I have several objects in our framework, that by the requirement need to provide event ObjectTerminated. The user of the framework can subscribe to this event and clean-up some unmanaged stuff, that he is using. These objects are designed to exist the whole life time of the application, and I'm not controlling their life. You can think of them as an array of singletons.
I want to write code like this:
class SomeWorkflowControlObject
{
public event EventHandler<> ObjectTerminated;
~SomeWorkflowControlObject()
{
if (ObjectTerminated != null) ObjectTerminated(this, null);
}
}
I am not sure, am I allowed to do that. What could go possibly wrong with such solution?
Updated:
What about Process.GetCurrentProcess().Exited ? Can I use it in such manner?
You should not do this. Basically, destructors do not exist in C#. What you have written is a finalizer, and the only thing a finalizer should ever do is to free unmanaged resources.
You are not allowed to access any other managed object at all, since the garbage collector might already have removed it. I do not think your null check is a sufficient guard against this situation; that object reference might still point to your (event) delegate, even if it's already gone.
So in short, don't do this.
Alternatives:
Subscribe to the Application.ApplicationExit event if you have a Windows Forms application.
You might want to consider implementing the IDisposable interface instead and then do something like this:
public class SomethingVeryLongLived : IDisposable
{
…
}
…
public static void Main()
{
using (var sth = new SomethingVeryLongLived(…))
{
Application.Run(new SomeForm(…));
} // <-- at this point, foo.Dispose() is guaranteed to be called.
}
Take note that even in the case of using IDisposable, it's probably not a good idea / design to trigger an event inside the object that is getting disposed, since disposed objects should no longer be accessed.
For this very reason, I would recommend you do the following:
Use a try…finally block:
public static void Main()
{
var sth = new SomethingVeryLongLived(…);
try
{
Application.Run(new SomeForm(…));
}
finally
{
SomethingVeryLongLived.Terminate();
}
}
This would seem best to me because you are not abusing the IDisposable interface, and it's very clear what the code does... there's no hidden meaning.
I believe the design of that framework is inherently flawed. Let me explain:
1. [...] several objects in our framework, that by the requirement need to provide event ObjectTerminated.
This doesn't make sense. If an object has been terminated, as the event's name suggests, then I would assume that it is already gone and that I can no longer access it. This raises two questions:
How does something dead trigger an event? It's like a corpse talking to you from its grave, "I am dead." Do you really want this?
Why would anyone else be interested in reacting to such an event, if the event sender is no longer supposed to be there? What is there to clean up after the corpse has already been buried?
2. I'm not controlling their life.
Who is controlling their lifetime, then? Why isn't it their responsibility to do, or trigger, the necessary clean up work at the appropriate moment? Let me further elaborate on this very point:
3. [...] can subscribe to this event and clean-up some unmanaged stuff [...]
Where is this unmanaged stuff, and which object is responsible for handling it? If it is your own object, then why doesn't your object dispose of it — why do you instead want to trigger an event, so that someone else can dispose of the stuff? It's like me carrying out my neighbour's garbage, instead of him doing it himself. (I'm not talking about an old lady there.)
Your class would make much more sense if it looked like this:
class SomeWorkflowControlObject : IDisposable
{
// the following event doesn't make sense, sorry.
// public event EventHandler<> ObjectTerminated;
private IntPtr _handleToUnmanagedResource;
~SomeWorkflowControlObject()
{
Dispose(explicitly: false);
}
public void Dispose()
{
Dispose(explicitly: true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool explicitly)
{
if (explicitly)
{
// free managed resources here; perhaps trigger an event 'Disposing'.
}
DisposeUnmanagedResource(_handleToUnmanagedResource);
}
}
That is, it is a wrapper around some unmanaged resource, for whose disposal it is responsible itself, and noone else. Thus, there is no longer a need to trigger an event, such that someone else can dispose the unmanaged resource, which should be hidden inside your object anyway.
Related
I read the excellent answer explaining how to use the Dispose pattern, and also why it works that way.
Proper use of the IDisposable interface
The post clearly states that you would want to use the Dispose pattern in two different scenarios:
get rid of unmanaged resources (because we have to)
get rid of managed resources (because we want to be helpful)
My question is:
When an object is subscribed to an external event for its entire lifetime, is it also common/good practice to unregister from that event in the Dispose method? Would you implement the IDisposable interface for that purpose?
Yes, you should.
This is the best way to indicate to consumers of your class that it has "resources" that must be released. (even though event subscriptions are technically not resources)
In many (most?) cases, an object will become eligible for garbage collection very soon after Dispose is called. For example, this will always be true for IDisposable objects instantiated with a using statement:
using(var myDisposableObject = ...)
{
...
} // myDisposableObject.Dispose() called here
// myDisposableObject is no longer reachable and hence eligible for garbage collection here
In this situation, I personally wouldn't clutter the code with removal of event subscriptions in the general case.
