Program.cs

using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Hosting;

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

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureAppConfiguration((hostingContext, configuration) =>
                {
                    IHostEnvironment env = hostingContext.HostingEnvironment;
                    configuration
                        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
                        .AddJsonFile($"secret.json", true, true);
                    IConfigurationRoot configurationRoot = configuration.Build();
                })
                .ConfigureWebHostDefaults(webBuilder => { webBuilder.UseStartup<Startup>(); })
        ;
    }
}