For example, if I want to cancel some operation in a Dispose() call (which can be called multiple times), then do I need to write
public void Dispose()
{
if (!cancellationTokenSource.IsCancellationRequested)
{
cancellationTokenSource.Cancel();
}
}
or is it enough with the simpler
public void Dispose()
{
cancellationTokenSource.Cancel();
}
(You are welcome to comment on whether it is wise or not to cancel things in a Dispose method, but that is not the point of this question.)
Yes.
But only if the CancellationTokenSource has not been disposed yet.
From the reference source:
ThrowIfDisposed();
// ...
// fast-path test to check if Notify has been called previously
if (IsCancellationRequested)
return;
This seems more a question about the Dispose pattern, then about CancellationToken or anything else. And I am uncertain if you implemented said pattern properly. Here is the official MS Document on the mater:
https://learn.microsoft.com/en-us/dotnet/standard/design-guidelines/dispose-pattern
And here my interpretation:
There is two levels of Disposing: Dispose and Finalizsation. As the code for both is very similar, often they are combiend into one function (in C# usually it is Dispose one). The main difference is if you relay it to contained classes. You always relay a Dispose call(the relay is usually what Dispose is about). You never relay a Finalization call (Finalisation is between that instance and the GC only).
There are also two cases: One in wich you handle Unmanaged resources directly. And one in wich you handle just another Disposeable class.
Unamanged resource directly
In this case the first thing you do is implement a Finalizer, so at least the GC can reliably clean this up. Then you implement IDisposeable as an additional feature so programmers can use stuff like the using pattern to have it cleaned up deterministic at runtime.
Handling something that implements IDisposeable
You have a resource that implements IDisposeable (say like a Filestream Reference). You implement IDisposeable in your class for the sole purpose of relaying the Dispose() call to said FileStream. This is the way more common case. It would guess it makes about 95-99% of all Dispose Implementations.
One thing to keep in mind here is that "Dispose" and "Finalize" often implies lower level cleanup. A SQLConenction you call dispose on will be closed first (if nessesary). A Filehandle you Dispose off will also first be closed. Even if calling cancellationTokenSource.Cancel was not repeatable, cancellationTokenSource.Dispose should call Cancel as part of it's operation and should be Repeatable. The class itself does implement IDisposeable. And if any class does, it is usually saver to just call Dispose rather then manually doing the cleanup manually via Cancel: https://learn.microsoft.com/en-us/dotnet/api/system.threading.cancellationtokensource?view=netframework-4.7.2
Related
A question for you all.
In my company, we are developing an application that runs inside Microsoft’s MVC framework.
The controller classes that we are implementing inherit from the MVC base class Controller. Example:
public class MyController: Controller
{
protected bool IsDisposed { get; set; }
… various methods…
}
The discussion we are having in the team centers around the Dispose() pattern. Essentially, this involves implementing the IDisposable interface, preferably according to the Microsoft-endorsed pattern.
See for example this link: http://msdn.microsoft.com/en-us/library/fs2xkftw%28v=vs.110%29.aspx
Interestingly, our controller classes do not own any resources, either managed or managed. As a result, the implementation of Dispose(bool) is greatly simplified:
protected override void Dispose(bool disposing)
{
IsDisposed = true;
base.Dispose(disposing);
}
There is some disagreement about the use (or need) of the IsDisposed property, that is used in the following method:
protected void ThrowIfDisposed()
{
if (IsDisposed) throw new ObjectDisposedException(“MyController”);
}
This method is then called at the beginning of every method that does "real" work. The thinking here is that a disposed object should not be used again, hence it should throw the ObjectDisposedException. The other opinion is that, since Dispose(bool) does “nothing” (other than setting the IsDisposed property and calling Dispose(bool) on the base class), the “disposed” object is not really in a state that is unusable, so there is no reason to throw. Therefore, there is no reason to even implement Dispose(bool).
An argument against this is that MyController should throw when it is disposed and one of its methods is called, so that its behaviour does not change should in future versions managed and/or unmanaged resources be added.
The argument against this last point is that MyController should never add any resources in future versions, but rather it should be derived from should the need to add resources occur in the future. An additional question is: why doesn’t the (library) class Controller implement ThrowIfDisposed() or something similar?
So, summarizing, faction one wants to implement Dispose(bool) and ThrowIfDisposed() as shown above, faction two thinks they are unnecessary and wants to do away with them.
I see merits in both viewpoints, cannot really make up my mind. Opinions?
