I have a C# object with a critical resource that needs to be flushed very specific points in time making it a bad candidate to leave around for the garbage collector to take care of whenever it gets around to it, how should I handle this? Is there something like C++'s delete operator that will let me manually kill the instance of this object when needed?
You are looking for IDisposable. Here is an example class that implements this.
class MyDisposableObject : IDisposable
{
public MyDisposableObject()
{
}
~MyDisposableObject()
{
Dispose(false);
}
private bool disposed;
private void Dispose(bool disposing)
{
if (!this.disposed)
{
if (disposing)
{
// Dispose of your managed resources here.
}
// Dispose of your unmanaged resources here.
this.disposed = true;
}
}
void IDisposable.Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
To use it, you can do something like this:
public void DoingMyThing()
{
using (MyDisposableObject obj = new MyDisposableObject())
{
// Use obj here.
}
}
The using keyword makes sure that the Dispose() method on IDisposable gets called at the end of its scope.
The IDisposable interface exists for deterministic destruction. There's a pattern for implementing it correctly on MSDN.
In tandem, you should also consider using the using statement when your object's lifetime does not span multiple scopes.
This is precisely what the IDiposable interface is for. You release the critical resources in the Dispose() method, and then leave the object around for the garbage disposer to deal with deallocating the memory.
Google for the IDisposable interface. This is the only mechanism available to you. It's tailor made if your critical resource is unmanaged. If it's a managed resource, could you be more specific about what needs to be "flushed".
The IDisposable interface was added to support deterministic destruction in C++/CLI, and you can use it from any .NET language. It's what you want.
If you are talking about a specific managed resource that you feel "MUST" be released at a specific time, you could specifcally call the Garbage Collectors' Collect method, after de-referencing the object, but there are performance considerations to be thought of, as normally the garbage collector knows when to collect items. And in general it is a bad idea.
As others are mentioning above, the IDisposable pattern is helpful for releasing unmanaged resources when needed.
NOTE: I'm going to repeat, you COULD call GC.Collect() but it is NOT a good thing, but a valid answer for the question!
Related
Can we use Dispose method without Implementing IDisposable Interface ?
You can name a method as Dispose and use it as an ordinary method without any restrictions:
public class MyClass {
public void Dispose() {
...
}
}
...
// using() emulation
MyClass m = null;
try {
m = new MyClass();
...
}
finally {
if (m != null)
m.Dispose();
}
but if you want using() syntax you have to implement IDisposable:
public class MyNextClass: IDisposable {
protected virtual void Dispose(Boolean disposing) {
...
}
public void Dispose() {
Dispose(true);
GC.SuppressFinalize(this);
}
}
...
// compare this with the using() emulation in the code above
using (MyNextClass n = new MyNextClass()) {
...
}
Anyway, even it's possible to name a method Dispose it's not a good practice to surprize (and finally irritate) developers (including you) who read your texts; so either implement IDisposable or find some other name for the method.
Nobody will stop you from doing that but it is very poor design because everybody knows about the idisposable interface. If you give a different meaning to this method, you are obfuscating your design to whoever takes a look at your code later. Maybe even yourself in a few years where you don't remember the reason why you did this .
Managed Objects are disposed automatically even if you do not implement IDisposable, IDisposable let you dispose which runtime won't dispose like files, open handles unmanaged code components.
Implementing a Dispose Method
The pattern for disposing an object, referred to as a dispose pattern,
imposes order on the lifetime of an object. The dispose pattern is
used only for objects that access unmanaged resources, such as file
and pipe handles, registry handles, wait handles, or pointers to
blocks of unmanaged memory. This is because the garbage collector is
very efficient at reclaiming unused managed objects, but it is unable
to reclaim unmanaged objects.
If you do not want to use the IDisposable pattern and want to have your own then I believe that is not recommended way as it one would have to discover that could be obvious using IDisposable.