For example, an ASP.NET Page or UserControl is IDisposable, and often handles events from other controls on the web page. There is no need to remove these event subscriptions when the Page or UserControl is disposed, and in fact I've never seen an ASP.NET application where this is done.
UPDATE
Other answerers suggest you should always unsubscribe to events in the Dispose method of an IDisposable class.
I disagree with this in the general case, though there may be application-specific situations where it is appropriate.
The logical conclusion is that any class that subscribes to events should be IDisposable, so that it can unsubscribe deterministically - I see no logical reason why this recommendation should only apply to classes that happen to own unmanaged resources. I don't think this is a good general recommendation for the following reasons:
Making a class IDisposable just so it can unsubscribe from events adds complexity for users of the class.
Unsubscribing from events in the Dispose method requires the developer to keep a track of event subscriptions that need to be removed - somewhat fragile as it's easy to miss one (or for a maintenance developer to add one).
In situations where a class subscribes to events from a long-lived publisher, it is probably more appropriate to use a Weak Event Pattern to ensure that the subscriber's lifetime is not affected by the event subscription.
In many situations (e.g. an ASP.NET Page class subscribing to events from its child controls), the lifetime of publisher and subscriber are closely related, so there is no need to unsubscribe.
I prefer to have a two-pronged approach:
(1) An explicit method to UnregisterFromExternalEvents();
(2) A call in Dispose() to that method.
This way any code that controls instances of your class can either explicitly unregister, or trust on Dispose to properly dispose and take care of such matters.
Yes, it would be good practice to unregistered all external events, but though not extremely necessary, due to loose coupling nature of events.
It removes the event entry point reference of the subscriber object from the event generator and yes will be helpful.
For the part to unsubscribe in the dispose method is also fine. The thumb rule of Dispose method is - "Dispose method should unload resources in a way that if the dispose is called more than once it still works, i.e you should free resource in dispose one and only one time. (which would require checks before disposing resources)"
Yes, it is an extremely good idea. An event publisher holds a reference to the event subscriber which will prevent the subscriber from being garbage collected. (See Do event handlers stop garbage collection from occurring?)
In addition, if your event handlers use the resources you are releasing, then the event handlers (which will continue to be called by the event publisher) may generate exceptions after the resources have been released.
It is important, therefore, to unregister from any events before releasing your resources, especially if you are making use of async or multiple threads, as it is possible in some scenarios for an event to be raised between releasing a resource and unregistering from that event.
The following code demonstrates this:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ReleaseEvents
{
class Program
{
public static event EventHandler SomethingHappened;
static void Main( string[] args )
{
using ( var l_dependent = new Dependent() )
{
SomethingHappened( null, EventArgs.Empty );
}
// Just to prove the point, garbage collection
// will not clean up the dependent object, even
// though it has been disposed.
GC.Collect();
try
{
// This call will cause the disposed object
// (which is still registered to the event)
// to throw an exception.
SomethingHappened( null, EventArgs.Empty );
}
catch ( InvalidOperationException e )
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine( e.ToString() );
}
Console.ReadKey( true );
}
}
class Dependent : IDisposable
{
private object _resource;
public Dependent()
{
Program.SomethingHappened += Program_SomethingHappened;
_resource = new object();
}
private void Program_SomethingHappened( object sender, EventArgs e )
{
if ( _resource == null )
throw new InvalidOperationException( "Resource cannot be null!" );
Console.WriteLine( "SomethingHappened processed successfully!" );
}
public void Dispose()
{
_resource = null;
}
}
}
The second time that the SomethingHappened event is raised, the Dependent class will throw an InvalidOperationException. You must unregister the event to prevent this from happening:
class Dependent : IDisposable
{
// ...
public void Dispose()
{
Program.SomethingHappened -= Program_SomethingHappened;
_resource = null;
}
}
I actually ran into this problem when I first tried to implement a MVVM architecture. When I was switching between ViewModels, I was simply releasing what I thought was my only reference (an ActiveViewModel property). I didn't realize that the events my ViewModel was subscribed to was keeping it in memory. As the application ran longer, it became slower and slower. Eventually I realized that the ViewModels I thought I had released were actually continuing to process events (expensively). I had to explicitly release the event handlers to fix this problem.
Thanks to this question I will write my classes as:
class Foo : IDisposable
{
public event EventHandler MyEvent;
/// <summary>
/// When disposing unsubscibe from all events
/// </summary>
public void Dispose()
{
if (MyEvent != null)
{
foreach (Delegate del in MyEvent.GetInvocationList())
MyEvent -= (del as EventHandler);
}
// do the same for any other events in the class
// ....
}
}
I am interested in knowing why static destructors are not allowed in C#. Note that I am not supposing that they exist in any other language.
I could have a class like this one.
class A
{
static A()
{
// here I can load a resource that will be avaliable for all instances
// of this class.
}
}
When the application ends I may need to release the resource.
So, the semantic of a static destructor could be the following: called when the application ends, for classes that contain it and were initialized in the app.
