I've seen so much C# code in my time as a developer that attempt to help the GC along by setting variables to null or calling Dispose() on classes (DataSet for example) within thier own classes Dispose() method that
I've been wondering if there's any need to implement it in a managed environment.
Is this code a waste of time in its design pattern?
class MyClass : IDisposable
{
#region IDisposable Members
public void Dispose()
{
otherVariable = null;
if (dataSet != null)
{
dataSet.Dispose();
}
}
#endregion
}
The GC does not call .Dispose() (It does, however, call the finalize ~MyClass() method, which you can provide a call to the Dispose() method to have resources automatically managed when the GC decides to clean up your class).
You must always provide a way of disposing internal resources such as DataSets to code that uses your classes (and ensure you actually call .Dispose() or wrap the constructor in a using). Using IDisposable on your classes that use internal resources is highly recommended.
From MSDN:
The primary use of this interface is
to release unmanaged resources. The
garbage collector automatically
releases the memory allocated to a
managed object when that object is no
longer used. However, it is not
possible to predict when garbage
collection will occur. Furthermore,
the garbage collector has no knowledge
of unmanaged resources such as window
handles, or open files and streams.
public void Dispose()
{
otherVariable = null;
if (dataSet != null)
{
dataSet.Dispose();
dataSet = null;
}
}
No, Dispose methods are not a waste of time.
The dispose pattern is there to allow a caller to clean up a class as soon as they have finished with it, rather than waiting for the GC to collect it. The delay doesn't matter much for plain heap memory, which is why basic classes like String don't implement it. What Dispose is useful for however is cleaning up unmanaged resources. Somewhere internally, the Dataset class is using an unmanaged resource, so it provides a dispose method to allow you to let it know when that unmanaged resource can be released.
If the pattern has been followed correctly Dataset will also have a finalizer (or some subclass will) which means that if you didn't dispose of it manually, eventually the GC would run, the finalizer would get called and the unmanaged resource would be cleaned up that way. This unmanaged resource might be important though, imagine if it was a file lock, or a database connection, you don't really want to hang around waiting for the GC to run before you can reuse your database connection. Dispose provides a deterministic way of cleaning up resources when they are finished rather than relying on the non-deterministic GC.
As for setting variables to null in a dispose method. It nearly all cases it would be pointless. setting a variable to null removes a reference to that variable, which will make it eligible for garbage collection (if that's the last reference), but as you are disposing of the class anyway, you are likely to be going out of scope for the containing class so the internal class will become eligible for collection anyway.
If you have member variables inside your class that are disposable that you created (not just references you hold), then you should always call dispose on them from your own class's dispose method, but don't bother setting them to null.
Not entirely. If you have member variables which are disposable, then you probably should dispose of it like that. Your object may live longer than the scope of the work it is doing as the garbage collector is not guaranteed to run at any particular time.
Setting managed variables to null is a waste of time though. The object won't get GC'd any faster.
Garbage truck comes to my area every week but it doesn't collect my garbage unless I put my garbage bin in a way that it can collect.
You should simply remove all unwanted event subscriptions, reference and clear unmanaged handlers. Then Garbage Collector will take care of the rest.
Below example show the general best practice to implement IDisposable interface. Reference : https://msdn.microsoft.com/en-us/library/system.idisposable.dispose(v=vs.110).aspx
public class DisposeExample
{
// A base class that implements IDisposable.
// By implementing IDisposable, you are announcing that
// instances of this type allocate scarce resources.
public class MyResource: IDisposable
{
// Pointer to an external unmanaged resource.
private IntPtr handle;
// Other managed resource this class uses.
private Component component = new Component();
// Track whether Dispose has been called.
private bool disposed = false;
// The class constructor.
public MyResource(IntPtr handle)
{
this.handle = handle;
}
// Implement IDisposable.
// Do not make this method virtual.
// A derived class should not be able to override this method.
public void Dispose()
{
Dispose(true);
// This object will be cleaned up by the Dispose method.
// Therefore, you should call GC.SupressFinalize to
// take this object off the finalization queue
// and prevent finalization code for this object
// from executing a second time.
GC.SuppressFinalize(this);
}
// Dispose(bool disposing) executes in two distinct scenarios.
// If disposing equals true, the method has been called directly
// or indirectly by a user's code. Managed and unmanaged resources
// can be disposed.
