Let's say I have a class with a object field. When Dispose() is called I would like to clear the reference to that object. The private field can only be set once, so ideally I would like it to be readonly, but if it is readonly there is a compile time error when I try to release the reference to the object during Dispose(). Ideally I would like to have a safe dispose AND mark the _value field as readonly. Is this possible or even necessary?
public class Foo : IDisposable
{
public Foo(object value)
{
_value = value;
}
public object Value { get { return _value; } }
private readonly object _value;
public void Dispose()
{
//Cleanup here
_value = null // causes compile time error
}
}
That's not necessary, even if it were possible.
Dispose is typically intended to clean up unmanaged resources, although there can be exceptions to that rule (note the comments). However, in this situation, you should allow the garbage collector to do its job. It will run indeterminately once the objects are considered to have no active roots. Under normal circumstances, you do not need to take action to force it. Just write your code with objects in properly limited scope and you will be fine.
Setting the reference to null does not in fact do anything. The garbage collector will clean up the object when there are no longer any references to it, which in this case since you are doing this in the Dispose method presumably the instance of Foo is about to no longer have any references to it and the difference in timing is probably not meaningful. Typically you implement the Dispose pattern because your type has as a member a class that itself implements IDisposable (in this case the type is Object, which does not implement IDisposable), or you have unmanaged resources that you would like to release deterministically. You can find a description of the Dispose pattern here. Note that if you create a readonly member variable of a type that implements IDisposable you can call the Dispose method on that object inside your Dispose method:
public class SomeClass : IDisposable
{
private Boolean mDisposed;
private readonly MemoryStream mStream = new MemoryStream(); // Could be any class that implements IDisposable
public void Dispose() {
Dispose(true);
GC.SuppressFinalize(this);
}
protected void Dispose(Boolean disposing) {
if (disposing & !mDisposed) {
mStream.Dispose(); // Could and should call Dispose
mDisposed = true;
}
return;
}
}
This works because the readonly nature is on the reference to the object, not the object itself.
That is not necessary nor correct. Wanting to do what you ask in your question seems to indicate you are somewhere accessing a disposed object, and that is wrong.
Rather than try to do what you have asked, you should perhaps implement IsDisposed (which btw is part of the standard Dispose pattern) and check it first.
As others have noted, the Dispose pattern is intended for releasing unmanaged resources.
This is an older post, but I face the same question regularly because I unsubscribe from objects that I subscribed to.
I do this unsubscription during dispose because those objects live longer than this object, and the way delegates are implemented means that the publisher keeps the subscriber alive, not the other way around like we would think.
BUT, when you unsubscribe from the events, you have the natural tendency to want to clear the reference to the publisher too, which really isn't such a big deal. I suggest you keep the 'readonly' constraint because it makes the rules clearer and the code more robust while the object is still alive. If it means you sit with the reference after dispose, that is okay becuase the object is now collectible.
Related
I have a class with a destructor that I get a null reference exception from because the variable I destroy is sometimes null.
Is this an appropriate use of the null conditional operator to be in the destructor?
I'm not even sure if this is an appropriate use of a destructor itself since it is not used to dispose of the actual object it's called on but rather a variable of it.
~clsSAPSettings()
{
mtbTemp?.Close();
}
This code was converted from VB6 so I'm trying to figure out how to handle this issue. Any information welcome.
Edit: The class mtbTemp belongs to implements IDisposable but doesn't have a finaliser/desctructor. It simply closes a connection used in an ORM model.
For anyone after a detailed explanation I found a great answer, Proper use of the IDisposable interface, it goes into detail about the use of finalizer and how garbage collection actually works.
