();
}
Thanks!
// Will the previously created object be destroyed and memory freed?
Potentially. As soon as you do this, you'll no longer be holding a reference to the original DataTable. As long as nothing else references this object, it will become eligible for garbage collection. At some point after that, the GC will run, and collect the object, which will in turn free it's memory.
This is true for static, instance, and local variables. The mechanism is identical in all of these scnearios.
Note that if the object you're referencing implements IDisposable, it's a good practice to call Dispose() on it before losing the reference. Technically, a properly implemented IDisposable implementation will eventually handle things properly, but native resources may be tied up until the GC collection occurs, which may not happen quickly. Note that this has nothing to do with (managed) memory, but is still a good practice.
That being said, your first example is a bad practice. While the memory will (eventually) get cleaned up, you're performing extra allocations that serve no purpose whatsoever, so it'd be better to not "double allocate" the variable.
The short answer is that all of this is handled by the garbage collector. The instances won't be immediately deleted, but they will be marked as "unreachable" and freed at a later time.
I suggest having a read through the Garbage Collection article at MSDN
Objects are reference variable in C#. That means they store memory references in them.
So when you re-assign an object, you simply, overwrite the earlier value (memory reference) that it held. And the earlier value is now qualified for Garbage Collection. Now its Garbage Collector's job to free that memory. Same is for all kinds of variables including static
Related
Or is it already to late if the finalize method is reached?
Basically I'm creating some code to log to a MySql database. Each log entry is represented by an object and stored in a queue until it gets flushed to the database in a batch insert / update. I figured it'd be inefficient to create a new object on the heap every time I wanted to write an entry (especially since I might want to write an entry or two in performance sensitive areas). My solution was to create a pool of objects and reuse them.
Basically I'm trying to not re-invent the wheel by letting the .Net Garbage Collector let me know when an object is no longer needed and can be added back to the pool. The problem is I need away to abort garbage collection from the destructor. Is that possible?
Can you? Yes.
Should you? No, it is almost certainly a terrible idea.
The general rule C# developers should remember is the following:
If you find yourself writing a finalizer, you probably did something wrong.
The memory allocators used by well-established managed VMs (such as the CLR or JVM) are extremely fast. One of the things that slows down the garbage collector in these systems is the use of customized finalizers. In an effort to optimize the runtime, you are actually giving up a very fast operation in favor of a much slower operation. Furthermore, the semantics of "bringing an object back to life" are difficult to understand and reason about.
Before you consider using a finalizer, you should understand everything in the following articles.
Never write a finalizer again (well, almost never)
DG Update: Dispose, Finalization, and Resource Management
Connection pooling is a feature virtually any major DB connection implementation is already going to natively support, so there is no reason to handle this manually. You'll be able to simply create a new connection for each operation and know that behind the scenes the connections will actually be pooled.
To answer the literal question that you asked, yes. You can ensure that an object is not going to be GCed after it is finalized. You can do so simply by creating a reference to it from some "live" location.
This is a really bad idea though. Take a look at this example:
public class Foo
{
public string Data;
public static Foo instance = null;
~Foo()
{
Console.WriteLine("Finalized");
instance = this;
}
}
public static void Bar()
{
new Foo() { Data = "Hello World" };
}
static void Main(string[] args)
{
Bar();
GC.Collect();
GC.WaitForPendingFinalizers();
Console.WriteLine(Foo.instance.Data);
Foo.instance = null;
GC.Collect();
GC.WaitForPendingFinalizers();
}
This will print out:
Finalized
Hello World
So here we had an object end up being finalized, and we then accessed it later on. The problem however is that this object has been marked as "finalized". When it is finally hit by the GC again it's not finalized a second time.
You could re-register for finalization in the destructor, like so:
~YourClass()
{
System.GC.ReRegisterForFinalize(this);
}
And from there you'd probably want something to reference so it doesn't get finalized again, but this is a way to do it.
http://msdn.microsoft.com/en-us/library/system.gc.reregisterforfinalize(v=vs.110).aspx
Most resources state that the garbage collector figures that out on its own based on references and that I shouldn't mess with it.
