Assembly Load Context throws errors in Blazor App 0.8

[fred-perkins]

Describe the bug

When trying to load an assembly using the AssemblyLoadContext.Default a NotImplementedException is thrown. The Static property Default and it's other static methods don't seem to have any implementations.

To Reproduce

Steps to reproduce the behavior:

1. Download Sample code from: fred-perkins/BlazorLoading
2. Run application
3. Select Fetch Assembly in the left panel
4. Click "Fetch Assembly using AssemblyLoadContext" Button
5. Observe error in console: WASM: [System.NotImplementedException] The method or operation is not implemented.

Expected behavior

AssemblyLoadContext should attempt to load the DLL in the default context.

Additional context

We need to be able to dynamically load libraries one at a time within Blazor. Using Assembly.Load to read from bytes presents a problem where assemblies can be loaded several times, which causes a InvalidCastException to be thrown when resolving a type from one of those libraries.
Using AssemblyLoadContext it is possible to avoid this, however it seems unsupported when running in Webassembly on Blazor - But functions within a console application.

This is likely a mono issue:
Looking at mono it seems there are two implementations of the class: AssemblyLoadContext Facade
& AssemblyLoadContext.

[fred-perkins]

@danroth27 - We spoke about this a week ago (Part of a derivco call). This is blocking us from migrating our framework to Blazor for our POC, as we need to be able to dynamically load libraries in the background. Is there a known alternative we could use other than Assembly.Load - Which doesn't solve our problem

[danroth27]

@fred-perkins Thanks for sharing this feedback with us!

@SteveSandersonMS @lewing Thoughts on this one? Do we have any support currently for dynamically loading .NET assemblies in a Blazor app? Is this really all part of supporting lazy loading of application areas?

[SteveSandersonMS]

Could you clarify what the problem with Assembly.Load is? If your code caches the results of loading the assembly, is that not sufficient to ensure you only load each one once?

[fred-perkins]

@SteveSandersonMS / @danroth27 So after a bit of testing I got Assembly.Load(byte[]) to work for me as i wanted. I think there was a misunderstanding about how Assembly.Load varies compared to the API on DotNet core - Where a byte array loaded assembly is anonymous and causes issues where assemblies would be loaded twice. This occurred when one was manually loaded, and the second where an assembly used a type for a dependent assembly - Which results in a InvalidCastException with duplicate types.
This means that for me to work around the issue I've had to use AssemblyLoadContext in .Net Core & Assembly.Load for the web assembly portion.

Ideally i'd prefer to use the AssemblyLoadContext Api across the board so i don't have to do fallback code in my .NetStandard libraries like the following:

public Assembly LoadFromStream(byte[] assemblySource)
{
    if (AssemblyLoadContext.Default != null)
    {
        return AssemblyLoadContext.Default.LoadFromStream(new MemoryStream(assemblySource))
    }
    return Assembly.Load(assemblySource);
}

I did have a dig through the mono source code in the end. It looks like they don't fully support the AssemblyLoadContext Api, and where they have partial support it only works for when you specify the assemblyName or filePath - The LoadFromStream throws a not supported exception as mentioned above.

[SteveSandersonMS]

Thanks for the update, @fred-perkins! I'm glad you have a workable solution for now.

Longer term, any inconsistencies between .NET Core and Mono are definitely worth reporting. In this specific case it sounds like Mono ideally would support more of AssemblyLoadContext, and might even want tweaks to ensure that Assembly.Load behaves the same as it does on .NET Core (even if that behavior isn't what you wanted in your scenario, it should still be consistent). If you were able to report any such API gaps or inconsistencies to http://github.com/mono/mono that would be really helpful.

[SteveSandersonMS]

I'll close this as external since it's really about the underlying runtime, not about Blazor.

[fred-perkins]

@SteveSandersonMS Awesome, thanks for the reply. I'll get a ticket raised with the Mono guys :)

[tylerhartwig]

@fred-perkins
Can you link to the related mono issue?

I'm struggling to properly resolve a type existing in Assembly A, that depends on Assembly B.

Both are loaded into the default context already, but I'm looking to ensure A uses the type from B (newly loaded at runtime, separate from the default context), rather than what's already loaded.