Please do not use any fields of reference types in the finalizer: the order in which GC (Garbage Collector) collects them is unpredictable and that's why
~clsSAPSettings()
{
mtbTemp?.Close();
}
code can well be performed by GC as followes:
Collect mtbTemp instance
Start collecting this instance:
Call ~clsSAPSettings()
Call mtbTemp?.Close(); i.e. call a method of the collected (destroyed) instance
and you'll have an unstable hard to find error. It seems that you are looking for a IDisposable interface:
public class clsSAPSettings: IDisposable {
private MyTemp mtbTemp;
...
protected virtual void Dispose(bool disposing) {
if (disposing) {
mtbTemp?.Close();
GC.SuppressFinalize(this);
}
}
public void Dispose() {
Dispose(true);
}
//TODO: do you really want finalizer?
~clsSAPSettings() {
Dispose(false);
}
}
Consider:
~clsSAPSettings()
{
mtbTemp?.Close();
}
The problem here isn't the null conditional usage. That doesn't by itself present any issues.
The biggest problem is that in a finalizer you should not touch any other object. When the finalizer fires, your object is toast. You no longer have any guarantees about the life of mtbTemp, including whether or not it has already been garbage collected, so you should not touch it. It might work; it might cause a temporary resurrection, or it might crash horribly.
The correct place to do something like this is in IDisposable.Dispose. In the Dispose method, that would be absolutely fine:
public void Dispose() // where your class : IDisposable
{
mtbTemp?.Close();
mtbTemp = null;
}
You probably don't need a finalizer at all. They are incredibly rare.
When you are closing streams or other unmanaged objects, you should use the Dispose pattern, rather than a destructor. You never know when that destructor will fire.
Regarding the use of the null conditional operator in a destructor: I see no problem with the operator itself. I do with referencing other objects that might already been destructed or in destruction.
Is it legal to call a method on disposed object? If yes, why?
In the following demo program, I've a disposable class A (which implements IDisposable interface).As far as I know, if I pass disposable object to using() construct, then Dispose() method gets called automatically at the closing bracket:
A a = new A();
using (a)
{
//...
}//<--------- a.Dispose() gets called here!
//here the object is supposed to be disposed,
//and shouldn't be used, as far as I understand.
If that is correct, then please explain the output of this program:
public class A : IDisposable
{
int i = 100;
public void Dispose()
{
Console.WriteLine("Dispose() called");
}
public void f()
{
Console.WriteLine("{0}", i); i *= 2;
}
}
public class Test
{
public static void Main()
{
A a = new A();
Console.WriteLine("Before using()");
a.f();
using ( a)
{
Console.WriteLine("Inside using()");
a.f();
}
Console.WriteLine("After using()");
a.f();
}
}
Output (ideone):
Before using()
100
Inside using()
200
Dispose() called
After using()
400
How can I call f() on the disposed object a? Is this allowed? If yes, then why? If no, then why the above program doesn't give exception at runtime?
I know that the popular construct of using using is this:
using (A a = new A())
{
//working with a
}
But I'm just experimenting, that is why I wrote it differently.
Disposed doesn't mean gone. Disposed only means that any unmanaged resource (like a file, connection of any kind, ...) has been released. While this usually means that the object doesn't provide any useful functionality, there might still be methods that don't depend on that unmanaged resource and still work as usual.
The Disposing mechanism exist as .net (and inheritly, C#.net) is a garbage-collected environment, meaning you aren't responsable for memory management. However, the garbage collector can't decide if an unmanaged resource has been finished using, thus you need to do this yourself.
If you want methods to throw an exception after the object has been diposed, you'll need a boolean to capture the dispose status, and once the object is disposed, you throw the exception:
public class A : IDisposable
{
int i = 100;
bool disposed = false;
public void Dispose()
{
disposed = true;
Console.WriteLine("Dispose() called");
}
public void f()
{
if(disposed)
throw new ObjectDisposedException();
Console.WriteLine("{0}", i); i *= 2;
}
}
The exception is not thrown because you have not designed the methods to throw ObjectDisposedException after Dispose has been called.
The clr does not automagically know that it should throw ObjectDisposedException once Dispose is called. It's your responsibility to throw an exception if Dispose has released any resources needed for successful execution of your methods.