// If disposing equals false, the method has been called by the
// runtime from inside the finalizer and you should not reference
// other objects. Only unmanaged resources can be disposed.
protected virtual void Dispose(bool disposing)
{
// Check to see if Dispose has already been called.
if(!this.disposed)
{
// If disposing equals true, dispose all managed
// and unmanaged resources.
if(disposing)
{
// Dispose managed resources.
component.Dispose();
}
// Call the appropriate methods to clean up
// unmanaged resources here.
// If disposing is false,
// only the following code is executed.
CloseHandle(handle);
handle = IntPtr.Zero;
// Note disposing has been done.
disposed = true;
}
}
// Use interop to call the method necessary
// to clean up the unmanaged resource.
[System.Runtime.InteropServices.DllImport("Kernel32")]
private extern static Boolean CloseHandle(IntPtr handle);
// Use C# destructor syntax for finalization code.
// This destructor will run only if the Dispose method
// does not get called.
// It gives your base class the opportunity to finalize.
// Do not provide destructors in types derived from this class.
~MyResource()
{
// Do not re-create Dispose clean-up code here.
// Calling Dispose(false) is optimal in terms of
// readability and maintainability.
Dispose(false);
}
}
public static void Main()
{
// Insert code here to create
// and use the MyResource object.
}
}
Related
In this answer I found,
Cleanup the unmanaged resources in the Finalize method and the
managed ones in the Dispose method, when the Dispose/Finalize pattern
has been used in your code.
And later I found this nice article about finalize and dispose and got a clear idea about them. The article has the following code(Page 3), to explain the concepts:
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose(false);
}
protected void Dispose(bool disposing)
{
if (disposing)
{
// Code to dispose the managed resources of the class
}
// Code to dispose the un-managed resources of the class
isDisposed = true;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
But below that, the same note (which I included in the beginning of this question) appears.
The Dispose/Finalize Pattern
Microsoft recommends that you implement both Dispose and Finalize when working with unmanaged resources. The correct sequence then would
be for a developer to call Dispose. The Finalize implementation would
run and the resources would still be released when the object is
garbage collected even if a developer neglected to call the Dispose
method explicitly. Francesco Balena writes in his blog "the
Dispose/Finalize pattern should be used only when your type invokes
unmanaged code that allocates unmanaged resources (including unmanaged
memory) and returns a handle that you must use eventually to release
the resource. Both dispose and finalize must chain up to their parent
objects by calling their parent's respective methods after they have
disposed or finalized their own members".
Simply put, cleanup the unmanaged resources in the Finalize method and the managed ones in the Dispose method, when the
Dispose/Finalize pattern has been used in your code.
Now I am confused again. In the entire article and in the code sample, it is shown that unmanaged resources should be freed in Dispose(). But then what is the relevance of that comment?
Edit:
As it is confirmed that this line :
Simply put, cleanup the unmanaged resources in the Finalize method and
the managed ones in the Dispose method, when the Dispose/Finalize
pattern has been used in your code
is erroneous, I edited this answer.
See its very simple.
If you are dealing with unmanaged resources - Implement both Dispose and Finalize. Dispose is to be called by developers to free up the resources as soon as they see it that its no longer needed for them. If they forget to call Dispose then Framework calls the finalize in its own GC cycle (usually will take its own sweet time).
If your object uses disposable objects internally - You implement Dispose() if you created and retained a reference to any object of a type which implements Dispose() and which you haven't already disposed.
If neither of the above is the case (you are NOT dealing with unmanaged resources nor your object uses disposable objects internally) - Then don't do anything. Don't implement Finalize nor Dispose.
Some classic examples:
System.IO.FileStream object manages the lock/stream handles to files. So it implements both dispose and finalize. If the developer disposes it then the other program can access it right away. If he forgets to dispose it then Framework finalize it and close the handles later in its GC cycle.
System.Text.StringBuilder dose not have any unmanaged resource. So no dispose no finalize.
