C#•2y ago

✅ I'm trying to understand WPF's source code. What is a WeakReference

WeakReference is a class that probably has applications other than in WPF (in particular, I'm trying to understand the Dispatcher class, wich I believe has to do with thread safety, but easiest things first). Can someone explain to me what is a WeakReference? The code of the class itself is not extensive, but it uses another concept I'm not comfortable with yet: IntPtr, and lots of attributes. Here's a link to WeakReference.cs https://referencesource.microsoft.com/mscorlib/system/weakreference.cs.html#https://referencesource.microsoft.com/mscorlib/system/weakreference.cs.html

19 Replies

333fred•2y ago

A WeakReference is a reference to an instance of a class that does not keep that instance alive

starlss•2y ago

Why the IntPtr?

333fred•2y ago

All references are pointers

starlss•2y ago

Ok, so if the only thing referencing an instance in memory is WeakReference, the GC will collect the object regardless?

333fred•2y ago

Correct It specifically does not keep the object alive

starlss•2y ago

What are the situations in which that behavior is desirable?

333fred•2y ago

Well, plenty of them. For example, let's take BindingBase.EnableThreadSynchronization That's a UI method that tells graphics libraries (like WPF or MAUI) that they need to access a specific collection using a given lock But they don't want that mapping to force the collection to stay alive: if the collection is unreferenced by any other locations, then the mapping can go away If they used regular references though, that mapping would force the collection to stay alive, consuming memory for no purpose

starlss•2y ago

ok, I think I understand it a bit more. The swarm of attributes is still very confusing.

333fred•2y ago

Don't worry about the attributes They're not relevant to you I would also not recommend using referencesource, that's the old .NET Framework code https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/WeakReference.cs,985a18841b9c6087 is the modern .NET version

starlss•2y ago

ok, alright, thanks. That seems much better. No more extern definitions I think

333fred•2y ago

Don't forget to $close the thread if there are no more questions

MODiX•2y ago

Use the /close command to mark a forum thread as answered

starlss•2y ago

wait, how likely do you think it will be for me to use WeakReferences? You see, I'm making a monogame, and I'm taking inspiration from WPF for the UI. I want to understand the entire UI and be able to simplify it and "implement it" in monogame

333fred•2y ago

Unlikely They're a very specialized tool

starlss•2y ago

ok thanks a lot! can I close now?

333fred•2y ago

Sure, it's your thread 🙂

starlss•2y ago

thank you

hime•2y ago

I've used it for memoization where the results are stored for that object as a weak reference https://stackoverflow.com/a/53299290

private static readonly ConditionalWeakTable<object, ConcurrentDictionary<string, object>> WeakCache = new();

public static TResult Memoized<T, TResult>(
    this object context, T arg, Func<T, TResult> f,
    [CallerMemberName] string? cacheKey = null) where T : notnull
{
    if (context == null) throw new ArgumentNullException(nameof(context));
    if (cacheKey == null) throw new ArgumentNullException(nameof(cacheKey));

    var objCache = WeakCache.GetOrCreateValue(context);

    var methodCache = (ConcurrentDictionary<T, TResult>) objCache
        .GetOrAdd(cacheKey, _ => new ConcurrentDictionary<T, TResult>());

    return methodCache.GetOrAdd(arg, f);
}

private static readonly ConditionalWeakTable<object, ConcurrentDictionary<string, object>> WeakCache = new();

public static TResult Memoized<T, TResult>(
    this object context, T arg, Func<T, TResult> f,
    [CallerMemberName] string? cacheKey = null) where T : notnull
{
    if (context == null) throw new ArgumentNullException(nameof(context));
    if (cacheKey == null) throw new ArgumentNullException(nameof(cacheKey));

    var objCache = WeakCache.GetOrCreateValue(context);

    var methodCache = (ConcurrentDictionary<T, TResult>) objCache
        .GetOrAdd(cacheKey, _ => new ConcurrentDictionary<T, TResult>());

    return methodCache.GetOrAdd(arg, f);
}

333fred•2y ago

Dammit man, you've unarchived the thread

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✅ I'm trying to understand WPF's source code. What is a WeakReference