Is it really needed to implement dispose pattern for managed resources only

Question

I've read carefully through this article and it seems to clearly state that the dispose pattern should be implemented in all cases of IDisposable implementation. I'm trying to understand why I need to implement the dispose pattern in cases when my class holds only managed resources (i.e. other IDisposable members or safe handles). Why cannot I just write

class Foo : IDisposable
{
    IDisposable boo;

    void Dispose()
    {
        boo?.Dispose();
    }
}

If it's definitely known that there are no unmanaged resources and there is no point to call Dispose method from finalizer as managed resources aren't being freed from finalizer?

Update: In order to add some clarity. The discussion seems to come down to the question whether it's needed to implement the dispose pattern for every base public non-sealed class which implements IDisposable or not. I however cannot find potential problems with hierarchy when a base class without unmanaged resources doesn't use dispose pattern while child classes which have unmanaged resources do use this pattern:

class Foo : IDisposable
{
    IDisposable boo;

    public virtual void Dispose()
    {
        boo?.Dispose();
    }
}

// child class which holds umanaged resources and implements dispose pattern
class Bar : Foo
{
    bool disposed;
    IntPtr unmanagedResource = IntPtr.Zero;

    ~Bar()
    {
        Dispose(false);
    }

    public override void Dispose()
    {
        base.Dispose();
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposed)
            return;

        if (disposing)
        {
            // Free any other managed objects here.
            //
        }
        // close handle

        disposed = true;
    }
}

// another child class which doesn't hold unmanaged resources and merely uses Dispose 
class Far : Foo
{
    private IDisposable anotherDisposable;

    public override void Dispose()
    {
        base.Dispose();
        anotherDisposable?.Dispose();
    }
}

Even more, for me it looks as better separation of concerns when implementations are responsible only for those things which they are aware of.

Diaz

What if a derived class adds unmanaged resources? – Solita

Even if it's only managed resources, one very common source of memory leaks are events. An event can only be set to null by the owner. – Ionosphere

@SLaks, so why cannot I make Dispose method virtual and implement dispose pattern in derived class in that case ? – Diaz

@Zer0, can't events be null-ed in the Dispose public method without using Dispose(bool disposing) ? – Diaz

"Why cannot I just write ...", as opposed to what? – Crossway

@quantificion you can and you should override Dispose in inheriting classes. Trying to fix bad code that leaks through event subscriptions with finalizers is insane (also it's not guaranteed by anything to work properly as finalizers could be called much later). – Loadstar

@quantificion the quote in my answer is taken from the beginning of the very same article. Also, as I mentioned, you can check vs autogenerated pattern (click on IDisposable in your code -> implement with pattern). – Loadstar

@Sergey.quixoticaxis.Ivanov, then the article seems to present contradictory statements as there is another quotation: You should implement this pattern for all base classes that implement Dispose() and are not sealed. – Diaz

@Diaz this article has been like this for ages sadly, it's probably one of the reasons why so much code implements finilizers (with destructor syntax, oh) in classes that have nothing to do with unmanaged resources or have unmanaged resources in their inheritors. I believe that the easiest way is to make a console application with three classes A, B : A, C : B and Console.WriteLine everywhere. As I believe that when literature seems contradictory, it's always easier to try yourself. – Loadstar

Given that the whole idea that Finalizers are sufficient to release unmanaged resources was mistaken, this whole pattern is probably a mistake. So accomodating finalizers in such a common pattern should never have been done. Introducing dispose(bool) lower down in the type hierarchy I think would work fine, and in retrospect is probably a better design. – Zama

@Sergey.quixoticaxis.Ivanov, I tried it and I know how it works, however the primary reason I raised the question is that dispose pattern applicability for specific cases, I highlighted in the question, seems to start having some dogmatic reasoning and I'm trying to clarify whether there are still some valid reasons which I'm missing or we need to admit that this isn't only way to cover specific scenarios. – Diaz

@LasseVågsætherKarlsen, As opposed to You should implement this pattern for all base classes that implement Dispose() and are not sealed, taken from there. – Diaz

@Diaz the only thing I can say about it is that dispose is currently being used for 2 independent scenarios in .Net: 1st original scenario of working with unmanaged resources (which, as I assume from your comment, you have understanding of) and 2nd unconventional (but widely utilized) scenario of emulating c++ destructor semantics. The dispose pattern is a solution defined for the former. That's why documentation is so focused on unmanaged resources. Dispose pattern is appliable for the later scenario, but, as far as I know and as far as I understand the docs, is not meant for it. – Loadstar

Answer 1

This

private class Foo : IDisposable
{
    IDisposable boo;

    public void Dispose()
    {
        boo?.Dispose();
    }
}

Is perfectly fine. As is

public sealed class Foo : IDisposable
{
    IDisposable boo;

    public void Dispose()
    {
        boo?.Dispose();
    }
}

What could go wrong if I have public not sealed base class implemented as above with virtual Dispose method ?

