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IDisposable Question
I have written a class and implemented the IDisposable interface.
I implemetned the Dispose method and put a Code Break in the method.
My assumption is it would have been called when the Class went out of scope due to C# Garabage collection.
I want the dispose method to close an unmanaged resource. I thought it would be more elegant than just calling the method LogOff() instead getting it called whenever the method went out of scope?
But it doesn't seem to get called or stop at the code break.
You need to explicitly call Dispose on any objects that implement IDisposable. If you use the using() {} code construct the compiler will automatically call Dispose at the end of the using block.
A good pattern is to also track via a private boolean field whether dispose has been called or not, and if not call it from the objects finalizer (and also call GC.SuppressFinalize() from your Dispose method assuming that you handle all finalization tasks from there also).
You should consider wrapping your interaction with your IDisposable class in a using statement. Doing so will allow you to specify when your object goes out of scope, and ensures the Dispose() method gets called.
For the correct syntax, see the example from the referenced MSDN article:
using System;
class C : IDisposable
{
public void UseLimitedResource()
{
Console.WriteLine("Using limited resource...");
}
void IDisposable.Dispose()
{
Console.WriteLine("Disposing limited resource.");
}
}
class Program
{
static void Main()
{
using (C c = new C())
{
c.UseLimitedResource();
}
Console.WriteLine("Now outside using statement.");
Console.ReadLine();
}
}
Short answer: Dispose() is called when you call it.
Long answer: take a look at using block. This is a syntax sugar that meant to be used together with IDisposable interface for pretty and safely disposing code, and is roughly equivalent to
Foo foo = new Foo();
try
{
// your code that uses foo
}
finally
{
foo.Dispose();
}
In other words foo is guaranteed to be disposed upon leaving using() scope.
Garbage collection does not happen immediately after a variable goes out of scope. The GC runs periodically and .NET has different "levels" of Garbage collection. Different levels get collected more frequently. If you want your object's dispose method to be called immediately, you should use a using statement
using (MyClass object = new MyClass())
{
//ensures Dipose is called, even if exceptions are thrown
}
My assumption is it would have been called when the Class went out of scope due to C# Garabage collection.
That's now how garbage collection works. There's two things going on here:
Garbage collection - this cleans up objects at some time after there's no longer any references left to them. This implies that the objects have gone out of scope (if they are locals), but notably GC doesn't say when this cleanup happens - it usually happens lazily when the system decides it needs to run a collection to free up more memory. The method that is called to clean up resources in this case is the finalizer, which in C# has the form ~Classname().
IDisposable: the problem with GC is that you have no control over when the finalizer is called, so IDisposable was introduced as a pattern to be used when you need resources to be cleaned up at a specific time and don't want to wait for a collect to happen. It's up to the caller code to call Dispose() as appropriate, there's no GC support. C# does have the using(){} syntax, which simplifies this, and calls Dispose() automatically at the end of the using block.
Related
Is it legal to call a method on disposed object? If yes, why?
In the following demo program, I've a disposable class A (which implements IDisposable interface).As far as I know, if I pass disposable object to using() construct, then Dispose() method gets called automatically at the closing bracket:
A a = new A();
using (a)
{
//...
}//<--------- a.Dispose() gets called here!
//here the object is supposed to be disposed,
//and shouldn't be used, as far as I understand.
If that is correct, then please explain the output of this program:
public class A : IDisposable
{
int i = 100;
public void Dispose()
{
Console.WriteLine("Dispose() called");
}
public void f()
{
Console.WriteLine("{0}", i); i *= 2;
}
}
public class Test
{
public static void Main()
{
A a = new A();
Console.WriteLine("Before using()");
a.f();
using ( a)
{
Console.WriteLine("Inside using()");
a.f();
}
Console.WriteLine("After using()");
a.f();
}
}
Output (ideone):
Before using()
100
Inside using()
200
Dispose() called
After using()
400
How can I call f() on the disposed object a? Is this allowed? If yes, then why? If no, then why the above program doesn't give exception at runtime?
I know that the popular construct of using using is this:
using (A a = new A())
{
//working with a
}
But I'm just experimenting, that is why I wrote it differently.
Disposed doesn't mean gone. Disposed only means that any unmanaged resource (like a file, connection of any kind, ...) has been released. While this usually means that the object doesn't provide any useful functionality, there might still be methods that don't depend on that unmanaged resource and still work as usual.
