HOW do i know when i need to dispose of something? Someone just mention i had several objects in my code that i need to dispose of. I had no idea i needed to dispose anything (this is my first week with C#). How do i know when i need to dispose an object? i was using http://msdn.microsoft.com/en-us/library/system.security.cryptography.hashalgorithm.aspx and i do not see any mention of dispose on the page or seen it mention in any other objs i was told i to dispose (by someone on SO).
I know i need to when something inherits IDisposable but HOW do i KNOW when it does inherit it?
Similar questions here:
When should I dispose my objects in .NET?
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Identify IDisposable objects
You should dispose anything that implements IDisposable. Just wrap it on an using:
using(var some = new Something())
{
//use normally
}
An easy way would be to type obj.disp and see if intellisense has a dispose method.
The class implements the interface IDisposable, that means that it has a Dispose method.
Not every class that implements IDisposable requires you to call Dispose, but most of them do. If you see that the class implements IDisposable (or has a Dispose method because it inherits the interface from a base class), you have two choises:
Dig deep in the documentation to find out why the class implements IDisposable, and if you really need to call Dispose.
Just call Dispose.
Either method is safe. If the Dispose method doesn't do anything, the call will be very quick. You can even call Dispose more than once without harm.
Even better then just calling the Dispose method is to use a using block:
using (FileStream s = File.OpenRead(path)) {
...
}
At the end bracket of the block the Dispose method is called automatically. The using block is implemented as a try...finally, so the Dispose method is guaranteed to be called even if an exception occurs in the block.
If an class implements IDisposable you should dispose of intances of that class. If it doesn't you don't. In this case HashAlgorithm derives from ICryptoTransform which derives from IDisposable. This means all instance of classes descending from HashAlgorithm must be disposed.
You should dispose of any object that implements the IDisposable interface.
public abstract class HashAlgorithm : ICryptoTransform,
IDisposable
Anything that has unmanaged resources (DB connections for example) should implement the IDisposable interface.
There are a couple of good reasons for this:
You know that the unmanaged resources (which are typically quite scarce) are going to be cleaned up. Usually these will be cleared up in the finalizer anyway, but due to how the GC has to tidy up objects with finalizers this could take a while.
If you implement the standard dispose pattern you save the GC a lot of work as it doesn't need to call the finalizer.
I know i need to when something
inherits IDisposable but HOW do i KNOW
when it does inherit it?
Assuming you're using Visual Studio. I usually right click on the type, then "Go To Definition". If I see that it, or any of its super classes, implement IDisposable, I make sure I call Dispose on it. This is typically done by wrapping it in a using block as others have mentioned.
"Will the last person to leave the room please turn out the lights?"
An object which implements IDisposable holds the information and impetus necessary to do some "clean-up" operations that should happen "sometime", but which can't happen while the object is still in use. If the object is totally abandoned, those clean-up operations won't happen. The system includes a custodian, with which objects can register when they are created; if an object is abandoned by absolutely everyone but the custodian, the custodian can ask the object to perform its cleanup actions before the custodian too abandons it. Note that for a variety of reasons, the custodian isn't 100% effective at handling abandoned objects. It is thus very desirable that, whenever possible, the last entity to hold a useful reference to an object dispose of it before abandoning the reference.
Related
I have a class hierarchy, each member of which may create IDisposable objects.
I added a List<IDisposable> property to the base class in this hierarchy, to which I add any disposable objects on creation. The root Dispose method iterates through this list and calls Dispose for each item in its list and clears the list. In the application, I explicitly call the top object's Dispose method, causing disposal to cascade through the hierarchy.
This works, but is there a better way? Am I unwittingly duplicating some functionality already present in the framework?
(Note - the objects in question have a lifetime that precludes just wrapping them in a using block or disposing of them in the same methodwhere they are created.)
Edit
Just for clarification - I'm only keeping those objects around that need to be kept. Some are disposed of in the same method where they are created, but many are used in such a way that this isn't possible.
No that is correct. IDisposable is designed to free up unmanaged resources and should be called as soon as possible after you have finished with the instance. It's a common misconception that this is unnecessary, or that the finailizer will do this automatically when the object is garbage collected. It does not.
The correct pattern for IDisposable is here, and included below for quick reference.
public class Resource : IDisposable
{
// Dispose() calls Dispose(true)
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
// NOTE: Leave out the finalizer altogether if this class doesn't
// own unmanaged resources itself, but leave the other methods
// exactly as they are.
~Resource()
{
// Finalizer calls Dispose(false)
Dispose(false);
}
// The bulk of the clean-up code is implemented in Dispose(bool)
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
// free managed resources
}
// free native resources here if there are any
}
}
So long as you implement the disposable pattern correctly (as described here), this method is fine.
As far as I know, only using statements have special support for IDisposable - there isn't anything in the framework that replicates what you are doing.
If you're talking about arbitrary IDisposable objects, I don't believe it exists.
The System.ComponentModel.Container class implements cascading Dispose, but requires the elements to implement IComponent. If you control your IDisposable objects you could make them implement IComponent - it only requires implementing a single property Site that can return null.