I am wondering if I can explicitly tell the garbage collector that it may dispose an object while still keeping a reference.
What I would like to do is to tell the garbage collector that I currently don't need an object anymore (but might again) and then at a later point when (if) I need the object again I would like to check if it has been disposed already. If it has I simply recreate it, if it hasn't I'd like to "unmark" it from garbage collection until I am done with it again.
Is this possible?
I plan to implement something similar to the Lazy<T> class. Pseudocode:
obj = new DisposeIfNecessary<LargeObject>(() => InitLargeObject());
obj.DoSomething(); // Initializes obj using InitLargeObject()
obj.DisposeIfNecessary(); // This is where the magic happens
... // obj might get disposed at some point
obj.DoAnotherThing(); // Might or might not call InitLargeObject() again
obj.Dispose(); // I will not need it again
The WeakReference class does exactly what you want, using the IsAlive property to check for state before using it.
You can get a "strong" reference to it again via the Target property, which will affect the reference count and stop it from being eligible for collection.
Also note that Dispose doesn't directly relate to garbage collection, so disposing an item (depending on the Dispose implementation) might make it unusable anyway - but again, that is nothing to do with GC. On a general practice note, as mentioned by #HansPassant, calling Dispose on an item (or generally anything claiming to dispose) and then attempting to use it again afterwards is a code smell (or just plain wrong as other developers will expect Dispose to be a last-call method marking the object as unusable from then on).
The WeakReference class will not be responsible for re-creating collected objects, but in conjunction with IsAlive you can handle that logic yourself.
Also, to the point in the comments, the GC doesn't do anything clever with WeakReference in terms of deciding when to collect it in terms of trying to leave WeakReference items until last; it will collect the underlying object as it would others during a run if it is eligible - no special handling and definitely no "cache" behaviour.
I was asked this question in an interview: How does memory leakage problem occur in C# as of all know Garbage Collector responsible for all the memory management related work? So how is it possible?
From MSDN:-
A memory leak occurs when memory is allocated in a program and is
never returned to the operating system, even though the program does
not use the memory any longer. The following are the four basic types of memory leaks:
In a manually managed memory environment: Memory is dynamically allocated and referenced by a pointer. The pointer is erased before the memory is freed. After the pointer is erased, the memory can no longer be accessed and therefore cannot be freed.
In a dynamically managed memory environment: Memory is disposed of but never collected, because a reference to the object is still active. Because a reference to the object is still active, the garbage collector never collects that memory. This can occur with a reference that is set by the system or the program.
In a dynamically managed memory environment: The garbage collector can collect and free the memory but never returns it to the operating system. This occurs when the garbage collector cannot move the objects that are still in use to one portion of the memory and free the rest.
In any memory environment: Poor memory management can result when many large objects are declared and never permitted to leave scope. As a result, memory is used and never freed.
Dim DS As DataSet
Dim cn As New SqlClient.SqlConnection("data source=localhost;initial catalog=Northwind;integrated security=SSPI")
cn.Open()
Dim da As New SqlClient.SqlDataAdapter("Select * from Employees", cn)
Dim i As Integer
DS = New DataSet()
For i = 0 To 1000
da.Fill(DS, "Table" + i.ToString)
Next
Although this code is obviously inefficient and not practical, it is meant to demonstrate that if objects are added to a collection (such as adding the tables to the DataSet collection), the objects are kept active as long as the collection remains alive. If a collection is declared at the global level of the program, and objects are declared throughout the program and added to that collection, this means that even though the objects are no longer in scope, the objects remain alive because they are still being referenced.
You may also check this reference:-
Identify And Prevent Memory Leaks In Managed Code
The above link gives a very good conclusion
Although .NET reduces the need for you to be concerned with memory,
you still must pay attention to your application's use of memory to
ensure that it is well-behaved and efficient. Just because an
application is managed doesn't mean you can throw good software
engineering practices out the window and count on the GC to perform
magic.
