Dispose Dos and Don'ts

re: Dispose Dos and Don'ts

Richard Blewett — Thu, 23 Sep 2004 16:58:00 GMT

In terms of ObjectDisposedException I'd argue that you should *always* throw it after a client has called Dispose (other than recalling the Dispose method). They don't know which methods access a resource you release and which don't (unless they have an uhealthy knowledge of your implementation).

Dispose to me means "I'm finished with you". So in that case they have no business calling your methods beyond that point.

re: Dispose Dos and Don'ts

Chris Lyon — Thu, 23 Sep 2004 17:21:00 GMT

Hi Richard

You could imagine a class that has a property like IsClosed, or a static method that you wouldn't want to throw. Also consider inherited methods (in particular methods that inherit from System.Object, like ToString()).

I hesitate to lay down "always do it this way..." rules, because there are always exceptions.

re: Dispose Dos and Don'ts

Richard Blewett — Thu, 23 Sep 2004 17:22:00 GMT

How could you end up in a race condition between the finalizer and dispose? If dispose is running, someone needs to have called it - presumable from a live root. So unless the GC has made a catastrophic error and decided you were garbage when you had a live root; or another object called your Dispose in their Finalizer (which they definitely shouldn't) - how is the Finalizer going to be running at the same time as Dispose?

re: Dispose Dos and Don'ts

Richard Blewett — Thu, 23 Sep 2004 17:25:00 GMT

OK - granted about statics - I wouldn't consider statics anyway as they are not involved with the instance being Disposed. But I guess the IsClosed is valid. I suppose I prefer to lay down an absolute and then let people argue why their situation is different ;-)

re: Dispose Dos and Don'ts

Chris Lyon — Thu, 23 Sep 2004 17:25:00 GMT

Richard, please read the comments from my previous blog entry and the links to Chris Brumme's blog for an explanation on this race condition.

re: Dispose Dos and Don'ts

Chris Lyon — Thu, 23 Sep 2004 17:32:00 GMT

Richard, you probably don't want to throw inside overloaded operators or methods inherited from interfaces, just to name a few more examples.

re: Dispose Dos and Don'ts

Richard Blewett — Thu, 23 Sep 2004 17:38:00 GMT

Ugghh - Resurrection

Is there actually a useful purpose for resurrection? I have to say its always looked like an abomination to me and so never really been that bright on my radar.

I find it slightly perverse to prefer guarding against someone resurrecting my object than against protecting against clean up failure in Dispose. To me the latter is a much more likely scenario and anyone resurrecting my object deserves everything they get ;-)

re: Dispose Dos and Don'ts

Chris Lyon — Thu, 23 Sep 2004 17:41:00 GMT

It's clear to me now that I need to dedicate a whole blog entry on resurrection :)

Unless resurrection is an expected scenario, guarding against it isn't really necessary (most BCL classes don't).

re: Dispose Dos and Don'ts

Richard Blewett — Thu, 23 Sep 2004 17:53:00 GMT

Chris, I can kind of see why throwing in an operator overload would be bizarre for a consumer, but why do you say an interface implementation? What if that implementation accesses the released resource?

re: Dispose Dos and Don'ts

Chris Lyon — Thu, 23 Sep 2004 18:00:00 GMT

Interfaces generally specify behaviour, not implementation, so the Interface is often seperate from the resource.

For example, IClomparable.CompareTo shouldn't necessarily throw if the object has been disposed of.

Again, these are guidelines, not hard rules.

re: Dispose Dos and Don'ts

Richard Blewett — Thu, 23 Sep 2004 20:52:00 GMT

Agreed that interface define behaviour, but the issue is whether the implementation can satisfy that behaviour once Dispose has been called. But as you say - they are only guidelines (have you seen Pirates of the Caribbean? ;-) )

I'm looking forward to your blog post on resurrection - it will be interesting to here about the BCL classes that have been built with it in mind.

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