Sounds like a situation where the visitor pattern could be appropriate. Although I never understand the claim that it extends your classes and leaves them unchanged, because I only know examples where classes should have an AcceptVisitor method or the like. BTW, it's not a pattern I like because it is complex and tends to clutter code.
If one object will create many other IDisposable objects and maintain ownership of them throughout its existence, the pattern you describe can be a good one. It may be enhanced by having your class implement method "T RegDispos<T>(T thing) where T:IDisposable;" which will add a disposable to the list and return it. One can thus take care of the creation and cleanup of a resource in the same statement, by replacing a statement like "someField = someDisposType.CreateThing();" with "someField = RegDispos(someDisposType.CreateThing());".
If your class doesn't expose a public constructor (requires outsiders use factory methods), and if you're either using vb.net or are willing to use thread-static fields, you can even combine initialization and cleanup with declaration (e.g. "var someField = RegDispos(someDisposType.CreateThing());"). For this to be safe, the constructor must be invoked within a try/catch or try/finally block that can call Dispose on created sub-objects if construction fails. Because field initializers in C# don't have access to constructor parameters (a weakness in the language, IMHO) the only way to implement such a pattern is to have a factory method create the list and put it into a thread-static variable which can then be read by a static RegDispos method.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Any sense to set obj = null(Nothing) in Dispose()?
I understand if this question is closed as a duplicate, but I'm having some trouble reconciling some posts on this topic.
First a little background. I have a class Foo as follows
public class Foo : IDisposable
{
private Dictionary<int, string> _reallyBigDictionary =
new Dictionary<int, string>();
public void Dispose()
{
_reallyBigDictionary = null;
}
}
Instances of Foo are known to have a limited scope (i.e. I know we're not keeping it around forever). Given it's instance's limited scope, I don't see how nulling out _reallyBigDictionary actually frees up memory sooner than the dispose. The way I understand it, these objects won't ever get cleaned up until garbage collection is run. At that time, references to the given instance of Foo will be null regardless, so I expect GC to reclaim that memory regardless.
These posts lead me to believe that there is no point in setting member variables to null:
Memory leak problems: dispose or not to dispose managed resources?
Is it better to destroy all objects or just let the garbage collector do the job?
This post makes me question otherwise:
Proper use of the IDisposable interface
Can anyone clarify this point for me? Is the IDisposable implementation really necessary here? Because I just can't convince myself it is.
You don't need IDisposable; here's why. In summary, you need to follow Rule 1: Don't implement IDisposable unless you need to.
There are only two situations when IDisposable does need to be implemented:
The class owns unmanaged resources.
The class owns managed (IDisposable) resources.
Since Dictionary<int, string> is not IDisposable, your class shouldn't be either.
By implementing the IDisposable interface you can scope the useage of resources with the Using statement.
Also many classes in the underying .NET Framework detect that a class is IDisposable and runs the Dispose() for you, absolving you of the responsibility of nulling your managed child objects.
EDIT ----
Because of comments and a markdown, I thought Id add to my answer :
Many many classes in the .NET Framework implement IDisposable - for example many Generic collection types. When you dispose of such a collection the Microsoft implementation will call your class's Dispose method, having detected that it implements IDisposable. Also, there is a case for setting a reference to an object to null genrally - including perhaps in your Dispose method. If you want to remove a reference to an object, set the reference to null. Other classes might want to maintain a reference to the same object, so the argument that setting a reference to null is wrong' is a flawed argument. Using your Dispose method to set the reference to null is no different than setting it to null in any other way - granted, but by doing it in your IDisposable implemnentation you can have a far more determinitic and error-proof way of ensuring that this happens. Implementing IDisposable is also an excellent way to implement scoping, e.g. for transactional algorithms etc. Implementaing IDisposable is usefull in many ways and neednt be avoided, as long as you know how and why it works. It always amazes me how few .NET developers truly understand the IDisposable interface, Finalizers, the garbage collector and .NET best practices in general
I don't think that disposal is necessary in this instance. It's not like it's an active connection that's going to stay open and keep using up resources. When you're done using it, it will manage itself.
If you really want to make sure that there are no memory leaks coming from that piece of code, you can try adding the following to the dispose method:
GC.Collect();
I have a method, which has a try/catch/finaly block inside. Within the try block, I declare SqlDataReader as follows:
SqlDataReader aReader = null;
aReader = aCommand.ExecuteReader();
In the finally block, the objects which are manually disposed of are those which are set at the class level. So objects in the method which implement IDisposable, such as SqlDataReader above, do they get automatically disposed of? Close() is called on aReader after a while loop executes to get the contents of the reader (which should be Dispose() as that calls Close()). If there is no call to Close(), would this object be closed/disposed of automatically when the method finishes or the object goes out of scope?
EDIT: I am aware of the using statement but there are scenarios which are confusing me.
No, objects are not automatically disposed when they go out of scope.
They're not even guaranteed to be disposed if/when they're garbage-collected, although many IDisposable objects implement a "fallback" finaliser to help ensure that they're eventually disposed.
