unable to add/update connection string in app.config

try
            {
                
                System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
                //ConnectionStringSettingsCollection connections = ConfigurationManager.ConnectionStrings;
                //ConfigurationPermission cp = new ConfigurationPermission(System.Security.Permissions.PermissionState.Unrestricted);
                ConnectionStringSettings css = new ConnectionStringSettings("cString3", "abcdefxyz");
                config.ConnectionStrings.ConnectionStrings.Add(css);
                config.Save();
                ConfigurationManager.RefreshSection("connectionStrings"); //May be this is unnecessary.
                foreach (ConnectionStringSettings connection in connections)
                {
                    string str = connection.Name;
                    string val = connection.ConnectionString;
                    MessageBox.Show(str + " " + val);
                    //textBox1.Text = str;
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

private static void CopyCurrentDataConfiguration(IDataConfiguration dataConfig)
       {
           // Load the configuration from the configuration file
           var configSource = ConfigurationSourceFactory.Create();

           // Retrieve the default datasource name
           var dataConfigurationSection = (DatabaseSettings)configSource.GetSection(
               DatabaseSettings.SectionName);
           var defaultDatabaseName = dataConfigurationSection.DefaultDatabase;

           CopyConnectionStrings(dataConfig, defaultDatabaseName);
       }

       private static void CopyConnectionStrings(IDataConfiguration dataConfig,string defaultDatabaseName)
       {
           // Add the configurations from our application config file to the builder
           foreach (ConnectionStringSettings settings in ConfigurationManager.ConnectionStrings)
           {
               try
               {
                   var configuredDatabase = dataConfig
                     .ForDatabaseNamed(settings.Name)
                     .ThatIs
                     .AnotherDatabaseType(settings.ProviderName);

                   if (settings.Name.Equals(defaultDatabaseName))
                   {
                       configuredDatabase.AsDefault();
                   }
               }
               catch { }
           }
       }

       public static void AddNewConnectionString(string databaseName, string connString)
       {
           var builder = new ConfigurationSourceBuilder();
           var dataConfig = builder.ConfigureData();
           CopyCurrentDataConfiguration(dataConfig);

           // Add our database connections to the builder
           dataConfig
             .ForDatabaseNamed(databaseName)
             .ThatIs
             .AnotherDatabaseType("Oracle.DataAccess.Client")
             .WithConnectionString(connString)
             .AsDefault();

           string conn = "your_connection_string";

           // Create a new configuration source, update it with the builder and make it the current configuration
           var newConfigSource = new DictionaryConfigurationSource();
           builder.UpdateConfigurationWithReplace(newConfigSource);
           EnterpriseLibraryContainer.Current = EnterpriseLibraryContainer.CreateDefaultContainer(newConfigSource);
       }