Add support for unloading Assembly with WPF control in .NET Core

[janvorli]

The .NET Core 3.0 has a new feature to enable unloading assemblies from memory, called unloadable AssemblyLoadContext. This feature is useful e.g. for implementing unloadable plugins.

In WPF, the MS.Internal.Resources.ResourceManagerWrapper holds on the assemblies containing controls forever (the assemblies are added to an internal container in the AppDomain.CurrentDomain.AssemblyLoad event handler and never removed).
So when the assembly is loaded into an unloadable AssemblyLoadContext, the resource container prevents the assembly from ever being unloaded, as the AssemblyLoadContext unloadability relies on GC and all references to the assembly need to go away before it can be unloaded.

To make WPF unloadability friendly, adding an ability to remove an assembly from the resource container would be needed.

It seems the following changes should be sufficient:

• In the existing AppDomain.CurrentDomain.AssemblyLoad event handler in the ResourceContainer , the resource manager hooks the AssemblyLoadContext.Unloading event for all assemblies that have IsCollectible property true. The AssemblyLoadEvent.GetLoadContext(Assembly) would be used to get the context to hook the event handler on from the assembly.
• When the Unloading event fires, the resource container removes the assembly from its cache

[weltkante]

probably related to #1700

[wall-code-solutions]

Is there anything new here? Is this still an issue that will be addressed in the near future?