How to read configuration settings before initializing a Host in ASP .NET Core?

You can clear the default sources added by CreateDefaultBuilder then add a pre-built IConfiguration with the AddConfiguration extension method.

public static void Main(string[] args)
{
    //...

    var configuration = new ConfigurationBuilder()
        .AddEnvironmentVariables()
        .AddCommandLine(args)
        .AddJsonFile("appsettings.json")
        .Build();

    //Do something useful with the configuration...

    var host = Host.CreateDefaultBuilder(args)
        .ConfigureAppConfiguration(builder =>
        {
            builder.Sources.Clear();
            builder.AddConfiguration(configuration);
        })
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseStartup<Startup>();
        })
        .Build();

    //...
}