可以在C＃中创建一个真正弱键字典吗？ [英] Is it possible to create a truely weak-keyed dictionary in C#?
问题描述
我正在努力弄清楚C＃的真实 WeakKeyedDictionary<，> 的细节，但我遇到困难。
I'm trying to nut out the details for a true WeakKeyedDictionary<,> for C#... but I'm running into difficulties.
我意识到这是一个不平凡的任务，但似乎无法声明一个 WeakKeyedKeyValuePair<，> （其中GC
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.
有两个主要问题我看到：
There are two main problems I see:
-
我迄今为止看到的每个实现都不会在收集密钥后修剪值。想想这个 - 使用这样一个字典的主要原因之一是防止这些值被保留（不仅仅是键！），因为它们是不可达到的，但是在这里它们被强引用所指向。
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，添加/删除足够的字典，最终会被替换，但是如果没有，那怎么办？
Yes, add/remove from the Dictionary enough and they'll eventually be replaced, but what if you don't?
没有一个假设的 WeakKeyedKeyValuePair<，> （或者另一种方法，告诉GC只有当该键可达时才标记该值）引用它的任何值钥匙永远不会被收集。存储任意值时，这是一个问题。
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可以以非理想/黑客方式解决：使用GC通知等待完整的GC完成，然后在另一个线程中修剪字典。这个我是半决定的。
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让我陷入困境。我意识到这很容易被所以不这样做所抵制，但是我有疑问 - 这个问题是否可以解决？
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< TKey，TValue>类别。
使编译器能够将对象字段动态地附加到托管对象。
Enables compilers to dynamically attach object fields to managed objects.
它本质上是一个字典，键和值都是 WeakReference ，只要密钥存活，该值就会保持活着。
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 和等于进行平等比较，它使用 ReferenceEquals 。
Note! This class does not use GetHashCode and Equals to do equality comparisons, it uses ReferenceEquals.
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