Background and Motivation

There is an internal struct System.Runtime.CompilerServices.DependentHandle that is currently only used by System.Runtime.CompilerServices.ConditionalWeakTable<TKey, TValue>. The struct allows for a dependency between two objects to be created by a third type without modifying the first two classes, and only using up a pointer's worth of memory in the third type.

I have personally find use for such a class, in my Weak Delegates system for example. While in there I had to use slightly convoluted tricks to make it work I believe a more proper implementation included in the BCL would be useful.

Proposed API

namespace System.Runtime.CompilerServices
{
    public struct DependentHandle
    {
        public DependentHandle(object? target, object? dependent);

        public bool IsAllocated { get; }

        public object? Target { get; set; }
        public object? Dependent { get; set; }

        (object? Target, object? Dependent) TargetAndDependent { get; }

        public void Free();
    }
}

Open Questions

Should DependentHandle implement IDisposable? If so, we should expose the same on GCHandle

Should the constructor be private and have a static Alloc method to match GCHandle?

Should TargetAndDependent be a property or a method?