c# - ASP.NET MVC 6: view components in a separate assembly

Question

I'd like to define view components (which are new in ASP.NET MVC 6) in a separate assembly from the MVC 6 web startup project so that I can reuse them in multiple web projects. A sample solution might look like this:

• BookStore.Components (houses common view components)
• BookStore.Web1 (references BookStore.Components)
• BookStore.Web2 (references BookStore.Components)

I created a new Class Library (Package) and created a view component inside. I also created the view following the nested folder convention. My BookStore.Components project looks like this:

When I try to invoke this view component from my web project:

@Component.Invoke("BookOfTheMonth")

...I get a 500 error with an empty content body. It seems like the ViewComponent class is discovered, but the razor view for the component isn't.

I also tried to extend DefaultViewComponentDescriptorProvider so that view components from the BookStore.Components assembly can be discovered:

Defined an AssemblyProvider

 public class AssemblyProvider : IAssemblyProvider
    {
        public IEnumerable<Assembly> CandidateAssemblies
        {
            get
            {
                yield return typeof(AssemblyProvider).Assembly;
                yield return typeof(BookStore.Components.BookOfTheMonthViewComponent).Assembly;
            }
        }
    }

Registered AssemblyProvider using Autofac

builder.RegisterType<AssemblyProvider>()
    .AsImplementedInterfaces();

builder.RegisterType<DefaultViewComponentDescriptorProvider>()
    .AsImplementedInterfaces();

I'm not sure if the registration of DefaultViewComponentDescriptorProvider above is needed or not, so I tried with and without it, but I still get a 500 error on a page where the view component is invoked.

How can I invoke a view component that lives in a separate assembly from the MVC6 web project?

Answer 1

Update 2017-03-09

Things have changed a bit in Visual Studio 2017 using MS Build. Luckily it's much simpler. Here's how to get this to work:

In the external assembly, add this to the csproj file:

<ItemGroup>
   <EmbeddedResource Include="Views/**/*.cshtml" />
</ItemGroup>

In the main web project, add this NuGet package: Microsoft.Extensions.FileProviders.Embedded

Then in Startup, add the external assembly to the list of File Providers:

    services.Configure<RazorViewEngineOptions>(options =>
    {
        options.FileProviders.Add(new EmbeddedFileProvider(
             typeof(SampleClassInAssembly).Assembly
             # Prior to .Net Standard 2.0
             # typeof(SampleClassInAssembly).GetTypeInfo().Assembly
        ));
    });

I'll leave the original answer below for now, in case people are still trying to get this to work with older versions of .Net Core and project.json.

================================================================

Here are the steps to make this work.

• Make sure your view structure in the components assembly is the same as your web project. Note that there was a mistake in the screenshot that I posted along with my question.

• Register CompositeFileProvider in Startup.cs of the web project:

  services.Configure<RazorViewEngineOptions>(options =>
  {
      options.FileProvider = new CompositeFileProvider(
          new EmbeddedFileProvider(
              typeof(BookOfTheMonthViewComponent).GetTypeInfo().Assembly,
              "BookStore.Components"
          ),
          options.FileProvider
      );
  });
  

Both CompositeFileProvider and EmbeddedFileProvider are new, so you'll need to get these from the aspnetvnext NuGet feed. I did this by adding this source:

Add the dependencies in project.json:

"Microsoft.AspNet.FileProviders.Composite": "1.0.0-*",
"Microsoft.AspNet.FileProviders.Embedded": "1.0.0-*",

Lastly, add this to the project.json of the Components assembly:

"resource": "Views/**"

That should be enough to get this working.

Here is a working demo: https://github.com/johnnyoshika/mvc6-view-components/tree/master

This answer was formulated from this discussion here: https://github.com/aspnet/Mvc/issues/3750

Update 2016-01-15 There is currently one painful problem with external view components. Any changes you make to the view cshtml file does not automatically get recompiled. Even a forced Visual Studio clean and rebuild doesn't do it. You need to change a .cs file in the components assembly in order to trigger a view recompilation, but it looks like this is something that will be corrected in the future. The reason for this problem is explained here: https://github.com/aspnet/Mvc/issues/3750#issuecomment-171765303