Interestingly, our controller classes do not own any resources, either managed or managed.
Then you do not need the IDisposable pattern.
This method [ ThrowIfDisposed() ] is then called at the beginning of every method that does “real” work.
The qustion here is: Why?
If you do want to track a usable/abandoned state then just don't call it IsDisposed.
why doesn’t the (library) class Controller implement ThrowIfDisposed() or something similar?
Because it isn't useful.
Go back to the beginning: why did somebody think this was necessary? What is it for?
It seems you can just rip it out.
If an object receives a request after being disposed, the request should not fail for any reason other than ObjectDisposedException--meaning it shouldn't cause a NullReferenceException, yield garbage data, crash the system, or do any other such thing. That does not imply that the request shouldn't succeed--merely that ObjectDisposedException should be the only mode of failure.
As for IsDisposed, I can't really see much use for it in most cases. Code which doesn't know whether an object is disposed generally won't know much else enough about the object's state to know whether it might be unusable for some other reason; consequently, code that doesn't know whether an object is disposed generally shouldn't do anything with it except--in some cases--to Dispose it, and in most of those cases where one would want to call Dispose on an object if it hadn't already been disposed, one should simply call Dispose unconditionally. There could be some value in having a method MayNeedDisposing which would return true for objects with non-trivial Dispose methods until they've been disposed, but such a method could unconditionally return false for objects with null Dispose methods [in cases where a class would e.g. register a callback purely for the purpose of calling Dispose on something, testing MayNeedDisposing beforehand could be a useful optimization].
Finally, with regard to whether a class or interface should implement or inherit IDisposable, the issue is not whether the type itself acquire resources, but rather whether the last surviving reference to something that does acquire resources may be stored in that type. For example, even though the vast majority of IEnumerator<T> implementations don't acquire resources, some do, and in many such cases the only references to such objects will be stored in variables of type IEnumerator<T>, held by code that has no idea whether the objects in question have any resources or not. Note that non-generic IEnumerator doesn't implement IDisposable but some implementations acquire resources. As a consequence, code which calls IEnumerable.GetEnumerator will be required to call Dispose on the returned enumerator if it implements IDisposable. The fact that IEnumerator doesn't inherit IDisposable doesn't absolve code which receives an IEnumerator implementation that also implements IDisposable from the responsibility of calling Dispose--it merely makes it harder for such code to carry out that responsibility.
Faction two is absolutely right. Don't do it.
Implementing IDisposable in the way described and for the reasons given is a violation of at least three good principles.
You ain't gonna need it. http://c2.com/cgi/wiki?YouArentGonnaNeedIt
Do the simplest thing that could possibly work. http://www.xprogramming.com/Practices/PracSimplest.html
Keep it simple stupid. https://en.wikipedia.org/wiki/KISS_principle
It's just wrong on every level. Don't do it.
I am investigating a sporadic crash in my app and still not sure what is causing it. It happens sometimes when I want to close my MDI form that contains an VB 6.0 or maybe VC 6.0 Text control - its wrapper for C# as Interop. While looking at the code I noticed I have some code like this:
private void Dispose(bool disposing)
{
if (!this.disposed)
{
if (disposing) //Disposing of everything: managed and unmanaged resources.
{
moTextEditor.Dispose(); --here
moTextEditor.Dispose(); --here
}
// Deal with our own & whatever other unmanaged resources (this used to be done in the finalizer, above)
SetLockedFields(false);
disposed = true;
}
}
so noticedthat moTextEditor.Dispose(); line getting repeated two times? What do you think? Can this be the issue?
The IDisposable interface documentation for Dispose specifically states:
If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times. Instance methods other than Dispose can throw an ObjectDisposedException when resources are already disposed.
If the object implements IDisposable properly this should not be an issue.
That being said, not all implementations of IDisposable follow the rules. I personally would say that object has a bug and should be corrected if calling Dispose multiple times is problematic.
The repeated dispose looks like a mistake, but normally that shouldn't cause any problems.
If implemented correctly, an object can be disposed more than once without harm.
Can Calling Dispose() more than one time on an object lead to a crash?
Yes.
Does Calling Dispose() more than one time on an object always lead to a crash?
No.
Basically, it depends entirely on the implementation. The fact that an object implements IDisposable simply means that it has a Dispose method. There are many guidelines about how you should implement it, but there are virtually no absolute rules.
So now we ask:
Is it considered a good practice to ensure that multiple calls to Dispose for my IDisposable object don't throw exceptions, cause crashes, or otherwise break stuff?
