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.
Related
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
I have a method Foo that returns a IDisposable object. Oftentimes when this method is called, the return value is used. However, in this case I need the method to be called but don't need the return value. Is the correct thing to do to immediately call Dispose on it? If so, does it matter which of the below I do?
Foo().Dispose();
or
using(Foo())
{
}
or just
Foo();
If an object implements IDisposable then you should call the Dispose method when you are finished with the object. Dispose is meant to release resources that should not be held until a GC (Garbage Collector) cycle (or things that are not covered by the GC). When the GC detects that an object is no longer used it will call the objects finalizer, but there is no way to predict when that will happen. Therefore, the IDisposable interface was created so that programmers could explicitly dispose of an object's resources (it doesn't delete the object, the GC still does that).
I think I would prefer
Foo().Dispose();
but it really doesn't matter (between the first two). If you just use "Foo();" then you are not respecting the Dispose pattern which would be incorrect. You may want to try to determine what sort of resources the object is creating, perhaps there is a way to get the information you need without creating an object that needs disposing.
Background:
I've a got a main object that live on my app's UI thread. This long-lived parent / container object contains a sub-object as a private member that is internally multi-threaded.
Based on some event published from the child / contained object I'd like the parent object to simply Dispose() the child and recreate it.
The contained object owns some unmanaged resources and well as some large managed memory buffers.
I could create an event handler on the top level object that does this, but this means that the object about to be Disposed will be in the call stack of the method call to about to replace it. (!)
This is because the event handler delegate will be called by one of the child object's state handling functions on it's own thread.
This seems...wrong. No?
In particular, the the child's FireAnEvent() method will resume execution after the delegate calls are processed, except that now execution will resume in the context of an already "disposed" object.
Intuitively, I can't see this leading to good things.
Question:
Is there an established C# pattern to destroy a contained object as a result of it's own event?
Or, is there GC magic that makes such a simple event handler good enough somehow?
Or, am I missing some key bit of understanding?
Calling IDisposable.Dispose() doesn't signal anything special to the .NET framework. The only thing you need to do is remove any references to the object you wish to remove. Once this is done and the object is out of the call stack, it will become a candidate for garbage collection.
Note that your object will not necessarily be garbage collected immediately, or even the next time the GC runs; it is merely an assumed eventuality.
The only purpose of IDisposable is to provide a standard means for requesting that an object clean itself up and release resources. You can hold a reference to a "disposed" object for as long as you like, which will prevent the GC from collecting the object. IDisposable.Dispose() is just another method; technically, you can make it do anything you want.
This question has a very nicely detailed answer that may help you understand IDisposable a bit more: Proper use of the IDisposable interface
as mentioned, an IDisposable object is nothing magical. It just lets you use the using shorthand, which is just a shorthand for:
try { // code in the using block }
catch{}
finally{
disposableObject.Dispose()
}
Have you considered incorporating a third type of object into the mix? It is ill advised that contained object be conscious of their container.
Roughly this would be your workflow:
contained object decides it should be restarted.
contained object frees resources.
contained object writes to a queue on a third object (not the containing object).
containing object accesses the queue when you feel you should create new contained objects and reinstantiates the objects. Alternatively adding to the queue raises an event to the container to empty it.
The third object might seem pointless but it would make your life a lot easier if you ever decided to refactor.
Objects which subscribe to events for the purpose of notification should be prepared to receive notifications at any time, even after they have been disposed. The purpose of a notification is to tell an object to do whatever it needs to do in response to something that has happened. If an object can't do anything useful in response to a notification, it should simply not do anything.
Further, the purpose of Dispose isn't to "destroy" an object, nor the resources it contains, but rather to release it from any obligations it may have had to outside objects, and allow it to release any outside entities from any obligations they might have had toward it. In many cases, an object will be useless once it releasing the services of outside entities that were committed to it, and thus objects which have been disposed cannot be expected to be useful; if, as is likely, a method which is called after Disposed cannot satisfy its duties because necessary outside entities have been released, it should throw ObjectDisposedException rather than failing some other way.
Putting these observations together, while many methods on a disposed object should throw ObjectDisposedException, notification event handler methods should not, since they're instructing the object to "do whatever you need to do to meet your obligations, given that something has happened". If an object has been disposed, it has no obligations. Thus, being disposed doesn't prevent an object from satisfying an event handler contract; instead, it allows the object to meet the contract by silently ("successfully") doing nothing.
