Question

I know that you can call an instance method that executes for each object. I also know that you can have a static method on the type that is callable from the type.

But how would one call a method that acts on every instance of a particular type (say, to set a member variable to zero, for example)?

Answer 1

C# doesn’t provide a direct mechanism to track all reachable objects and there’s almost never a good reason to want such automatic tracking functionality (rather than, say, an explicit pool that you manage yourself).

But to answer your requirement directly, you’ll need to:

1. Manually track all the reachable instances of the type (perhaps with a set of weak-references to prevent the tracker itself from extending the objects’ lifetimes).
2. Provide a mechanism to (say) perform a side-effect on each member of this set.
3. Get rid of the associated weak-reference from the set when an object is destroyed. This is necessary to prevent a weak-reference leak.

All of this will have to be done in a thread-safe manner.

public class Foo
{
    private static readonly HashSet<WeakReference> _trackedFoos = new HashSet<WeakReference>();
    private static readonly object _foosLocker = new object();

    private readonly WeakReference _weakReferenceToThis;

    public static void DoForAllFoos(Action<Foo> action)
    {
        if (action == null)
            throw new ArgumentNullException("action");

        lock (_foosLocker)
        {
            foreach (var foo in _trackedFoos.Select(w => w.Target).OfType<Foo>())
                action(foo);
        }
    }

    public Foo()
    {
       _weakReferenceToThis = new WeakReference(this);

        lock (_foosLocker)
        {
            _trackedFoos.Add(_weakReferenceToThis);
        }
    }

    ~Foo()
    {
        lock (_foosLocker)
        {
            _trackedFoos.Remove(_weakReferenceToThis);
        }
    }
}

Are you sure you need all of this though? This is all really strange and non-deterministic (will be heavily impacted by when garbage-collection occurs).