This question already has answers here:
Proper use of the IDisposable interface
(20 answers)
Closed 5 years ago.
What is the best practice for when to implement IDisposable?
Is the best rule of thumb to implement it if you have one managed object in the class, or does it depend if the object was created in the class or just passed in? Should I also do it for classes with no managed objects at all?
If you mean unmanaged objects then yes, you should be implementing it whenever you have one or more unmanaged resource you are handling in your class. You should also be using the pattern when you are possibly holding on to objects that are IDisposable themselves, and make sure to dispose of them when your class is disposed.
(agreed that this question has already been asked enough times as to run a small printer out of ink should they be printed...)
While everyone has mentioned (unmanaged) resources, I have another thing to add: I use it when I need to eliminate event handler hookups that would otherwise prevent a class from going out of scope and being garbage collected.
As an example, I have a service which gets injected in to a child view, that child view will subscribe to various async finished type events on the service. The owner of that child view has no idea what the concrete type is, it simply has it as an interface. The service may go out of scope at some arbitrary point in the future, and I don't want it hanging around not being GC'ed. Upon getting rid of that child view, the owner will call Dispose on it to give it the chance to unhook any event handlers. Here is a slightly contrived (and very pseudo code) example, note how the interface for the child view also implements IDisposable.
public class OwnerView {
public OwnerView() {
_childView = new ChildView(myServiceReference);
}
public void CloseChildView() {
if (childView != null) {
_childView.Close();
_childView.Dispose();
}
_childView = null;
}
private IChildView _childView;
}
public class ChildView : IChildView {
public ChildView(MyService serviceRef) {
_serviceRef = serviceRef;
_serviceRef.GetSettingsAsyncFinished += new EventHandler(someEventHandler);
}
public void IDisposable.Dispose() {
_serviceRef -= someEventHandler;
}
private MyService _serviceRef;
}
public interface IChildView : IDisposable {
void DoSomething();
... etc ...
}
There are far more authoritative comments about this from others on SO, like Do event handlers stop garbage collection from occuring? and Garbage collection when using anonymous delegates for event handling. You may also want to check out this codeproject article.
I think the docs are pretty clear about what IDisposable is good for.
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.
It even has an example. In that example the class that implements IDisposable contains a handle. A handle needs to be freed when it no longer used. This is done in the Dispose() method. So the user of that class sees that it implements IDisposable and knows that the class needs to be explictily disposed when it is no longer needed (so the handle can be freed). It is a best practise (i.e. rule) that you should always call Dispose() on IDisosable instances when the instance is no longer needed.
You should implement IDisposable when your class holds resources that you want to release when you are finished using them.
When your class contains unmanaged objects, resources, opened files or database objects, you need to implement IDisposable.
If it has properties which also need to be disposed.
Related
I've been reading about IDisposable interface lately (this topic was quite useful Proper use of the IDisposable interface) and also about usage of using statement (topic Uses of "using" in C#). Then I started wondering what if I should somehow free my own collection class from memory.
class User{
private string username {set;get;}
private string password {set;get;}
...
}
Should it implement IDisposable interface?
class User : IDisposable
{
...
public void Dispose()
{
this.Dispose();
}
}
so it could be freed from memory? Or does GC do it automaticly and I shouldn't even bother about it.
As far as I understand it's important to free unmanaged resources like DB connections and such but what about those collection classes. Since I use them quite frequently It really started to bug me.
tl;dr;
should I implement IDisposable on User class?
Kind Regards.
edit: thanks everyone for the replies!
Or does GC do it automaticly and I shouldn't even bother about it.
This. Unless you have unmanaged resources (either directly or by way of a reference to something else which is disposable), you almost certainly shouldn't implement IDisposable.
Your current implementation would just call itself:
public void Dispose()
{
this.Dispose();
}
... so assuming you don't really want to call this.Dispose(), what would you want to do when Dispose() is called? It's not like disposal causes garbage collection - so what action do you want to take? If the answer is "nothing" then you probably shouldn't be implementing IDisposable. (The exception here is if this is designed to be a base class and you're expecting some derived classes to require disposal... that's a more complex scenario.)