The Disposing mechanism exist as .net (and inheritly, C#.net) is a garbage-collected environment, meaning you aren't responsable for memory management. However, the garbage collector can't decide if an unmanaged resource has been finished using, thus you need to do this yourself.
If you want methods to throw an exception after the object has been diposed, you'll need a boolean to capture the dispose status, and once the object is disposed, you throw the exception:
public class A : IDisposable
{
int i = 100;
bool disposed = false;
public void Dispose()
{
disposed = true;
Console.WriteLine("Dispose() called");
}
public void f()
{
if(disposed)
throw new ObjectDisposedException();
Console.WriteLine("{0}", i); i *= 2;
}
}
The exception is not thrown because you have not designed the methods to throw ObjectDisposedException after Dispose has been called.
The clr does not automagically know that it should throw ObjectDisposedException once Dispose is called. It's your responsibility to throw an exception if Dispose has released any resources needed for successful execution of your methods.
A typical Dispose() implementation only calls Dispose() on any objects that it stores in its fields that are disposable. Which in turn release unmanaged resources. If you implement IDisposable and not actually do anything, like you did in your snippet, then the object state doesn't change at all. Nothing can go wrong. Don't mix up disposal with finalization.
The purpose of IDisposable is to allow an object to fix the state of any outside entities which have, for its benefit, been put into a state that is less than ideal for other purposes. For example, an Io.Ports.SerialPort object might have changed the state of a serial port from "available for any application that wants it" to "only usable by one particular Io.Ports.SerialPort object"; the primary purpose of SerialPort.Dispose is to restore the state of the serial port to "available for any application".
Of course, once an object that implements IDisposable has reset entities that had been maintaining a certain state for its benefit, it will no longer have the benefit of those entities' maintained state. For example, once the state of the serial port has been set to "available for any application", the data streams with which it had been associated can no longer be used to send and receive data. If an object could function normally without outside entities being put into a special state for its benefit, there would be no reason to leave outside entities in a special state in the first place.
Generally, after IDisposable.Dispose has been called on an object, the object should not be expected to be capable of doing much. Attempting to use most methods on such an object would indicate a bug; if a method can't reasonably be expected to work, the proper way to indicate that is via ObjectDisposedException.
Microsoft suggests that nearly all methods on an object which implements IDisposable should throw ObjectDisposedException if they are used on an object which has been disposed. I would suggest that such advice is overbroad. It is often very useful for devices to expose methods or properties to find out what happened while the object was alive. Although one could give a communications class a Close method as well as a Dispose method, and only allow one to query things like NumberOfPacketsExchanged after a close but not after a Dispose, but that seems excessively complicated. Reading properties related to things that happened before an object was Disposed seems a perfectly reasonable pattern.
Calling Dispose() doesn't set the object reference to null, and your custom disposable class doesn't contain any logic to throw an exception if its functions are accessed after Dispose() has been called so it is of course legal.
In the real world, Dispose() releases unmanaged resources and those resources will be unavailable thereafter, and/or the class author has it throw ObjectDisposedException if you try to use the object after calling Dispose(). Typically a class-level boolean would be set to true within the body of Dispose() and that value checked in the other members of the class before they do any work, with the exception being thrown if the bool is true.
A disposer in C# is not the same as a destructor in C++. A disposer is used to release managed (or unmanaged) resources while the object remains valid.
Exceptions are thrown depending on the implementation of the class. If f() does not require the use of your already disposed objects, then it doesn't necessarily need to throw an exception.
In C++, I'm used to using the sentry pattern to ensure that resources acquired are properly released when the block or function exits (see here for example). I used this, for example, for grabbing the graphics state, and then I can do whatever I want to that state, and it gets put back when the sentry object (reference) goes out of scope.
Now I'm using C#, and the same trick doesn't work, because the destructor doesn't get called until who-knows-when later.
Is there some OTHER method that is guaranteed to fire when an object's last reference is released? Or do I just have to remember to call some Restore() or Apply() or DoYourThing() method before returning from any method where I would otherwise use a sentry?
