Can the .NET garbage collector collect an object that is a member to a class?

Question

I have a class that contains a pointer to an object inheriting off IDisposable

class Foo 
{
    private Bar bar;

    private test() 
    {
        bar = CreateBarFactory();
    }

};

bar never is used beyond when it is set, in test();

I'm wondering if the GC is permitted to dispose of bar even if the Foo instance is still around?

if so is there a way I can prevent this? bar is a pointer to an object that contains a ref count, if it's Disposed() then the ref is released causing some functionality to be disabled

Answer 1

If the instance of foo is still around (i.e. someone is holding a reference to it.. keeping it alive), bar will not be collected. It will be treated as 'reachable' by the GC i.e. in use.

Members of reachable objects are also considered to be reachable.

Answer 2

Please don't mix up terminology. C# does have pointers, but that's not what you have there. You have a reference. Importantly, references do affect the garbage collector, whereas pointers, in and of themselves, do not.(*)

So long as the instance of Foo is reachable, it's reference to bar will keep that object reachable too. If Foo and Bar have finalizers, then, in the finalizer of Foo is the one circumstance where you shouldn't assume anything about bar - it may have already been finalized itself.

Even though, as you've indicated, no other method will be called that accesses bar, the GC and JIT don't perform this sort of analysis. For object references, the entire object is considered to be reachable, and any references it contains are followed and the objects located similarly are marked as reachable. This has to be done so that Reflection based access to an object will never obtain an invalid reference. This is also why, if your object has allocated a large helper object that you will no longer use, it can be (in this limited circumstance) useful to set the reference to null.

The only lifetime analysis that does consider what future code will run is the code within a single method body, with respect to the local variables. A local reference variable within a method is not sufficient to keep an object alive if no further references to that variable will occur. This is worked out collaboratively between the JIT and GC.

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(*) people sometimes think that pointers keep objects alive. This is not strictly true. The act of pinning, which can yield a pointer, will keep an object alive, but there's nothing stopping you from keeping a pointer beyond the time that the object was pinned. Of course, de-referencing the pointer at any such time would be unsafe...

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And, another terminology issue:

I'm wondering if the GC is permitted to dispose of bar

The GC doesn't dispose of anything. Disposal occurs when an object implements IDisposable and the user of the object calls Dispose on it (either directly or via e.g. a using block).

The GC doesn't call Dispose. It may call a finalizer, if one is defined on an object (and so my warning above would apply) and it then collects the object.