Your semantic is one possible one, but I guess you have not checked all effects it would have on the language. In most (more or less) dynamic languages I know, destruction is a much more complicated topic that it looks like. Why not call the destructor when the class is not referenced anymore? Or if the assembly is unloaded? In what order should destructor be called? ...?
If you just want to execute some code when the application ends, have a look at the .Net documentation. There are easier and more reliable ways to do so.
So, the semantic of a static destructor could be the following: - be
called when the application ends, on class that contains it and was
charged in the app.
Your semantic relies on your program doing an specific action at the end of execution and this is far more difficult to correctly handle than just a piece of code that runs at the end of normal execution of the process.
Think about transactions or file management operations. You have to manually handle crashes and unexpected termination of the process and try recovering at next run anyway, so then having an static destructor wouldn't help that much. .NET managed world favors upon using patterns instead of that. Now, if you're having serious problems with this, try to attach an event handler to the DomainUnloaded event on the AppDomain and perform your cleanup there.
You can, also, give a try to the singleton dispose way:
class A : IDisposable
{
public static A Instance {get; private set;}
public static A()
{
Instance=new A();
}
public void MethodA() {...}
public void Dispose()
{
//...
}
~A()
{
// Release your hard resources here
}
}
And then, use the safe method:
A.Instance.MethodA();
I have a class that handles events from a WinForms control. Based on what the user is doing, I am deferencing one instance of the class and creating a new one to handle the same event. I need to unsubscribe the old instance from the event first - easy enough. I'd like to do this in a non-proprietary manner if possible, and it seems like this is a job for IDisposable. However, most documentation recommends IDisposable only when using unmanaged resources, which does not apply here.
If I implement IDisposable and unsubscribe from the event in Dispose(), am I perverting its intention? Should I instead provide an Unsubscribe() function and call that?
Edit: Here's some dummy code that kind of shows what I'm doing (using IDisposable). My actual implementation is related to some proprietary data binding (long story).
class EventListener : IDisposable
{
private TextBox m_textBox;
public EventListener(TextBox textBox)
{
m_textBox = textBox;
textBox.TextChanged += new EventHandler(textBox_TextChanged);
}
void textBox_TextChanged(object sender, EventArgs e)
{
// do something
}
public void Dispose()
{
m_textBox.TextChanged -= new EventHandler(textBox_TextChanged);
}
}
class MyClass
{
EventListener m_eventListener = null;
TextBox m_textBox = new TextBox();
void SetEventListener()
{
if (m_eventListener != null) m_eventListener.Dispose();
m_eventListener = new EventListener(m_textBox);
}
}
In the actual code, the "EventListener" class is more involved, and each instance is uniquely significant. I use these in a collection, and create/destroy them as the user clicks around.
Conclusion
I'm accepting gbjbaanb's answer, at least for now. I feel that the benefit of using a familiar interface outweighs any possible downside of using it where no unmanaged code is involved (how would a user of this object even know that?).
If anyone disagrees - please post/comment/edit. If a better argument can be made against IDisposable, then I'll change the accepted answer.
Yes, go for it. Although some people think IDisposable is implemented only for unmanaged resources, this is not the case - unmanaged resources just happens to be the biggest win, and most obvious reason to implement it. I think its acquired this idea because people couldn't think of any other reason to use it. Its not like a finaliser which is a performance problem and not easy for the GC to handle well.
Put any tidy-up code in your dispose method. It'll be clearer, cleaner and significantly more likely to prevent memory leaks and a damn sight easier to use correctly than trying to remember to un-do your references.
The intention of IDisposable is to make your code work better without you having to do lots of manual work. Use its power in your favour and get over some artificial "design intention" nonsense.
I remember it was difficult enough to persuade Microsoft of the usefulness of deterministic finalisation back when .NET first came out - we won the battle and persuaded them to add it (even if it was only a design pattern at the time), use it!
My personal vote would be to have an Unsubscribe method in order to remove the class from events. IDisposable is a pattern intended for deterministic release of unmanaged resources. In this case you not managing any unmanaged resources and therefore should not be implementing IDisposable.
IDisposable can be used to manage event subscriptions but probably shouldn't. For an example I point you to WPF. This is a library rife with events and event handlers. Yet virtually no class in WPF implements IDisposable. I would take that as an indication that events should be managed another way.
One thing that bothers me about using IDisposable pattern for unsubscribe from events is Finalization issue.
Dispose() function in IDisposable is supposed to be called by the developer, HOWEVER, if it isn't called by the developer, it is a understood that the GC will call this function (by the standard IDisposable pattern, at least). In your case, however, if you don't call Dispose no one else will - The event remains and the strong reference holds GC from calling the finalizer.
The mere fact that Dispose() won't be called automatically by GC seems to me enough to not to use IDisposable in this case. Perhaps it calls for a new application specific interface that says that this type of object must have a Cleanup function called to be disposed by GC.