C# has finalizers, like C++, that are called when an object is destroyed. They are in the same form as C++, that is:
~ClassName()
{
}
As you know, though, C# does not have deterministic memory management, and so this isn't really a great guarantee. AS a direct result, you should not use finalizers for releasing unmanaged resources in C#. Instead, we use the IDisposable interface. This exposes a single method on the implementor, Dispose, which is intended to be called when you want to release unmanaged resources.
public class MyDisposable : IDisposable
{
public void Dispose()
{
// get rid of some expensive unmanaged resource like a connection
}
}
We can also use the using sugar to allow semantic invocation of Dispose() when the using block terminates (gracefully or not).
using(var disposable = new MyDisposable)
{
// Do something with the disposable
} // Dispose is invoked here
Should you find yourself using finalizers and Dispose, you can consider using the GC.SuppressFinalize method, although that's a bit out of my scope. There was a really good discussion elsewhere on StackOverflow about this here
This can be used to perform RAII-esque trickery, of course, such as
using(var guard = new Guard(resource))
{
}
C# provides the using block for this case, which works on any object that implements IDisposable.
For example, if you have a type class Foo : IDisposable then you can do this:
using (new Foo(/* ... */)) {
// Your code that depends on this resource.
}
This is equivalent to:
Foo x = new Foo(/* ... */);
try {
// Your code
} finally {
if (x != null) {
((IDisposable)x).Dispose();
}
}
Except, of course, that you don't have access to x. You can, however, gain such access if you need it by creating a variable in the using block:
using (Foo foo = new Foo(/* ... */)) {
// Your code that uses "foo"
}
The standard way of creating a class that can be disposed of this way or by the garbage collector is:
class Foo : IDisposable {
protected virtual void Dispose(bool disposing) {
// Your cleanup code goes here.
}
public void Dispose() {
GC.SuppressFinalize(this);
Dispose(true);
}
~Foo() {
Dispose(false);
}
}
To implement deterministic deallocation of unmanaged resources, you would use a Dispose pattern. For many cases you can combine this with a using block if you want this to occur within a specific scope.
File handles or handles to a graphics device would usually be considered a unmanaged resource. These are resources that the .NET framework does not track references for and does not automatically deallocate.
If it is managed resource, just references to C# objects/lists, then usually deterministic deallocation is not necessary, and the garbage collector can usually handle these more efficiently than you can. Don't worry so much about when managed resources are deallocated. If you no longer have a reference to them, then they should be of no concern. The concern comes when you are done with something but somehow have a lingering reference to it that is preventing the GC from deallocating it.
It sounds like what you're describing is objects that implement the IDisposable interface for C#. the IDisposable has a Dispose() method, that when defined properly on your own objects, is used to release resources.
Additionally C# does have destructors defined as such:
public class Foo
{
public Foo()
{
// constructor
}
~Foo() // destructor
}
when wrapping IDisposable objects in a using block, the Dispose() method is automatically called, the other option is to throw it in a finally block (when not using... using)
The easiest way to do this is enclosing the sentry object in a using statement. So something like this.
using (sentry = new Sentry())
{
//do your stuff here
}
After this block, sentry will have been released and out of scope.
I have a class that spawns another UI thread and does it's thing. I need to abort that thread and clean up whenever my parent class is destroyed. So how do I know when my parent class is destoryed?
Coming from C++ my first thought was to put this in the destructor. But C# doesn't really have any destructors - only finalizers and dispose - which from what I understand - may or may not be called (I guess it's a mood thing for the GC??).
That's great and simple - if you may or may not want to release your resources.
But where do you put code that ABSOLUTELY POSITIVELY MUST BE EXECUTED whenever an object is destroyed?
You put it in Dispose (implementing the IDisposable interface) and then ensure that Dispose is called when the object is no longer required. There's a language construct that does just this:
using (var foo = new Foo())
{
// Do something with foo.
}
foo.Dispose will be called at the end of the using block. This is equivalent to:
{
var foo = new Foo();
try
{
// Do something with foo.
}
finally
{
foo.Dispose();
}
}
Note that Dispose is not called automatically when the object leaves scope; you need to do it yourself, using either a using block or by calling it explicitly.
You should, however, provide a finalizer in Foo that calls Dispose, so that if the object isn't disposed before the GC gets to it, you aren't left with unreleased resources:
~Foo()
{
Dispose();
}
The idea behind the IDisposable pattern is that it tells you unambiguously when a class needs disposing. Here's an article that describes how to implement it properly (accounting for possible descendant classes).
There is no way to guarentee that something "ABSOLUTELY POSITIVELY [WILL] BE EXECUTED" when an object is destroy (try pulling the plug on your PC --- finalizers ain't gonna be called)
The best you can hope for is the finalizer -- defined in C# using C++ destructor syntax. Although, you be better off implementing IDisposable, and using a using{} block.