From the docs:

Because the order in which the garbage collector destroys managed objects during finalization is not defined, calling this Dispose overload with a value of false prevents the finalizer from trying to release managed resources that may have already been reclaimed.

Accessing a managed object that has already been reclaimed, or accessing its properties after it's been Disposed (perhaps by another finalizer) will cause an Exception to be raised in the Finalizer, which is bad:

If Finalize or an override of Finalize throws an exception, and the runtime is not hosted by an application that overrides the default policy, the runtime terminates the process and no active try/finally blocks or finalizers are executed. This behavior ensures process integrity if the finalizer cannot free or destroy resources.

So if you had:

   public  class Foo : IDisposable
    {
        IDisposable boo;

        public virtual void Dispose()
        {
            boo?.Dispose();
        }
    }
    public class Bar : Foo
    {
        IntPtr unmanagedResource = IntPtr.Zero;
        ~Bar()
        {
            this.Dispose();
        }

        public override void Dispose()
        {
            CloseHandle(unmanagedResource);
            base.Dispose();
        }

        void CloseHandle(IntPtr ptr)
        {
            //whatever
        }
    }

~Bar -> Bar.Dispose() -> base.Dispose() -> boo.Dispose() But boo may have been reclaimed by the GC.

Zama

just to add to this: the full dispose pattern is only applicable if your class is directly responsible for any unmanaged resources -- then you'd need a finalizer to free them in case someone forgot to call Dispose. If your class does not have any unmanaged resources (or it is using SafeHandle to manage them), then you do not need a finalizer and the above implementation of IDisposable is all you need. – Coreencorel

@Coreencorel What I believe this answer is implying is it's safe to forgo the dispose pattern when you're sure the class can't be inherited from. Otherwise it's entirely plausible there exists unmanaged resources in something that derives from your class. In that case you do need the dispose pattern (including finalizers). – Ionosphere

@Ionosphere no you do not, and you can easily check that you do not. As Brandon said, only the classes that are directly responsible for unmanaged resources need it. And ofc you can easily check it. – Loadstar

@Loadstar Maybe you misunderstood me. You don't need a finalizer on a class without unmanaged resources. But you do need to follow the dispose pattern. There's no easy way (outside reflection) that a class can easily detect if derived classes have unmanaged resources. Plus you should be following the dispose pattern even without unmanaged resources (events are the first thing to come to mind). – Ionosphere

What could go wrong if I have public not sealed base class implemented as above with virtual Dispose method ? – Diaz

@Zer0, Could you elaborate on why having Dispose(bool disposing) is important for the events cleanup? – Diaz

@Ionosphere and there is no need to know. If you're disposing your class, the chain of Dispose->base.Dispose will clean up everything. If it's finilization then finilizers will be called by CLR for all classes in the hierarchy thus again cleaning everything up. – Loadstar

@Diaz I was referring to the virtual part of the dispose pattern, not the bool indicating whether it's disposing of managed or unmanaged resources. I'll post an answer that shows a bad implementation of Dispose I see all too often that will cause memory leaks. – Ionosphere

I still can't get a point. Why badly implemented derived class is a justification for imposing the dispose pattern into a base class. We can and should implement dispose pattern for Bar as it holds unmanaged resources but why should it have an influence on how Foo is going to be implemented ? – Diaz

"why should it have an influence on how Foo is going to be implemented?" Because when you write a class that's open for extension, you need to make it possible to extend that class without introducing bugs. – Zama

But dispose pattern in base class doesn't actually prevent from introducing bugs in a derived class, it doesn't impose any restrictions on the child classes implementations. If someone doesn't apply dispose pattern where it's needed for whatever reason, the probability that he/she will start doing that only because it's applied in base class it pretty similar to probability that this won't happen. – Diaz