I think disposable is for anything that GC can't take care of automatically, and event references count in my book. Here is a helper class I came up with.
public class DisposableEvent<T> : IDisposable
{
EventHandler<EventArgs<T>> Target { get; set; }
public T Args { get; set; }
bool fired = false;
public DisposableEvent(EventHandler<EventArgs<T>> target)
{
Target = target;
Target += new EventHandler<EventArgs<T>>(subscriber);
}
public bool Wait(int howLongSeconds)
{
DateTime start = DateTime.Now;
while (!fired && (DateTime.Now - start).TotalSeconds < howLongSeconds)
{
Thread.Sleep(100);
}
return fired;
}
void subscriber(object sender, EventArgs<T> e)
{
Args = e.Value;
fired = true;
}
public void Dispose()
{
Target -= subscriber;
Target = null;
}
}
which lets you write this code :
Class1 class1 = new Class1();
using (var x = new DisposableEvent<object>(class1.Test))
{
if (x.Wait(30))
{
var result = x.Args;
}
}
One side effect, you must not use the event keyword on your events, since that prevents passing them as a parameter to the helper constructor, however, that seems to not have any ill effects.
Another option would be to use weak delegates or something like WPFs weak events, instead of having to unsubscribe explicitly.
P.S. [OT] I consider the decision to only provide strong delegates the single most expensive design mistake of the .NET platform.
From all that i read about disposables i would argue that they were really mainly invented for solving one problem: freeing unmanaged system resources in a timely manner. But still all examples that i found are not only focussed on the topic of unmanaged resources, but also have another property in common:
Dispose is called just to speed up a process that otherwise would have occured later on automatically (GC -> finalizer -> dispose)
Calling a dispose method that unsubscribes from an event however would never occur automatically, even if you would add a finalizer that would call your dispose. (at least not as long as the event owning object exists - and if it would be called you wouldn't benefit from the unsubscription, since the event owning object would also be gone anyway)
So the main difference is that events somehow build an object graph that can't be collected, since the event handling object suddenly becomes referenced of the service that you just wanted to reference/use. You suddenly are forced to call Dispose - no automatic disposal is possible. Dispose would thereby get a subtle other meaning than that found in all examples where a Dispose call - in dirty theory ;) - isn't necessary, since it would get called automaically (at some time)...
Anyway.
Since the disposable pattern is something that already is pretty complicated (deals with finalizers that are hard to get right and many guidelines / contracts) and more importantly in most points has nothing to do with the event back referencing topic, i would say it would be easier to get that separeted in our heads by just not using that metaphor for something that could be called "unroot from object graph" / "stop" / "turn off".
What we want to achieve is to disable / stop some behaviour (by unsubscribing from an event).
It would be nice to have a standard interface like IStoppable with a Stop() method, that by contract is just focussed on
getting the object (+ all its own stoppables) disconnected from events of any objects that it didn't create by its own
so that it won't get called in implicit event style manner any longer (therefore can be perceived as stopped)
can be collected as soon any traditional references onto that object are gone
Let's call the only interface method that does the unsubscription "Stop()". You would know that the stopped object is in a acceptable state but only is stopped. Maybe a simple property "Stopped" would also be a nice to have.
It even would make sense to have an interface "IRestartable" that inherits from IStoppable and additionally has a method "Restart()" if you just want to pause a certain behaviour that certainly will be needed again in the future, or to store a deleted model object in a history for later undo recovery.
After all writing i have to confess to have just seen an example of IDisposable somewhere over here: http://msdn.microsoft.com/en-us/library/dd783449%28v=VS.100%29.aspx
But anyway until i get every detail and the original motivation of IObservable i would say it is not the best use case example
since again it is a pretty complicated system around it and we only have a small problem over here
and it might be that one of the motivations of that that whole new system is to get rid of events in the first place, which would result in a sort of stack overflow concerning th e original question
But it seems they are on some right track. Anyway: they should have used my interface "IStoppable" ;) since i strongly believe there is a difference in
Dispose: "you should call that method or something might leak if the GC happens to late" ....
and
Stop: "you have to call this method to stop a certain behaviour"
IDisposable is firmly about resources, and the source of enough problems to not muddy the waters further I think.
I'm voting for an Unsubscribe method on your own Interface too.
One option may be not to unsubscribe at all - just to change what the subscription means. If the event handler could be made smart enough to know what it's meant to do based on the context, you don't need to unsubscribe in the first place.
That may or may not be a good idea in your particular case - I don't think we've really got enough information - but it's worth considering.
No, you're not preventing the intention of IDisposable. IDisposable is intended as an all-purpose way to ensure that when you're done using an object, you can proactively clean up everything tied to that object. It doesn't have to be only unmanaged resources, it can include managed resources too. And an event subscription is just another managed resource!
A similar scenario that frequently arises in practice is that you will implement IDisposable on your type, purely in order to ensure you can call Dispose() on another managed object. This is not a perversion either, it is just tidy resource management!
In C#, if a class, such as a manager class, does not have resources, is there any benefit to having it : IDisposable?
