C# Will the typical IDisposable pattern not cause resource leak in some condition

Question

Below is a typical IDisposable implementation for C# class which has managed and unmanaged resources both. My question is, Can there be a situation where ~DisposableObject() destructor method gets called before the Dispose() method of the class for any given object. And if that is possible, will it not set the flag disposed to false, and the managed resources will never get a chance to be disposed off because Dispose(bool) method does not do anything if disposed flag is set to false.

public class DisposableObject : IDisposable
    {
        private bool disposed;
        private ManagedResource mgdRs;
        private UnmanagedResource unMgdRs;

        public DisposableObject()
        {
            mgdRs = new ManagedResource();
            unMgdRs = new UnmanagedResource();
        }

        ~DisposableObject()
        {
            this.Dispose(false);
        }
    
        public void Dispose()
        {    
             this.Dispose(true);
             GC.SuppressFinalize(this);       
        }

        protected virtual void Dispose(bool disposing)
        {
            if (!this.disposed)
            {
                if (disposing)
                {
                    // clean up managed resources
                    //call dispose on your member objects
                    mgdRs.Dispose();
                }

                // clean up unmanaged resources
                unMgdRs.Destroy();
                this.disposed = true;
            }
        }
    }
}

https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-dispose i find your a little hard to follow the ms example one is a little easier.. i dont think they exactly the same. when i follow the ms one its eaiser and make sense strugging to see if ur has an issue. basically if its already be called then handle. — Seabizkit, Oct 01 '20 at 19:08
You're asking if it's possible for the finalizer to run before `Dispose()` is called? Then yes, because it is legal for a program to create a disposable object and then never call `Dispose()` on it: nothing in the language _requires_ `Dispose()` to be called. If `ManagedResource` has its own finalizer then that finalizer _may_ have run by the time the finalizer for `DisposableObject` runs. There isn't really anything you can do about that because [finalizers might not ever run anyway](https://ericlippert.com/2015/05/18/when-everything-you-know-is-wrong-part-one/). — Jeff, Oct 01 '20 at 19:08
destructors can be called an any time.... even way before you think it is — Seabizkit, Oct 01 '20 at 19:09
Discussion related to the modified document @Seabizkit linked: [The Dispose guidelines really should be updated to point developers in the right direction](https://github.com/dotnet/docs/issues/8463). It's long, but you have to read it all. — Jimi, Oct 01 '20 at 19:14
I find the official guidance pretty confusing and always refer to Stephen Cleary's advice ([Stephen's blog](https://blog.stephencleary.com/2009/08/how-to-implement-idisposable-and.html) and [this codeproject article](https://www.codeproject.com/Articles/29534/IDisposable-What-Your-Mother-Never-Told-You-About) on IDisposable. In this case: refactor so that your class only handles either managed or unmanaged resources, then follow the guidelines in the linked article. — jeroenh, Oct 03 '20 at 09:11

Joel Coehoorn · Accepted Answer · 2020-10-02T14:34:24.730

2

Yes, it is possible for the finalizer to be called first. However, there are three things to understand here.

First, it's very rare in C# to need to implement the full IDisposable pattern. The full pattern is only needed when you have both managed and original unmanaged resources in the same type.

Most unmanaged resources you deal with already have a managed wrapper you rely on instead, and therefore there is no need to implement another finalizer/destructor. Additionally, developers implementing an original unmanaged resource requiring a finalizer will often limit themselves to just that one resource, such that there are no managed resources in the same type to create the conflict from the question. In this way, the unmanaged resource becomes managed, and now can be included as part of a simpler pattern.

The second thing to understand is the finalizer runs as part of garbage collection. Given the finalizer is running, we know the unmanaged resources are cleaned up, and only managed resources remain. The very definition of a managed resource is something that the garbage collector will take of. Therefore, if the finalizer runs too soon, causing the Dispose() call to miss the managed resources, that's okay, because the garbage collector is already running and will take care of it.

This brings me to my third thing. Not only may the finalizer be called first, but when that happens its very likely the finalizer is called only, because it would be highly unusual for anything but the garbage collector to invoke the finalizer. If the garbage collector is running for an object, there won't be any reference remaining which might be used to also call Dispose(), whether intentionally or implicitly. And again, that's okay: the finalizer is running, so we know unmanaged resources will be cleaned up, and the garbage collector is running, so we know managed resources will also be cleaned up.

edited Oct 02 '20 at 14:34

answered Oct 01 '20 at 19:12

Joel Coehoorn

399,467
113
570
794

1

im pretty sure that is incorrect. garbage collector will not collect unmanaged resources like u suggest in ur last comment. if that was the case then it wouldn't be unmanaged. #tag – Seabizkit Oct 01 '20 at 19:15
@Seabizkit I'm not suggesting the garbage collector will take care of unmanaged resources. I'm reminding the OP it's only **managed** (without the "un") resources left orphaned if the finalizer runs too soon, and garbage collector _will_ take care of those. – Joel Coehoorn Oct 01 '20 at 19:16
1

"Yes, it is possible (though rare) for the finalizer to be called first." How is this possible? Is there any documented case? I don't think finalizer can anytime be executed before `Dispose()`. Calling dispose requires a hard reference. Of course I'm not talking about an object ressurection case and zombie objects - it's obviously possible in this case. – Adassko Oct 01 '20 at 19:17
1

