C# .net Query of Run time Connection

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C#

C#4.0

i wanna change the app config file runtime in c#.net
4.0 version my code is:------------------
-----------------------------------

var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            int initialCount = ConfigurationManager.ConnectionStrings.Count;
            //string connStringName = txtConnName.Text;
            string serverName = txtserver.Text;
            string databaseName = txtdatabase.Text;
            string userId = txtUsername.Text;
            string password = txtPassword.Text;
            var connectionStringBuilder = new SqlConnectionStringBuilder
            {
                DataSource = serverName,
                InitialCatalog = databaseName,
                UserID = userId,
                Password = password
            };
            var csSetting = new ConnectionStringSettings(connectionStringBuilder.ConnectionString, "System.Data.SqlClient");
            //var csSetting = new ConnectionStringSettings(connStringName, connectionStringBuilder.ConnectionString, "System.Data.SqlClient");
            var csSection = config.ConnectionStrings;
            csSection.ConnectionStrings.Add(csSetting);
            config.Save(ConfigurationSaveMode.Modified, true);
            ConfigurationManager.RefreshSection("connectionString");
            int finalCount = ConfigurationManager.ConnectionStrings.Count;

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this code working 3.5 but it is not working in 4.0
please help me its Urgent
...................Thank you