Yes.
Does everyone actually follow that guideline?
No.
Is that actually my problem here?
*shrug * It's hard to say. We don't know how that object specifically implements Dispose.
BDotA,
Generally it wouldn't make your application to crash, but you need to consider what the Dispose() method of moTextEditor is doing.
I've read through this post about disposing of datasets and I still have a question about the destructor. I know that post basically says that you don't need to dispose of Datasets, Datatables, and Dataviews, but my dataset is MASSIVE, so I want to release that memory ASAP. So, my question, should I include a destructor even though the dataset will be disposed when my objects' dispose method is called? Also, explain to me again why the "bool disposing" is needed.
public DEditUtil(DataSet dsTxData)
{
this.dsTxData = dsTxData;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
dsTxData.Dispose();
disposed = true;
}
}
~DEditUtil()
{
Dispose(false);
}
Yes, in general you should implement the full IDisposable pattern whenever either of the following is true:
You have unmanaged resources being allocated by your class, or
You have managed resources that implement IDisposable (which implies that they, in turn, have unmanaged resources)
The presence of the finalizer (the general CLR term for what C++/C# call a "destructor") is to handle cases where your Dispose method is not called for some reason. The boolean value being passed in to your protected Dispose() method indicated if you are being called from within the public Dispose, or from within your finalizer.
If your public Dispose method is being called, that call stack is deterministic: your dispose method is being called directly, so you can safely call methods (including Dispose) on your child objects.
If you are inside of the finalizer, then you have no idea what's going on with other objects that are also being garbage-collected. In general, it may not be safe to call methods on managed objects your control from within your finalizer.
So, the boolean value basically says: "if true, dispose everything; if false, only dispose my unmanaged resources and let everyone else deal with theirs."
The memory used by your DataSet object will be available for garbage collection as soon as it is not referenced anymore by the code.
The garbage collector will make that memory available to the program at a later (non determinate) time.
Both things do not depend of having or not a destructor or calls to Dispose, so the answer is no - you don't need a destructor.
No, you do not need any other method call here, it's already enough what you did.
Dispose will be called by the runtime and you will free resource allocated, the cleanup let's leave up to GC to decide how and when to do it.
If you have really huge problems with memory you can try to cal GC.Collect() to enforce the collection of the garbage, that usually works, but it's never a good practise to use it in that way, so try to avoid it as much as possible.
EDIT
According to the comments, it's important to pay attention on execution flow in your case, cause the DataSet cleanup will be done only if it's not disposed==false and disposing == true, which from the code provided, will be a case only during esplicit call from the code.
Very seldom should user-written classes ever use finalizers (or C# destructors) for any purpose other than to log failures to call Dispose. Unless one is delving deep into the particulars of how finalizers work, and exactly what is or is not guaranteed guaranteed about the context in which they run, one should never call any other object's Dispose method within a finalizer. In particular, if one's object's Finalize() method is running, any IDisposable objects to which it holds a reference will usually fall into one of the following categories:
Someone else still has a reference to that object and expects it to be usable, so calling `Dispose` would be bad.
The object cannot be safely disposed within a finalizer thread context, so calling `Dispose` would be bad.
The object would have kept the present object alive if there were anything meaningful for its `Dispose` handler to do; the fact that the present object's `Finalize` method is running implies that there's no longer any need to call `Dispose` on the other object (this scenario can occur with events).
The object has already had its `Finalize` method called, so calling `Dispose` would be at best superfluous.
The object is scheduled to have its `Finalize` method called, so calling `Dispose` would likely be superfluous.
Although there are a few cases where an object might need to clean up another IDisposable object within a Finalize method, using Finalize properly in such cases is tricky, and using it improperly is apt to be worse than not using it at all. Among other things, Finalize generally only runs when an entity requests an IDisposable and wrongfully fails to call Dispose before abandoning it. It's usually better to focus one's efforts on making sure that Dispose gets properly before an object is abandoned, than on trying to properly handle buggy consumer code.
Lets say I have a class that associates itself with another class. It would look something like the following:
public class DisposableClassOne : IDisposable
{
private class mDisposableClassTwo;
public DisplosableClassOne(DisposableClassTwo dcTwoInjected)
{
mDisposableClassTwo = dcTwoInjected;
}
public void Dispose()
{
// Should I dispose here? or make caller dispose of dcTwoInjected
//mDisposableClassTwo.Dispose();
}
}
Should I call the Dispose method of mDisposableClassTwo or should I make the caller handle it like this?
using(DisposableClassTwo dcTwoInjected = new DisposableClassTwo())
using(DisposableClassOne dcOne = new DisposableClassOne(dcTwoInjected))
{
// do stuff with dcOne
}
I'm thinking making the caller handle it is the best way to go, but I think by putting the call in the Dispose method it guarantees that it will get called. Is there a better way to handle this?
