Support Collectible Dynamic Assemblies

[jonorossi]

.NET Core 3.0 introduces support for unloading assemblies after loading them into a custom AssemblyLoadContext via the new Unload method and constructor with isCollectible parameter. The custom AssemblyLoadContext is set up to only contain the assemblies you want with everything else being in the default context. Obviously this is a .NET Core 3.0 feature, and .NET Framework will never support it; .NET Standard also does not define System.Runtime.Loader classes. Unfortunately it looks like dynamic assemblies were missed (https://github.com/dotnet/corefx/issues/38426) and support for them won't make the cut for .NET Core 3.0.

• https://docs.microsoft.com/en-us/dotnet/standard/assembly/unloadability-howto
• https://github.com/dotnet/samples/tree/master/core/tutorials/Unloading
• https://github.com/dotnet/coreclr/issues/552

.NET Framework 4.0 introduced AssemblyBuilderAccess.RunAndCollect which allows dynamic assemblies to be garbage collected when no reference is held. RunAndCollect has a whole heap of restrictions on what can be emitted into the assembly (e.g. no COM interop), but I think we could fairly trivially add opt-in for this per ModuleScope to make it disposable.

• https://docs.microsoft.com/en-us/dotnet/framework/reflection-and-codedom/collectible-assemblies

I don't know how useful this feature would actually be since we don't get any requests for it, but logging this as an enhancement to raise the topic since it is possible. It sounds like it RunAndCollect could be helpful for #472.

[stakx]

I don't know how useful this feature would actually be since we don't get any requests for it [...]

There has been at least one StackOverflow question which seems related to just this.

we could fairly trivially add opt-in for this per ModuleScope to make it disposable [...]. It sounds like it RunAndCollect could be helpful for #472.

I haven't thought it through all the way, one thing that perhaps would have to be pointed out in the documentation is that dynamic assemblies could be collected only once their associated ProxyGenerator is no longer referenced, nor any proxy objects created by that generator.

[jonorossi]

I haven't thought it through all the way, one thing that perhaps would have to be pointed out in the documentation is that dynamic assemblies could be collected only once their associated ProxyGenerator is no longer referenced, nor any proxy objects created by that generator.

Yep, that was my intention. There are heaps of limitations to the feature and it would really only fit the scenario of plugins or something similar where you dump everything and start again without restarting the process, most applications obviously don't work this way.

I only logged the issue after reading the .NET Core 3.0 release notes and doing a little bit of investigation into how it works, so thought I'd capture my notes.

[xlievo]

I admire your efforts and persistence over the years, as well as the rigorous way to discuss issues! Thank

[mhascak]

Hello, I´m currently extending plugins system which based on AspNetBoilareplate framework and this enhancement will help me to create more dynamic dev worklow. When plugins DLLs changes in CORE project I desperately need to unload / reload this plugins assemblies., It works thanks custom AssemblyLoadContext, but after that I have this error:

Castle.DynamicProxy - A non-collectible assembly may not reference a collectible assembly.

So I vote +1 for this. Tahanks!

[jairbubbles]

Hi @jonorossi, I stumbled into that issues through NSubstitute. Rather than an opt-in I was wondering if we couldn't use RunAndCollect when we detect that the current assembly is collectible (see https://learn.microsoft.com/en-us/dotnet/api/system.reflection.assembly.iscollectible?view=net-7.0).

Something like:

Assembly.GetExecutingAssembly().IsCollectible ? AssemblyBuilderAccess.RunAndCollect : AssemblyBuilderAccess.Run;

Using collectible assemblies is not common so I feel like it would have limited repercussions.

[SongOfYouth]

If we use AssemblyLoadContext and set isCollectible to true, when call ProxyGenerator.CreateInterfaceProxyWithoutTarget, we get a error with "A non-collectible assembly may not reference a collectible assembly".

[stakx]

"A non-collectible assembly may not reference a collectible assembly"

If this is generally true, then it would seem to be the death sentence for this enhancement, since (I assume) assemblies that use DynamicProxy will typically not be collectible. I wonder if the feature would still be worth having (as an opt-in) for the few remaining cases (like @mhascak's, see above).

[SongOfYouth]

"A non-collectible assembly may not reference a collectible assembly"

If this is generally true, then it would seem to be the death sentence for this enhancement, since (I assume) assemblies that use DynamicProxy will typically not be collectible. I wonder if the feature would still be worth having (as an opt-in) for the few remaining cases (like @mhascak's, see above).

In some scenarios, it's needed. for example, the component using DynamicProxy is just a plugin in my application, and i want to load and hot unload it at runtime, sometimes for dynamic update. I used DynamicProxy to create http client proxy, before, all my components is collectible plugins, but now they are not.