Simple example:
public interface IBoxManager
{
int addBox(Box b);
}
public class BoxManager : IBoxManager
{
public int addBox(Box b)
{
using(dataContext db = new dataContext()){
db.Boxes.add(b);
db.SaveChanges();
}
return b.id;
}
}
Will there be any benefit in memory use when using BoxManager if it also implements IDisposable? public class BoxManager : IBoxManager , IDisposable
For example:
BoxManager bm = new BoxManager();
bm.add(myBox);
bm.dispose();//is there benefit to doing this?
There are only 2 reasons for implementing IDisposable on a type
The type contains native resources which must be freed when the type is no longer used
The type contains fields of type IDisposable
If neither of these are true then don't implement IDisposable
EDIT
Several people have mentioned that IDisposable is a nice way to implement begin / end or bookended operations. While that's not the original intent of IDisposable it does provide for a very nice pattern.
class Operation {
class Helper : IDisposable {
internal Operation Operation;
public void Dispose() {
Operation.EndOperation();
}
}
public IDisposable BeginOperation() {
...
return new Helper() { Operation = this };
}
private void EndOperation() {
...
}
}
Note: Another interesting way to implement this pattern is with lambdas. Instead of giving an IDisposable back to the user and hoping they don't forget to call Dispose have them give you a lambda in which they can execute the operation and you close out the operation
public void BeginOperation(Action action) {
BeginOperationCore();
try {
action();
} finally {
EndOperation();
}
}
There won't be a scrap of difference between the disposable and non-disposable version if you don't explicitly make use of the Dispose() method.
While your code wouldn't benefit from implementing IDisposable, I can't agree with other opinions here that state that IDisposable is only meant to (directly or indirectly) free native resources. IDisposable can be used whenever the object needs to perform clean up task at the end of it's lifetime span. It's extremely useful together with using.
A very popular example: in ASP.NET MVC Html.BeginForm returns an IDisposable. On creation, the object opens the tag, when Dispose is called it closes it. No native resources involved, yet still a good use of IDisposable.
No, there will be no benefit if you don't do something useful like releasing unmanaged resources that your class might hold in the Dispose method.
One major point of confusion, which may not be applicable in your case but arises often, is what exactly constitutes a "resource". From the perspective of an object, an unmanaged resource is something which an outside entity () is "doing"(*) on its behalf, which that outside entity will keep doing--to the detriment of other entitites--until told to stop. For example, if an object opens a file, the machine which hosts the file may grant that object exclusive access, denying everyone else in the universe a chance to use it unless or until it gets notified that the exclusive access isn't needed anymore.
(*) which could be anything, anywhere; possibly not even on the same computer.
(**) or some way in which the the behavior or state of an outside entity is altered
If an outside entity is doing something on behalf of an object which is abandoned and disappears without first letting the entity know its services are no longer required, the outside entity will have no way of knowing that it should stop acting on behalf of the object which no longer exists. IDisposable provides one way of avoiding this problem by providing a standard means of notifying objects when their services are not required. An object whose services are no longer required will generally not need to ask any further favors from any other entities, and will thus be able to request that any entities that had been acting on its behalf should stop doing so.
To allow for the case where an object gets abandoned without IDisposable.Dispose() having been called first, the system allows objects to register a "failsafe" cleanup method called Finalize(). Because for whatever reason, the creators of C# don't like the term Finalize(), the language requires the use of a construct called a "destructor" which does much the same thing. Note that in general, Finalize() will mask rather than solve problems, and can create problems of its own, so it should be used with extreme caution if at all.
A "managed resource" is typically a name given to an object which implements IDisposable and usually, though not always, implements a finalizer.
No, if there are no (managed or unmanaged) resources there is no need for IDisposable either.
Small caveat: some people use IDisposable to clean up eventhandlers or large memory buffers but
you don't seem to use those
it's a questionable pattern anyway.
From my personal experience (confirmed with discussion and other posts here) I would say, that there could be a situations where your object use massive amount of events, or not massive amount but frequent subscriptions and unsubscription from the event which sometimes leads to that the object is not garbage collected. In this case I in Dispose unsubscribe from all events my object subscribed before.
Hope this helps.
IDisposable is also great if you want to benefit the using () {} syntax.
In a WPF project with ViewModels, I wanted to be able to temporarily disable NotifyPropertyChange events from raising. To be sure other developers will re-enable notifications, I wrote a bit of code to be able to write something like:
using (this.PreventNotifyPropertyChanges()) {
// in this block NotifyPropertyChanged won't be called when changing a property value
}
The syntax looks okay and is easily readable. For it to work, there's a bit of code to write. You will need a simple Disposable object and counter.
public class MyViewModel {
private volatile int notifyPropertylocks = 0; // number of locks
protected void NotifyPropertyChanged(string propertyName) {
if (this.notifyPropertylocks == 0) { // check the counter
this.NotifyPropertyChanged(...);
}
}
protected IDisposable PreventNotifyPropertyChanges() {
return new PropertyChangeLock(this);
}
public class PropertyChangeLock : IDisposable {
MyViewModel vm;
// creating this object will increment the lock counter
public PropertyChangeLock(MyViewModel vm) {
this.vm = vm;
this.vm.notifyPropertylocks += 1;
}
// disposing this object will decrement the lock counter
public void Dispose() {
if (this.vm != null) {
this.vm.notifyPropertylocks -= 1;
this.vm = null;
}
}
}
}
There are no resources to dispose here. I wanted a clean code with a kind of try/finally syntax. The using keyword looks better.
is there any benefit to having it : IDisposable?
