How is appsettings.json loaded in ASP.NET Core

Question

The NET Core 2.x Web API project template provides a Program.cs and Startup.cs (among other things by default).

If you put a breakpoint inside the constructor of Startup, and add a watch you can see the values loaded from appsettings.json.

Startup.cs and Program.cs are not explicitly loading appsettings.json, so it must be happening in the call to CreateDefaultBuilder or Build in Program.

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

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

I have looked the Microsoft source code on https://github.com/aspnet/Configuration and https://github.com/aspnet/Hosting, in particular the WebHostBuilder, but I can't see where it happens.

Where is the code that loads the appsettings.json?

Answer 1

appsettings.json is loaded by the call to CreateDefaultBuilder located in Microsoft.AspNetCore.WebHost.

 //snip
 config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
.AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: true);
 //snip

Source code available on GitHub - https://github.com/aspnet/AspNetCore/blob/master/src/DefaultBuilder/src/WebHost.cs#L165