AppSettingManager

    public class AppSettingManager
    {
        public static bool Update(string key, string value)
        {

            try
            {
                var config = Create();
                if (config == null)
                {
                    return false;
                }
                var isModified = !string.IsNullOrEmpty(Get(key));
                if (isModified)
                {
                    config.AppSettings.Settings.Remove(key);
                }
                // Add an Application Setting.
                config.AppSettings.Settings.Add(key, value);
                // Save the changes in App.config file.
                config.Save(ConfigurationSaveMode.Modified);
                // Force a reload of a changed section.
                ConfigurationManager.RefreshSection("appSettings");
                return true;
            }
            catch (Exception)
            {
                return false;
            }

        }

        public static bool Add(string key, string value)
        {
            try
            {
                var config = Create();
                if (config == null)
                {
                    return false;
                }
                config.AppSettings.Settings.Add(key, value);
                config.Save(ConfigurationSaveMode.Modified);
                ConfigurationManager.RefreshSection("appSettings");
                return true;
            }
            catch (Exception)
            {
                return false;

            }

        }

        public static string Get(string key)
        {
            var config = Create();
            if (config == null)
            {
                return null;
            }
            return config.AppSettings.Settings[key].Value;
        }

        private static Configuration Create()
        {
            try
            {
                return ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            }
            catch (Exception)
            {
                return null;
            }
        }
    }