Disposing my System.IDisposable object in my finalizer

Question

There are several discussions here on StackOverflow about what to do if my object manages other managed objects that implement System.IDisposable.

Note: Below I am not talking about unmanaged code. I fully understand the importance of cleaning up unmanaged code

Most of the discussions say that if your object owns another managed object that implements System.IDisposable, then you should also implement System.IDisposable, in which case you should call the Dispose() of the disposable objects your object holds. This would be logical, because you don't know whether the disposable object you own uses unmanaged code. You only know that the creator of the other object thought it would be wise if you'd call Dispose as soon as you don't need the object anymore.

A very good explanation of the Disposable pattern was given here on StackOverflow, edited by the community wiki:

Proper use of the IDisposable interface

Quite often, and also in the mentioned link I read:

"You don't know the order in which two objects are destroyed. It is entirely possible that in your Dispose() code, the managed object you're trying to get rid of is no longer there."

This baffles me, because I thought that as long as any object holds a reference to object X, then object X will not and cannot be finalized.

Or in other words: as long as my object holds a reference to object X I can be certain that object X is not finalized.

If this is true, then why could it be, that if I hold the reference to my object until my finalize, the object I refer to is already finalized?

Answer 1

After all the answers, I created a small program that shows what Jodrell wrote (thank you Jodrell!)

• An object can be garbage collected as soon as it will not be used, even if I have a reference to it
• This will only be done if not debugging.

I wrote a simple class that allocates unmanaged memory and a MemoryStream. The latter one implements System.IDisposable.

According to everyone on StackOverflow I should implement System.IDisposable and free the unmanaged memory as well as Dispose the managed memoryStream if my Dispose is called, but if my finalizer is called I should only free the unmanaged memory.

I write some diagnostic console messages

class ClassA : System.IDisposable
{
    IntPtr memPtr = Marshal.AllocHGlobal(1024);
    Stream memStream = new MemoryStream(1024);
        
    public ClassA()
    {
        Console.WriteLine("Construct Class A");
    }
    
    ~ClassA()
    {
        Console.WriteLine("Finalize Class A");
        this.Dispose(false);
    }
    
    public void Dispose()
    {
        Console.WriteLine("Dispose()");
        this.Dispose(true);
        GC.SuppressFinalize(this);
    }
    
    public void Dispose(bool disposing)
    {
        Console.WriteLine("Dispose({0})", disposing.ToString());
        if (!this.IsDisposed)
        {
            if (disposing)
            {
                Console.WriteLine("Dispose managed objects");
                memStream.Dispose();
            }

            Console.WriteLine("Dispose unmanaged objects");
            Marshal.FreeHGlobal(memPtr);                
        }
    }

    public bool IsDisposed { get { return this.memPtr == null; } }
}

This program follows the Dispose Pattern as described numerous times, a.o. here in stackoverflow in Proper use of the IDisposable interface

by the way: for simplicity I've left out exception handling

A simple console program creates the object, doesn't use it, but keeps the reference to it and forces the garbage collector to collect:

private static void TestFinalize()
{
    ClassA a = new ClassA() { X = 4 };

    Console.WriteLine("Start Garbage Collector");
    GC.Collect();
    GC.WaitForPendingFinalizers();
    Console.WriteLine("Done");
}

Note that variable a holds a reference to the object until the end of the procedure. I forget to Dispose, so my finalizer should take care of disposing

Call this method from your main. Run the (release) build from the debugger and run it via a command prompt.

• If run from the debugger then the object stays alive until the end of the procedure, so until after the garbage collector has finished collecting
• If run from a command prompt, then the object is finalized before the procedure ends, even though I still have a reference to the object.

So Jodrell is right:

• unmanaged code needs Dispose() and Finalize, use Dispose(bool)

• Managed disposable objects need Dispose(), preferably via Dispose(bool). In Dispose(bool) only call Dispose() of managed objects if disposing

• don't trust the debugger: it makes that objects are finalized on different moments than without debugger

Answer 2

In most cases when Finalize is called on an object which holds references to one or more IDisposable objects, one or more of the following will apply:

1. The other object has already been cleaned up, in which case calling Dispose is at best useless.
2. The other object has been scheduled to be finalized as soon as possible, but hasn't yet, in which case calling Dispose is probably unnecessary.
3. The other object's cleanup code cannot be used safely within the finalizer threading context, in which case calling Dispose could likely be disastrous.
4. The other object is still being used by code elsewhere, in which case calling Dispose could likely be disastrous.

There are a few situations where code knows enough about IDisposable objects that it's dealing with to know either that none of the above apply, or that it should trigger cleanup despite the above; those situations, however, may be better served by having the other objects supply a method other than Dispose which the object being finalized can call.

Answer 3

Quoting Eric Lippert's, When everything you know is wrong, part two

Myth: Keeping a reference to an object in a variable prevents the finalizer from running while the variable is alive; a local variable is always alive at least until control leaves the block in which the local was declared.

{   
     Foo foo = new Foo();
     Blah(foo);  // Last read of foo
     Bar();
     // We require that foo not be finalized before Bar();
     // Since foo is in scope until the end of the block,
     // it will not be finalized until this point, right?
}

The C# specification states that the runtime is permitted broad latitude to detect when storage containing a reference is never going to be accessed again, and to stop treating that storage as a root of the garbage collector. For example, suppose we have a local variable foo and a reference is written into it at the top of the block. If the jitter knows that a particular read is the last read of that variable, the variable can legally be removed from the set of GC roots immediately; it doesn’t have to wait until control leaves the scope of the variable. If that variable contained the last reference then the GC can detect that the object is unreachable and put it on the finalizer queue immediately. Use GC.KeepAlive to avoid this.

Why does the jitter have this latitude? Suppose the local variable is enregistered into the register needed to pass the value to Blah(). If foo is in a register that Bar() needs to use, there’s no point in saving the value of the never-to-be-read-again foo on the stack before Bar() is called. (If the actual details of the code generated by the jitter is of interest to you, see Raymond Chen’s deeper analysis of this issue.)

Extra bonus fun: the runtime uses less aggressive code generation and less aggressive garbage collection when running the program in the debugger, because it is a bad debugging experience to have objects that you are debugging suddenly disappear even though the variable referring to the object is in scope. That means that if you have a bug where an object is being finalized too early, you probably cannot reproduce that bug in the debugger!

See the last point in this article for an even more horrid version of this problem.

Answer 4

If done properly, you don't have to worry about disposing objects that are already disposed. Every implementation of Dispose should just not do anything if it had been disposed before.

So indeed, you cannot know whether any child objects have been disposed or finalized already (because the order of finalization is random, see other post), but you can safely call their Dispose method anyway.

Answer 5

The truth is somewhere between the two:

• The object can't be garbage collected, so the possibility of the object no longer "being there" isn't true
• An object can be finalized when there are no longer any references to it from other non-finalizable objects.

If object X refers to object Y, but both are finalizable, then it's entirely possible for object Y to be finalized before object X, or even for them to be finalized concurrently.

If your assumption were correct, then you could create two objects which refer to each other (and have finalizers), and they could never be garbage collected because they could never be finalized.