All unused resources will be Garbage collected eventually when the program unloads or after separate interval if the resources are no longer used.
Its a better practice to clean/dispose used resources(variables, objects) when you no longer wish to use them to free memory.
As said in the other answers, the only times you have to implement IDisposable is when you deal with unmanaged resources or class members that are IDisposable themselves (in this case you should dispose them in your own Dispose method).
Another scenario you might see is when people want to use the using { } syntactic sugar to automatically call some method at the end of the variable scope without using the try { } finally { } form.
I.e.:
public class MyObject : IDisposable
{
public void Foo()
{
}
public void Dispose()
{
// Something they want to call after the use of an instance of MyObject
}
}
...
using (var myObj = new MyObject())
{
myObj.Foo();
}
instead of
public class MyObject
{
public void Foo()
{
}
public void MethodToCallAfterUse()
{
// Something they want to call after the use of an instance of MyObject
}
}
var myObj = new MyObject();
try
{
myObj.Foo();
}
finally
{
myObj.MethodToCallAfterUse();
}
If an object asks an outside entity to "do something" (perform an action, reserve a resource, etc.) on its behalf until further notice, and that outside entity might continue to exist after the object requesting its services has ceased to be useful, then the object should ensure that the outside entity receives notice when its services are no longer required. The IDisposable resource exists as a standard means by which an object can say "I may be obligated to let outside entities know when I don't need their services; let me know when my services will no longer be needed, so I can satisfy my obligation to tell any outside entities that I no longer need their services."
In .NET, once no references to an object exist anywhere in the universe, the object itself will likewise cease to exist. If the only object which knows of an obligation to do perform some action ceases to exist without having performed the action, the action will not get performed. If an object has no obligations, however, having it simply cease to exist once there are no references is just fine.
I think so. But take a look at a built-in class in ASP.NET:
public sealed class HttpPostedFile
{
public Stream InputStream { get; } // Stream implements IDisposable
// other properties and methods
}
Suppose I have an instance of HttpPostedFile called file. Since there is no Dispose method to explicitly invoke, file.InputStream.Dispose() won't be invoked until it's destructed, which I think goes against the original intention of IDisposable. I think the correct implementation should contain a standard IDisposable implementation. So, if one of the members implements IDisposable, the class needs to implement it too.
What are your opinions? It seems to be a bit complicated.
In general, you should implement IDisposable if you own the resource represented by the property - see this question for a discussion on this subject.
I'd say that because HttpPostedFile is instantiated during processing of an HTTP request, it doesn't own the stream, and hence doesn't dispose it. The stream will be disposed when the HTTP request processing finishes.
If your class creates one or more IDisposable objects and holds the only references to them, then your class should almost certainly implement IDisposable and dispose the IDisposable objects it created. If one or more IDisposable objects will be passed into the constructor of your class, then you need to consider a few scenarios:
Your creator may want to keep using the IDisposable after you're done with it, and will certainly know when it's no longer needed (the semantics of your class would let him know you're done with it).
Your creator won't want to use the IDisposable after you're done with it, and may not know when you're going to be done with it.
Your class may be used in some circumstances corresponding to (1) above, and in some circumstances (2), but your creator will know in advance which circumstance applies.
Your creator can't predict whether he's going to want to keep using the object after you're done with it.
For scenario #1, there's no need for you to implement IDisposable, though it might not be a bad idea to implement a do-nothing IDisposable handler and have your consumers use it, in case another scenario applies in future.
For scenario #2, your object should take ownership of the IDisposable, and should Dispose it when done. I don't really like having objects take unconditional ownership of IDisposables; I prefer to implement things as in #3.