It doesn't look so in your specific example, however: there is one good reason to implement IDisposable even if you don’t have any IDisposable fields: your descendants might.
This is one of the big architectural problems of IDisposable highlighted in IDisposable: What your mother never told you about resource deallocation. Basically, unless your class is sealed you need to decide whether your descendants are likely to have IDisposable members. And this isn't something you can realistically predict.
So, if your descendants are likely to have IDisposable members, that might be a good reason to implement IDisposable in the base class.
Short answer would be no. However, you can smartly use the nature of the Dispose() executement at the end of the object lifecycle. One have already gave a nice MVC example (Html.BeginForm)
I would like to point out one important aspect of IDisposable and using() {} statement. At the end of the Using statement Dispose() method is automatically called on the using context object (it must have implemented IDisposable interface, of course).
There one more reason that no one mentioned (though it's debateful if it really worth it):
The convension says that if someone uses a class that implement IDisposable, it must call its Dispose method (either explicitly or via the 'using' statement).
But what happens if V1 of a class (in your public API) didn't need IDisposable, but V2 does? Technically it doesn't break backward compatibility to add an interface to a class, but because of that convension, it is! Because old clients of your code won't call its Dispose method and may cause resources to not get freed.
The (almost) only way to avoid it is to implement IDisposable in any case you suspect that you'll need it in the future, to make sure that your clients always call your Dispose method, that some day may be really needed.
The other (and probably better) way is to implemet the lambda pattern mentioned by JaredPar above in the V2 of the API.
Let's say I want to enforce a rule:
Everytime you call "StartJumping()" in your function, you must call "EndJumping()" before you return.
When a developer is writing their code, they may simply forget to call EndSomething - so I want to make it easy to remember.
I can think of only one way to do this: and it abuses the "using" keyword:
class Jumper : IDisposable {
public Jumper() { Jumper.StartJumping(); }
public void Dispose() { Jumper.EndJumping(); }
public static void StartJumping() {...}
public static void EndJumping() {...}
}
public bool SomeFunction() {
// do some stuff
// start jumping...
using(new Jumper()) {
// do more stuff
// while jumping
} // end jumping
}
Is there a better way to do this?
Essentially the problem is:
I have global state...
and I want to mutate that global state...
but I want to make sure that I mutate it back.
You have discovered that it hurts when you do that. My advice is rather than trying to find a way to make it hurt less, try to find a way to not do the painful thing in the first place.
I am well aware of how hard this is. When we added lambdas to C# in v3 we had a big problem. Consider the following:
void M(Func<int, int> f) { }
void M(Func<string, int> f) { }
...
M(x=>x.Length);
How on earth do we bind this successfully? Well, what we do is try both (x is int, or x is string) and see which, if any, gives us an error. The ones that don't give errors become candidates for overload resolution.
The error reporting engine in the compiler is global state. In C# 1 and 2 there had never been a situation where we had to say "bind this entire method body for the purposes of determining if it had any errors but don't report the errors". After all, in this program you do not want to get the error "int doesn't have a property called Length", you want it to discover that, make a note of it, and not report it.
So what I did was exactly what you did. Start suppressing error reporting, but don't forget to STOP suppressing error reporting.
It's terrible. What we really ought to do is redesign the compiler so that errors are output of the semantic analyzer, not global state of the compiler. However, it's hard to thread that through hundreds of thousands of lines of existing code that depends on that global state.
Anyway, something else to think about. Your "using" solution has the effect of stopping jumping when an exception is thrown. Is that the right thing to do? It might not be. After all, an unexpected, unhandled exception has been thrown. The entire system could be massively unstable. None of your internal state invariants might be actually invariant in this scenario.
Look at it this way: I mutated global state. I then got an unexpected, unhandled exception. I know, I think I'll mutate global state again! That'll help! Seems like a very, very bad idea.
Of course, it depends on what the mutation to global state is. If it is "start reporting errors to the user again" as it is in the compiler then the correct thing to do for an unhandled exception is to start reporting errors to the user again: after all, we're going to need to report the error that the compiler just had an unhandled exception!
If on the other hand the mutation to global state is "unlock the resource and allow it to be observed and used by untrustworthy code" then it is potentially a VERY BAD IDEA to automatically unlock it. That unexpected, unhandled exception might be evidence of an attack on your code, from an attacker who is dearly hoping that you are going to unlock access to global state now that it is in a vulnerable, inconsistent form.