@Adassko `var f = new FileStream(path);` pretty much guarantees that finalizer will be executed before `Dispose` (which will likely never be called as one not using `using` is unlikely to dispose objects either). – Alexei Levenkov Oct 01 '20 at 19:49
@JoelCoehoorn - Thanks for answering clearly, that this(Destructor before Dispose(true)) situation is possible, however it is very rare. Now in this light, should I move the if(disposing){} statement out of the if(!this.disposed){} statement inside Dispose(bool) method. This will allow managed resources to be disposed off correctly even in the rare incident of Destructor getting called before Dispose(true). – anands Oct 01 '20 at 19:52
1

@AnandSrivastava Does your situation really have _original_ unmanaged resources involved at all? This is ***very rare***. – Joel Coehoorn Oct 01 '20 at 20:27
@AlexeiLevenkov: _"pretty much guarantees that finalizer will be executed before Dispose (which will likely never be called..."_ -- please clarify. Naturally, a scenario where `Dispose()` is not called at all does mean the finalizer will be called before. But that's not an interesting or relevant scenario. The interesting and relevant scenario is code that somehow has a reference to an object so that it can call `Dispose()`, and yet the finalizer was executed anyway. This does not seem possible. That the reference is reachable will prevent the object from being put in the finalizer queue. – Peter Duniho Oct 01 '20 at 20:42
@PeterDuniho I don't think the question (and the answer) actually limits discussion to cases when one calls `Dispose` (directly or with `using`). I agree that it is impossible (or at very least unlikely - I can't come up with any such case) that finalizer is called before Dispose *if* `Dispose` actually written out in the code for that object. – Alexei Levenkov Oct 01 '20 at 21:06
1

@Alexei: ah...I see how your reading of the question is different from mine. FWIW, based on _"because Dispose(bool) method does not do anything if disposed flag is set to false"_, I take as a given that the OP is in fact _specifically_ asking about the `Dispose()` method actually being called (but after the finalizer), and not about the more general scenario of the finalizer being called without `Dispose()` having been called first (which should never happen anyway, since a correct `Dispose()` will call `SuppressFinalize()`) nor ever being called. – Peter Duniho Oct 01 '20 at 21:52
@PeterDuniho Theoretically you could call Dispose after the finalizer runs with a weak reference, with unsafe/unmanaged code, by reviving an object from the finalizer, or other contrived situations, which is why the answer is correct to say it would be very unusual for Dispose to be called after the finalizer, but not impossible. – Servy Oct 02 '20 at 15:03
It's also possible (though again, awkward and very unusual) to call the finalizer manually. – Joel Coehoorn Oct 02 '20 at 15:08
@Servy: all of those examples would involve code explicitly bypassing the usual mechanisms. I guess until the OP clarifies, they _could_ mean what you think, but it seems unlikely. Why would they ask about something that they would have to know is actually happening explicitly? (The weak reference is a bad example, by the way, because if the finalizer has run, trying to retrieve the target reference will throw an exception.) – Peter Duniho Oct 02 '20 at 16:04
@PeterDuniho The weak reference still works. It will throw after the GC actually destroys the object, which happens at some arbitrary point in time after the finalizer runs. The object is still technically rooted and alive when in the finalizer queue or when the finalizer is running. As for "That not being what the question is asking", the question *presupposes* that you can dispose the object after the finalizer, and asks why that is not demonstrating a flaw in the pattern. You said their assumption was invalid, I've explained why it is valid (just probably not worth accounting for). – Servy Oct 02 '20 at 16:19
@PeterDuniho - You have captured the question accurately. I am very new to C# however I have some experience in C++. In C++ all the resource acquired by the object are cleaned up inside destructor method. In my post here, I had taken the object dispose code from MS documentation and the way the method was written, I was wondering if there could ever be a situation where I end up not cleaning up managed resources because of Destructor getting called before the Dispose(of IDisposable) method implemented inside the class. – anands Oct 04 '20 at 10:40
@AnandSrivastava If you're coming from C++, you have probably seen the C# equivalent is a finalizer... but you may not have seen yet that you almost never have to actually write them. – Joel Coehoorn Oct 04 '20 at 14:18
@JoelCoehoorn - Oh ok, this is other way round in C++, there is no way possible that any method of the object is called after the Destructor(or Finalizer in C# lingo) has been called. So to confirm my understanding, In normal/legitimate C# program the Finalizer/Destructor will be the last method to be called on an object. However there are only some and highly unusual scenarios where Dispose or any other method may be called after Finalizer/Destructor. – anands Oct 04 '20 at 19:14
@AnandSrivastava If you're new to C#, the best thing you could do is forget finalizers even exist. The point of managed language is you let the garbage collector think about clean-up. – Joel Coehoorn Oct 04 '20 at 20:29
@JoelCoehoorn - Can you help me understand why I got down voted(-1). – anands Oct 11 '20 at 13:56
@anands I'm not sure, but it may be because there are many other questions about IDisposable, so someone felt this was a duplicate. – Joel Coehoorn Oct 11 '20 at 16:22

C# Will the typical IDisposable pattern not cause resource leak in some condition

1 Answers1