Just holding on to a reference to an object when you actually don't have any use for it anymore is a good way to leak. Particularly so when you store it in a static variable, that creates a reference that lives for the life of the AppDomain unless you explicitly set it back to null. If such a reference is stored in a collection then you can get a true leak that can crash your program with OOM. Not usual.
Such leaks can be hard to find. Particularly events can be tricky that way, they will get the event source object to add a reference to the event handler object when you subscribe an event handler. Hard to see in C# code because you never explicitly pass this when you subscribe the event. If the event source object lives for a long time then you can get in trouble with all of the subscriber objects staying referenced.
The tricky ones do require a memory profiler to diagnose.
Example would be a Class Child containing ClickEventHandler method subscribed to an Event ClickEvent of another class Parent.
GC of Child class would be blocked until Parent class goes out of scope..Even if Child goes out of scope it won't be collected by GC until Parent goes out of scope
All such subscribers subscribing to a Broadcaster(Event) would not be collected by GC until the broadcaster goes out of scope.
So, its a one way relation
Broadcaster(ClickEvent) -> Subscribers(ClickEventHandler)
GC of all the ClickEventHandlers would be blocked until ClickEvent goes out of scope!
As example:
You have application with main form and static variable Popups[] collectionOfPopups.
Store all application popups objects into static array and never delete them.
So with each new popup will take memory and GC will never release it.
Try to read how GC works
http://msdn.microsoft.com/en-us/library/ee787088.aspx
This will explains everything.
There can be multiple reasons, but here is one:
Consider two classes:
class A
{
private B b;
}
class B
{
private A a;
}
If you create an object for each of those classes, and cross-link them, and after that both those objects go out of the scope, you will still have link to each of them within another one. It is very difficult for GC to catch these sorts of crosslinks, and it may continue to believe that both objects are still in use.
Lets say I have something like:
public class Item
{
public string Code;
public List<string> Codes = new List<string>();
}
private void SomeMethod()
{
List<Item> Items = new List<Item>();
for (int i = 0; i < 10; i++)
{
Item NewItem = new Item();
NewItem.Code = "Something " + i.ToString();
NewItem.Codes.Add("Something " + i.ToString());
Items.Add(Item);
}
//Do something with Items
}
I am instantiating Item and not free'ing it because I need to have access to it in the list later on (rough example).
What I am wondering is when SomeMethod() has finished executing, will Items (and the contents of it - including the List<>) be de-referenced and allow the garbage collector to clean up the memory as and when it runs? Basically, will this section of code cause any memory leaks or should everything be de-referenced when SomeMethod() has finished processing.
My understanding is that when nothing holds a reference to an object it will be garbage collected so in my mind this code should be Ok but I just wanted to make sure I understand correctly.
EDIT:
If I was to add one of the objects Items is holding into another list that would still be in scope (a global list for example). What would happen?
Once your variable Items goes out of scope, the garbage collector will indeed dispose of it (and its contents, as your code is written) at its leisure.
The garbage collector will collect anything that is no longer reachable by code. Since you will no longer be able to reach your Items list or any of the Items contained within the list, then the garbage collector will collect them at some point. Your understanding of the garbage collector is correct.
Ok,
List<Item> Items
is declared in the scope of the method. So when the method ends, it goes out of scope and is the list is dereferenced.
The memory will be released at some point after that, when the Garbage Collecter sees fit.
