No equivalent of AddControllersAsServices() for VC's and TagHelpers

[dazinator]

If you have an external assembly, contianing a Controller, and a ViewComponent, you can register the controller with MVC by calling AddControllersAsServices()

However there is no equivalent for the ViewComponent.
The default behaviour seems to be if you have a project reference to the assembly then the VC is picked up, but if you don't have a project reference to it (i.e ILibraryManager is not aware of it) - because for example, it's a standalone "plugin" assembly, then VC's are not detected.

Could you add a method similar to AddControllersAsServices() but for VC's?

Or perhaps consider adding some sort of "IncludeMvcServicesFromAssemblies()` method, that will make sure the assemblies are treated as candidates for all types of services that MVC looks for - i.e Controllers, VC's, and whatever else.

[dazinator]

• And here is a repro showing the issue, that you can load an assembly at runtime, register its controllers, but not the VC's: https://github.com/dazinator/mvc6-view-components/tree/issue-noprojectreferences

[danroth27]

We should also look at adding a more general mechanism for injecting everything from an assembly: #4089.

[danroth27]

We should also support constructor injection in view components by default like we do for controllers.

[danroth27]

And we should do the same for Tag Helpers.

[dazinator]

Not sure if this is related, have raised #4100 to do with ViewModels not being resolved when they also live in the external assembly alongiside the razor view. It seems whatever is responsible for compiling the razor view does not include the assembly that the view is in itself (embedded within assembly) when attempting to resolve Model's..

[rynowak]

@dazinator - Razor uses your project's referenced assemblies as its inputs for compilation. We need to figure out a good all-up way to handle these scenarios.

What's stopping you from referencing this assembly at compile time?

Can you enumerate all assemblies you want to "include" at startup time?

[dazinator]

@rynowak

What's stopping you from referencing this assembly at compile time?

I'm building an application that is designed to be extensible, as in plugins / extensions can be installed at runtime, after the application is deployed. So far, the only blockers I have found are that VC's don't work without a project reference, and neither does Razor View compilation.

Can you enumerate all assemblies you want to "include" at startup time?

Yep. At startup time, my application registers an IAssemblyProvider that returns the union of the DefaultAssembyProvider plus my additional "plugin" assemblies. This doesn't seem to effect Razor Compilation or VC Discovery though.

[davidfowl]

Then you need to hook more than just the assembly provider, in rc2 you can more easily add assemblies to the razor compilation. In rc1 it's a bit cumbersome but possible.

[rynowak]

after the application is deployed

Yep. At startup time ...

These two aren't the same (at least not in my mind). Are you saying that you want to add assemblies after startup? Or is the set of assemblies fixed at startup time?

This doesn't seem to effect Razor Compilation or VC Discovery though.

VC's use the assembly provider. If it's not finding your extra assemblies, this is something weird. https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/DefaultViewComponentDescriptorProvider.cs#L49

Then you need to hook more than just the assembly provider, in rc2 you can more easily add assemblies to the razor compilation. In rc1 it's a bit cumbersome but possible.

We should still make this easier if it's just adding more "MVC" assemblies.

[dazinator]

@rynowak

VC's use the assembly provider. If it's not finding your extra assemblies, this is something weird. https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/DefaultViewComponentDescriptorProvider.cs#L49

You are right! Implementing an IAssemblyProvider that includes my VC assembly has actually fixed this issue, I just wasn't testing correctly! :)

We should still make this easier if it's just adding more "MVC" assemblies.

Yes please, so it's as easy as AddControllersAsServices() !

These two aren't the same (at least not in my mind). Are you saying that you want to add assemblies after startup? Or is the set of assemblies fixed at startup time?

When the application starts up, the set of assemblies is fixed, but it consists of the assemblies that were project referenced (that ILibraryManager knows about) and the set that have been placed in some location through the concept of installing an extension into the application which happens at runtime, after the application has been deployed somewhere. Installing an extension basically dumps additional assemblies somewhere, and restarts the application, so that on startup the new assemblies are detected, and the MVC components can be correctly registered with the framework. It's this second part - correctly registering these assemblies with the MVC framework that has been challenging, as some things have API's like AddControllersAsServices and some things require that you implement IAssemblyProvider or some other hook :)

[dazinator]

@davidfowl

In rc1 it's a bit cumbersome but possible.

Are you able to point me in the right direction for how to do this with RC 1 as I'd love to get that working?

[rynowak]

#4089 - iterating on design in this area

[rynowak]

@dazinator - sorry, I missed your comment here.

@pranavkm - how can you add more assemblies as references for Razor compilation at startup time?

[pranavkm]

@dazinator, there's a callback on RazorViewEngineOptions that lets you configure the Roslyn compilation context:

var previous = options.CompilationCallback;
options.CompilationCallback = (context) => 
{
    if (previous != null)
    {
        previous(context);
    }

    var references = myAssemblies.Select(Assembly.Load).ToArray();
    context.Compilation = context.Compilation.AddReferences(references);
};

[dazinator]

@pranavkm - options.CompilationCallback doesn't appear to be a thing in 6.0.0-rc1-final. Is there an alternative for RC1 or would we have to upgrade to perhaps RC2? or Core?

[pranavkm]

We added that as part of RC2.Looking at the code, there isn't a hook to configure compilation in RC1. Perhaps you could copy https://github.com/aspnet/Mvc/blob/6.0.0-rc1/src/Microsoft.AspNet.Mvc.Razor/Compilation/RoslynCompilationService.cs in to your application for the time being and remove it once you upgrade to RC2?

[javiercn]

Reopened until I check-in the tag helpers part