i've read msdn and various posts about dispose pattern, and there are still a couple of things i don't understand. I've written the following code to test the dispose pattern. Please note that there aren't unmanged resources, i'm using vs2008 and .net 3.5 :
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void tryDispose()
{
//test 1 : allocate resource and leave manage it to gc
BL.myclass df = new BL.myclass();
//test 2 : try to force garbage collecting
//GC.Collect();
//test 3 : call dispose myself
//using (BL.myclass df = new BL.myclass())
//{
//}
}
private void button1_Click(object sender, EventArgs e)
{
tryDispose();
}
this is my disposable class:
class myclass: IDisposable
{
private StronglyTypedDs myDS;
private bool _disposed;
public myclass()
{
using (myDSTableAdapter docDocadpt = new myDSTableAdapter())
{
myDS = new StronglyTypedDs();
docDocadpt.Fill(myDS.TheTable);
}
}
#region IDisposable Members
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
~myclass()
{
Dispose(false);
}
protected virtual void Dispose(bool disposing)
{
if (!_disposed)
{
if (disposing)
{
if (myDS != null)
myDS .Dispose();
myDS = null;
}
}
_disposed = true;
}
#endregion
}
The results are :
test 1a - just instantiating myclass, the destructor is commentend since myclass doesn't contains unmanaged resources : myclass.dispose is not called, even if i close the application (whose dispose is executed instead) . So what's the state of the dataset once i close the application ?
test 1b - uncommenting destructor, it's ok all disposes are called when i close the application.
test 2a and 2b - i do the above test just calling gc.collect : the behaviour is identical to test 1a and 1b respectively
test 3 - everything works fine (of course)
many posts says that, if myclass doesn't contains unmanaged resources, i don't need to add the destructor; yet in my tests , if i don't add the destructor , myclass.dispose doesn't get called when i close the application. I haven't waited for the gc to run by itself (if i remember correctly gc.collect doesn't guarantes the class instance is deallocated) to check if it will call myclass.dispose .
So what's the correct implemention : always define e destructor or avoid it if myclass contains only managed resources ?
If i had filled all generations levels, would had the gc called myclass dispose or not without having implemented a destructor?
Finally i've noticed that if i define a destructor , but don't declare the class as implementing IDisposable, the disposing chain works anyway. This could make sense since the destructor might be translated to finalize in IL. But i find it really confusing : is it some kind of "implicit" interface implementation i don't know? gc can dispose the item but users can't
thank you in advance
Stefano
Trust your Garbage Collector. Managed resources will get cleaned up, eventually. There is no need for a finalizer or implementing IDisposable unless you have some external resource that needs to be released.
This typically means that you only should implement IDisposable when you either:
Are wrapping a native resource. (In this case, you probably also want a finalizer.)
Are encapsulating a class which implements IDisposable. (In this case, you want IDisposable, but don't need to implement a finalizer/destructor.)
Part of the confusion with IDisposable, in my opinion, is that it covers quite a few use cases, and the proper implementation changes depending on what you're doing (ie: wrapping native resources, wrapping other IDisposable classes, or using Factored types). To address this, I've written a multiple part series on IDisposable - it might help clarify some of this for you.
The correct pattern is to only use a finalizer when your class contains unmanaged resources. As to relying on the GC to dispose of your managed objects, don't. The IDisposable contract makes it clear that this object needs to be disposed.
Ok i think i've understood, referring to my example its correct to implement a dispose because the dataset is global to my class and implements IDisposable , while i don't need the finalizer because there aren't unmanaged resources.
Even if i "forget" to dispose some managed resource in my class dispose method, the gc will collect it at some point. The dispose method is just an utility i provide to other classes/developers for managed resources, a must with the finalizer if i wrap unmanaged resources .
i'll read the articles you have provided asap, but in the mean time i've the last question : when gc will free memory owned by my class and its resources ? when someone calls dispose or when it will run (it will free memory instead of moving it to next generation ) ?
thank you everybody for your answers and examples
I wrote a brief seris entitled How to Implement IDisposable and Finalizers: 3 Easy Rules. It describes a much simpler approach that Microsoft themselves have followed since the 2.0 version of the BCL.
The "official" pattern is needlessly complex and needlessly confusing.
Your code is correct, you've implemented it exactly like it is documented in the MSDN library.
You'll need to take a second look though. Reason what happens when the destructor (aka finalizer) runs. The disposing argument will be false, the protected Dispose method does nothing. This is entirely normal, finalizers should only ever release unmanaged resources. You don't have any. It is extraordinary rare to ever have an unmanaged resource in your own code. They belong in the nice wrapper classes available in .NET to turn an unmanaged operating resource into a nice managed class. If you find yourself thinking you need a finalizer, you'll be wrong 99.99% of the time. Even if you do wrap an unmanaged resource, you should use one of the SafeHandle wrappers. And rely on their finalizers.