There are two ways of handling #3. The one I prefer is for your object to take a parameter (either a Boolean or an enum) along with the IDisposable, indicating whether it is supposed to take ownership of the IDisposable. Your class unconditionally implements IDisposable; the implementation disposes of any objects it has taken ownership of, but not those it hasn't. An alternative is to have two subclasses with a common base class - one subclass implements IDisposable and the other does not. I prefer the former pattern, because it allows for the addition of a method to replace an IDisposable with a new one (of which it may or may not take ownership). For example, if I were implementing a control with an Image property, I would have a SetImage method which with a parameter to specify whether the control should own the passed-in image; that method would Dispose the old image if it owned it, and could then either take ownership of the new image or not.
bool OwnMyImage;
Image MyImage;
void SetImage(Image NewImage, bool TakeOwnership)
{
IDisposable oldDisposable; // Could reuse one variable for multiple IDisposables
if (OwnMyImage)
{
oldDisposable = Threading.Interlocked.Exchange(MyImage, null);
if (oldDisposable != null)
{
oldDisposable.Dispose();
}
}
OwmMyImage = TakeOwnership;
MyImage = NewImage;
}
Scenario #4 is complicated; the best way to handle it is probably for your object to implement IDisposable by raising a Disposed event. Your creator can use that event to do either Dispose the object if you were the last one using it, or adjust a flag or counter so that other code will know the object shouldn't be left undisposed on your behalf.
It depends.
Stream is also implemented by TextStream (possibly on top of StringBuilder), so no unmanaged resources are required.
HttpPostedFile may not use any unmanaged resources at all, so it's safe to postpone deconstruction until the garbage collector sees fit.
For too long I let the garbage collector do its magic, removing all responsibilities from my self.
Sadly it never turned into an issue... So I never gave a second thought to the subject.
Now when I think about it I don't really understand what the "dispose" function really does and how and when it should be implemented.
The same question for finalize...
And a last question...
I have a class pictureManipulation : when I need to save/resize/change format ... I start a new instance of that class use its objects and... well let the garbage collection kill the instance
class student
{
public void displayStudentPic()
{
PictureManipulation pm = new PictureManipulation();
this.studentPic = pm.loadStudentImage(id);
}
}
Class Test
{
student a = new Student();
a.displayStudentPic();
// Now the function execution is ended... does the pm object is dead? Will the GC will kill it?
}
Regarding your class Student
Do I need a Dispose() ?
Assuming the Picture class is IDisposable: Yes. Because a Student object 'owns' the studentPic and that makes it responsible for cleaning it up. A minimal implementation:
class Student : IDisposable
{
private PictureClass studentPic;
public void Dispose()
{
if (studentPic != null)
studentPic.Dispose();
}
...
}
And you now use a Student object like:
void Test
{
using (Student a = new Student())
{
a.displayStudentPic();
} // auto Dispose by using()
}
If you can't/don't use a using(){} block, simply call a.Dispose(); when you're done with it.
But please note that the (far) better design here would be to avoid keeping a picture object inside your Student object. That sets off a whole chain of responsibilities.
Do I need a Finalizer?
No. Because when a Student object is being collected, its studentPic object is guaranteed to be collected in the same run. A Finalizer (destructor) would be pointless but still expensive.
You only need to implement the Dispose method if your type holds some unmanaged resources like DB connections, file handles, etc. or if some of the objects that are held by your type implement the IDisposable interface. Here is a few points you should consider when implementing the standard Dispose pattern:
if your object doesn’t hold any IDisposable objects or unmanaged resources (DB connection, for example) then you don’t need to implement the IDisposable or finalizer at all
if your object holds references to IDisposable objects, then call Dispose() on these objects in the Dispose method
if your object doesn’t hold any unmanaged resources then don’t implement a finalizer, the Garbage Collector won’t attempt to finalize your object (which has a performance hit) unless you have implemented a finalizer.
if your object holds unmanaged resources, clean them up in the finalizer without re-writing any of the cleanup code in the Dispose(bool) method already.
You need to take care about object disposal if it holds resources other than just memory held by the object itself.
For instance, if your object abstracts a file, you must be in control when the file is released, or, you will mess things up very bad: your app finished using it and it will still be locked, until GC disposes your object.
To know how to do it properly, read manuals about dispose and finalize as well as the using(){} clause.
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
}
}