Is there any guidance or best practices around who should call Dispose() on disposable objects when they have been passed into another object's methods or constuctor?
Here's a couple of examples as to what I mean.
IDisposable object is passed into a method (Should it dispose of it once its done?):
public void DoStuff(IDisposable disposableObj)
{
// Do something with disposableObj
CalculateSomething(disposableObj)
disposableObj.Dispose();
}
IDisposable object is passed into a method and a reference is kept (Should it dispose of it when MyClass is disposed?):
public class MyClass : IDisposable
{
private IDisposable _disposableObj = null;
public void DoStuff(IDisposable disposableObj)
{
_disposableObj = disposableObj;
}
public void Dispose()
{
_disposableObj.Dispose();
}
}
I'm currently thinking that in the first example the caller of DoStuff() should dispose of the object as it probably created the object. But in the second example it feels like MyClass should dispose of the object as it keeps a reference to it. The problem with this is that the calling class might not know MyClass has kept a reference and therefore might decide to dispose of the object before MyClass has finished using it.
Are there any standard rules for this sort of scenario? If there are, do they differ when the disposable object is being passed into a constructor?
P.S.: I have posted a new answer (containing a simple set of rules who should call Dispose, and how to design an API that deals with IDisposable objects). While the present answer contains valuable ideas, I have come to believe that its main suggestion often won't work in practice: Hiding away IDisposable objects in "coarser-grained" objects often means that those need to become IDisposable themselves; so one ends up where one started, and the problem remains.
Is there any guidance or best practices around who should call Dispose() on disposable objects when they have been passed into another object's methods or constuctor?
Short answer:
Yes, there is much advice on this topic, and the best that I know of is Eric Evans' concept of Aggregates in Domain-Driven Design. (Simply put, the core idea as applied to IDisposable is this: Encapsulate the IDisposable in a coarser-grained component such that it is not seen by the outside and is never passed to the component consumer.)
Moreover, the idea that the creator of an IDisposable object should also be in charge of disposing it is too restrictive and often won't work in practice.
The rest of my answer goes into more detail on both points, in the same order. I'll finish off my answer with a few pointers to further material that is related to the same topic.
Longer answer — What this question is all about in broader terms:
Advice on this topic is usually not specific to IDisposable. Whenever people talk about object lifetimes and ownership, they are referring to the very same issue (but in more general terms).
Why does this topic hardly ever arise in the .NET ecosystem? Because .NET's runtime environment (the CLR) performs automatic garbage collection, which does all the work for you: If you no longer need an object, you can simply forget about it and the garbage collector will eventually reclaim its memory.
Why, then, does the question come up with IDisposable objects? Because IDisposable is all about the explicit, deterministic control of a (often sparse or expensive) resource's lifetime: IDisposable objects are supposed to be released as soon as they are no longer needed — and the garbage collector's indeterminate guarantee ("I'll eventually reclaim the memory used by you!") simply isn't good enough.
Your question, re-phrased in the broader terms of object lifetime and ownership:
Which object O should be responsible for ending the lifetime of a (disposable) object D, which also gets passed to objects X,Y,Z?
Let's establish a few assumptions:
Calling D.Dispose() for an IDisposable object D basically ends its lifetime.
Logically, an object's lifetime can only be ended once. (Never mind for the moment that this stands in opposition to the IDisposable protocol, which explicitly permits multiple calls to Dispose.)
Therefore, for the sake of simplicity, exactly one object O should be responsible for disposing D. Let's call O the owner.
Now we get to the core of the issue: Neither the C# language, nor VB.NET provide a mechanism for enforcing ownership relationships between objects. So this turns into a design issue: All objects O,X,Y,Z that receive a reference to another object D must follow and adhere to a convention that regulates exactly who has ownership over D.
Simplify the problem with Aggregates!
The single best advice that I have found on this topic comes from Eric Evans' 2004 book, Domain-Driven Design. Let me cite from the book:
Say you were deleting a Person object from a database. Along with the person go a name, birth date, and a job description. But what about the address? There could be other people at the same address. If you delete the address, those Person objects will have references to a deleted object. If you leave it, you accumulate junk addresses in the database. Automatic garbage collection could eliminate the junk addresses, but that techncal fix, even if available in your database system, ignores a basic modeling issue. (p. 125)
See how this relates to your issue? The addresses from this example are the equivalent to your disposable objects, and the questions are the same: Who should delete them? Who "owns" them?
