Why call Dispose()? Memory leak won't occur?

Question

Edit: My question isn't getting the main answer that I was looking for. I wasn't clear. I would really like to know two things:

1. Can NOT calling Dispose() cause memory leaks?
2. What's the worst thing that can happen if you have a large program and never call Dispose() on any of your IDisposable objects?

I was under the impression that memory leaks could occur if Dispose() isn't called on IDisposable objects.

Per the discussion on this thread, my perception was incorrect; a memory leak will NOT occur if Dispose() isn't called.

Why ever bother calling Dispose() then? Is it just to free the resource immediately, instead of sometime later? What's the worst thing that can happen if you have a large program and never call Dispose() on any of your IDisposable objects?

Elo

Its down to how lazy you want to be, and how violent you want the garbage collector to be. – Gaillardia

Dispose() is merely for the deterministic release of unmanaged resources. Nothing more. I'd really hate to wait for the garbage collector to close a file for use elsewhere. – Prehuman

possible duplicate of Proper use of the IDisposable interface – Alcides

@JohnSaunders Thank you for that link. That's an excellent answer. I just pasted it into a document (four pages long) that I'm about to read... – Elo

You need to define "memory leak" to get answer - it is very hard to get conventional "memory leak" (memory never freed till process end) in managed code. – Handmaid

@AlexeiLevenkov I guess what I'm referring to is memory that never gets freed, meaning the application would need to be restarted in order to free the memory on a server. – Elo

I've added answer on memory related issues, the duplicate proposed by @JohnSaunders goes in details for other fallouts of not calling Dispose. – Handmaid

It's not a duplicate, IMHO, because the other question focuses more to unmanaged resources, this question is about all kind of side effects when not calling dispose. – Mcburney

Answer 1

Dispose is used to release non-managed resources. This could mean memory, if a class allocated non-managed memory, but it is more often native objects and resources like open files and database connections.

You often need to call Dispose on a class that itself doesn't have any non-managed resources, but it does contain another class that is disposable and may have non-managed resources.

It's also sometimes useful for developers to implement dispose to ensure deterministic finalization--guaranteeing the order in which resources are freed.

Also note that classes that implement dispose often also have a finalizer to release resourcdes if Dispose is not called. Objects with a finalizer have a different life-cycle than classes without one. When they are ready for GC, the GC will see that they have a finalizer and instead of immediately collecting the object when the GC is ready to, it puts it into the finalization queue. This means that the object lives for one extra GC iteration. When you call dispose, the implementation usually (but is not required to) calls GC.SuppressFinalize() which means the finalizer no longer needs to be called.

If a class implements IDisposable, you should always call Dispose().

Plucky

That's helpful, but I'm still a bit confused. Can memory leaks occur due to not calling Dispose()? – Elo

IDisposable has nothing to do with memory leaks. – Alcides

Should every IDisposable class implement a finalizer/destructor to protect against the possibility of consuming code not calling Dispose()? – Elo

@BobHorn, good question. Only IDisposable classes that actually themselves have non-managed references should implement a finalizer. In those classes, you need to release the non-managed resources. However, it's not safe to follow references to any managed resources in a finalizer because the finalizer does not guarantee any order of finalization or collection and it could be running the finalizer on a class whose references have already been collected. This can lead to unexpected null reference exception or resurrection. tinyurl.com/gcresurrection – Plucky

@SamuelNeff: During Finalize, any objects to which the object being finalized has a direct or indirect reference is guaranteed to exist. What is not guaranteed is whether any such objects have had their finalizers run already. It is in some cases entirely appropriate for a finalizer to access other objects' members; the key requirement is whether the members are designed to be accessed from a finalizer context. – Crystallization

@supercat, do you you have a reference for that? The only thing directly on point that I could find is this: msdn.microsoft.com/en-us/library/b1yfkh5e(v=vs.71).aspx "The Finalize method should not reference any other objects". – Plucky

@SamuelNeff: If one reads any articles about the details of how the finalization mechanism works including things like the "freachable" queue, such things are described in detail. More significantly, one of the primary purposes of using a garbage-collector is to make it impossible for code to access a dangling reference. This means that as long as there is any potential execution path via which code could get a reference to something, the object must continue to exist. It's possible that in the early design stages of .net there was a plan to have the system... – Crystallization

...null out all reference-type fields of any object which becomes eligible for finalization before placing it on the freachable queue. In fact, being able to specify (e.g. via attributes) that certain fields should be nulled out before making an object freachable (with the effect that the objects referenced by those fields would not have to be kept alive an extra generation) could be a useful feature, but having all fields invalidated when an object becomes freachable would greatly complicate any type of cleanup which would require e.g. flushing a buffer before closing a communications pipe. – Crystallization

@SamuelNeff: The pattern of flushing a buffer before closing a pipe, btw, demonstrates that it may be as a practical matter necessary for a finalizable class to hold arrays. It also demonstrates another issue: it's possible (and not hard) to design finalizable class which encapsulates an unmanaged pipe and buffers data for it; a finalizable class which wraps a managed pipe and buffers data, however, will be harder, since it will have to ensure that the managed pipe does not get finalized before the wrapper class can flush its data. That's doable, but it's tricky. – Crystallization

@supercat, Thanks for the details. I'm not arguing or saying you're wrong, you make a lot of sense and sounds very reasonable. I would appreciate an authoritative source if you have one. I looked and couldn't find one. – Plucky

Answer 2

While some other answers seem to be suggesting that you can get away with not calling it, this is really bad advice. You should always call Dispose on any IDisposable resource.