A typical Dispose() implementation only calls Dispose() on any objects that it stores in its fields that are disposable. Which in turn release unmanaged resources. If you implement IDisposable and not actually do anything, like you did in your snippet, then the object state doesn't change at all. Nothing can go wrong. Don't mix up disposal with finalization.
The purpose of IDisposable is to allow an object to fix the state of any outside entities which have, for its benefit, been put into a state that is less than ideal for other purposes. For example, an Io.Ports.SerialPort object might have changed the state of a serial port from "available for any application that wants it" to "only usable by one particular Io.Ports.SerialPort object"; the primary purpose of SerialPort.Dispose is to restore the state of the serial port to "available for any application".
Of course, once an object that implements IDisposable has reset entities that had been maintaining a certain state for its benefit, it will no longer have the benefit of those entities' maintained state. For example, once the state of the serial port has been set to "available for any application", the data streams with which it had been associated can no longer be used to send and receive data. If an object could function normally without outside entities being put into a special state for its benefit, there would be no reason to leave outside entities in a special state in the first place.
Generally, after IDisposable.Dispose has been called on an object, the object should not be expected to be capable of doing much. Attempting to use most methods on such an object would indicate a bug; if a method can't reasonably be expected to work, the proper way to indicate that is via ObjectDisposedException.
Microsoft suggests that nearly all methods on an object which implements IDisposable should throw ObjectDisposedException if they are used on an object which has been disposed. I would suggest that such advice is overbroad. It is often very useful for devices to expose methods or properties to find out what happened while the object was alive. Although one could give a communications class a Close method as well as a Dispose method, and only allow one to query things like NumberOfPacketsExchanged after a close but not after a Dispose, but that seems excessively complicated. Reading properties related to things that happened before an object was Disposed seems a perfectly reasonable pattern.
Calling Dispose() doesn't set the object reference to null, and your custom disposable class doesn't contain any logic to throw an exception if its functions are accessed after Dispose() has been called so it is of course legal.
In the real world, Dispose() releases unmanaged resources and those resources will be unavailable thereafter, and/or the class author has it throw ObjectDisposedException if you try to use the object after calling Dispose(). Typically a class-level boolean would be set to true within the body of Dispose() and that value checked in the other members of the class before they do any work, with the exception being thrown if the bool is true.
A disposer in C# is not the same as a destructor in C++. A disposer is used to release managed (or unmanaged) resources while the object remains valid.
Exceptions are thrown depending on the implementation of the class. If f() does not require the use of your already disposed objects, then it doesn't necessarily need to throw an exception.
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.
PROBLEM DESCRIPTION:
I have this class (FooClass) that implements IDisposable which takes another disposable (OtherDisposableClass) as an argument of its constructor.
I create an instance of OtherDisposableClass
I create an instance of FooClass and pass the instance OtherDisposableClass to it
The instance of OtherDisposableClass should not be mutateable so I mark it readonly
I can't get rid of the reference to the instance of OtherDisposableClass in my Dispose method
QUESTION
What is the best thing to do here? Should we
Not mark the instance of OtherDisposableClass as readonly so we can set it to 'null' in the Dispose method
Mark OtherDisposableClass as readonly and do not remove the reference to it ever again.
Something else
Please elaborate your answer. Thank you
public class FooClass : IDisposable
{
private readonly OtherDisposableClass _disposable;
private readonly string _imageSource;
private readonly string _action;
private readonly string _destination;
private bool _isInitialized;
public FooClass(OtherDisposableClass disposable)
{
_disposable = disposable;
}
~FooClass()
{
// Finalizer calls Dispose(false)
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
// Not possible because _disposable is marked readonly
_disposable = null;
}
}
The thing that does the creation of OtherDisposableClass should be disposing of it... leave it alone in FooClass (you can have it readonly if you like).
Setting it to null will have no effect anyway, it will only set the reference from within FooClass to null (which is about to be disposed anyway).
EDIT
From the code given it looks like you don't even need to implement IDisposable here - this is just over complicating things. If you created an instance of OtherDisposableClass within the constructor of FooClass then you could implement IDisposable and simply call OtherDisposableClass.Dispose() within the FooClass.Dispose().
