AspNetCore RazorEngine will not use FileProviders configured in derived IOptions<RazorViewEngineOptions> - razor

Question

Use Case
A separated template engine that doesn't interfere in any way with normal Razor operation in an AspNetCore (2.X) web app.
Problem
Whilst trying to implement the above, I've created a whole bunch of derived wrapper classes based on RazorViewEngine, RazorViewCompilerProvider, DefaultRazorPageFactoryProvider, DefaultRazorViewEngineFileProviderAccessor and RazorViewEngineOptions in an effort that these can be registered with DI and injected whilst not having side affects in the normal Razor code path. I've succeeded except for one annoying issue, whereby I still need to configure my custom FileProvider (TemplateRepository) within the normal RazorViewEngineOptions rather than my wrapper class.
e.g. In the below code from Startup.cs, even though the file provider is specified in my custom Options object, and that is what is injected into the wrapper classes, the TemplateRepository is not called for a View request unless the second service.Configure is also included (using RazorViewEngineOptions).
services.Configure<TemplateOptions>(options =>
{
options.ViewLocationExpanders.Add(new TemplateNameExpander());
options.ViewLocationFormats.Add("{0}");
options.AreaViewLocationFormats.Add("{0}");
options.FileProviders.Clear();
options.FileProviders.Add(new TemplateRepository(new SqlConnectionFactory(configuration)));
});
services.Configure<RazorViewEngineOptions>(
options =>
{
options.FileProviders.Add(new TemplateRepository(new SqlConnectionFactory(configuration)));
});
This would suggest to me that somewhere in the RazorViewEngine dependency tree the RazorViewEngineOptions is being injected somewhere, but I cannot find it.
Full Source # GitHub

Answer 1

It seems that you have defined your custom RazorViewEngine but you do not tell MVC to use it.
Try to add below codes to add TemplateRazorEngine to MVC view engine.
services.Configure<TemplateOptions>(options =>
{
options.ViewLocationExpanders.Add(new TemplateNameExpander());
options.ViewLocationFormats.Add("{0}");
options.AreaViewLocationFormats.Add("{0}");
options.FileProviders.Clear();
options.FileProviders.Add(new TemplateRepository(new SqlConnectionFactory(configuration)));
});
services.Configure<MvcViewOptions>(options => {
var engine = services.BuildServiceProvider().GetRequiredService<TemplateRazorEngine>();
options.ViewEngines.Add(engine);
});

Answer 2

Late to the party but maybe you will use this in the future. I've also tried a lot of options and in the end I came to the conclusion that child containers would solve my particular issue. Unfortunately the AspNetCore container doesn't support them so I had to implement something quick that might not work in your case. Another option would be to use StructureMap or any other container that supports this functionality.
public class ChildServiceProvider : IServiceProvider, IDisposable
{
private readonly IServiceProvider _child;
private readonly IServiceProvider _parent;
public ChildServiceProvider(IServiceProvider parent, IServiceProvider child)
{
_parent = parent;
_child = child;
}
public ChildServiceProvider(IServiceProvider parent, IServiceCollection services)
{
_parent = parent;
_child = services.BuildServiceProvider();
}
public void Dispose()
{
(_child as IDisposable)?.Dispose();
}
public object GetService(Type serviceType)
{
return _child.GetService(serviceType) ?? _parent.GetService(serviceType);
}
}
And this is how I used it
public class Startup : IStartup
{
public IServiceProvider ChildServiceProvider { get; set; }
IServiceProvider IStartup.ConfigureServices(IServiceCollection services)
{
// Define a wrapper for the RazorViewEngine and it as a singleton
services.AddSingleton<CustomRazorEngine>(serviceProvider =>
{
// get the RazorViewEngine from the childContainer
var razorViewEngine = ChildServiceProvider.GetRequiredService<IRazorViewEngine>();
return new CustomRazorEngine(razorViewEngine);
});
return services.BuildServiceProvider();
}
public void Configure(IApplicationBuilder app)
{
ChildServiceProvider = CreateChildServiceProvider(app);
app.UseMvc();
}
IServiceProvider CreateChildServiceProvider(IApplicationBuilder parentApp)
{
// create the child container from the parentApp and register
// the custom RazorViewEngineOptions that you need for the isolated templating engine
// and whatever custom services that you need
var server = parentApp.ApplicationServices.GetRequiredService<IServer>();
var webHost = WebHost.CreateDefaultBuilder()
.ConfigureServices(services =>
{
services.AddOptions();
services.Configure<RazorViewEngineOptions>(opts =>
{
opts.FileProviders.Clear();
opts.FileProviders.Add(new CustomFileProvider());
});
services.AddMvc();
})
.Build();
return new ChildServiceProvider(parentApp.ApplicationServices, webHost.Services);
}
}
And the custom classes that you would need to implement
public class CustomFileProvider : IFileProvider
{
}
public class CustomRazorEngine
{
private readonly IRazorViewEngine _razorViewEngine;
public CustomRazorEngine(IRazorViewEngine razorViewEngine)
{
_razorViewEngine = razorViewEngine;
}
}
This was tested with dotnet 2.2 but haven't thoroughly tested it to be 100% that there are no performance issues or other hidden ones.
Also would be curious to know if you found another solution :)