Some .NET objects have what's called a "finalizer" - something you can define on your own classes as well, but that you rarely see done in a typical C# programmer's code. The finalizer is what runs when the garbage collector destroys the object, and sometimes it will call Dispose - but only if the implementer of the class made it that way.

The best practice is to always Dispose - no matter what. There are plenty of libraries I've used where not calling Dispose on a resource results in a memory leak, a connection leak, a operating system resource leak, or other kinds of horribleness. And the garbage collector will not reconcile the problem, because they don't implement any custom finalizer.

See related: Will the Garbage Collector call IDisposable.Dispose for me?

Mcmaster

Answer 3

The convention is that if an object implements IDisposable you should call Dispose() or use the "using" pattern. The difference between Dispose() and waiting for the destructor (finalizer) to execute is that Dispose() is called right away and can be used for freeing some important resources like db connections, files, devices, unmanaged oejects, etc.

So to summarize - if it is IDisposable - Dispose() it!

Oratory

Answer 4

Not calling Dispose will not ever (*see note 2 on wrong implementation) cause traditional "memory leak" (memory is never freed till end of the process).

The "only" thing that will happen in relation to memory is it will be freed in non-deterministic moment in the future.

One interesting case of non Dispose objects is when very small managed objects hold large amounts of unmanaged memory (i.e. allocated with some flavor of Win32 memory management functions i.e. HeapAlloc ). In this case managed memory manager may not be able to detect memory pressure properly to trigger Gen2 GC and (especially in case of x86 - 32bit process) it may prematurely fail to allocate managed memory for your process. The other issue in this case is fragmentation of address space by "waiting for GC to be de-allocated" (again mostly in x86 case) - when smaller chunks of native memory are allocated with some relatively large space between them preventing allocation of large blocks needed for managed memory management.

Notes:

1. This answer explicitly talks about true memory leaks/memory allocation issues cased by not disposing IDisposable object managing memory. While it is true that there are no "true memory leaks" caused by such practice most people will consider growing memory usage as memory leak (similar to storing large amount of objects in static list/dictionary for lifetime of an application).
2. One can create object that manages native memory and incorrectly implements IDisposable pattern. In this case it is possible to really leak native memory (irrespective of calling Dispose).
3. In most cases objects that implement IDisposable don't managed memory at all. For most practical C# programs native resources managed by such objects are handles for system resources like files, bitmaps, fonts, synchronization objects or COM native objects. Not disposing them in timely manner will cause other issues.

Dispose all objects properly. There is no excuse not to.

Handmaid

+1 This is helpful. Memory "leaks" won't occur if you don't call Dispose(), however, memory could be used for longer than necessary, which could cause problems. – Elo

Failure to call Dispose may cause an unbounded memory leak even within the realm of managed code. An object which subscribes to an event from another object and never unsubscribes will have its memory lifetime extended to that of the latter object. If during the lifetime of a long-lived objects an unbounded number of objects are created that subscribe to events from it and are abandoned without unsubscribing, the number of useless objects that must be kept in memory will grow without bound. – Crystallization

@Crystallization - good point, but note that what you describe is not true memory leak (unmanged memory that will never be freed due to lost pointer), but indeed unbounded memory usage. – Handmaid

@AlexeiLevenkov: I would use the term "memory leak" to describe a program which requires an unbounded amount of memory to handle a bounded amount of semantically-meaningful state. If an OS does not free an application's memory on exit, even a single unreleased allocation would constitute a memory leak; if, however, the OS frees all allocations on exit (I can't think of any modern ones that don't) I can't think of any meaningful definition of "leak" other than the above, and it applies just as well to managed and unmanaged code. – Crystallization

Answer 5

Dispose() is intended to free resources that the garbage collector won't free, such as database connections. These resources should also be freed in the finalizer, however the finalizer is much slower than the Dispose() method.

Wellmeaning

Answer 6

For me:

Dispose can be used in using() scope. This can help me to determine the lifespan of a IDisposeable component. I usually use this in StreamWriter/Reader or SqlConnection classes.

Another use of Dispose is it can end a lifespan of a component explicitly. Such as calling Form.Dispose() in C# winform will close the form. However, for SqlConnection, people said that just calling Dispose alone without explicitly calling Close won't guarantee the connection to be closed. Advised to call both Close and Dispose. I haven't tried this though.

And another thing, after Dispose() is called, the GC can immediaetly free the memory, because they know that the object lifespan is end, rather than waiting for the lifespan to end.

Similiar question may be C# disposing IDisposable

Iredale

Answer 7

Can NOT calling Dispose() cause memory leaks?

Yes, of course. Below is just one example.

Assume you have a main window in your application and you create a child control that has an event subscription to the main window. You unsubscribe from them on Dispose. If you don't dispose, main window can hold the reference to your child control until you close the application.

What's the worst thing that can happen if you have a large program and never call Dispose() on any of your IDisposable objects?

The worse case is not releasing some unwanted memory until you close the application.

And the other question is, what if you never implement IDisposable or finalization when they are required?

The worse case of having a memory leak is holding on to that memory until you restart the PC. This can happen only if you have un-managed resources and you don't implement dispose/finalize. If you implement Idisposable interface and implement the finalizer, finalization process will execute the Dispose for you.

Another reason why you should call Dispose is to Suppress the finalization.

As I've indicated before if there is any object with Finalize method and you didn't call the Dispose. That object can reside in memory for two GC cycles. In the first cycle, it enqueue that instance to the finalization queue and finalization happens after GC process. So, only next GC cycle can release that memory.

Graciagracie