I updated my question to include example you given with proper implementations of Bar. I'm still looking for a strong reasoning to not do things this way and do always as article prescribes. – Diaz

There is some weird statement in the linked article, which seems to be the root of all this mess: “Managed objects that consume large amounts of memory or consume scarce resources. Freeing these objects explicitly in the Dispose method releases them faster than if they were reclaimed non-deterministically by the garbage collector”. How would you do that? Calling Dispose() is only possible, if the object implements IDisposable. And the method IDisposable.Dispose() is only for freeing unmanaged resources. Is reclaiming managed objects while being finalizer-reachable really possible? – Clower

Answer 2

I haven't seen this particular usage of Dispose mentioned yet, so I thought I'd point out a common source of memory leaks when not using the dispose pattern.

Visual Studio 2017 actually complains about this via static code analysis that I should "implement the dispose pattern". Do note I'm using SonarQube and SolarLint, and I don't believe Visual Studio will catch this alone. FxCop (another static code analysis tool) likely will, although I didn't test that.

I note the below code to showcase the dispose pattern is also there to protect against something like this, which has no unmanaged resources:

public class Foo : IDisposable
{
    IDisposable boo;

    public void Dispose()
    {
        boo?.Dispose();
    }
}

public class Bar : Foo
{
    //Memory leak possible here
    public event EventHandler SomeEvent;

    //Also bad code, but will compile
    public void Dispose()
    {
        someEvent = null;
        //Still bad code even with this line
        base.Dispose();
    }
}

The above illustrates very bad code. Don't do this. Why is this horrific code? That's because of this:

Foo foo = new Bar();
//Does NOT call Bar.Dispose()
foo.Dispose();

Let's assume this horrific code was exposed in our public API. Consider the above classes used by consumers of it:

public sealed class UsesFoo : IDisposable
{
    public Foo MyFoo { get; }

    public UsesFoo(Foo foo)
    {
        MyFoo = foo;
    }

    public void Dispose()
    {
        MyFoo?.Dispose();
    }
}

public static class UsesFooFactory
{
    public static UsesFoo Create()
    {
        var bar = new Bar();
        bar.SomeEvent += Bar_SomeEvent;
        return new UsesFoo(bar);
    }

    private static void Bar_SomeEvent(object sender, EventArgs e)
    {
        //Do stuff
    }
}

Are the consumers perfect? No.... UsesFooFactory should probably also unsubscribe from the event. But it does highlight a common scenario where an event subscriber outlives the publisher.

I've seen events cause countless memory leaks. Especially in very large or extreme high performance codebases.

I can also hardly count how many times I've seen objects live long past their time of disposal. This is a very common way many profilers find memory leaks (disposed objects still held onto by a GC Root of some kind).

Again, overly simplified example and horrible code. But it's really never good practice to call Dispose on an object and not expect it to dispose the entire object, whether it has been derived from a million times or not.

Edit

Please note this answer is intentionally only addressing managed resources, showcasing that the dispose pattern is useful in this scenario as well. This purposefully does not address the use case for unmanaged resources as I felt there was a lack of focus on managed only uses. And there are many other good answers on here that talk about that.

I will, however, note a few quick things that are important when it comes to unmanaged resources. The above code might not address unmanaged resources, but I want to make clear it does not contradict how they should be handled.

It's extremely important to use finalizers when your class is responsible for unmanaged resources. Briefly, finalizers are automatically called by the Garbage Collector. So it gives you a reasonable guarantee that it always gets called at some point in time. It's not bulletproof, but a far cry from hoping user code calls Dispose.

This guarantee is not true of Dispose. An object can be reclaimed by the GC without Dispose ever being called. That's the key reason finalizers are used for unmanaged resources. The GC itself only handles managed resources.

But I will also note it's equally important finalizers should not be used to clean up managed resources. There are countless reasons why (it's the GC's job to do this after all) but one of the largest drawbacks to using finalizers is delaying garbage collection on an object.

The GC, seeing an object is free to reclaim but has a finalizer, will delay the collection by placing the object in the finalizer queue. This adds significant unnecessary lifetime to an object, plus more pressure on the GC.

Finally I'll note that finalizers are non-deterministic for this reason, despite having similar syntax to something like a destructor in C++. They are very different beasts. You should never rely upon a finalizer to clean up unmanaged resources at a specific point in time.

