Setting Objects to Null/Nothing after use in .NET

Question

Should you set all the objects to null (Nothing in VB.NET) once you have finished with them?

I understand that in .NET it is essential to

Answer 1

this kind of "there is no need to set objects to null after use" is not entirely accurate. There are times you need to NULL the variable after disposing it.

Yes, you should ALWAYS call .Dispose() or .Close() on anything that has it when you are done. Be it file handles, database connections or disposable objects.

Separate from that is the very practical pattern of LazyLoad.

Say I have and instantiated ObjA of class A. Class A has a public property called PropB of class B.

Internally, PropB uses the private variable of _B and defaults to null. When PropB.Get() is used, it checks to see if _PropB is null and if it is, opens the resources needed to instantiate a B into _PropB. It then returns _PropB.

To my experience, this is a really useful trick.

Where the need to null comes in is if you reset or change A in some way that the contents of _PropB were the child of the previous values of A, you will need to Dispose AND null out _PropB so LazyLoad can reset to fetch the right value IF the code requires it.

If you only do _PropB.Dispose() and shortly after expect the null check for LazyLoad to succeed, it won't be null, and you'll be looking at stale data. In effect, you must null it after Dispose() just to be sure.

I sure wish it were otherwise, but I've got code right now exhibiting this behavior after a Dispose() on a _PropB and outside of the calling function that did the Dispose (and thus almost out of scope), the private prop still isn't null, and the stale data is still there.

Eventually, the disposed property will null out, but that's been non-deterministic from my perspective.

The core reason, as dbkk alludes is that the parent container (ObjA with PropB) is keeping the instance of _PropB in scope, despite the Dispose().