Okay, you want to get rid of the destructor. It isn't healthy to leave it in, it keeps the object in memory longer than necessary. When you do, you'll cut it down to:
public void Dispose()
{
if (myDS != null) myDS.Dispose();
}
Which is the boiler-plate implementation of most any Dispose() method, just call the Dispose method of members of the class. You can completely omit it if you don't have any members with a Dispose() method.
Next, you do misunderstand how the Dispose() method gets called. It is not automatic. Which is the point of having it in the first place, releasing resources automatically is already taken care of by the garbage collector. The Dispose() method is there for you to call, either with the using statement or calling it directly. So that you can release the resource early instead of waiting for the garbage collector finalizer thread to get around to it. Which can take a while. Call it when you know that your program won't be using the object anymore.
If your DataSet is actively used by the form then you cannot dispose it until the form closes. Call the class' Dispose() method in a FormClosed event handler. Or, better, open the form's Designer.cs file, cut-and-paste the Dispose() method you find in there and move it to the form's source code file. And add the dispose call. I know that's a bit confuzzling, but the only time it's okay to edit the designer file.
The main purpose of IDisposable is to have a consistent standard interface you can dispose of unmanaged resources with, that is if you didn't do something to ensure Dispose() was called these resources would hang around after the app was closed. Also understood by the using() syntax, that is using will implement the following block for you:
DisposableType someDisposable = new DisposableType();
try
{
// Do whatever
}
finally
{
((IDisposable)someDisposable).Dispose();
}
This is all implemented in a nice design like so:
using(DisposableType someDisposable = new DisposableType())
{
// Do whatever
}
I have the following class
public class Presenter: IDisposable
{
public IView View
{get;private set;}
//snip other object reference, all managed
public Presenter(IView view)
{
View=view;
View.MouseUp += MouseUpEvent;
}
public void MouseUpEvent()
{
//do whatever you want to do on mouse up
}
public void Dispose()
{
View.MouseUp -= MouseUpEvent;
// no more manual disposing
}
}
The question now is, am I implement Dispose() method correctly? Or do I need to manually dispose all the other managed objects just because I have explicilty define Dispose()?
I reckon that the GC is smart enough to do its own disposing ( except the event subscription) even without me manually doing it. Am I right?
If you go with the choice of subscribing in the constructor, then this looks reasonable. I would echo Josh's sentiments that it may not be the best approach. On the other hand, it may be the simplest way of achieving your goal, which is always a good thing. I'm not going to pretend to be an expert on UI patterns: having raised the concern, I'll assume that this is the way you want to work, and address the question itself :)
I personally find Josh's prescribed pattern1 overly complex for simple scenarios - your approach is fine, with just one change: make your class sealed. If you don't want to seal the class, you should go for the Dispose(bool) option (but without the finalizer) because subclasses may also need to dispose of things, and may need a finalizer. Without the possibility of a derived type, life is simpler (as it so often is).
You don't need to do anything with other members just because you now implement IDiposable for that one reason.
So, do you need to derive any further from this class?
1 I do understand that this is the standard recommended pattern, although I'd recommend that you read the advice of Joe Duffy et al for even more details - it can all get very complicated.
Personally, I would avoid hooking/unhooking the event in the constructor and dispose. Instead I would add code to the View get/set accessors and add them there. But if the Presenter is disposed while a View is attached, I would not bother trying to clean that up. You can explicitly detach the View from the presenter if you need explicit detaching.
Having said that, here's what I know about IDisposable.
The recommended approach to implementing IDisposable is to have a protected Dispose(bool) method where you take action. The reason is, you want to distinguish between an explicit disposal and a disposal caused by finalization (garbage collection.)
When you are being disposed because of an explicit Dispose() call, it's ok to touch managed objects and you are expected to dispose of anything you've created that also needs disposing. So you do this only when disposing=true.
But if someone (you) forgets to call Dispose and the finalizer is called, you're being disposed after garbage collection (disposing=false) and you don't want to touch any managed objects because they may already be finalized. The only thing you need to free up in this case is unmanaged resources like Win32 handles and such.
Finally, when Dispose() is explicitly called you'll notice I called GC.SupressFinalize(this) which is a performance hint for the garbage collector. It lets it know that the object doesn't need to be finalized when it is collected. Finalization isn't cheap.
class MyObject : IDisposable {
~MyObject() {
Dispose(false);
}
public void Dispose() {
Dispose(true);
GC.SupressFinalize(this);
}
protected virtual void Dispose(bool disposing) {
if (disposing) {
// dispose of managed resources
}
// dispose of unmanaged resources
}
}
Here is code from MSDN. I don't understand why the work isn't just done in the regular Dispose() method here. What is the purpose of having the Dispose(bool) method? Who would ever call Dispose(false) here?
public void Dispose()
{
Dispose(true);
// Use SupressFinalize in case a subclass
// of this type implements a finalizer.