As far as the pattern is concerned what it means to
// Code to dispose the managed resources of the class
is that call the Dispose methods of any .NET objects that you have as components inside that class
And
// Code to dispose the un-managed resources of the class
Means to close the raw handles and pointers. Here is your updated code with examples
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose(false);
}
protected void Dispose(bool disposing)
{
if (!isDisposed)
{
if (disposing)
{
// Code to dispose the managed resources of the class
internalComponent1.Dispose();
internalComponent2.Dispose();
}
// Code to dispose the un-managed resources of the class
CloseHandle(handle);
handle = IntPtr.Zero;
isDisposed = true;
}
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
Here is an old question explaining it
If a Foo has resources which will benefit from deterministic cleanup, but none that can be usefully cleaned up in a finalizer, it should implement IDisposable but should not override Finalize or have a destructor. If a class holds multiple resources, and at least one can be cleaned up in a finalizer, then each discrete resource that could be cleaned up in a finalizer should be encapsulated into its own Finalizer/destructor-equipped object (which may be defined in a protected nested class), and the class that would contain those resources should hold references to the wrapper objects. Once that is done, the outer class will fit the pattern for classes with a Dispose method but no finalizer/destructor.
I have two code samples and I want to know what is the deifference between them and which is better to use as best practice and performance wise:
using (TestForm test = new TestForm())
{
test.ShowDialog();
}
and the other is:
TestForm test = null;
try
{
test = new TestForm();
test.ShowDialog();
}
catch(Exception ex)
{
}
finally
{
test = null;
}
The IDisposable interface defines the Dispose method, as well as the possibility to use the "using" syntax. The dispose method of a class can be implemented to release resources, close database connections and any sort of finalizing and cleanup. Just setting the class instance to null won't execute any of the code defined in the dispose method. As a generic rule, if a class implements IDisposable, dispose should be called when you're finished with the class instance.
Dispose() is for freeing unmanaged resources. This may be done in a finalizer as well (which might call Dispose()) but don't rely on it. If it isn't done, then you leak unmanaged resources.
Setting a reference to null only means that a particular reference no longer points to that object. It can live on quite a while after that (or even indefinitely if you have another reference – well, if you have multiple reference to an object you Dispose()d of, then it gets ugly, probably).
Generally, always call Dispose() on IDiposables when you're done with them. It's easier if you wrap them into a using statement:
using (var foo = new SomeDiposableObject()) {
// do something with foo
}
The IDisposable pattern is a mechanism to timely free unmanaged and managed resources that an object may be consuming.
The typical way the pattern is implemented is as follows:
public void Dispose() //Implementes the IDisposable interface
{
this.Dispose(true);
GC.SupressFinalize(this); //All resources have been released, no need to run the finalizer. We make the GC's life a little easier;
}
protected void Dispose(bool disposing)
{
if (disposing)
{
//Relesase managed resources.
}
//Release unmanaged resources.
}
~MyDisposableObject() //finalizer
{
this.Dispose(false)
}
The thing to note here is that the release of resources through the Dispose method is very similar to what you logically would expect to find in a finalizer. It is not done directly in the finalizer due to two main reasons:
the finalizer is not executed in a deterministic order. That is why we do not dispose managed resources from the finalizer (Dispose(false)), as some or all the managed resources held by the object might have been finalized before the object itself. This is not true with unmanaged resources as, by definition, they will never be finalized by the GC.
We do not know when the finalizer is run (it is up to the GC).
The basic idea is that an object implementing IDisposable is a sign for any consumer saying: "hey, I'm holding on to a certain amount of unmanaged and/or managed resources that will eventually be released when the GC decides I'm no longer useful, but if you need those resources back in a timely way, call Dispose() and I'll be happy to oblige.".
On the other hand, setting a reference variable to null is not freeing any resources at all. If the reference you have removed from the object was the only one to said object, then the object will eventually be collected by the GC and managed and unmanaged resources will be freed (when, is anyones guess).
If there were more live references still pointing to the object, then the object would live one and no resources whatsoever would be freed.
Assuming this as the traditional Dispose pattern (taken from devx but seen on many websites)
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose(false);
}
protected void Dispose(bool disposing)
{
if (disposing)
{
// Code to dispose the managed resources of the class
}
// Code to dispose the un-managed resources of the class
isDisposed = true;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
I don't understand why we call GC.SupressFinalize(this). This requires me to write my own managed resource disposal, including nulling my references? I'm a bit lost, I must admit. Could someone shed some light on this pattern?
Ideally, I would like to only dispose my unmanaged resources and let the GC do the managed collecting by itself.
Actually, I don't even know why we specify a finalizer. In any case, the coder should call dispose himself, now shouldn't he? If that's just a fallback mechanism, I'd remove it.
The IDisposable pattern is used so that the object can clean up its resources deterministically, at the point when the Dispose method is called by the client code.