I'm going to disagree with Eric: when to do this or not depends on the circumstances. At one point, I was reworking my a large code base to include acquire/release semantics around all accesses to a custom image class. Images were originally allocated in unmoving blocks of memory, but we now had the ability to put the images into blocks that were allowed to be moved if they hadn't been acquired. In my code, it is a serious bug for a block of memory to have slipped past unlocked.
Therefore, it is vital to enforce this. I created this class:
public class ResourceReleaser<T> : IDisposable
{
private Action<T> _action;
private bool _disposed;
private T _val;
public ResourceReleaser(T val, Action<T> action)
{
if (action == null)
throw new ArgumentNullException("action");
_action = action;
_val = val;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
~ResourceReleaser()
{
Dispose(false);
}
protected virtual void Dispose(bool disposing)
{
if (_disposed)
return;
if (disposing)
{
_disposed = true;
_action(_val);
}
}
}
which allows me to do make this subclass:
public class PixelMemoryLocker : ResourceReleaser<PixelMemory>
{
public PixelMemoryLocker(PixelMemory mem)
: base(mem,
(pm =>
{
if (pm != null)
pm.Unlock();
}
))
{
if (mem != null)
mem.Lock();
}
public PixelMemoryLocker(AtalaImage image)
: this(image == null ? null : image.PixelMemory)
{
}
}
Which in turn lets me write this code:
using (var locker = new PixelMemoryLocker(image)) {
// .. pixel memory is now locked and ready to work with
}
This does the work I need and a quick search tells me I needed it in 186 places that I can guarantee won't ever fail to unlock. And I have to be able to make this guarantee - to do otherwise could freeze a huge chunk of memory in my client's heap. I can't do that.
However, in another case where I do work handling encryption of PDF documents, all strings and streams are encrypted in PDF dictionaries except when they're not. Really. There are a tiny number of edge cases wherein it is incorrect to encrypt or decrypt the dictionaries so while streaming out an object, I do this:
if (context.IsEncrypting)
{
crypt = context.Encryption;
if (!ShouldBeEncrypted(crypt))
{
context.SuspendEncryption();
suspendedEncryption = true;
}
}
// ... more code ...
if (suspendedEncryption)
{
context.ResumeEncryption();
}
so why did I choose this over the RAII approach? Well, any exception that happens in the ... more code ... means that you are dead in the water. There is no recovery. There can be no recovery. You have to start over from the very beginning and the context object needs to be reconstructed, so it's state is hosed anyway. And by comparison, I only had to do this code 4 times - the possibility for error is way, way less than in the memory locking code, and if I forget one in the future, the generated document is going to be broken immediately (fail fast).
So pick RAII when you absolutely positively HAVE to have the bracketed call and can't fail.
Don't bother with RAII if it is trivial to do otherwise.
If you need to control a scoped operation, I would add a method which take an Action<Jumper> to contain the required operations on the jumper instance:
public static void Jump(Action<Jumper> jumpAction)
{
StartJumping();
Jumper j = new Jumper();
jumpAction(j);
EndJumping();
}
An alternative approach that would work in some circumstances (i.e. when the actions can all happen at the end) would be to create a series of classes with a fluent interface and some final Execute() method.
var sequence = StartJumping().Then(some_other_method).Then(some_third_method);
// forgot to do EndJumping()
sequence.Execute();
Execute() can chain back down line and enforce any rules (or you can build the closing sequence as you build the opening sequence).
The one advantage this technique has over others is that you aren't limited by scoping rules. e.g. if you want to build the sequence based on user inputs or other asynchronous events you can do that.
Jeff,
what you're trying to achieve is generally referred to as Aspect Oriented Programming (AOP). Programming using AOP paradigms in C# is not easy - or reliable... yet. There are some capabilities built directly into the CLR and .NET framework that make AOP possible is certain narrow cases. For example, when you derive a class from ContextBoundObject you can use ContextAttribute to inject logic before/after method calls on the CBO instance. You can see examples of how this is done here.
Deriving a CBO class is annoying and restrictive - and there is another alternative. You can use a tool like PostSharp to apply AOP to any C# class. PostSharp is far more flexible than CBOs because it essentially rewrites your IL code in a postcompilation step. While this may seem a bit scary, it's very powerful because it allows you to weave in code in almost any way you can imagine. Here's a PostSharp example built on your use scenario:
using PostSharp.Aspects;
[Serializable]
public sealed class JumperAttribute : OnMethodBoundaryAspect
{
public override void OnEntry(MethodExecutionArgs args)
{
Jumper.StartJumping();
}
public override void OnExit(MethodExecutionArgs args)
{
Jumper.EndJumping();
}
}
class SomeClass
{
[Jumper()]
public bool SomeFunction() // StartJumping *magically* called...
{
// do some code...
} // EndJumping *magically* called...
}
PostSharp achieves the magic by rewriting the compiled IL code to include instructions to run the code that you've defined in the JumperAttribute class' OnEntry and OnExit methods.
