Changes from version 2.x

The big change from version 2.0 was that, as of 2.1, the configuration is done by convention—that is, the process of adding appsettings.json JSON files (generic and optional per environment) and all that is hidden from the users.

This is defined in the WebHost.CreateDefaultBuilder method. You can, however, still build your own ConfigurationBuilder and add whatever you like to it. To do this, you call the ConfigureAppConfiguration method, as described in Chapter 1, Getting Started with ASP.NET Core, and illustrated in the following code block:

Host    .CreateDefaultBuilder(args)
    .ConfigureAppConfiguration(builder =>
    {
        var jsonSource = new JsonConfigurationSource { Path = 
            "appsettings.json" };
        builder.Add(jsonSource);
    })
    .ConfigureWebHostDefaults(builder =>
    {
        builder.UseStartup<Startup>();
    });

Or, if you just want to add a single entry to the configuration that is built by default (or, to the one you're modifying), you call the UseSettings extension method, as follows:

Host    .CreateDefaultBuilder(args)
    .ConfigureWebHostDefaults(builder =>
    {
        builder.UseSetting("key", "value");
        builder.UseStartup<Startup>();
    });

So, when the Startup class is instantiated, it will get passed an IConfiguration object that is built from the code that you put in here.

After seeing how the application configuration is done, let's see how we can do the same for the host.