The finaliser is only there as a fallback in case the client code fails to call Dispose for some reason.
If the client code calls Dispose then the clean-up of resources is performed there-and-then and doesn't need to be done again during finalisation. Calling SuppressFinalize in this situation means that the object no longer incurs the extra GC cost of finalisation.
And, if your own class only uses managed resources then a finaliser is completely unnecessary: The GC will take care of any managed resources, let those resources themselves worry about whether they need a fallback finaliser. You should only consider a finaliser in your own class if it directly handles unmanaged resources.
SuppressFinalize only suppresses any custom finalizer.
It does not alter any other GC behavior.
You never need to explicitly null out references. (Unless you want them to be collected early)
There is no difference between a class without any finalizer and an instance on which you've called SuppressFinalize.
Calling SuppressFinalize prevents an extra call to Dispose(false), and makes the GC somewhat faster. (finalizers are expensive)
Note that classes without unmanaged resources should not have a finalizer. (They should still call SuppressFinalize, unless they're sealed; this allows inherited classes to add unmanaged resources)
The SuppressFinalize call exists in case some derived class decides to add a finalizer. If a normal dispose completes successfully, finalization won't be necessary; even if a derived class decides to add one, the SuppressFinalize call will prevent it from executing and interfering with garbage collection.
To understand why this is important, you should think of finalization not as being part of garbage collection, but rather something that happens before it. When a class registers for finalization (automatic on creation, if it overrides Finalize) it is put into a special list called the Finalization Queue. No object in the Finalization Queue, nor any object referenced directly or indirectly by an object in the queue, can be garbage-collected, but if any object in the finalization queue is found to have no rooted references other than from the queue, the object will be pulled from the queue and the finalizer will run. While the finalizer is being dispatched, the object will not be collectable (since a reference will exist during the dispatch); once the finalizer is complete, there will usually not be any references to the object anymore, so it (and objects referenced thereby) will usually be collectable.
Personally, I think the SuppressFinalize is silly, since I can think of no good reason why a derived class should ever have a finalizer. If a derived class is going to add souse unmanaged resources(*) which the parent class will know nothing about, another class should be created for the purpose of holding those resources; the parent class should hold a reference to that. That way, the parent class itself will not need finalization, and objects which are referenced by the parent class won't be needlessly blocked from garbage collection.
From Msdn :
"
This method sets a bit in the object header, which the system checks when calling finalizers. The obj parameter is required to be the caller of this method.
Objects that implement the IDisposable interface can call this method from the IDisposable.Dispose method to prevent the garbage collector from calling Object.Finalize on an object that does not require it.
"
So it prevents an extra call from the GC. If it is called from within the the finalizer method, when object is being finalized, then it wont do anything, as it is already being finalised.
Otherwise, the GC is allowed to reclaim memory, without finalisation of the object, thus making things faster.
As noted on MSDN executing the Finalize method is costly. By calling dispose you've already self finalized your class so the finalizer doesn't need to be called. The finalizer is implemented in case the Dispose is never called directly by your code (or whoever 'owns' the instance).
// If the monitor.Dispose method is not called, the example displays the following output:
// ConsoleMonitor instance....
// The ConsoleMonitor class constructor.
// The Write method.
// The ConsoleMonitor finalizer.
// The Dispose(False) method.
// Disposing of unmanaged resources.
//
// If the monitor.Dispose method is called, the example displays the following output:
// ConsoleMonitor instance....
// The ConsoleMonitor class constructor.
// The Write method.
// The Dispose method.
// The Dispose(True) method.
// Disposing of managed resources.
// Disposing of unmanaged resources.
From https://msdn.microsoft.com/en-us/library/system.gc.suppressfinalize(v=vs.110).aspx
I have few Question for which I am not able to get a proper answer .
1) Why should we call SuppressFinalize in the Dispose function when we don't have a destructor .
2) Dispose and finalize are used for freeing resources before the object is garbage collected. Whether it is managed or unmanaged resource we need to free it , then why we need a condition inside the dispose function , saying pass 'true' when we call this overridden function from IDisposable:Dispose and pass false when called from a finalize.
See the below code I copied from net.
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose(false);
}
protected void Dispose(bool disposing)
{
if (disposing)
{
// Code to dispose the managed resources of the class
}
// Code to dispose the un-managed resources of the class
isDisposed = true;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
what if I remove the boolean protected Dispose function and implement the as below.