Evans goes on to suggest Aggregates as a solution to this design problem. From the book again:
An Aggregate is a cluster of associated objects that we treat as a unit for the purpose of data changes. Each Aggregate has a root and a boundary. The boundary defines what is inside the Aggregate. The root is a single, specific Entity contained in the Aggregate. The root is the only member of the Aggregate that outside objects are allowed to hold references to, although objects within the boundary may hold references to each other. (pp. 126-127)
The core message here is that you should restrict the passing-around of your IDisposable object to a strictly limited set ("aggregate") of other objects. Objects outside that aggregate boundary should never get a direct reference to your IDisposable. This greatly simplifies things, since you no longer need to worry whether the greatest part of all objects, namely those outside the aggregate, might Dispose your object. All you need to do is make sure that the objects inside the boundary all know who is responsible for disposing it. This should be an easy enough problem to solve, as you'd usually implement them together and take care to keep the aggregate boundaries reasonably "tight".
What about the suggestion that the creator of an IDisposable object should also dispose it?
This guideline sounds reasonable and there's an appealing symmetry to it, but just by itself, it often won't work in practice. Arguably it means the same as saying, "Never pass a reference to an IDisposable object to some other object", because as soon as you do that, you risk that the receiving object assumes its ownership and disposes it without your knowing.
Let's look at two prominent interface types from the .NET Base Class Library (BCL) that clearly violate this rule of thumb: IEnumerable<T> and IObservable<T>. Both are essentially factories that return IDisposable objects:
IEnumerator<T> IEnumerable<T>.GetEnumerator()
(Remember that IEnumerator<T> inherits from IDisposable.)
IDisposable IObservable<T>.Subscribe(IObserver<T> observer)
In both cases, the caller is expected to dispose the returned object. Arguably, our guideline simply doesn't make sense in the case of object factories... unless, perhaps, we require that the requester (not its immediate creator) of the IDisposable releases it.
Incidentally, this example also demonstrates the limits of the aggregate solution outlined above: Both IEnumerable<T> and IObservable<T> are way too general in nature to ever be part of an aggregate. Aggregates are usually very domain-specific.
Further resources and ideas:
In UML, "has a" relationships between objects can be modelled in two ways: As aggregation (empty diamond), or as composition (filled diamond). Composition differs from aggregation in that the contained/referred object's lifetime ends with that of the container/referrer. Your original question has implied aggregation ("transferable ownership"), while I've mostly steered towards solutions that use composition ("fixed ownership"). See the Wikipedia article on "Object composition".
Autofac (a .NET IoC container) solves this problem in two ways: either by communicating, using a so-called relationship type, Owned<T>, who acquires ownership over an IDisposable; or through the concept of units of work, called lifetime scopes in Autofac.
Regarding the latter, Nicholas Blumhardt, the creator of Autofac, has written "An Autofac Lifetime Primer", which includes a section "IDisposable and ownership". The whole article is an excellent treatise on ownership and lifetime issues in .NET. I recommend reading it, even to those not interested in Autofac.
In C++, the Resource Acquisition Is Initialization (RAII) idiom (in general) and smart pointer types (in particular) help the programmer get object lifetime and ownership issues right. Unfortunately, these are not transferrable to .NET, because .NET lacks C++'s elegant support for deterministic object destruction.
See also this answer to the question on Stack Overflow, "How to account for disparate implementation needs?", which (if I understand it correctly) follows a similar thought as my Aggregate-based answer: Building a coarse-grained component around the IDisposable such that it is completely contained (and hidden from the component consumer) within.
A general rule is that if you created (or acquired ownership of) the object then it is your responsibility to dispose it. This means that if you receive a disposable object as a parameter in a method or constructor you usually should not dispose it.
Note that some classes in the .NET framework do dispose objects that they received as parameters. For example disposing a StreamReader also disposes the underlying Stream.
This is a follow-up to my previous answer; see its initial remark to learn why I am posting another.
My previous answer got one thing right: Each IDisposable should have an exclusive "owner" who will be responsible for Dispose-ing of it exactly once. Managing IDisposable objects then becomes very comparable to managing memory in unmanaged code scenarios.
