Does assigning to Nothing cause Dispose to be invoked?

Question

I recently saw some VB .NET code as follows:

Dim service = ...
Try
   ...
   service.Close()
Finally
   service = Nothing
End Try

Does assigning Nothing to service do anything? If it is a garbage collection issue, I am assuming that when "service" went out of scope the referenced object would be garbage collected and the dispose method called on the object.

It seems to me that assigning this variable Nothing can't really do anything, as there could be another reference to the object around so the reference counts haev to be checked anyways.

Answer 1

Looks like the answer lies in the response to this post: http://stackoverflow.com/questions/175454/loops-and-garbage-collection

Answer 2

NO!

You're seeing old VB6 code, where assigning Nothing reduced the reference count on COM objects.

Answer 3

It only releases the reference, which may mean that the object is available for garbage collection (there could still be other variables referencing the same object). If the object implements IDisposable, you need to call Dispose explicitly, otherwise you may have a resource leak.

Answer 4

Assinging NULL to a reference in .NET does not help to clean the object away. It might help the garbage collector to run a little quicker in some corner cases but that's not important at all. It does not call dispose, either (when dealing with a disposable)

I love to assign NULL anyways to explicitly state that I won't use that other object anymore. So it has much more to do with catching bugs (you'll get a nullreference exception instead of possibly calling into some other object - which might fail or even silently create some side effects.)

So assigning NULL after closing another object (File or whatever) is a "code cleanliness" thing that eases debugging, it's not a help to the garbage collector (except in some really strange corner cases - but when you need to care about that you WILL know more about the garbage collector than you ever wanted to know anyways ...)

Answer 5

In most situations assigning null (Nothing) to a reference makes no difference to garbage collection what so ever.

The garbage collector doesn't care about scope, it only cares about usage. After the point in the code where the object is used the last time, the garbage collector knows that it can collect it because it won't be used any more.

Assigning null to the reference doesn't count as using the object, so the last point of usage is before that code. That means that when you clear the reference the garbage collector may already have collected the object.

(In debug mode though the usage of a variable is expanded to it's scope, so that the debugger can show the value of the variable throughout it's scope.)

Answer 6

As everybody has already said, setting to nothing does not force garbage collection, if you want to force GC then you would be far better to use the using ke word

   Using objA As A = New A()
       objA.DoSomething()
   End Using

You still don't need to set to nothing as the End Using tells the Garbage collection that the object is no longer to be used