How to delete an instance of a class within that instance

Question

class Foo
{
    int Data;
    public Foo(int data)
    {
        Data = data;
    }
    public void Diminish()
    {
        this.Data--;
    }
}
Foo Foo1 = new Foo(5);
Foo1.Diminish();

Is there Any way delete Foo1 When Data is 0 within Foo1?

(Free up the memory used by that instance)

Answer 1

As some comments and at least one answer explain, a normal reference in a .NET program is exactly what keeps that object alive. It is not possible to "delete" the object explicitly in the way you seem to mean.

An object will remain alive as long as it is "reachable" (i.e. it is possible to start with a "rooted" reference and find the reference to that object). Once it is no longer reachable (which could happen even when a reference to the object exists, as long as that reference can't be reached from a rooted reference…e.g. a circular reference between two objects does not prevent garbage collection of those two objects, as long as the only reference to each is from the other).

All of this is discussed in detail in documentation references provided elsewhere.

Now, all that said, depending on what you're actually trying to do, you might find that the WeakReference<T> class is what you need. This class provides a way to reference objects without preventing them from being garbage-collected. You can combine this with reference-counting and an object repository to maintain non-weak references to keep the object alive to produce the results you want.

For example:

class RefCounted
{
    private static readonly HashSet<RefCounted> _alive = new HashSet<RefCounted>();

    private int _referenceCount;

    public RefCounted()
    {
        AddRef();
    }

    public AddRef()
    {
        if (_referenceCount == 0)
        {
            // the collection ensures a strong reference to the object, to prevent
            // the object from being garbage-collected even if the only other
            // references are weak references.
            _alive.Add(this);
        }
        _referenceCount++;
    }

    public Release()
    {
        if (--_referenceCount)
        {
            // no longer need to force the object to stay alive; if the only remaining
            // references to the object are weak references, then the garbage
            // collector may collect the object.
            _alive.Remove(this);
        }
    }
}

Then you can use it like this:

WeakReference<RefCounted> o = new WeakReference<RefCounted>(new RefCounted());
RefCounted r;

if (o.TryGetTarget(out r))
{
    // do stuff with r

    // done with the object? Then call:

    r.Release();
}

It is important to note that this is not precisely what you're asking for. The above still does not give you deterministic control over the actual garbage collection of the object. Furthermore, once you retrieve the target of the weak reference into e.g. a local variable, you now hold a strong reference to the object for the lifetime of that local variable (i.e. up to the last point that variable is used) and the object cannot be collected during that lifetime, even if the reference count goes to 0 (though at that point the object will of course be collected once that variable no longer exists or is reachable).

But if you really have a scenario where you need to use some type of reference counting to track and in some way control the lifetime of an object, the above is how you'd do it. You would have to store the WeakReference<T> objects rather than direct references to T, retrieving the target reference at each place in the code where you need it.

Of course, there remains the question of why do you think you need reference counting. And even if you need reference counting, why should the reference count be directly related to the lifetime of the object? Your question does not explain these aspects, and so I can't really address them directly. I will say that reference counting is not needed at all for effective management of managed objects in .NET programs. As long as you don't actively interfere with the maintenance and use of references to managed objects, the .NET garbage collection system will do a far better job of dealing with the lifetimes of objects than you or I could.

Answer 2

C# is a managed language, you can't "delete" anything, once something can no longer be referenced the value clean up will be handled by the Garbage Collector.

There are edge cases where you need to handle clean up, or be more aware of how you are handling creation:

• Instances of classes that implement IDisposable interface should be disposed of by calling the Dispose() method or wrapped in a using statement
• Unmanaged resource wrappers, such as IntPtr may need to be cleaned up using Marshal.FreeHGlobal(IntPtr) or Marshal.FreeCoTaskMem(IntPtr) if you are required to manage them and you're making a call to unmanaged code
• Static (and possibly long living) values will never be un-referenceable unless you actively set variables to null or remove items from collections

Answer 3

An object never decides when to "delete" or garbage collect itself. It can't know what else has a reference to it. When there are no references left to an object then it gets garbage collected.

In the scenario described in the question - suppose you had a List<Foo>() and then one instance of Foo spontaneously decided, based on its own internal logic, that it should no longer exist? I don't know what that would even look like - all references to it suddenly become null?

If that were possible, the defects it would cause would be beyond all mortal comprehension. We have enough trouble when two threads modify the same object or property because of improper/nonexistent locking, and so from the viewpoint of one thread something has changed when it shouldn't.

But this would create similar scenarios even in single-threaded applications.

var myFoo = new Foo(1);
myFoo.Diminish();
var x = myFoo.Data; // Null reference exception! WTH - it nullified itself!

A fundamental concept of OOP is that classes don't need to know too much about the internal state of other classes. But now you'd need to "know" that a class could suddenly choose to opt-out and jump off the cliff into the GC, and constantly null-check objects that already weren't null.

It's a very good thing that this is not possible.

Answer 4

From MSDN:

The .NET Framework's garbage collector manages the allocation and release of memory for your application. Each time you create a new object, the common language runtime allocates memory for the object from the managed heap. As long as address space is available in the managed heap, the runtime continues to allocate space for new objects. However, memory is not infinite. Eventually the garbage collector must perform a collection in order to free some memory. The garbage collector's optimizing engine determines the best time to perform a collection, based upon the allocations being made. When the garbage collector performs a collection, it checks for objects in the managed heap that are no longer being used by the application and performs the necessary operations to reclaim their memory.

So the garbage collector will take care of releasing the object when it goes out of scope unless you need to release some expensive (memory consumption) resource. You can sets the reference of the instance to null, this will terminate the object, unless there is some other reference to the object:

Foo Foo1 = new Foo(5);
Foo1.Diminish();
if(Foo1.Data == 0)
{
    Foo1 = null;
}

Answer 5

An object instance will remain "alive" as long as any piece of code references it. So, it's something you cannot really control from within a class.