Whether in your case PostSharp/AOP is a better alternative than "repurposing" the using statement is unclear to me. I tend to agree with #Eric Lippert that the using keyword obfuscates important semantics of your code and imposes side-effects and semantic menting to the } symbol at the end of a using block - which is unexpected. But is this any different than applying AOP attributes to your code? They also hide important semantics behind a declarative syntax ... but that's sort of the point of AOP.
One point where I whole-heartedly agree with Eric is that redesigning your code to avoid global state like this (when possible) is probably the best option. Not only does it avoid the problem of enforcing correct usage, but it can also help avoid multithreading challenges in the future - which global state is very susceptible to.
I don't actually see this as an abuse of using; I'm using this idiom in different contexts and never had problems... especially given that using is only a syntactic sugar. One way I use it to set a global flag in one of third party libraries I use, so that the change is reverted when finishing operations:
class WithLocale : IDisposable {
Locale old;
public WithLocale(Locale x) { old = ThirdParty.Locale; ThirdParty.Locale = x }
public void Dispose() { ThirdParty.Locale = old }
}
Note you don't need to assign a variable in using clause. This is enough:
using(new WithLocale("jp")) {
...
}
I slightly miss C++'s RAII idiom here, where the destructor is always called. using is the closest you can get in C#, I guess.
We have done almost exactly what you propose as a way to add method trace logging in our applications. Beats having to make 2 logging calls, one for entering and one for exiting.
Would having an abstract base class be helpful? A method in the base class could call StartJumping(), the implementation of the abstract method that child classes would implement, and then call EndJumping().
I like this style and frequently implement it when I want to guarantee some tear down behaviour: often it is much cleaner to read than a try-finally. I don't think you should bother with declaring and naming the reference j, but I do think you should avoid calling the EndJumping method twice, you should check if it's already been disposed. And with reference to your unmanaged code note: it's a finalizer that's typically implemented for that (although Dispose and SuppressFinalize are typically called to free up the resources sooner.)
I've commented on some of the answers here a bit about what IDisposable is and isn't, but I will reiterate the point that IDisposable is to enable deterministic cleanup, but does not guarantee deterministic cleanup. i.e. It's not guaranteed to be called, and only somewhat guaranteed when paired with a using block.
// despite being IDisposable, Dispose() isn't guaranteed.
Jumper j = new Jumper();
Now, I'm not going to comment on your use of using because Eric Lippert did a far better job of it.
If you do have an IDisposable class without requiring a finalizer, a pattern I've seen for detecting when people forget to call Dispose() is to add a finalizer that's conditionally compiled in DEBUG builds so that you can log something whenever your finalizer is called.
A realistic example is a class that's encapsulates writing to a file in some special way. Because MyWriter holds a reference to a FileStream which is also IDisposable, we should also implement IDisposable to be polite.
public sealed class MyWriter : IDisposable
{
private System.IO.FileStream _fileStream;
private bool _isDisposed;
public MyWriter(string path)
{
_fileStream = System.IO.File.Create(path);
}
#if DEBUG
~MyWriter() // Finalizer for DEBUG builds only
{
Dispose(false);
}
#endif
public void Close()
{
((IDisposable)this).Dispose();
}
private void Dispose(bool disposing)
{
if (disposing && !_isDisposed)
{
// called from IDisposable.Dispose()
if (_fileStream != null)
_fileStream.Dispose();
_isDisposed = true;
}
else
{
// called from finalizer in a DEBUG build.
// Log so a developer can fix.
Console.WriteLine("MyWriter failed to be disposed");
}
}
void IDisposable.Dispose()
{
Dispose(true);
#if DEBUG
GC.SuppressFinalize(this);
#endif
}
}
Ouch. That's quite complicated, but this is what people expect when they see IDisposable.
The class doesn't even do anything yet but open a file, but that's what you get with IDisposable, and the logging is extremely simplified.
public void WriteFoo(string comment)
{
if (_isDisposed)
throw new ObjectDisposedException("MyWriter");
// logic omitted
}
Finalizers are expensive, and the MyWriter above doesn't require a finalizer, so there's no point adding one outside of DEBUG builds.
With the using pattern I can just use a grep (?<!using.*)new\s+Jumper to find all places where there might be a problem.
With StartJumping I need to manually look at each call to find out if there is a possibility that an exception, return, break, continue, goto etc can cause EndJumping to not be called.
I dont think you want to make those methods static
You need to check in the dispose if end jumping has alredy been called.
If I call start jumping again what happens?
You could use a reference counter or a flag to keep track of the state of the 'jumping'. Some people would say IDisposable is only for unmanaged resources but I think this is ok. Otherwise you should make start and end jumping private and use a destructor to go with the constructor.
class Jumper
{
public Jumper() { Jumper.StartJumping(); }
public ~Jumper() { Jumper.EndJumping(); }
private void StartJumping() {...}
public void EndJumping() {...}
}