Here FooClass is created with a dependency on OtherFooClass - OtherFooClass will outlive FooClass - therefore FooClass should be GC'd first and then there will be no reference to OtherDisposableClass remaining - which will allow it to be GC'd
The IDisposable interface allows you to release unmanaged resources in a deterministic way. Unmanaged resources can be file handles, database connections etc. Disposing an object does not release any memory. Memory is only reclaimed by the garbage collector and you have very little control of when and how that is done. Disposing an object does not free any memory.
Still there is a connection between garbage collection and IDisposable. If your class directly controls an unmanaged resource it should release this resource during finalization. However, having a finalizer on a class has a performance impact and should be avoided if possible. FooClass does not directly control an unmanaged resource and the finalizer is not needed. However, to enforce the deterministic release of unmanaged resources it is important the FooClass.Dispose calls OtherDisposableClass.Dispose when Dispose is called explictly (when disposing is true). If Dispose is called from the finalizer you should not call methods on other managed objects because these objects may already have been finalized.
So to sum it up:
Garbage collection and reclaiming of memory is distinct from releasing unmanaged resources.
The Dispose method should always release unmanaged resource.
The Dispose method should call Dispose on aggregated objects but only when called directly (not when called from the finalizer).
It should be possible to call Dispose multiple times without any problems.
There is no point in setting an IDisposable field to null in the Dispose method.
Avoid implementing finalizers if you do not control any unmanaged resources directly.
When implementing IDisposable you have to take inheritance into consideration because both the base class and the derived class may want to call Dispose on fields and/or release unmanaged resources. Sealing a class can avoid a lot of this complexity.
To elaborate on the point about setting the field to null:
I assume that your code is sane. E.g., only FooClass has a permanent reference to OtherDisposableClass and both objects will after disposal be unusable (e.g. throw ObjectDisposedException in all public methods except Dispose).
If you set the reference to OtherDisposableClass to null then that instance becomes eligible for garbage collection because there are no references to the instance. However, after disposing FooClass you surely will not keep an reference to that instance because it is unusable. This means that both the FooClass and the OtherDisposableClass are no longer reachable from a GC root and the entire object graph is now eligible for garbage collection. So there is no need to sever the link between FooClass and OtherDisposableClass because they will become eligible for garbage collection at the same time.
In VB.NET, if a type which implements IDisposable has any variables that are declared WithEvents, it is generally a good idea--and is sometimes very important--to set those variables to null in the Dispose method, since failure to do so will mean that objects to which those variables referred will be left holding back-references to the object that was disposed. That can cause serious memory leaks in the scenario where many short-lived objects subscribe to events from a long-lived object.
Although C# does not allow VB.NET-style WithEvents declarations, it's possible to code properties in C# which behave similarly:
// Handle a property Wobbler which identifies an object whose Wibbled event
// I want to handle with my WibbleWobble method.
Woozle _wobble;
Woozle Wobbler {
get { return _wobbler; }
set {
var wasWobbler = _wobbler
if (wasWobbler == value) return; // Don't unsubcribe and resubscribe same object
if (wasWobbler != null)
wasWobbler.Wibbled -= WibbleWobble; // Unsubscribe old event
if (value != null)
value.Wibbled += WibbleWobble; // Subscribe new event
_wobbler = value;
}
};
If one uses a pattern such as that (it can be a nice way to ensure subscribes get paired up with unsubscribes), one should set Wobbler to null in one's Dispose method, to ensure that any event that was attached to it gets detached.
Other than the aforementioned scenarios, it generally doesn't matter whether Dispose sets things to null, since the only thing one should be doing with the objects referred to by those fields is disposing them and cancelling their events, calling Dispose multiple times on a well-coded object should be harmless. There are a few cases, however, were setting a field to null in dispose may be helpful. Some collections store data using an array along with an indication of which slots hold meaningful data; if a collection "removes" an item from the array by indicating a slot doesn't hold anything meaningful, but doesn't null out the reference, that item and anything to which it holds a direct or indirect reference will be kept alive by the GC. If disposed objects null out their references to other objects, the amount of data kept alive by the collection may be minimized.