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose();
}
public void Dispose()
{
// Code to dispose the managed resources of the class
// Code to dispose the un-managed resources of the class
isDisposed = true;
// Call this since we have a destructor . what if , if we don't have one
GC.SuppressFinalize(this);
}
}
I'm going out on a limb here, but... most people don't need the full-blown dispose pattern. It's designed to be solid in the face of having direct access to unmanaged resources (usually via IntPtr) and in the face of inheritance. Most of the time, neither of these is actually required.
If you're just holding a reference to something else which implements IDisposable, you almost certainly don't need a finalizer - whatever holds the resource directly is responsible for dealing with that. You can make do with something like this:
public sealed class Foo : IDisposable
{
private bool disposed;
private FileStream stream;
// Other code
public void Dispose()
{
if (disposed)
{
return;
}
stream.Dispose();
disposed = true;
}
}
Note that this isn't thread-safe, but that probably won't be a problem.
By not having to worry about the possibility of subclasses holding resources directly, you don't need to suppress the finalizer (because there isn't one) - and you don't need to provide a way of subclasses customising the disposal either. Life is simpler without inheritance.
If you do need to allow uncontrolled inheritance (i.e. you're not willing to bet that subclasses will have very particular needs) then you need to go for the full pattern.
Note that with SafeHandle from .NET 2.0, it's even rarer that you need your own finalizer than it was in .NET 1.1.
To address your point about why there's a disposing flag in the first place: if you're running within a finalizer, other objects you refer to may already have been finalized. You should let them clean up themselves, and you should only clean up the resources you directly own.
Here are the main facts
1) Object.Finalize is what your class overrides when it has a Finalizer. the ~TypeName() destructor method is just shorthand for 'override Finalize()' etc
2) You call GC.SuppressFinalize if you are disposing of resources in your Dispose method before finalization (i.e. when coming out of a using block etc). If you do not have a Finalizer, then you do not need to do this. If you have a Finalizer, this ensures that the object is taken off of the Finalization queue (so we dont dispose of stuff twice as the Finalizer usually calls the Dispose method as well)
3) You implement a Finalizer as a 'fail safe' mechanism. Finalizers are guaranteed to run (as long as the CLR isnt aborted), so they allow you to make sure code gets cleaned up in the event that the Dispose method was not called (maybe the programmer forgot to create the instance within a 'using' block etc.
4) Finalizers are expensive as Types that have finalizers cant be garbage collected in a Generation-0 collection (the most efficient), and are promoted to Generation-1 with a reference to them on the F-Reachable queue, so that they represent a GC root. it's not until the GC performs a Generation-1 collection that the finalizer gets called, and the resources are released - so implement finalizers only when very important - and make sure that objects that require Finalization are as small as possible - because all objects that can be reached by your finalizable object will be promoted to Generation-1 also.
Keep the first version, it is safer and is the correct implementation of the dispose pattern.
Calling SuppressFinalize tells the GC that you have done all the destruction/disposing yourself (of resources held by your class) and that it does not need to call the destructor.
You need the test in case the code using your class has already called dispose and you shouldn't tell the GC to dispose again.
See this MSDN document (Dispose methods should call SuppressFinalize).
1. Answer for the first question
Basically, you don't have to call SuppressFinalize method if your class doesn't have a finalize method (Destructor). I believe people call SupressFinalize even when there is no finalize method because of lack of knowledge.
2. Answer for the second question
Purpose of the Finalize method is to free un-managed resources. The most important thing to understand is that, Finalize method is called when the object is in the finalization queue. Garbage collector collects all the objects that can be destroy. Garbage Collector adds objects those have got finalization to the finalization queue before destroy. There is another .net background process to call the finalize method for the objects those are in the finalization queue. By the time that background process execute the finalize method, that particular object's other managed reference may have been destroyed. Because there is no specific order when it comes to the finalization execution. So, the Dispose Pattern wants to make sure that finalize method do not try to access managed objects. That's why managed objects are going in side "if (disposing)" clause which is unreachable for the finalize method.
You should always call SuppressFinalize() because you might have (or have in the future) a derived class that implements a Finalizer - in which case you need it.
Let's say you have a base class that doesn't have a Finalizer - and you decided not to call SuppressFinalize(). Then 3 months later you add a derived class that adds a Finalizer. It is likely that you will forget to go up to the base class and add a call to SuppressFinalize(). There is no harm in calling it if there is no finalizer.
