CODEWITHSUNDEEP
c#.netdisposedestructorfinalizer
Daniel Peñalba
Daniel Peñalba

Reputation: 31857

Two questions about Dispose() and destructors in C#

I have a question about how to use Dispose() and destructors. Reading some articles and the MSDN documentation, this seems to be the recommended way of implementing Dispose() and destructors.

But I have two questions about this implementation, that you can read below:

class Testing : IDisposable
{
    bool _disposed = false;

    protected virtual void Dispose(bool disposing)
    {
        if (!_disposed) // only dispose once!
        {
            if (disposing)
            {
                // Not in destructor, OK to reference other objects
            }
            // perform cleanup for this object
        }
        _disposed = true;
    }

    public void Dispose()
    {
        Dispose(true);

        // tell the GC not to finalize
        GC.SuppressFinalize(this);
    }

    ~Testing()
    {
        Dispose(false);
    }
}

GC.SupressFinalize(this) on Dispose()

When the programmer uses using or calls Dispose() explicity, our class is calling to GC.SupressFinalize(this). My question here is:

Finalizing without a Dispose() call

Suppose that the GC is going to clean our object but the programmer did not call Dispose()

Thanks in advance

Upvotes: 16

Views: 3717

Answers (3)

Henk Holterman
Henk Holterman

Reputation: 273244

1. What does SuppressFinalize do?

It unregisters the object from the finalizer list, ie when the GC later collects the object it will ignore the presence of the destructor. This is a big gain in performance since the destructor would otherwise require the object's collection, and that of everything it references, to be delayed.

2. Why don't we dispose [managed] resource at this point? In other words, why can't we free [managed] resources on destructor?

You could, but it is certain to be pointless: the object you're in has become unreachable so all those owned managed resources are unreachable too. They will be Finalized and collected by the GC in the same run and calling Dispose() on them is unnecessary but not totally without risk or cost.

2a What code must be executed in the if inside, and what outside?

Inside the if(disposing), call _myField.Dispose()

In other words, Dispose of managed resources (objects with a Dispose)

Outside, call code to cleanup (close) unmanaged resources, like Win32API.Close(_myHandle).

Note that when you don't have unmanaged resources, as will usually be the case (look up SafeHandle), you don't need a destructor and therefore no SuppressFinalize.

And that makes that the complete (official) implementation of this pattern is only needed because of the possibility that Test is inherited from.
Note that the Dispose(bool) is protected. When you declare your class Testing to be sealed, it is totally safe and conforming to omit the ~Testing().

Upvotes: 13

supercat
supercat

Reputation: 81159

The vast majority of the time when an object which owns IDisposable resources is finalized, at least one of the following statements will apply to each of those resources:

  1. It has already been finalized, in which case no cleanup is necessary.
  2. Its finalizer hasn't yet run, but is scheduled to do so, in which case no cleanup is necessary.
  3. It can only be cleaned up within a particular thread (which isn't the finalizer thread), in which case the finalizer thread must not try to clean it up.
  4. It may still be in use by someone else, in which case the finalizer thread must not try to clean it up.

In some of the rare circumstances where none of the above apply, cleanup within the finalizer might be appropriate, but one shouldn't even consider it unless one has first examined the above four possibilities.

Upvotes: 1

NOtherDev
NOtherDev

Reputation: 9672

First part:

When GC.SupressFinalize(this) is called, GC is informed that the object has already freed its resources and it can be garbage-collected as any other object. And yes, finalization and "destructors" are the same thing in .NET.

Second part:

Finalization is done by a separate thread and we have no control on time and order of finalization, so we don't know whether any other objects are still available or are already finalized. For this reason, you can't reference other object outside the disposing block.

Upvotes: 2

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