perhostjson.cs

    public class PerHostJsonConfigBuilder : ConfigurationBuilder
    {
        public string Directory { get; set; }
        JObject hostConfig = null;

        public override void Initialize(string name, NameValueCollection config)
        {
            base.Initialize(name, config);

            JObject LoadByHostName(string host)
            {
                string appName = config["appName"] ?? null;
                if (appName != null) appName = "." + appName;

                var configName = $"{host}{appName ?? ""}.config.json";

                var fileName = Path.Combine(System.Web.Hosting.HostingEnvironment.ApplicationPhysicalPath,
                    Directory,
                    configName);

                if (File.Exists(fileName))
                {
                    System.Diagnostics.Trace.TraceInformation($"Loaded config {configName}");
                    return JObject.Parse(File.ReadAllText(fileName));
                }

                return null;
            }

            hostConfig = LoadByHostName(config["default"] ?? "Production") ?? LoadByHostName(Environment.MachineName);
        }

        public override ConfigurationSection ProcessConfigurationSection(ConfigurationSection configSection)
        {
            IEnumerable<(string key, string value)> HostSettings(string section)
            {
                if (hostConfig == null) yield break;

                var appSettings = hostConfig[section];

                if (appSettings != null && appSettings is JObject settings)
                {
                    foreach (var property in settings.Properties())
                    {
                        yield return (property.Name, property.Value.ToString());
                    }
                }
            }
            
            if (configSection is AppSettingsSection app)
            {
                foreach (var setting in HostSettings("AppSettings"))
                {
                    app.Settings.Remove(setting.key);
                    app.Settings.Add(setting.key, setting.value);
                }
            }

            if (configSection is ConnectionStringsSection connection)
            {
                foreach (var setting in HostSettings("ConnectionStrings"))
                {
                    var connectionString = connection.ConnectionStrings[setting.key];

                    if (connectionString != null)
                    {
                        connectionString.ConnectionString = setting.value;
                    }
                }
            }

            return base.ProcessConfigurationSection(configSection);
        }
    }