My suggested IDisposable pattern is posted here: How to properly implement the Dispose Pattern
Why do some people use the Finalize method over the Dispose method?
In what situations would you use the Finalize method over the Dispose method and vice versa?
The finalizer method is called when your object is garbage collected and you have no guarantee when this will happen (you can force it, but it will hurt performance).
The Dispose method on the other hand is meant to be called by the code that created your class so that you can clean up and release any resources you have acquired (unmanaged data, database connections, file handles, etc) the moment the code is done with your object.
The standard practice is to implement IDisposable and Dispose so that you can use your object in a using statment. Such as using(var foo = new MyObject()) { }. And in your finalizer, you call Dispose, just in case the calling code forgot to dispose of you.
Others have already covered the difference between Dispose and Finalize (btw the Finalize method is still called a destructor in the language specification), so I'll just add a little about the scenarios where the Finalize method comes in handy.
Some types encapsulate disposable resources in a manner where it is easy to use and dispose of them in a single action. The general usage is often like this: open, read or write, close (Dispose). It fits very well with the using construct.
Others are a bit more difficult. WaitEventHandles for instances are not used like this as they are used to signal from one thread to another. The question then becomes who should call Dispose on these? As a safeguard types like these implement a Finalize method, which makes sure resources are disposed when the instance is no longer referenced by the application.
Finalize is the backstop method, called by the garbage collector when it reclaims an object. Dispose is the "deterministic cleanup" method, called by applications to release valuable native resources (window handles, database connections, etc.) when they are no longer needed, rather than leaving them held indefinitely until the GC gets round to the object.
As the user of an object, you always use Dispose. Finalize is for the GC.
As the implementer of a class, if you hold managed resources that ought to be disposed, you implement Dispose. If you hold native resources, you implement both Dispose and Finalize, and both call a common method that releases the native resources. These idioms are typically combined through a private Dispose(bool disposing) method, which Dispose calls with true, and Finalize calls with false. This method always frees native resources, then checks the disposing parameter, and if it is true it disposes managed resources and calls GC.SuppressFinalize.
Finalize gets called by the GC when this object is no longer in use.
Dispose is just a normal method which the user of this class can call to release any resources.
If user forgot to call Dispose and if the class have Finalize implemented then GC will make sure it gets called.
Finalize
Finalizers should always be protected, not public or private so that the method cannot be called from the application's code directly and at the same time, it can make a call to the base.Finalize method
Finalizers should release unmanaged resources only.
The framework does not guarantee that a finalizer will execute at all on any given instance.
Never allocate memory in finalizers or call virtual methods from finalizers.
Avoid synchronization and raising unhandled exceptions in the finalizers.
The execution order of finalizers is non-deterministic—in other words, you can't rely on another object still being available within your finalizer.
Do not define finalizers on value types.
Don't create empty destructors. In other words, you should never explicitly define a destructor unless your class needs to clean up unmanaged resources and if you do define one, it should do some work. If, later, you no longer need to clean up unmanaged resources in the destructor, remove it altogether.
Dispose
Implement IDisposable on every type that has a finalizer
Ensure that an object is made unusable after making a call to the Dispose method. In other words, avoid using an object after the Dispose method has been called on it.
Call Dispose on all IDisposable types once you are done with them
Allow Dispose to be called multiple times without raising errors.
Suppress later calls to the finalizer from within the Dispose method using the GC.SuppressFinalize method
Avoid creating disposable value types
Avoid throwing exceptions from within Dispose methods
Dispose/Finalized Pattern
Microsoft recommends that you implement both Dispose and Finalize when working with unmanaged resources. The Finalize implementation would run and the resources would still be released when the object is garbage collected even if a developer neglected to call the Dispose method explicitly.
Cleanup the unmanaged resources in the Finalize method as well as Dispose method. Additionally call the Dispose method for any .NET objects that you have as components inside that class(having unmanaged resources as their member) from the Dispose method.
There're some keys about from the book MCSD Certification Toolkit (exam 70-483) pag 193:
destructor ≈(it's almost equal to) base.Finalize(), The destructor is converted into an override version of the Finalize method that executes the destructor’s code and then calls the base class’s Finalize method. Then its totally non deterministic you can't able to know when will be called because depends on GC.
If a class contains no managed resources and no unmanaged resources, it shouldn't implement IDisposable or have a destructor.