Ionosphere

Hands down, it's very bad code. Could you please elaborate on why one should write ~Base() { } in the base class, if the base does not have unmanaged resources itself? `cause I still seem to misunderstand your comments. – Loadstar

The problem with code above is flawed implementation of derived class. Now having dispose pattern in the base class how would it enforce to implement a derived class correctly ? In other words how would the dispose pattern fix the problem ? – Diaz

@Diaz To answer your question, if the base class properly implements the dispose pattern but the derived classes do not, then you are correct, it's still bad code. The entire chain needs to implement the pattern. That said, if the base class does not implement the dispose pattern correctly, all derived classes have problems they can't overcome alone. – Ionosphere

Setting an event's invocation list to null is pointless. That is unless you plan on disposing of the object and yet holding onto rooted references to it for an extended period of time, which you absolutely should not be doing (and if you did, that would be the bug, not omitting nulling out of the classes fields). Nulling out fields of an object that is about to go out of scope anyway is just redundant. They're about to no longer be rooted references, so there's no need. – Xenocrates

Additionally, the fix to the problem of "A derived class isn't calling the base class's dispose method" is for the derived class to call base.Dispose(), not for the base class to implement "the dispose pattern", as that doesn't solve the problem at all. – Xenocrates

@Loadstar Nowhere in the documentation does it state the base class must have a finalizer. Nor did I mean to imply that. – Ionosphere

@Ionosphere "Otherwise it's entirely plausible there exists unmanaged resources in something that derives from your class. In that case you do need the dispose pattern (including finalizers)" it's your quote. – Loadstar

@Loadstar That should probably read "(including finalizers where necessary)". – Ionosphere

If I understand you correctly your statement is mainly about virtualization of full chain of cleanup calls. In that case what is the difference between virtualization of Dispose and Dispose(bool disposing) and what is the point of preferring latter over former apart from unification with unmanaged resources cleanup. – Diaz

@Ionosphere ah, ok) Sorry, I've been spending lots of time removing code like ~A() { Dispose(false); } // just in case, so it's a painful theme for me. – Loadstar

@Xenocrates that doesn't fix this scenario Base foo = new Derived(); foo.Dispose();. The correct fix is to have a virtual Dispose method. Which is part of the dispose pattern. So no, simply editing the code above to call base.Dispose() is still bad code. I will edit. – Ionosphere

@Ionosphere And how is it bad? Just saying, "it's bad" doesn't mean anything. – Xenocrates

@Xenocrates Updated the answer. In short, when I call Dispose on an object I do expect that object to be disposed of. I really don't care what type variable I'm referencing when doing so. That's why virtual methods exist. And why the dispose pattern uses them. – Ionosphere

All you're talking about is perfectly valid, but I still cannot find in your answer a correlation to dispose pattern which is not only about Dispose virtualization but about specific signatures and orders of calls as well. – Diaz

@Diaz This answer does not cover unmanaged resources and finalizers intentionally. I purposefully only wanted to highlight that the pattern is useful even if you're not using unmanaged objects. Which does include the usage of a virtual dispose method. Some may find it helpful. – Ionosphere

@Ionosphere Making the method virtual doesn't do anything to fix the problem though. You still need to call the base class's method whether the method is virtual or not. "The dispose pattern" doesn't do anything to fix that problem. The fix is to call the base class method regardless of which style you're using. And your answer is just wrong. Your "bad" code does in fact dispose of the base class's resources. Virtual methods exist so that when a given method is called on an instance statically typed to the base class, the derived class can completely control the behavior. – Xenocrates

"But it does highlight a common scenario where an event subscriber outlives the publisher. I've seen events cause countless memory leaks." But it doesn't. The publisher goes out of scope, so it gets cleaned up by the GC, so it doesn't extend the lifetime of the subscriber, even if there is no dispose code to wipe the event. – Xenocrates

"But it's really never good practice to call Dispose on an object and not expect it to dispose the entire object" That's true, but the dispose pattern doesn't affect whether or not this is true. It's up to the derived classes to derive the type correct regardless of how the base class is implemented, for the type to function properly. The base class can't force the derived classes to do that, or prevent them from providing a buggy extension of the class. – Xenocrates

Zer0 I take your point. If you want to support your class being derived from then you should make the Dispose method virtual. But you still wouldn’t need to implement the full Dispose pattern – Coreencorel

@Zer0, I'm not sure why you're referring finalizes and unmanfged resources. The pattern itself may exist without these things and this is clearly stated in the article. I however have some doubts regrading reasoning of applying it when simpler ways to achieve the same result exist (the pattern can be applied too, but what is the point to over-complicate), that's primary reason I raised the question. I updated my answer with example of applying of the dispose pattern only when it's needed. – Diaz

Answer 3

You probably got it wrong. You don't need to implement finilizer if you don't have unmanaged resources. You can check it by using automatic pattern implementation in Visual Studio (it will even generate the comments saying that you should uncomment the finalizer ONLY if you use unmanaged resources).