If the class you are creating logically owns(1) the constructor injected resource then it should dispose of it internally. If it does not own the resource, then it should do nothing, and rely on the consumer to decide when it should be disposed.
If your class owns a reference to an unmanaged resource, you may also need to implement a finalizer (destructor) since there is no guarantee that anyone will ever call your Dispose method at all.
In general, you want to avoid cases where the caller must decide when they should dispose of an object passed to the constructor of a class. It's possible for the caller to dispose of the resource prematurely (or hold on to it longer than necessary). This is not always an easy thing to achieve in certain designs ... sadly, the Disposable Object pattern doesn't always compose cleanly with itself.
(1) By ownership I mean that your class controls the lifetime of the resource which it is passed and no other code holds a reference to it. If any other code (whose lifetime is not tied to the lifetime of your class) holds a reference to the resource and will use it independently, then you are not the owner of that resource.
I think it is a life-time decision you have to make in your design.
Is DisposableClassOne Disposable BECAUSE it references a DisposableClassTwo?
And,
Does a DisposableClassTwo have a lifetime independent of DisposableClassOne?
To me, the answers to these two questions varies in each class design. The StreamReader/Writer is a great example of the first question being 'yes' and the second 'no' - no one would expect to use a Stream inside a StreamReader once the Reader is done with it, so Reader disposes it.
But what if DisposableClassTwo was some other resource - maybe a file refernece you passed to several classes in turn to 'do something to'. In that case, you don't want it disposed until you're ready and DisposableClassOne might be long gone by then.
The standard I'm familiar with is to use the Disposable pattern. Have a virtual Dispose(bool disposing) method. The Dispose() method calls Dispose(true) and the finalizer calls Dispose(false).
Then, in the main disposal method:
if (disposing)
{
// Dispose of the referenced object, as well.
}
StreamWriter & StreamReader follow this pattern. If you explicitely call Dispose(), they also dispose the underlying Stream. If you let the finalizer do your cleanup, they leave it alone.
Alternatively, you could add a property: DisposeChild, if true, dispose the child object, if not, leave it alone.
I also agree with rcravens. Under almost all circumstances, an object should be disposed within the same scope where it was instantiated.
To use IDisposable objects properly, one must follow the principle "Would the last one out please turn off the lights". There are a few nice scenarios, and an icky one:
The creator of an object gives it to another object (the "recipient") but the creator of the first object controls the recipient's lifetime, and can know when the recipient will no longer need the passed object. In that case, the creator of the original object takes care of Dispose.
The creator of an object gives it to the recipient, and the recipient knows that nobody else will use the object once it's been handed over. In that case, the recipient will call Dispose when it's done with the object.
An object recipient may sometimes be used in ways that match #1 and sometimes in ways that match #2, but the supplier of the object will know when it's given which situation applies. This situation may be handled by telling the recipient whether or not the recipient should Dispose of the object it is given.
The supplier and recipient will both want to use the object, but may have no idea which is going to outlive the other. This is the toughest situation.
I tend to like the approach suggested in #3. Sometimes situation #4 applies, though. One way of handling that is for the recipient of the object to fire an event when it's done with the object. That event can Interlocked.Decrement a counter indicating how many objects are still using the passed object. Once that counter hits zero, the object may be deleted.
It is better not to dispose any external references, those would be disposed automatically by the caller itself.
See this thread, in which it is suggested that only member variable needs to be disposed:
Finalize/Dispose pattern in C#
The concept of "cleanup by owner" is far too common to be ignored. Ownership must be more than just coincidence.
If you want software that is easier to maintain, avoid side effects that may be incorrectly interpreted. The Stream vs StreamReader example is a great one... both are IDisposable, and as the caller, I'm making both, so in no way should I ever give myself a pass at disposing either of them. Doing so would violate the abstraction, tying my calling code's knowledge to the particulars of the stream class' implementation (which could change over time, academically).
The easiest answer is to have the two usings(), even if you then go and make (and document!) that the second IDisposable will dispose of the first.
Which one do I call?
Is it necessary to call both?
