How can I add a custom JSON file into IConfiguration?

As of ASP.NET Core 5.0 it is as simple as adding a call to ConfigureAppConfiguration() in the call to CreateDefaultBuilder(args) inside Program.cs like so:

From

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });

to

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureAppConfiguration((hostingContext, config) =>
            {
                config.AddJsonFile("CustomSettings.json", optional: false, reloadOnChange: false);
            })
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });

For .Net Core 2.2, you need to modify Program.cs:

Before

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

After

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
            WebHost.CreateDefaultBuilder(args)

            //This is how you attach additional JSON files
            .ConfigureAppConfiguration((hostingContext, config) =>
            {
                config.AddJsonFile("customSettings.json", optional: false, reloadOnChange: false);
            })
            //End of update
            .UseStartup<Startup>();

For the latest amendments and to add other kinds of custom settings, please refer to Microsoft documentation at the following article.

You can do this by using the Options pattern:

On ASP.NET Core 2, register the config file on Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        var configuration = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)

             // custom config file
            .AddJsonFile("myappconfig.json", optional: false, reloadOnChange: false)
            .Build();

        BuildWebHost(args, configuration).Run();
    }

    public static IWebHost BuildWebHost(string[] args, IConfiguration config) =>
        WebHost.CreateDefaultBuilder(args)
            .UseConfiguration(config)
            .UseStartup<Startup>()
            .Build();
}

Create a class that matches with your config file:

public class MyAppConfig
{
    public string SomeConfig { get; set; }

    public int NumberConfig { get; set; }
}

Register it on ConfigureServices

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

    services.Configure<MyAppConfig>(Configuration);
}

Then, just access it in your Controller:

[Route("api/[controller]")]
public class ValuesController : Controller
{
    private readonly MyAppConfig _appConfig;

    public ValuesController(IOptions<MyAppConfig> optionsAccessor)
    {
        if (optionsAccessor == null) throw new ArgumentNullException(nameof(optionsAccessor));
        _appConfig = optionsAccessor.Value;
    }

    // GET api/values/5
    [HttpGet("{id}")]
    public string Get(int id)
    {
        return _appConfig.SomeConfig;
    }
}

If you are not in ASP.NET Core 2 yet, the process is almost the same. You just need to add the custom config file on Startup.cs. The rest is basically the same.