The dispose pattern is used only for objects that access unmanaged resources.

If you design a base class and some of the inheriting classes access unmanaged resources, the inheriting classes will deal with it themselves by overriding Dispose(bool) and defining the finalizer.

It's explained in this article, all finalizers will be called anyway if not suppressed. And if suppressed everything would have been released by the chain of Diapose(true) calls in the first place.

Loadstar

I took this from the article, please see here, marked as important. Not sure how else it can be interpreted. I, btw didn't mean specifically Finalize, but dispose patter at all which also can imply an implementation without a finalizer. – Diaz

@Diaz the quote in my answer is taken from the very same article. Also, as I've already mentioned, check VS's auto implementation. – Loadstar

Answer 4

If Dispose() is implemented using a public virtual method, derived classes that expect to override that method can do so and all will be well. If, however, anything on the inheritance chain implements IDisposable.Dispose() via means other than overriding the public virtual Dispose() method, that can make it impossible for a sub-derived class to implement its own IDisposable.Dispose() while still being able to access the parent implementation.

The Dispose(bool) pattern can be used regardless of whether or not there is a public Dispose() method, and thus avoids the need to have separate patterns for the cases where a class does or does not expose a public Dispose() method. The GC.SuppressFinalize(this) could generally be replaced by a GC.KeepAlive(this), but for classes without finalizers the cost is about the same. In the absence of that call, the finalizers for any objects to which a class holds a reference could get triggered while the class' own Dispose method is running. Not a likely scenario, nor one that would usually cause problems even if it occurs, but passing this to GC.KeepAlive(Object) or GC.SuppressFinalize(Object) makes such quirky situations impossible.

Cortney

IMHO, it should not be a problem, as the references are not meant to be used anyway: finalizer calls Dispose(false) and it should touch only unmanaged resources. – Loadstar

@Sergey.quixoticaxis.Ivanov: I'm not clear what you mean. Race conditions between finalizers and Dispose are rare and usually don't cause problems, but calling a function that is guaranteed to prevent premature finalization of an object and everything to which it holds a reference is easier than trying to ensure that failure to do so can never cause trouble. – Cortney

I mean when you have class A { private B b; private UnmanagedResource ur; }, if you come to Dispose(false) from finalizer, you should not access b at all (only clean your own unmanaged resources in A), thus whether b is already finilized or not is of no importance. Maybe I misunderstand what you mean. – Loadstar

@Sergey.quixoticaxis.Ivanov: Given something like class wrapper { int resource; void closeResource() { apiCleanupFunction(resource); void doSomething() { otherApiFunction(resource); } } class myThing { wrapper it; void Dispose() { it.doSomething(); it.closeResource(); } if there were no GC.KeepAlive(), anywhere, the JIT could replace myThing.Dispose with { int rsrc= it.resource; otherApiFunction(rsrc); someApiFunction(rsrc);} with myThing becoming eligible for finalize() as soon as it.resource is copied to rsrc, which could occur before otherApiFunction() completes. – Cortney

@Sergey.quixoticaxis.Ivanov: If the wrapper class is written properly, it should include its own GC.KeepAlive() or GC.SuppressFinalize() calls to prevent itself from being finalized before execution returns from closeResource(), but if there are no keep-alive calls anywhere weird and wacky things can happen. If the outer class includes a GC.SuppressFinalize() and its Dispose method gets called, that will take prevent these problems even if the inner class is missing a keep-alive that would otherwise be needed. – Cortney

I understand the logic, but it looks like one of the infinite scenarios of "how to increase the chances of things working with broken wrappers/base classes/you name it". If everything implements Dispose pattern and correctly suppresses finilization on itself, there's no need to worry about it, and, I believe, the original question was about the pattern, not about the multitude of broken code snippets. Just in case: I didn't downvote, 'cause your input is still on topic, IMHO. – Loadstar