<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
    <section name="hibernate-configuration"
             type="NHibernate.Cfg.ConfigurationSectionHandler, NHibernate" />
  </configSections>

  <connectionStrings>
    <add name="Main" connectionString="Data Source=.\sqlexpress;Initial Catalog=TEST;Integrated Security=True;"
      providerName="System.Data.SqlClient" />
  </connectionStrings>

  <hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
    <session-factory>
      <property name="dialect">NHibernate.Dialect.MsSql2005Dialect</property>
      <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
      <property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>
      <property name="connection.connection_string_name">Main</property>
      <property name="default_schema">dbo</property>
      <!-- ... -->
    </session-factory>
  </hibernate-configuration>
</configuration>

private void ConnStrMoveCodeToCfg()
{
  Configuration configuration = ConfigurationManager.OpenExeConfiguration( ConfigurationUserLevel.None );
  configuration.ConnectionStrings.ConnectionStrings["Main"].ConnectionString = connectionString_;
  configuration.Save( ConfigurationSaveMode.Full );
}

sessionFactory_ = new Configuration().Configure().BuildSessionFactory();