You are resposible for ensuring that any IDisposable objects are disposed, preferably by wrapping them in a using block.
You should use a using {...} block to wrap your IDisposable objects in - the Dispose() method (which for SqlDataReader passes off to the Close() method) will be called when the using block ends. If you do not use using, the object will not be automatically disposed when it goes out of scope - it will be up to the object finalizer, if it has one, to get rid of resources when it is garbage collected
using (SqlDataReader aReader = aCommand.ExecuteReader())
{
// ... do stuff
} // aReader.Dispose() called here
I agree with all of the above. You should make sure you call Dispose() yourself, and the easiest way to to this is with the using statement (you can also do this yourself in the finally block - this is more verbose, but sometimes necessary). If you don't do this you can find your application leaking unmanaged resources such as handles, or even unmanaged memory, especially if somewhere underneath all of this some COM components are being used, or calls are being made into the Win32 API. This can obviously lead to performance and stability problems, as well as excessive resource usage.
Just because objects that implement IDisposable "should" implement a finaliser that calls their Dispose(bool disposing) method to free unmanaged resources, is no guarantee that this will happen, so you definitely should not rely on it. See, for example, http://msdn.microsoft.com/en-us/library/b1yfkh5e%28VS.71%29.aspx for more information on this point.
Also, something else to bear in mind, is that if your type has members that are disposable, your type should either implement IDisposable (unless the lifecycle of those members is managed by another type, which obviously might get messy), or, if you only use such members in one method, or to implement one particular piece of functionality, you should consider making them local variables/parameters in the methods that use them.
The Dispose pattern doesn't make any guarantees about which objects will call Dispose on which other objects; it may happen sometimes, but you shouldn't care. Instead, it's your responsibility to make sure Dispose() is called for all IDisposable objects. The best way to do that is with the using statement. For example:
using (SqlDataReader aReader = aCommand.ExecuteReader())
{
// your code
}
I am puzzled by the statement "In the finally block, the objects which are manually disposed of are those which are set at the class level." By objects set at the class level, do you mean fields? You probably shouldn't be disposing of these within a ordinary method, because then the life-time of the fields is unpredictable, and depends on which methods you happened to have called. It would be better to implement IDisposable and dispose of fields in your Dispose method.
Might the Using statement help?
In a class I am writing, I am using an object to set some of its properties in a custom class I am writing.
This is being done in the constructor, but the class has a Dispose() method.
I have never actually used an object in a constructor which has a Dispose() method/implements IDisposable. Should I wrap this in a using(...) statement or should I implement a destructor/finalizer?
My imagination has made me ask this: This class is part of a third party closed source API. How can I find out what needs to be disposed of?
Thanks
If your reference to the object is local to the constructor then just wrap it in a using statement.
If your reference to the object is a class member then your class should implement IDisposable as well, with it's Dispose() method calling Dispose() on the object.
Just to add in response to the other parts of your question:
You should generally only implement a finalizer where you are using unmanaged resources which might need to be cleaned up if your program terminates abnormally. Any managed objects will be tidied up by the GC at some point. Don't rely on a finalizer to dispose of any managed objects as you can't predict when it will be executed.
You don't need to care about what needs to be disposed of. You should trust that where a class implements IDisposable, it takes care of everything through that pattern. If it's badly coded then you may encounter problems, but I'd be optimistic in this case.
I'd always opt for wrapping IDisposables in a Using block where possible as I think it's neater than trying to call the Dispose() method explicitly.
if you don't need the object outside the constructor, why don't you use the using statement ?
If there is a special "destruction" code - using is not enough and you need to call it's disposal code from some place...
I started messing around with FXCop against some of my assemblies and noticed something interesting that I'm trying to wrap my head around.
"Types that declare disposable members
should also implement IDisposable. If
the type does not own any unmanaged
resources, do not implement a
finalizer on it."
I have my own class that contains some controls that in turn inherit from System.Web.UI.WebControls.Button. It's not complaining about my inherited buttons, but the class that wraps them is.
What's the impact or danger here?
If your class contains IDisposable types, but your class is not IDisposable, then it is possible that the IDisposable types within your class are not disposed / freed on time, when an instance of your class is no longer needed.
Dispoble types should be disposed as soon as you no longer need them, so that resources are freed. If you do not do that, you'll have to wait until the GC kicks in, then the resources will be freed as well.
As you know, when you finish using a disposable object, you should call its Dispose method.
When you inherit from these controls, it is still possible to call the Dispose method. But if you make a wrapper, then the user of your wrapper class should be able to call Dispose.
public void Dispose() {
button.Dispose();
// any other thing that is disposable
}
What's the nature of the class that contains the buttons?
Normally I would only expect a class that's a composite control to have other controls as members. Such a class would normally inherit directly or indirectly from System.Web.UI.CompositeControl or System.Web.UI.WebControl, and the controls you have as members should be added to the Controls collection.
Doing so will ensure they are disposed properly and should get rid of the FxCop warning.
Post more details of the class in question if this doesn't help.