If the class has only managed resources, it should implement IDisposable but it shouldn't have a destructor. (When the destructor executes, you can’t be sure managed objects still
exist, so you can’t call their Dispose() methods anyway.)
If the class has only unmanaged resources, it needs to implement IDisposable and needs a destructor in case the program doesn’t call Dispose().
Dispose() method must be safe to run more than once. You can achieve that by using a variable to keep track of whether it has been run before.
Dispose() should free both managed and unmanaged resources.
The destructor should free only unmanaged resources. When the destructor executes, you
can’t be sure managed objects still exist, so you can’t call their Dispose methods anyway. This is obtained by using the canonical protected void Dispose(bool disposing) pattern, where only managed resources are freed (disposed) when disposing == true.
After freeing resources, Dispose() should call GC.SuppressFinalize, so the object can
skip the finalization queue.
An Example of a an implementation for a class with unmanaged and managed resources:
using System;
class DisposableClass : IDisposable
{
// A name to keep track of the object.
public string Name = "";
// Free managed and unmanaged resources.
public void Dispose()
{
FreeResources(true);
// We don't need the destructor because
// our resources are already freed.
GC.SuppressFinalize(this);
}
// Destructor to clean up unmanaged resources
// but not managed resources.
~DisposableClass()
{
FreeResources(false);
}
// Keep track if whether resources are already freed.
private bool ResourcesAreFreed = false;
// Free resources.
private void FreeResources(bool freeManagedResources)
{
Console.WriteLine(Name + ": FreeResources");
if (!ResourcesAreFreed)
{
// Dispose of managed resources if appropriate.
if (freeManagedResources)
{
// Dispose of managed resources here.
Console.WriteLine(Name + ": Dispose of managed resources");
}
// Dispose of unmanaged resources here.
Console.WriteLine(Name + ": Dispose of unmanaged resources");
// Remember that we have disposed of resources.
ResourcesAreFreed = true;
}
}
}
99% of the time, you should not have to worry about either. :) But, if your objects hold references to non-managed resources (window handles, file handles, for example), you need to provide a way for your managed object to release those resources. Finalize gives implicit control over releasing resources. It is called by the garbage collector. Dispose is a way to give explicit control over a release of resources and can be called directly.
There is much much more to learn about the subject of Garbage Collection, but that's a start.
The finalizer is for implicit cleanup - you should use this whenever a class manages resources that absolutely must be cleaned up as otherwise you would leak handles / memory etc...
Correctly implementing a finalizer is notoriously difficult and should be avoided wherever possible - the SafeHandle class (avaialble in .Net v2.0 and above) now means that you very rarely (if ever) need to implement a finalizer any more.
The IDisposable interface is for explicit cleanup and is much more commonly used - you should use this to allow users to explicitly release or cleanup resources whenever they have finished using an object.
Note that if you have a finalizer then you should also implement the IDisposable interface to allow users to explicitly release those resources sooner than they would be if the object was garbage collected.
See DG Update: Dispose, Finalization, and Resource Management for what I consider to be the best and most complete set of recommendations on finalizers and IDisposable.
Diff between Finalize and Dispose methods in C#.
GC calls the finalize method to reclaim the unmanaged resources(such as file operarion, windows api, network connection, database connection) but time is not fixed when GC would call it. It is called implicitly by GC it means we do not have low level control on it.
Dispose Method: We have low level control on it as we call it from the code. we can reclaim the unmanaged resources whenever we feel it is not usable.We can achieve this by implementing IDisposal pattern.
The summary is -
You write a finalizer for your class if it has reference to unmanaged
resources and you want to make sure that those unmanaged resources
are released when an instance of that class is garbage collected
automatically. Note that you can't call the Finalizer of an object explicitly - it's called automatically by the garbage collector as and when it deems necessary.
On the other hand, you implement the IDisposable interface(and
consequently define the Dispose() method as a result for your class) when your class
has reference to unmanaged resources, but you don't want to wait for
the garbage collector to kick in (which can be anytime - not in
control of the programmer) and want to release those resources as
soon as you are done. Thus, you can explicitly release unmanaged resources by calling an object's Dispose() method.
Also, another difference is - in the Dispose() implementation, you should release managed resources as well, whereas that should not be done in the Finalizer. This is because it's very likely that the managed resources referenced by the object have already been cleaned up before it's ready to be finalized.
