public static class ConfigurationManagerRequired
    {
        static ConfigurationManagerRequired()
        {
            AppSettings = new AppSettingsRequired();
        }

        public static AppSettingsRequired AppSettings { get; }

        public class AppSettingsRequired
        {
            public string this[string keyName]
            {
                get
                {
                    if (ConfigurationManager.AppSettings.AllKeys.Contains(keyName))
                    {
                        var value = ConfigurationManager.AppSettings[keyName];
                        if (string.IsNullOrEmpty(value))
                        {
                            throw new Exception($"AppSettings key {keyName} is empty");
                        }

                        return value;
                    }
                    else
                    {
                        throw new Exception($"AppSettings key {keyName} is not found");
                    }
                }
            }
        }
    }