[RFC] Provide a mechanism to handle WeakReference deduplication in the framework

[Therzok]

Steps to Reproduce

1. Create a treeview, where a node has a reference to its children and its parent
2. To avoid memory leaks, the back-reference to the parent has to be WeakReference

class Node
{
  WeakReference<Node> _parent;
  public Node(Node parent) {
    _parent = new WeakReference(parent);
  }
}

In a case of 1 parent with 5 children, there will be 5 WeakReference allocations for the same object. An alternative is to provide the WeakReference as a parameter, and that would complicate most of the code involved.

Proposal:

class Node
{
  WeakReference<Node> _parent;
  public Node(Node parent) {
    _parent = parent.WeakSelf();
  }
}

The existence of this API in Xamarin.Mac would allow us to have a sure way of grabbing a deduplicated reference to an NSObject. While it can be provided by our application libraries, other users of Xamarin.Mac that do not directly reference it would probably hit this.

@rolfbjarne proposed using a ConditionalWeakTable. The problem here is that we lose type safety

ConditionalWeakTable<NSObject, object> cwt = new();

public static WeakReference<T> WeakSelf<T>(this T target) where T:NSObject
{
  return (WeakReference<T>)cwt.GetValue(target, key => new WeakReference<T>(key));
}

[mandel-macaque]

@spouliot @rolfbjarne comments?

[Therzok]

Hmmm, no, the initial concern I had still exists.

You have to lose type safety, because the same object objects can be stored as either base or derived classes.

NSString x = ...;
NSObject y = x;

var wr1 = x.WeakSelf();
var wr2 = y.WeakSelf();
// type mismatch because WeakReference is not variant

[Therzok]

So, it would be something like, which is not nice at all.

ConditionalWeakTable<NSObject, WeakReference> cwt = new();

public static WeakReference WeakSelf(this NSObject target)
{
  return (WeakReference)cwt.GetValue(target, key => new WeakReference(key));
}

[rolfbjarne]

// type mismatch because WeakReference is not variant

Maybe you could implement your own variant version of WeakReference (based off GCHandle)?

[Therzok]

EDITED: The end implementation is not the problem here, I think. Having support for this in the framework to allow multiple different users to deduplicate all weak references of one object is really important in many cases, since allocations are going to be heavy for duplicate weakreferences, multiple weak gchandles created and just having a WeakSelf extension would make everyone's life easier.