是否可以在 C# 中创建一个真正的弱键字典? [英] Is it possible to create a truely weak-keyed dictionary in C#?

问题描述

我正在尝试为 C# 编写一个真正的 WeakKeyedDictionary<,> 的详细信息...但我遇到了困难.

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

我意识到这是一项不平凡的任务，但似乎无法声明 WeakKeyedKeyValuePair<,> (其中 GC 仅在键可访问时遵循值引用)使它看起来不可能.

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.

我看到了两个主要问题:

There are two main problems I see:

1. 到目前为止，我看到的每个实现都不会在收集键后修剪值.想想看 - 使用这样一个 Dictionary 的主要原因之一是防止这些值被保留(不仅仅是键！)，因为它们无法访问，但在这里它们被强引用指向.

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.

是的，从字典中添加/删除足够多，它们最终会被替换，但如果你不这样做呢?

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

如果没有假设的 WeakKeyedKeyValuePairWeakKeyedKeyValuePair<,>(或另一种告诉 GC 仅在键可访问时标记值的方法)，任何引用它的键的值都不会集.这是存储任意值时的问题.

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.

问题 1 可以通过一种相当不理想/骇人听闻的方式解决:使用 GC 通知等待完整的 GC 完成，然后继续在另一个线程中修剪字典.这个我还可以.

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.

但问题 2 让我很困惑.我意识到这很容易被所以不要那样做"所反驳，但这让我想知道 - 这个问题甚至可以解决吗?

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?

推荐答案

看看 ConditionalWeakTable类.

使编译器能够将对象字段动态附加到托管对象.

Enables compilers to dynamically attach object fields to managed objects.

它本质上是一个字典，其中键和值都是 WeakReference，只要 key 存在，value 就会保持活跃.

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.

注意！这个类不使用GetHashCode和Equals做相等比较，它使用ReferenceEquals.

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

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