How can MEF AssemblyCatalogs be used with custom AssemblyLoadContexts

Question

I am working on an ASP.NET Core API that is extensible with Plug-Ins. Because the Plug-In Assemblies need to be unloadable, I've started loading them into separate, collectible, AssemblyLoadContexts .

The Implementation basically looks like this:

// Creating a new Context for the Plugin.
AssemblyLoadContext context = new AssemblyLoadContext(directoryInfo.FullName, true);
...

// Loading every Assembly the Plugin uses into the Context.
Assembly assembly = context.LoadFromAssemblyPath(fileInfo.FullName);
...

// Creating AssemblyCatalogs with the Assemblies.
AssemblyCatalog assemblyCatalog = new AssemblyCatalog(assembly);

This works and I can use the Assemblies normally. However, the AssemblyCatalogs all have unpopulated Parts Properties and thus will not compose.

Now, if I load the Assemblies into the Default Context ( AssemblyLoadContext.Default ), everything works as expected, except the unloading of course.

What am I missing? Thanks for pointing me in the right direction:)

Answer 1

Okay, turns out I've made a mistake. My Plug-In Project had CopyLocalLockFileAssemblies set to true and because of this, copied the NuGet dependencies to the output directory. Since I would then just copy the output to the directory the API loads its Plug-Ins from, the API tried to load these referenced libraries as well. Now, both the API and Plug-In Project reference System.ComponentModel.Composition and - long story short - the DLL was loaded twice, in different versions. The API was using 6.0.0 and the Plug-In was using 4.0.0 .

Because of this, the attribute comparison (MEF finds its exports by looking for the ExportAttribute ) would later fail which is why the AssemblyCatalog was not able to find the Exports.