Get configuration value before services are configured

Question

I'd like to get a value from my json configuration file before dependency injection is configured / before I call "Build()" on my service collection.

If I want to configure services differently depending on a value in my json configuration file, how would I do that?

Another way to phrase this is how do I get a configuration value from an IConfigurationRoot implementation?

In this code, how would I set the value of "useService1" depending on a value in my configuration file?

    class Program
    {
        static void Main(string[] args)
        {
            var configuration = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("appsettings.json")
                .Build();

            var serviceCollection = new ServiceCollection();

            var useService1 = true; // how do I get a value from my configuration file here?
            if (useService1)
                serviceCollection.AddSingleton<IMyService>(new Service1());
            else
                serviceCollection.AddSingleton<IMyService>(new Service1());

            var serviceProvider = serviceCollection.BuildServiceProvider();

            var myService = serviceProvider.GetService<IMyService>();

            myService.DoWork();
        }

        interface IMyService
        {
            void DoWork();
        }

        class Service1 : IMyService
        {
            public void DoWork()
            {
                Console.WriteLine("1 doing work");
            }
        }

        class Service2 : IMyService
        {
            public void DoWork()
            {
                Console.WriteLine("2 doing work");
            }
        }
    }

Answer 1

I'm able to get nested appsettings.json configuration values using a semicolon to specify nested values.

appsettings.json:

{
  "TopLevel1":  "Value1",
  "TopLevel2": {
    "SecondLevel1": "Value2",
    "SecondLevel2": {
      "ThirdLevel": "Value3"
    }
  }
}

class Program
{
    static void Main(string[] args)
    {
        var configuration = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json")
            .Build();

        var value1 = configuration["TopLevel1"];
        var value2 = configuration["TopLevel2:SecondLevel1"];
        var value3 = configuration["TopLevel2:SecondLevel2:ThirdLevel"];

    }
}