Is it possible to create a truely weak-keyed dictionary in C#?

Question

I'm trying to nut out the details for a true WeakKeyedDictionary<,> for C#... but I'm running into difficulties.

I realise this is a non-trivial task, but the seeming inability to declare a WeakKeyedKeyValuePair<,> (where the GC only follows the value reference if the key is reachable) makes it seemingly impossible.

There are two main problems I see:

1. Every implementation I've so far seen does not trim values after keys have been collected. Think about that - one of the main reasons for using such a Dictionary is to prevent those values being kept around (not just the keys!) as they're unreachable, but here they are left pointed to by strong references.

   Yes, add/remove from the Dictionary enough and they'll eventually be replaced, but what if you don't?

2. Without a hypothetical WeakKeyedKeyValuePair<,> (or another means of telling the GC to only mark the value if the key is reachable) any value that refers to it's key would never be collected. This is a problem when storing arbitrary values.

Problem 1 could be tackled in a fairly non-ideal/hackish way : use GC Notifications to wait for a full GC to complete, and then go along and prune the dictionary in another thread. This one I'm semi-ok with.

But problem 2 has me stumped. I realise this is easily countered by a "so don't do that", but it has me wondering - is this problem even possible to solve?

Answer 1

Have a look at the ConditionalWeakTable<TKey, TValue> Class.

Enables compilers to dynamically attach object fields to managed objects.

It's essentially a dictionary where both the key and the value are a WeakReference, and the value is kept alive as long as the key is alive.

Note! This class does not use GetHashCode and Equals to do equality comparisons, it uses ReferenceEquals.