ConnectionStringsSection Class

// Create a connectionn string element and add it to 
// the connection strings section. 
static ConnectionStrings()
{
    // Get the application configuration file.
    System.Configuration.Configuration config =
            ConfigurationManager.OpenExeConfiguration(
            ConfigurationUserLevel.None);

    // Get the current connection strings count. 
    int connStrCnt = 
        ConfigurationManager.ConnectionStrings.Count;

    // Create the connection string name.  
    string csName = 
        "ConnStr" + connStrCnt.ToString();

    // Create a connection string element and 
    // save it to the configuration file. 

    // Create a connection string element.
    ConnectionStringSettings csSettings =
            new ConnectionStringSettings(csName,
            "LocalSqlServer: data source=127.0.0.1;Integrated Security=SSPI;" +
            "Initial Catalog=aspnetdb", "System.Data.SqlClient");

    // Get the connection strings section.
    ConnectionStringsSection csSection =
        config.ConnectionStrings;

    // Add the new element.
    csSection.ConnectionStrings.Add(csSettings);

    // Save the configuration file.
    config.Save(ConfigurationSaveMode.Modified);

}