how to detect AssemblyLoadContext been gc

[srxqds]

how to detect AssemblyLoadContext been gc

[elgonzo]

Why is it necessary to know when an AssemblyLoadContext instance is being collected?
Wouldn't it suffice to listen to the Unloading event?

[srxqds]

no, we need no when assemblyloadcontext not being used.
the alc unloading the type object in this alc will be using

[huoyaoyuan]

Unloading will prevent further usage of the ALC. It will be fully unloaded when all existing usages are dead.
You can use a weak reference to the ALC, and check the weak reference when GC happens. Something like this:

alc.Unloading += alc => _ = new UnloadSentinal(alc);

class UnloadSentinal(AssemblyLoadContext alc)
{
    private readonly WeakReference<AssemblyLoadContext> wr = new(alc);

    ~UnloadSentinal()
    {
        if (!wr.TryGetTarget(out _))
        {
            // Real unload handling
        }
        else
        {
            GC.ReRegisterForFinalize(this);
        }
    }
}