public TPDataContext() :
            base(global::TPAPI.Properties.Settings.Default.TruePotentialConnectionString, mappingSource)
    {
        OnCreated();
    }

Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
config.ConnectionStrings.ConnectionStrings["TPAPI.Properties.Settings.TruePotentialConnectionString"].ConnectionString = txtConnectionString.Text;
config.Save(ConfigurationSaveMode.Modified, true);
ConfigurationManager.RefreshSection("connectionStrings");

TPDataContext db = new TPDataContext(connStr);