ConfigureAppConfiguration.cs

WebHost.CreateDefaultBuilder(args)
                .UseStartup<Startup>()
                .ConfigureAppConfiguration((builderContext, config)=> {
                    IHostingEnvironment env = builderContext.HostingEnvironment;
                    config
                        .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
                        .AddJsonFile("appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: true);
                })
                .Build();