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;
             }
         }
     }