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
// If you need thread safety, use a lock around these
// operations, as well as in your methods that use the resource.
if (!_disposed)
{
if (disposing) {
if (_resource != null)
_resource.Dispose();
Console.WriteLine("Object disposed.");
}
// Indicate that the instance has been disposed.
_resource = null;
_disposed = true;
}
}
The finalizer would call Dispose(false) - in which case you don't touch any of the other managed resources (which may already have been finalized).
Personally I don't follow this pattern often - because I only very, very rarely need a finalizer, and it's also rare for me to write a non-sealed IDisposable implementation. If you're writing a sealed class without a finalizer, I would go for a simple implementation.
This is to allow the finalizer to work property, as well as to allow subclasses which derive from your class to dispose properly.
If you want more detailed info, I wrote a 5 part blog series on IDisposable, and covered the subclassing issue in detail in the Subclass from an IDisposable Class article.
Regarding the answer,
Your Dispose(disposing) method shouldn't explicitly free resources if it is called from finalizer, since these resources can be already freed by GC.
It's missing an important word. This should really say:
Your Dispose(disposing) method shouldn't explicitly free finalizable (i.e. managed) resources if it is called from finalizer, since these resources can be already freed by GC. Only native resources should be released in a Finalizer.
I'm pretty sure that the poster meant this but just wasn't explicit enough in the post : )
Your Dispose(disposing) method shouldn't explicitly free resources if it is called from finalizer, since these resources can be already freed by GC.
So, Dispose(disposing) should check whether it was called manually or from GC and acts appopriately.
I have been programming in .NET for four years (mostly C#) and I use IDiposable extensively, but I am yet to find a need for a finaliser. What are finalisers for?
A finalizer is a last ditch attempt to ensure that something is cleaned up correctly, and is usually reserved for objects that wrap unmanaged resources, such as unmanaged handles etc that won't get garbage collected.
It is rare indeed to write a finalizer. Fortunately (and unlike IDisposable), finalizers don't need to be propagated; so if you have a ClassA with a finalizer, and a ClassB which wraps ClassA, then ClassB does not need a finalizer - but quite likely both ClassA and ClassB would implement IDisposable.
For managed code, IDisposable is usually sufficient. Even if you don't clean up correctly, eventually the managed objects will get collected (assuming they are released).
Finalizers are only for freeing unmanaged resources like GDI bitmap handles for example. If you don't allocate any unmanaged resources then you don't need finalizers. In general it's a bad idea to touch any managed object in a finalizer because the order of finalization is not guaranteed.
One other useful technique using a finalizer is to assert that Dispose has been called when the application is required to do so. This can help catch coding errors in a DEBUG build:
void Dispose()
{
GC.SuppressFinalize(this);
}
#if DEBUG
~MyClass()
{
Debug.Fail("Dispose was not called.");
}
#endif
Finalizers are meant as a mechanism to release resources not controlled by garbage collector, like an unmanaged handle. While Dispose might do it, it isn't guaranteed that the consumer will call it.
Finalizers are for cleaning up resources if they were not disposed.
IE, nothing enforces that you ever call Dispose(), but Finalizers are called automatically by the garbage collector.
This functionality should not be relied upon, as there is no guarantee when (or if) garbage collection will get to your object.
Wikipedia says:
...a finalizer is a piece of code that
ensures that certain necessary actions
are taken when an acquired resource...
is no longer being used [because the
owning object has been garbage
collected]
And if you're not using a finaliser when you're writing IDisposables you've quite possibly got memory leaks, because there's no guarantee an owner is actually going to call Dispose().
MS themselves recommend you write something similar to this into your implementers:
public void Dispose()
{
this.Dispose(true);
}
protected virtual void Dispose(bool disposing)
{
if (!this.isDisposed)
{
if (disposing)
{
GC.SuppressFinalize(this);
}
}
//Dispose of resources here
this.isDisposed = true;
}
~DisposableSafe()
{
this.Dispose(false);
}
private bool isDisposed = false;
Personally, I can't stand the copy-paste so I tend to wrap that in an abstract class for reuse.