aspnetcore: how to return a view from middleware

Question

I'm working at a middleware for aspnetcore2.0 where I want to execute some razor view.
Actually I need a error handling middleware which would show nice pages from razor views. I know that it's possible to do with UseStatusCodePagesWithReExecute based on status codes. But I need a more general approach - handle an exception in my middleware to delegate (in some cases) it to an error view.

I realized that DeveloperExceptionPageMiddleware does something similar to what I need. But I can't understand how it works even after digging into its sources.

Here is the place where that middleware returns a view - https://github.com/aspnet/Diagnostics/blob/dev/src/Microsoft.AspNetCore.Diagnostics/DeveloperExceptionPage/DeveloperExceptionPageMiddleware.cs#L206

But I can't understand what kind of view it is. It's nor a razor page (as it has no @page directive) neither an mvc view (but i'm not sure).

In the project there're two files for that view: ErrorPage.cshtml and ErrorPage.Designer.cs. How that Designer.cs was created? It looks like a generated file. But thanks to it there's a normal class in the project (ErrorPage) which can be used explicitly. It inherits Microsoft.Extensions.RazorViews.BaseView class from Microsoft.Extensions.RazorViews.Sources package.

So the middleware just execute that view:

var errorPage = new ErrorPage(model);
return errorPage.ExecuteAsync(context);

How can it be achieved in my project?

Answer 1

UPDATE [2018.06]: Please note that the post was written for .NET Core 2.0 times, there're breaking changes for RazorEngine in .NET Core 2.1.

It turned out that it's pretty easy to do. Aspnet prjoect has an internal tool called RazorPageGenerator (see https://github.com/aspnet/Razor/tree/dev/src/RazorPageGenerator) which can be used to compile views. After compilation with this tool we'll get normal classes which can be used in middlewares.

But before we need to get RazorPageGenerator and slightly customize it.

1.Create a new console project

dotnet new console -o MyRazorGenerator

2.put NuGet.config inside this folder

<configuration>
  <config>
    <add key="globalPackagesFolder" value="./packages" />
  </config>
  <packageSources>
    <add key="aspnetcore-dev" value="https://dotnet.myget.org/F/aspnetcore-dev/api/v3/index.json " />
  </packageSources>
</configuration>

3.Add the following in csprj (as dotnet add package doesn't support installing pre-prelease packages)

<ItemGroup>
  <PackageReference Include="RazorPageGenerator" Version="2.1.0-*" />
  <PackageReference Include="Microsoft.AspNetCore.Mvc.Razor.Extensions" Version="2.1.0-*" />
  <PackageReference Include="Microsoft.AspNetCore.Razor.Language" Version="2.1.0-*" />
</ItemGroup>

4.restore dotnet restore to check you got RazorPageGenerator

5.add into Program.cs:

    public static int Main(string[] args)
    {
        if (args == null || args.Length < 1)
        {
            Console.WriteLine("Invalid argument(s).");
            return 1;
        }

        var rootNamespace = args[0];
        var targetProjectDirectory = args.Length > 1 ? args[1] : Directory.GetCurrentDirectory();

        var razorEngine = RazorPageGenerator.Program.CreateRazorEngine(rootNamespace, builder => {
            FunctionsDirective.Register(builder);
            InheritsDirective.Register(builder);
            SectionDirective.Register(builder);
        });
        var results = RazorPageGenerator.Program.MainCore(razorEngine, targetProjectDirectory);

        foreach (var result in results)
        {
            File.WriteAllText(result.FilePath, result.GeneratedCode);
        }

        Console.WriteLine();
        Console.WriteLine($"{results.Count} files successfully generated.");
        Console.WriteLine();
        return 0;
    }

6.Now we have our own generator and can compile views

7.Create a Razor View (.cshtml)

8.run our generator to compile view:

dotnet run --project .\MyRazorPageGenerator\MyRazorPageGenerator.csproj Croc.XFW3.Web .\Middleware

here I assume that the view is inside Middleware\Views folder.

9.Generator creates a file like ErrorPage.Designer.cs (if view was ErrorPage.cshtml) which we can use:

    public async Task Invoke(HttpContext context)
    {
        try
        {
            await _next.Invoke(context);
            if (context.Response.StatusCode == StatusCodes.Status404NotFound)
            {
                var statusCodeFeature  = context.Features.Get<IStatusCodePagesFeature>();
                if (statusCodeFeature == null || !statusCodeFeature.Enabled)
                {
                    if (!context.Response.HasStarted)
                    {
                        var view = new ErrorPage(new ErrorPageModel());
                        await view.ExecuteAsync(context);
                    }
                }
            }
        }
    }

Here we're returning our view in case of 404 error and absense of StatusCodePagesMiddleware. Can be useful for embedded UI in libs.

The generated code uses staff which should be added into your project. To get it we need to acquire nuget package Microsoft.Extensions.RazorViews.Sources. Again it’s not on nuget.org so we need to install it from https://dotnet.myget.org/feed/aspnetcore-dev/package/nuget/Microsoft.Extensions.RazorViews.Sources.