Incidentally, note that the code above is not thread safe, and cannot very cleanly be made thread-safe without locking, even if the subscribe/unsubscribe methods are thread safe and one uses only accesses _wobbler via Interlocked methods. If _wobbler is updated before subscriptions/unsubscriptions are processed, then if Wobbler is to some object George in one thread and to something else in another, the second thread might send George an unsubscribe request before the first thread has sent its subscription request (resulting in a dangling subscription). If it's not updated before processing subscriptions/unsubscriptions, then two simultaneous attempts to set Wobbler to George could result in George being given two consecutive subscription requests (which could eventually result in a dangling subscription).
You are correct in setting you member IDisposable to null in Dispose(bool disposing), see: https://msdn.microsoft.com/en-us/library/ms244737.aspx
As you cannot if the field is marked readonly the solution is I am afraid to remove the readonly.
I have an object that has a disposable object as a member.
public class MyClass
{
private MyDisposableMember member;
public DoSomething
{
using (member = new MyDisposableMember())
{
// Blah...
}
}
}
There can be many methods in MyClass, all requiring a using statement. But what if I did this instead?
public class MyClass
{
private MyDisposableMember member = new MyDisposableMember();
public DoSomething
{
// Do things with member :)
}
~MyClass()
{
member.Dispose();
}
}
As you can see, member is being disposed in the destructor. Would this work? Are there any problems with this approach?
Ideally, Dispose() should have already been called prior to finalization. It would be better to follow the typical dispose pattern, and allow the user to Dispose() the object properly, and have the finalizer Dispose of it if dispose has not already been called.
In this case, since you're encapsulating an IDisposable, you really don't need to implement the finalizer at all, though. (At the the point of finalization, your encapsulated member will get finalized, so there's no need to finalize your object - it just adds overhead.) For details, read this blog article I wrote on encapsulating an IDisposable.
You should probably make MyClass implement IDisposable. Inside the Dispose() method, call member.Dispose();. That way the programmer can have control over when the member gets disposed.
DO NOT DO THAT!
The GC will do that for you (indirectly as the object to dispose or another one will contain a destructor)
MyDisposableMember might even be disposed by the GC even before you dispose it - what happens then might not be what you intended to do.
Even worse: Adding a destructor (or finalizer) to a class costs additional time when disposing of the object (much more time as the object will stay in memory for at least one collection cyclus and maybe even promoted to the next generation).
Therfore, it would be completely useless and even backfire.
In your first example the member is not really part of the object's state since you're instantiating it every time it's used and disposing it right after. Since it's not part of the state don't model it as such just use a local variable when needed.
In more general you should put all disposal logic in Dispose() and implement IDisposable then use you class together with using or try-finally
The only thing I see wrong (and it isn't an error) is the fact that in a using statement you explicitly dispose of the object at that point in time (when your function / method is called). The destructor cannot be called they are invoked automatically. So at this point it may take some time for member to be disposed of. Better to implement the IDisposeable interface for MyClass.
Following the Microsoft pattern is your best bet so the users of your class have full control over when it is disposed.
public class MyClass : IDisposable
{
private MyDisposableMember member = new MyDisposableMember();
public DoSomething
{
// Do things with member :)
}
~MyClass()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (disposing) // Release managed resources
{
member.Dispose();
}
// Release unmanaged resources
}
}
When a finalizer runs, one of the following will be true about almost any IDisposable object to which it holds a reference:
The object will have already had its finalizer run, in which case calling Dispose on the object will be at best useless.
The object will not have had its finalizer run, but its finalizer will be scheduled to run, so calling Dispose on the object will be useless.
The object will still be in use by something other than the object being finalized, so calling Dispose on it would be bad.
There are a few situations where calling Dispose in a finalizer might be useful, but most situations fit the cases listed above, which all have a common feature: the finalizer shouldn't call Dispose.