.NET's predecessor technology, the Component Object Model (COM), used the following protocol for memory management responsibilities between objects:
"In-parameters must be allocated and freed by the caller.
"Out-parameters must be allocated by the called; they are freed by the caller […].
"In-out-parameters are initially allocated by the caller, and then freed and reallocated by the one called, if necessary. As is true for out parameters, the caller is responsible for freeing the final returned value."
(There are additional rules for error cases; see the page linked to above for details.)
If we were to adapt these guidelines for IDisposable, we could lay down the following…
Rules regarding IDisposable ownership:
When an IDisposable is passed into a method via a regular parameter, there is no transfer of ownership. The called method can use the IDisposable, but must not Dispose it (nor pass on ownership; see rule 4 below).
When an IDisposable is returned from a method via an out parameter or the return value, then ownership is transferred from the method to its caller. The caller will have to Dispose it (or pass on ownership over the IDisposable in the same way).
When an IDisposable is given to a method via a ref parameter, then ownership over it is transferred to that method. The method should copy the IDisposable into a local variable or object field and then set the ref parameter to null.
One possibly important rule follows from the above:
If you don't have ownership, you must not pass it on. That means, if you received an IDisposable object via a regular parameter, don't put the same object into a ref IDisposable parameter, nor expose it via a return value or out parameter.
Example:
sealed class LineReader : IDisposable
{
public static LineReader Create(Stream stream)
{
return new LineReader(stream, ownsStream: false);
}
public static LineReader Create<TStream>(ref TStream stream) where TStream : Stream
{
try { return new LineReader(stream, ownsStream: true); }
finally { stream = null; }
}
private LineReader(Stream stream, bool ownsStream)
{
this.stream = stream;
this.ownsStream = ownsStream;
}
private Stream stream; // note: must not be exposed via property, because of rule (2)
private bool ownsStream;
public void Dispose()
{
if (ownsStream)
{
stream?.Dispose();
}
}
public bool TryReadLine(out string line)
{
throw new NotImplementedException(); // read one text line from `stream`
}
}
This class has two static factory methods and thereby lets its client choose whether it wants to keep or pass on ownership:
One accepts a Stream object via a regular parameter. This signals to the caller that ownership will not be taken over. Thus the caller needs to Dispose:
using (var stream = File.OpenRead("Foo.txt"))
using (var reader = LineReader.Create(stream))
{
string line;
while (reader.TryReadLine(out line))
{
Console.WriteLine(line);
}
}
One that accepts a Stream object via a ref parameter. This signals to the caller that ownership will be transferred, so the caller does not need to Dispose:
var stream = File.OpenRead("Foo.txt");
using (var reader = LineReader.Create(ref stream))
{
string line;
while (reader.TryReadLine(out line))
{
Console.WriteLine(line);
}
}
Interestingly, if stream were declared as a using variable: using (var stream = …), compilation would fail because using variables cannot be passed as ref parameters, so the C# compiler helps enforce our rules in this specific case.
Finally, note that File.OpenRead is an example for a method that returns a IDisposable object (namely, a Stream) via the return value, so ownership over the returned stream is transferred to the caller.
Disadvantage:
The main disadvantage to this pattern is that AFAIK, noone uses it (yet). So if you interact with any API that doesn't follow the above rules (for example, the .NET Framework Base Class Library) you still need to read the documentation to find out who has to call Dispose on IDisposable objects.
In general, once you're dealing with a Disposable object, you're no longer in the ideal world of managed code where lifetime ownership is a moot point. Resultantly, you need to consider what object logically "owns", or is responsible for the lifetime of, your disposable object.
Generally, in the case of a disposable object that is just passed into a method, I would say no, the method should not dispose the object because it's very rare for one object to assume ownership of another object and then be done with it in the same method. The caller should be responsible for disposal in those cases.
There is no automatic answer that says "Yes, always dispose" or "No, never dispose" when talking about member data. Rather, you need to think about the objects in each specific case and ask yourself, "Is this object responsible for the lifetime of the disposable object?"
The rule of thumb is that the object responsible for creating a disposable owns it, and thus is responsible for disposing it later. This doesn't hold if there's an ownership transfer. For example:
public class Foo
{
public MyClass BuildClass()
{
var dispObj = new DisposableObj();
var retVal = new MyClass(dispObj);
return retVal;
}
}
Foo is clearly responsible for creating dispObj, but it's passing the ownership to the instance of MyClass.