Although it is not common to use this method, you can define a "destructor" in C# by using the ~ character, like so:
class Parent
{
~Parent() // destructor
{
// cleanup statements...
}
}
Note:
The destructor implicitly calls Finalize on the base class of the
object.
(source)
You make your class a Component and the containing class a Container.
See: C# - What is a component and how is it typically used?
Other answers seem to be busy with "going out of scope" use case. But "whenever my parent class is destroyed"? If "parent class" is a "containing object", components are the right tool.
May I know if the "objectToBeDisposed" will be disposed if its reference are kept by the "useDisposableObject"?
using(IDisposable objectToBeDisposed = new ObjectImplementedIDisposable ()){
ChildObject useDisposableObject = new ChildObject(objectToBeDisposed);
.... doing stuff with useDisposableObject;
}
Yes, in your example it will be disposed, because the using block explicitly calls the Dispose() method when leaving that block.
As an aside the referencing object is within the using block scope and will go out of scope at the end of the using block, which will avoid the potential issue of accessing a disposed object if the referencing object tried to use it once it was disposed.
Update thanks to #dlev
Disposal, finalization and memory deallocation are 3 totally different things in the .NET world.
Disposal is only an "application logic" thing, it is implemented through the disposable pattern.
If you implement the interface IDisposable, your object can be used with the using keyword.
The using keyword will translate in a simple way:
using (IDisposable xxx = new MyObject())
{
....
}
means
{
IDisposable xxx = new MyObject();
try
{
....
}
finally
{
if (xxx != null)
xxx.Dispose();
}
}
There is no other magic in this. That's all.
Object finalization is instead called by garbage collector when the objects should be freed.
The time when it happens and the order between destructors is unpredictable.
class myclass { ~myclass() { this code will be called by garbage collector, i cannot call it } }
The syntax is very similar to C++ destructors but the meaning is a lot different.
And then memory deallocation.
Memory deallocation is not connected to object disposal!
You can keep forever a reference to a disposed object. Disposal is just something related to application logic.
Destructors can also be cancelled or called more than once by garbage collector (see GC.ReRegisterForFinalize and GC.SuppressFinalize).
The presence of a destructor in a class slow down a lot the garbage collector, and inside destructor, you can do almost nothing except releasing unmanaged resources.
Disposal instead as I said is an application thing, is just a method, and is not related to garbage collector.
"Disposing" an object has nothing to do with which references are kept around. IDisposable itself is just an interface, nothing more.
When a class implements that interface, a user of the class can call a public void Dispose() method on the object. When the user of the class places an instance of the class in a using block, the Dispose() method will automatically get called. When an Enumerator class implements IDisposable and is used in a foreach loop, it will also have Dispose() called on it.
This has absolutely nothing to do with which references are kept around, or the automatic garbage collection system.
However, IDisposable also has a design "contract" that goes with it. This contract is a pattern that classes that implement IDisposable must follow to have correct program behavior.
The contract is that once you've called Dispose(), you should never call any property or method on the object again, otherwise you should expect an ObjectDisposedException to be thrown. The only thing that is allowed to be called on that object after that point is Dispose() again, or the object finalizer (via garbage collection).
Because of this, people will sometimes set an reference to an IDisposable class to null after they have disposed it, so they don't accidentally call methods on the already-disposed object. If you placed the reference in a using block, you don't have to worry about this; The compiler won't even let you touch the reference again. If you try, you will get an error and your program will fail to compile.
This contract is not built into the language or CLR itself - it must manually be adhered to by the implementer of a class.
Yes, it will be. I might be wrong, but I don't think #John Weldon's explanation is not accurate though. You can only call using on an IDisposable implementing class, and when the using exits, it will call Dispose() on the object.
The part where I don't agree is the 'because the referencing object is within the using block scope and will go out of scope at the end of the using block' part. The objects declared in scope will not always be disposed of when the using exits. If you take this example
JustAClass myInstance = new JustAClass();
using ( MyDisposableClass theUsingInstance = new MyDisposableClass() )
{
AnotherClass innerInstance = new AnotherClass();
myInstance.ItsOtherClass = innerInstance;
myInstance.ItsWhatever = theUsingInstance;
}
At the end of this code, myInstance.ItsOtherClass is not null. innerInstance has been created in the using section and won't be garbage collected since there is still a referenced. However, the object refered by myInstance.ItsWhatever has been disposed of, since it's the using target.
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