Is it possible to manually mark/unmark an object for garbage collection?

Question

Most resources state that the garbage collector figures that out on its own based on references and that I shouldn't mess with it.

I am wondering if I can explicitly tell the garbage collector that it may dispose an object while still keeping a reference.

What I would like to do is to tell the garbage collector that I currently don't need an object anymore (but might again) and then at a later point when (if) I need the object again I would like to check if it has been disposed already. If it has I simply recreate it, if it hasn't I'd like to "unmark" it from garbage collection until I am done with it again.

Is this possible?

I plan to implement something similar to the Lazy<T> class. Pseudocode:

obj = new DisposeIfNecessary<LargeObject>(() => InitLargeObject());
obj.DoSomething(); // Initializes obj using InitLargeObject()
obj.DisposeIfNecessary(); // This is where the magic happens

... // obj might get disposed at some point

obj.DoAnotherThing(); // Might or might not call InitLargeObject() again
obj.Dispose(); // I will not need it again

Answer 1

The WeakReference class does exactly what you want, using the IsAlive property to check for state before using it.

You can get a "strong" reference to it again via the Target property, which will affect the reference count and stop it from being eligible for collection.

Also note that Dispose doesn't directly relate to garbage collection, so disposing an item (depending on the Dispose implementation) might make it unusable anyway - but again, that is nothing to do with GC. On a general practice note, as mentioned by @HansPassant, calling Dispose on an item (or generally anything claiming to dispose) and then attempting to use it again afterwards is a code smell (or just plain wrong as other developers will expect Dispose to be a last-call method marking the object as unusable from then on).

The WeakReference class will not be responsible for re-creating collected objects, but in conjunction with IsAlive you can handle that logic yourself.

Also, to the point in the comments, the GC doesn't do anything clever with WeakReference in terms of deciding when to collect it in terms of trying to leave WeakReference items until last; it will collect the underlying object as it would others during a run if it is eligible - no special handling and definitely no "cache" behaviour.