One thing I decided to do before I knew much about .NET programming, but it still seems a good idea, is have a constructor that accepts an IDisposable also accept a Boolean which says whether ownership of the object is going to be transferred as well. For objects which can exist entirely within the scope of using statements, this generally won't be too important (since the outer object will be disposed within the scope of the Inner object's Using block, there's no need for the outer object to dispose the inner one; indeed, it may be necessary that it not do so). Such semantics can become essential, however, when the outer object will be passed as an interface or base class to code which doesn't know of the inner object's existence. In that case, the inner object is supposed to live until the outer object is destroyed, and thing that knows the inner object is supposed to die when the outer object does is the outer object itself, so the outer object has to be able to destroy the inner one.
Since then, I've had a couple of additional ideas, but haven't tried them. I'd be curious what other people think:
A reference-counting wrapper for an IDisposable object. I haven't really figured out the most natural pattern for doing this, but if an object uses reference counting with Interlocked increment/decrement, and if (1) all code that manipulates the object uses it correctly, and (2) no cyclic references are created using the object, I would expect that it should be possible to have a shared IDisposable object which gets destroyed when the last usage goes bye-bye. Probably what should happen would be that the public class should be a wrapper for a private reference-counted class, and it should support a constructor or factory method which will create a new wrapper for the same base instance (bumping the instance's reference count by one). Or, if the class needs to be cleaned up even when wrappers are abandoned, and if the class has some periodic polling routine, the class could keep a list of WeakReferences to its wrappers and check to ensure that at least some of them still exist.
Have the constructor for an IDisposable object accept a delegate which it will call the first time the object is disposed (an IDisposable object should use Interlocked.Exchange on the isDisposed flag to ensure it's disposed exactly once). That delegate could then take care of disposing any nested objects (possibly with a check to see if anyone else still held them).
Does either of those seem like a good pattern?
I am working on a class that deals with a lot of Sql objects - Connection, Command, DataAdapter, CommandBuilder, etc. There are multiple instances where we have code like this:
if( command != null )
{
command.Dispose();
}
if( dataAdapter != null )
{
dataAdapter.Dispose();
}
etc
I know this is fairly insufficient in terms of duplication, but it has started smelling. The reason why I think it smells is because in some instances the object is also set to null.
if( command != null )
{
command.Dispose();
command = null;
}
I would love to get rid of the duplication if possible. I have come up with this generic method to dispose of an object and set it to null.
private void DisposeObject<TDisposable>( ref TDisposable disposableObject )
where TDisposable : class, IDisposable
{
if( disposableObject != null )
{
disposableObject.Dispose();
disposableObject = null;
}
}
My questions are...
Is this generic function a bad idea?
Is it necessary to set the object to null?
EDIT:
I am aware of the using statement, however I cannot always use it because I have some member variables that need to persist longer than one call. For example the connection and transaction objects.
Thanks!
You should consider if you can use the using statement.
using (SqlCommand command = ...)
{
// ...
}
This ensures that Dispose is called on the command object when control leaves the scope of the using. This has a number of advantages over writing the clean up code yourself as you have done:
It's more concise.
The variable will never be set to null - it is declared and initialized in one statement.
The variable goes out of scope when the object is disposed so you reduce the risk of accidentally trying to access a disposed resource.
It is exception safe.
If you nest using statements then the resources are naturally disposed in the correct order (the reverse order that they were created in).
Is it necessary to set the object to null?
It is not usually necessary to set variables to null when you have finished using them. The important thing is that you call Dispose when you have finished using the resource. If you use the above pattern, it is not only unnecessary to set the variable to null - it will give a compile error:
Cannot assign to 'c' because it is a 'using variable'
One thing to note is that using only works if an object is acquired and disposed in the same method call. You cannot use this pattern is if your resources need to stay alive for the duration of more than one method call. In this case you might want to make your class implement IDisposable and make sure the resources are cleaned up when your Dispose method is called. I this case you will need code like you have written. Setting variables to null is not wrong in this case but it is not important because the garbage collector will clean up the memory correctly anyway. The important thing is to make sure all the resources you own are disposed when your dispose method is called, and you are doing that.
A couple of implementation details:
You should ensure that if your Dispose is called twice that it does not throw an exception. Your utility function handles this case correctly.
