How can “IConfiguration object is registered with DI by default” in Startup.cs in .NET Core 2

Question

In, aspnet core 2, when we want to to Access configuration during startup, we can do it thanks to dependency injection as far as I understood.

For this code in below, how can we pass IConfiguration object to this class with benefits of Dependency Injection ;

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/Home/Error");
        }

        app.UseStaticFiles();

        app.UseMvc(routes =>
        {
            routes.MapRoute(
                name: "default",
                template: "{controller=Home}/{action=Index}/{id?}");
        });
    }
}

I saw a sentence:

" The IConfiguration object is registered with DI by default."

How can we succeed this?

In program.cs there is a code like this, do we set IConfiguration object in startup.cs class with this code below? I thought .UseStartup() code carries out this job(passing IConfiguration object to Startup class) for us. But how?

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
        .ConfigureAppConfiguration(SetupConfiguration)
          .UseStartup<Startup>();

    private static void SetupConfiguration(WebHostBuilderContext ctx, IConfigurationBuilder builder)
    {
        //Remove the default configuration options
        builder.Sources.Clear();

        builder.AddJsonFile("config.json", false, true).
            AddEnvironmentVariables();
    }
}

I would be so happy if someone can explain this with deep details.

Answer 1

The dependency injection is handled internally for you for the IConfiguration instance. The technical details are easy to find if you browse around the source code . In this case, there is a helper class that mimics a basic DI container, you can see this here .