Does Garbage Collector Call Dispose()

Does garbage collector call Dispose()?

The GC does not call Dispose, it calls your finalizer (which you should make call Dispose(false)).

Please look at the related posts on the side or look up the C# best practices for the Dispose pattern (The docs on IDisposable explain it quite well IIRC.)

Will the Garbage Collector call IDisposable.Dispose for me?

The .Net Garbage Collector calls the Object.Finalize method of an object on garbage collection. By default this does nothing and must be overidden if you want to free additional resources.

Dispose is NOT automatically called and must be explicity called if resources are to be released, such as within a 'using' or 'try finally' block

see http://msdn.microsoft.com/en-us/library/system.object.finalize.aspx for more information

Should I call GC.Collect() in Dispose() method?

The dispose never called Garbage Collector automatically, the dispose design to free un-managed resource. The Garbage Collector execution is scheduled processes which will run in after specified time.

When you call .Net Garbage Collector, it calls the Object.Finalize method of an object on garbage collection to free manage resource, that's why your memory use count show less number

Will .Net call Dispose for me in this case?

Here's everything you'll need to know about IDisposable: CLR Inside Out:
Digging into IDisposable

In short, Dispose will not be called by the Garbage Collector, but if the object has a finalizer, the finalizer will be called. All well-designed implementations of IDisposable that hold unmanaged resources will have a finalizer that does the same as the Dispose method does.

However, if you wait for the Garbage Collector to clear up your memory, you will get non-deterministic behaviour, and it's also likely that your application will use memory in a non-optimal way. Not disposing of disposable resources is likely to adversely affect performance.

However, in F#, it's often easy to ensure that objects are automatically disposed of: just use the use keyword instead of a let binding.

However, that doesn't work in this particular example because Foo is a class, which means that disposable isn't a value, but rather a field. That means it doesn't go out of scope until the object itself goes out of scope, but AFAIK, there's no specific language construct to support that (neither is there in C#).

Here's how you can implement the Dispose idiom for Foo:

open System

type Foo() =
    let disposable:IDisposable = // create an IDisposable
    abstract member Dispose : disposing : bool -> unit
    default this.Dispose disposing =
        if disposing then disposable.Dispose ()
    interface IDisposable with
        member this.Dispose () = 
            this.Dispose true
            GC.SuppressFinalize this

The only part that doesn't quite fit into the 'standard' Dispose idiom is that the virtual Dispose method (bool -> unit) isn't protected, because F# doesn't support that (AFAIK).

Calling Dispose will not clean up the memory used by an object C#?

GC never calls Dispose method, as soon as you do it manually in finalizer

The workflow of GC is pretty simple, but a lot of people get confused, let's put things to their places

Dispose is not related to GC. Dispose is a method, that is defined in interface IDisposable, and like other interfaces, classes can implement that method and give users ability to call that method. GC doesn't pay attention to this interface at all.
As soon as we have an automatic memory management in .NET, and we know that GC does his job well, and it clears all managed resources automatically, accordingly there is no reason to provide user Dispose method for clearing managed resources. So generally Dispose is used for clearing unmanaged resources, but no one prevents you to unsubscribe from global events as an example, or call Dispose for nested objects (note that global events are almost always bad practice :))

On the other hand, GC knows about Finalizer, and if Finalizer exists GC postpones object disposition until next collection, and call Finalizer before clearing memory. To have disposition code in one place, programmers usually create Dispose(bool) and call this method from Finalizer and IDisposable.Dispose method. Here you need to be aware that you are not allowed to access managed resources, if the call come from Finalizer, because GC doesn't guarantee any disposition order, and that resources might already be in invalid state. This is called dispose pattern.

Regarding GC call in production, yes it is not recommended, again because generally GC does his work well, and since GC.Collect is expensive operation, there is no reason to bother with GC engine, before we decide to do automatic collection.

C# disposable question

Would the garbage collector automatically free the unmanaged resources (whatever, actually) associated with some IDisposable instance if, for example, I forgot to write using statement?

Usually, but not necessarily. The author of a disposable resource needs to do the right thing.

Obviously, I don't know when this would happen, but is it fine to leave the IDisposable to the GC when I don't care about those resources and I'm fine with that they will be disposed eventually?

Your question presupposes a falsehood. The garbage collector never calls Dispose, ever. Rather, the garbage collector calls the destructor (or "finalizer" if you prefer). It is the destructor which might or might not call Dispose for the forgetful user.

Your question also indicates a bad attitude. You might not care if the resources are freed late but another program certainly might care! What if your customer is running two programs that both try to access the same unmanaged resource, one written by you and one written by someone else? Be a good citizen; release your scarce resources as soon as you are done with them so that other programs can use them. That's the real purpose of "using" -- it's to be polite by ensuring that scarce resources are reclaimed quickly.

A correct implementation of the Dispose pattern will ensure that the destructor cleans up the unmanaged resources if the user forgets to call Dispose, and will ensure that the destructor does NOT clean up the resources if they remember.

If you are the person writing the implementation of Dispose for a class that owns unmanaged resources it is your responsibility to get the destructor code right for the case where the user does not call Dispose correctly. The correct way to write this code is extremely well documented; follow the pattern. See Jon's comments for some helpful links.

Note also that if you are writing such a class then you are required to also make it work in scenarios in which it is impossible for the user to call Dispose. For example, suppose the constructor allocates two unmanaged resources, and between the allocation of the first and second, an exception is thrown and caught outside of the constructor. There's then no way for the user to ever call Dispose because the assignment of the new object reference happens after the constructor runs successfully, and the constructor never finished running successfully. How is the first unmanaged resource freed? Only the destructor can free it. The destructor has to be robust in the face of such scenarios; the object destructed might never have been fully constructed so you cannot rely on any invariant of the constructor.

How garbage collection works on object references?

I may be off, but you seem to have a misunderstanding of Dispose and garbage collection. An object will be garbage collected once all references to it are gone, in a non-deterministic way. Dispose will usually get rid of the unmanaged resources, so the object is ready to be garbage collected. In your first example, you Disposed the object, theoretically making it unusable, but it still exists on the heap and you still have a reference to it, both A and B. Once those go out of scope, the garbage collector may reclaim that memory, but not always. In example 2, a Bitmap A is placed on the heap, then you return a reference of it, and set B to that reference. Then you dispose it and B goes out of scope. At that point no more references to it exist, and it will be garbage collected at a later point.

---

Related Topics