You should ensure that the relevant methods on your object raise an ObjectDisposedException if your object has already been disposed.
You should implement IDisposable in the class that owns these fields. See my blog post on the subject. If this doesn't work well, then the class is not following OOP principles, and needs to be refactored.
It is not necessary to set variables to null after disposing them.
If you're objects have a lot of cleanup to do, they might want to track what needs to be deleted in a separate list of disposables, and handle all of them at once. Then at teardown it doesn't need to remember everything that needs to be disposed (nor does it need to check for null, it just looks in the list).
This probably doesn't build, but for expanatory purposes, you could include a RecycleBin in your class. Then the class only needs to dispose the bin.
public class RecycleBin : IDisposable
{
private List<IDisposable> _toDispose = new List<IDisposable>();
public void RememberToDispose(IDisposable disposable)
{
_toDispose.Add(disposable);
}
public void Dispose()
{
foreach(var d in _toDispose)
d.Dispose();
_toDispose.Clear();
}
}
I assume these are fields and not local variables, hence why the using keyword doesn't make sense.
Is this generic function a bad idea?
I think it's a good idea, and I've used a similar function a few times; +1 for making it generic.
Is it necessary to set the object to null?
Technically an object should allow multiple calls to its Dispose method. (For instance, this happens if an object is resurrected during finalization.) In practice, it's up to you whether you trust the authors of these classes or whether you want to code defensively. Personally, I check for null, then set references to null afterwards.
Edit: If this code is inside your own object's Dispose method then failing to set references to null won't leak memory. Instead, it's handy as a defence against double disposal.
I'm going to assume that you are creating the resource in one method, disposing it in another, and using it in one or more others, making the using statement useless for you.
In which case, your method is perfectly fine.
As far the second part of your question ("Is setting it to null necessary?"), the simple answer is "No, but it's not hurting anything".
Most objects hold one resource -- memory, which the Garbage collection deals with freeing, so we don't have to worry about it. Some hold some other resource as well : a file handle, a database connection, etc. For the second category, we must implement IDisposable, to free up that other resource.
Once the Dispose method is called, both categories because the same : they are holding memory. In this case, we can just let the variable go out of scope, dropping the reference to the memory, and allowing to GC to free it eventually -- Or we can force the issue, by setting the variable to null, and explicitly dropping the reference to the memory. We still have to wait until the GC kicks in for the memory to actually be freed, and more than likely the variable will go out of scope anyway, moments after to set it to null, so in the vast majority of cases, it will have no effect at all, but in a few rare cases, it will allow the memory to be freed a few seconds earlier.
However, if your specific case, where you are checking for null to see if you should call Dispose at all, you probably should set it to null, if there is a chance you might call Dispose() twice.
Given that iDisposable does not include any standard way of determining whether an object has been disposed, I like to set things to null when I dispose of them. To be sure, disposing of an object which has already been disposed is harmless, but it's nice to be able to examine an object in a watch window and tell at a glance which fields have been disposed. It's also nice to be able to have code test to ensure that objects which should have been disposed, are (assuming the code adheres to the convention of nulling out variables when disposing of them, and at no other time).
Why don't you use using C# construct? http://msdn.microsoft.com/en-us/library/yh598w02.aspx
Setting to null if not required.
Others have recommended the using construct, which I also recommend. However, I'd like to point out that, even if you do need a utility method, it's completely unnecessary to make it generic in the way that you've done. Simply declare your method to take an IDisposable:
private static void DisposeObject( ref IDisposable disposableObject )
{
if( disposableObject != null )
{
disposableObject.Dispose();
disposableObject = null;
}
}
You never need to set variables to null. The whole point of IDisposable.Dispose is to get the object into a state whereby it can hang around in memory harmlessly until the GC finalises it, so you just "dispose and forget".
I'm curious as to why you think you can't use the using statement. Creating an object in one method and disposing it in a different method is a really big code smell in my book, because you soon lose track of what's open where. Better to refactor the code like this:
using(var xxx = whatever()) {
LotsOfProcessing(xxx);
EvenMoreProcessing(xxx);
NowUseItAgain(xxx);
}
I'm sure there is a standard pattern name for this, but I just call it "destroy everything you create, but nothing else".
I answered this in my question and answer here:
Implementing IDisposable (the Disposable Pattern) as a service (class member)
It implemented a simplistic, reusable component that works for any IDisposable member.