Request instance within ConfigureServices - dependency-injection

Question

Within ConfigureServices I'm setting up a number of services (this works). In the same method I'm trying to configure a custom file provider (for Razor). It looks like this:
services.AddMvc()
.AddRazorOptions(options =>
{
options.FileProvider = new CustomFileProvider(???);
});
CustomFileProvider has a few dependencies (that are all configured), but how can I ask the DI to give me an instance of CustomFileProvider right after all the services have been configured?
From what I can see the DI only injects in constructors, but in my case I need a "please give me an instance of CustomFileProvider right here" option.

Answer 1

If you want to be able to resolve services from the container while configuring some options, you need to leverage the IConfigureOptions<TOption> infrastructure, which most of MVC uses to set up the option defaults. See RazorViewEngineOptionsSetup for an example.
First, add the services you need to the collection:
services.AddSingleton<IDependency1, Dependency1>();
services.AddSingleton<IDependency2, Dependency2>();
Then, implement the setup class:
public class CustomFileProviderRazorViewEngineOptionsSetup : ConfigureOptions<RazorViewEngineOptions>
{
public CustomFileProviderRazorViewEngineOptionsSetup(IServiceProvider serviceProvider)
: base(options => ConfigureRazor(options, serviceProvider))
{
}
private static void ConfigureRazor(RazorViewEngineOptions options, IServiceProvider serviceProvider)
{
// Alternative 1 - Resolve each service and new up the instance.
var dependency1 = serviceProvider.GetService<IDependency1>();
var dependency2 = serviceProvider.GetService<IDependency2>();
options.FileProviders.Add(new CustomFileProvider(dependency1, dependency2));
// Alternative 2 - Same as alternative 1, but with moar magic ;)
options.FileProviders.Add(ActivatorUtilities.CreateInstance<CustomFileProvider>(serviceProvider));
// Alternative 3 - Just resolve CustomFileProvider from the service provider.
// This requires it to be registered first, of course.
options.FileProviders.Add(serviceProvider.GetService<CustomFileProvider>());
}
}
This is resolved from the container, so it'll allow you to inject an IServiceProvider, which can again be used to resolve the services you need. You could optionally add the CustomFileProvider to the container and resolve that directly instead, like in alternative 3.
Finally, add the setup class to the service collection:
services.TryAddEnumerable(
ServiceDescriptor.Transient<
IConfigureOptions<RazorViewEngineOptions>,
CustomFileProviderRazorViewEngineOptionsSetup>());
This will add the setup to the options builder pipeline, which means it'll run with the other registered instances of IConfigureOptions<RazorViewEngineOptions> to setup the options object.