Why is it always necessary to implement IDisposable on an object that has an IDisposable member?

Question

From what I can tell, it is an accepted rule that if you have a class A that has a member m that is IDisposable, A should implement IDisposable and it should call m.Dispose(

Answer 1

But if Image has followed conventions and implemented a finalizer and I don't care about the timely release of resources, what's the point?

You've missed the point of Dispose entirely. It's not about your convenience. It's about the convenience of other components that might want to use those unmanaged resources. Unless you can guarantee that no other code in the system cares about the timely release of resources, and the user doesn't care about timely release of resources, you should release your resources as soon as possible. That's the polite thing to do.

In the classic Prisoner's Dilemma, a lone defector in a world of cooperators gains a huge benefit. But in your case, being a lone defector produces only the tiny benefit of you personally saving a few minutes by writing low-quality, best-practice-ignoring code. It's your users and all the programs they use that suffer, and you gain practically nothing. Your code takes advantage of the fact that other programs unlock files and release mutexes and all that stuff. Be a good citizen and do the same for them. It's not hard to do, and it makes the whole software ecosystem better.

UPDATE: Here is an example of a real-world situation that my team is dealing with right now.

We have a test utility. It has a "handle leak" in that a bunch of unmanaged resources aren't aggressively disposed; it's leaking maybe half a dozen handles per "task". It maintains a list of "tasks to do" when it discovers disabled tests, and so on. We have ten or twenty thousand tasks in this list, so we very quickly end up with so many outstanding handles -- handles that should be dead and released back into the operating system -- that soon none of the code in the system that is not related to testing can run. The test code doesn't care. It works just fine. But eventually the code being tested can't make message boxes or other UI and the entire system either hangs or crashes.

The garbage collector has no reason to know that it needs to run finalizers more aggressively to release those handles sooner; why should it? Its job is to manage memory. Your job is to manage handles, so you've got to do that job.

Answer 2

Firstly, there's no guaranteeing when an object will be cleaned up by the finalizer thread - think about the case where a class has a reference to a sql connection. Unless you make sure this is disposed of promptly, you'll have a connection open for an unknown period of time - and you won't be able to reuse it.

Secondly, finalization is not a cheap process - you should be making sure that if your objects are disposed of properly you're calling GC.SuppressFinalize(this) to prevent finalization happening.

Expanding on the "not cheap" aspect, the finalizer thread is a high-priority thread. It will take resources away from your main application if you give it too much to do.

Edit: Ok, here's a blog article by Chris Brummie about Finalization, including why it is expensive. (I knew I'd read loads about this somewhere)

Answer 3

If you don't care about the timely release of resources, then indeed there is no point. If you can be sure that the code is only for your consumption and you've got plenty of free memory/resources why not let GC hoover it up when it chooses to. OTOH, if someone else is using your code and creating many instances of (e.g.) MyImage, it's going to be pretty difficult to control memory/resource usage unless it disposes nicely.

Answer 4

Many classes require that Dispose be called to ensure correctness. If some C# code uses an iterator with a "finally" block, for example, the code in that block will not run if an enumerator is created with that iterator and not disposed. While there a few cases where it would be impractical to ensure objects were cleaned up without finalizers, for the most part code which relies upon finalizers for correct operation or to avoid memory leaks is bad code.

If your code acquires ownership of an IDisposable object, then unless either the object's cleass is sealed or your code creates the object by calling a constructor (as opposed to a factory method) you have no way of knowing what the real type of the object is, and whether it can be safely abandoned. Microsoft may have originally intended that it should be safe to abandon any type of object, but that is unrealistic, and the belief that it should be safe to abandon any type of object is unhelpful. If an object subscribes to events, allowing for safe abandonment will require either adding a level of weak indirection to all events, or a level of (non-weak) indirection to all other accesses. In many cases, it's better to require that a caller Dispose an object correctly than to add significant overhead and complexity to allow for abandonment.

Note also, btw, that even when objects try to accommodate abandonment it can still be very expensive. Create a Microsoft.VisualBasic.Collection (or whatever it's called), add a few objects, and create and Dispose a million enumerators. No problem--executes very quickly. Now create and abandon a million enumeartors. Major snooze fest unless you force a GC every few thousand enumerators. The Collection object is written to allow for abandonment, but that doesn't mean it doesn't have a major cost.

Answer 5

But if Image has followed conventions and implemented a finalizer and I don't care about the timely release of resources, what's the point?

Then there isn't one, if you don't care about timely release, and you can ensure that the disposable object is written correct (in truth I never make an assumption like that, not even with MSs code. You never know when something accidentally slipped by). The point is that you should care, as you never know when it will cause a problem. Think about an open database connection. Leaving it hanging around, means that it isn't replaced in the pool. You can run out if you have several requests come in for one.

Nothing says you have to do it if you don't care. Think of it this way, it's like releasing variables in an unmanaged program. You don't have to, but it is highly advisable. If for no other reason the person inheriting from the program doesn't have to wonder why it wasn't taken care of and then try and clear it up.

Answer 6

Even if you don't care in this particular case, you should still follow the standard because you will care in some cases. It's much easier to set a standard and follow it always based on specific guidelines than have a standard that you sometimes disregard. This is especially true as your team grows and your product ages.