As an aside, since Items is declared in a "local" scope I'd prefer to call it items.
all the variables in the method are in a stackframe and all these variable will be destoried along with the stackframe after the method was executed.
that is said after SomeMethod() executed, the variable Items will nolonger exists, thus new List() will be marked "can be collected".
the second question is that, there is a globle list variable hold the reference of one of the object that in the variable Items, then after the SomeMethod() executed, the List in the SomeMethod will be marked as "can be collected", and all the other object beside the item that was referenced by the globle list variable will also be makred as "can be collected"
the reason is, the list actually hold the reference that point to the exact object in the heap
So the shared object can not be collected due to the reason that it is referenced by the globleList
In order to keep things sane and keep its job, GC has to promise two things:
Garbage will be collected. This seems pretty obvious, but if the GC doesn't promise to clean up managed objects without you having to tell it to, it's rather useless.
ONLY garbage will be collected. You don't have to worry about the GC cleaning up objects that are still in play.
So, once Items goes out of scope (that is, once the function returns), it's no longer reachable by running code, so it's eligible for collection without you having to do anything. Since the items in the list are no longer reachable either (their only link was in the list), they're eligible too. If you returned a reference to an entry in the list, though, that object is still in play and can't be collected yet. GC will do the right thing.
In fact, GC almost always does the right thing. There are really only three major cases you have to worry about:
If you're buiding a container (like if you're making your own List workalike). Any objects that no longer "exist" in the container as far as the user is concerned, should generally be set to null so the GC doesn't see them as reachable through your collection and erroneously keep them alive. If you remove an item from the collection, for example, null out the reference or overwrite it with another.
With large (>~85KB?) objects. They'll typically stick around in memory til they crowd out everything else, at which point a full GC cycle runs. (Normally, only certain likely-to-be-discarded objects are checked during a cycle. Full collections check pretty much everything, which takes significantly longer but might free more memory.)
If you're using IDisposable objects or native/unmanaged resources. Some incompetents don't know how to implement IDisposable correctly. If you're dealing with a library created by such an incompetent, then you'll need to ensure that you Dispose stuff, or only ever use it within a using block, or things can get really weird. (If you only use the .net API, you're pretty safe. But disposing is still good manners.)
For pretty much all other occasions, GC just works. Trust it.
The question is self explanatory :
Foo objfoo;
try
{
objfoo = new Foo();
..........
.........
}
catch
{
}
finally
{
objfoo = null;
}
Is it necessary to free objects like this ?
Note: Setting a local / field to null is not freeing the value. It is instead removing a reference to the value which may or may not make it elligable for collection during the next GC cyle.
To answer the question, no it is not necessary. The JIT`er will calculate the last time a local is used and will essentially remove the local as one of the object's GC roots at that time. Nulling the local out will not speed up this process.
Raymond Chen did an excellent article on this very subject
http://blogs.msdn.com/b/oldnewthing/archive/2010/08/10/10048149.aspx
You dont need to as objFoo will be eligible for garbage collection (as per your code) once you leave the try block. objfoo =null just sets the pointer to null, if any other code had a pointer to the object then it wont be garbage collected
Assigning null to a variable will not delete the object it points to.
You don't need to worry about memory cleanup. C# will do it automatically for you, if you're using .NET.
In C#, usually not, because your objects will be freed by the Garbage Collector. Setting it to null later might even extend its lifetime because (depending how the compiler optimizes your code) it is still referenced at that point.
An exception is if the object implements IDisposable, which should be disposed using its Dispose() method. But you usually wrap those in using-blocks.
No , its not necessary , let the CLR do its job.
Unless the objects are accessing unmanaged resources (files, network shares, database connections ...) there is no need to do this, since they will be marked for garbage collection once they are out of scope.
If they are using resources (the StreamReader class for example) it's necessary to dispose of the objects yourself, but that's best accomplished by using a using statement.
Automatic Garbage Collection means you do not need to free memory resources, i.e. objects created with new.
You may need to free other resources you have allocated, eg locking mutexes and maybe context specific to your own project, for which you have the finally block.
There' typically no need to do this. If your function is about to exit then your object will automatically become eligible for garbage collection.
However, if you've got a really long function or it's going to do some blocking operation after the finally block then by setting the variable to null you make it eligible for garbage collection immediately. This might be a good idea if Foo manages a large amount of data, for example.