Will the other throw an exception if I have already called one of them?
Close() and Dispose(), when called on a MemoryStream, only serve to do two things:
Mark the object disposed so that future accidental usage of the object will throw an exception.
Possibly1 release references to managed objects, which can make the GC's job a bit easier depending on the GC implementation. (On today's GC algorithms it makes no real difference, so this is a point for an academic discussion and has no significant real-world impact.)
MemoryStream does not have any unmanaged resources to dispose, so you don't technically have to dispose of it. The effect of not disposing a MemoryStream is roughly the same thing as dropping a reference to a byte[] -- the GC will clean both up the same way.
Which one do I call? Is it necessary to call both?
The Dispose() method of streams delegate directly to the Close() method2, so both do exactly the same thing.
Will the other throw an exception if I have already called one of them?
The documentation for IDisposable.Dispose() specifically states it is safe to call Dispose() multiple times, on any object3. (If that is not true for a particular class then that class implements the IDisposable interface in a way that violates its contract, and this would be a bug.)
All that to say: it really doesn't make a huge difference whether you dispose a MemoryStream or not. The only real reason it has Close/Dispose methods is because it inherits from Stream, which requires those methods as part of its contract to support streams that do have unmanaged resources (such as file or socket descriptors).
1 Mono's implementation does not release the byte[] reference. I don't know if the Microsoft implementation does.
2 "This method calls Close, which then calls Stream.Dispose(Boolean)."
3 "If an object's Dispose method is called more than once, the object must ignore all calls after the first one."
None of the above. You needn't call either Close or Dispose.
MemoryStream doesn't hold any unmanaged resources, so the only resource to be reclaimed is memory. The memory will be reclaimed during garbage collection with the rest of the MemoryStream object when your code no longer references the MemoryStream.
If you have a long-lived reference to the MemoryStream, you can set that reference to null to allow the MemoryStream to be garbage collected. Close and Dispose free neither the steam buffer nor the MemoryStream object proper.
Since neither Stream nor MemoryStream have a finalizer, there is no need to call Close or Dispose to cause GC.SuppressFinalize to be called to optimize garbage collection. There is no finalizer to suppress.
The docs for MemoryStream put it this way:
This type implements the IDisposable interface, but does not actually have any resources to dispose. This means that disposing it by directly calling Dispose() or by using a language construct such as using (in C#) or Using (in Visual Basic) is not necessary.
You can use the using block for this. It will automatically call Dispose when it goes outside of its scope.
Example:
using (MemoryStream ms = new MemoryStream())
{
// Do something with ms..
}
// ms is disposed here
Hope this helped.
Use using block so that your object is disposed if its implements IDisposable interface
the following code is Stream.Dispose from reflector as you can see, you don't need to close if you dispose (which is implicit when using using)
public void Dispose()
{
this.Close();
}
Which one do I call?
Any of them.
Is it necessary to call both?
No, either one is sufficient.
Will the other throw an exception if I have already called one of them?
No, disposable pattern declares that subsequent calls to Dispose don't cause negative effects.
Calling Close() will internally call Dispose() to release the resources.
See this link for more information:
msdn
Calling only Dispose() will do the trick =)
In .NET 3.5 (haven't checked other versions), methods are called in the following order when disposing a MemoryStream:
Stream.Dispose()
simply calls Close
Stream.Close()
calls Dispose(true), then GC.SuppressFinalize(this)
MemoryStream.Dispose(true)
sets _isOpen , _writable, and _expandable flags to false
Stream.Dispose(true)
closes async event if active
As a first solution, it's recommended to use using statements wherever possible. This is described here: http://msdn.microsoft.com/en-us/library/yh598w02.aspx
When the lifetime of an IDisposable object is limited to a single method, you should declare and instantiate it in the using statement. The using statement calls the Dispose method on the object in the correct way, and (when you use it as shown earlier) it also causes the object itself to go out of scope as soon as Dispose is called. Within the using block, the object is read-only and cannot be modified or reassigned.
Coming to the question now, as others suggested in most .NET framework classes, there is no difference between Close() and Dispose() and it doesn't matter which of the two methods you call. You should call one but not both. However, there are exceptions.
There are exceptions; for example, System.Windows.Forms.Form and System.Data.SqlClient.SqlConnection have different behavior for Close() and Dispose().
For complete details, you can check here: https://blogs.msdn.microsoft.com/kimhamil/2008/03/15/the-often-non-difference-between-close-and-dispose/