How to write in app.config file

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C#

Am using C# windows application. i used following code to write in app.cofig file to change connection string.but those chnages are not permanent. For next login am getting previous values instead of new values.

how to get new connection string for every login?

Configuration config = ConfigurationManager.OpenExeConfiguration( ConfigurationUserLevel.None );
           config.ConnectionStrings.ConnectionStrings.Add( new ConnectionStringSettings("MyConnectionString", String.Format("DataSource={0};InitialCatalog={1}; IntegratedSecurity={2}","testing", "testing2", "Testing6")));
           config.Save( ConfigurationSaveMode.Modified, true);
           ConfigurationManager.RefreshSection("connectionStrings");
           MessageBox.Show( ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString);

help me..