For a class that uses unmanaged resources, the best practice is to define both - the Dispose() method and the Finalizer - to be used as a fallback in case a developer forgets to explicitly dispose off the object. Both can use a shared method to clean up managed and unmanaged resources :-
class ClassWithDisposeAndFinalize : IDisposable
{
// Used to determine if Dispose() has already been called, so that the finalizer
// knows if it needs to clean up unmanaged resources.
private bool disposed = false;
public void Dispose()
{
// Call our shared helper method.
// Specifying "true" signifies that the object user triggered the cleanup.
CleanUp(true);
// Now suppress finalization to make sure that the Finalize method
// doesn't attempt to clean up unmanaged resources.
GC.SuppressFinalize(this);
}
private void CleanUp(bool disposing)
{
// Be sure we have not already been disposed!
if (!this.disposed)
{
// If disposing equals true i.e. if disposed explicitly, dispose all
// managed resources.
if (disposing)
{
// Dispose managed resources.
}
// Clean up unmanaged resources here.
}
disposed = true;
}
// the below is called the destructor or Finalizer
~ClassWithDisposeAndFinalize()
{
// Call our shared helper method.
// Specifying "false" signifies that the GC triggered the cleanup.
CleanUp(false);
}
The best example which i know.
public abstract class DisposableType: IDisposable
{
bool disposed = false;
~DisposableType()
{
if (!disposed)
{
disposed = true;
Dispose(false);
}
}
public void Dispose()
{
if (!disposed)
{
disposed = true;
Dispose(true);
GC.SuppressFinalize(this);
}
}
public void Close()
{
Dispose();
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
// managed objects
}
// unmanaged objects and resources
}
}
The main difference between Dispose and Finalize is that:
Dispose is usually called by your code. The resources are freed instantly when you call it. People forget to call the method, so using() {} statement is invented. When your program finishes the execution of the code inside the {}, it will call Dispose method automatically.
Finalize is not called by your code. It is mean to be called by the Garbage Collector (GC). That means the resource might be freed anytime in future whenever GC decides to do so. When GC does its work, it will go through many Finalize methods. If you have heavy logic in this, it will make the process slow. It may cause performance issues for your program. So be careful about what you put in there.
I personally would write most of the destruction logic in Dispose. Hopefully, this clears up the confusion.
Class instances often encapsulate control over resources that are not managed by the runtime, such as window handles (HWND), database connections, and so on. Therefore, you should provide both an explicit and an implicit way to free those resources. Provide implicit control by implementing the protected Finalize Method on an object (destructor syntax in C# and the Managed Extensions for C++). The garbage collector calls this method at some point after there are no longer any valid references to the object.
In some cases, you might want to provide programmers using an object with the ability to explicitly release these external resources before the garbage collector frees the object. If an external resource is scarce or expensive, better performance can be achieved if the programmer explicitly releases resources when they are no longer being used. To provide explicit control, implement the Dispose method provided by the IDisposable Interface. The consumer of the object should call this method when it is done using the object. Dispose can be called even if other references to the object are alive.
Note that even when you provide explicit control by way of Dispose, you should provide implicit cleanup using the Finalize method. Finalize provides a backup to prevent resources from permanently leaking if the programmer fails to call Dispose.
I searched the answer to this question a lot today. I will share my learnings here. My answer is based on this link, because it has the clearest explanation I have seen.
When your objects has access to unmanaged resources, you have to manually release those resources. This can be done via IDisposable or finalizer meaning they both release unmanaged resources.
Rule of thumb:
Implement IDisposable to release unmanaged resources and caller code must call Dispose method. If caller forgets to call Dispose() method, you still can provide a method to release those unmanaged resources. First option is using safe handle to wrap unmanaged resource. Second option is defining a finalizer. Using safe handle is recommended way in this case.
I think this link is the clearest answer to this question. I do not know why people provide complex explanations to this question on the internet. It made me feel confused until I find that link.
As we know dispose and finalize both are used to free unmanaged resources..
but the difference is finalize uses two cycle to free the resources , where as dispose uses one cycle..
To answer on the first part you should provide examples where people use
different approach for the exact same class-object.
Otherwise it is difficult (or even strange) to answer.
As for the second question better read first this
Proper use of the IDisposable interface
which claims that
It's your choice! But choose Dispose.
In other words: The GC only knows about finalizer (if any. Also known as destructor to Microsoft).
A good code will attempt to cleanup from both (finalizer and Dispose).