string FileName = "hibernate.cfg.xml";

            XmlDocument xmldoc = new XmlDocument();

            xmldoc.Load(FileName);

            XmlNodeList nodeList = xmldoc.DocumentElement.ChildNodes[0].ChildNodes;

foreach (XmlNode nl in nodeList)

            {

if (nl.Name == "property" && nl.Attributes["name"].Value == "connection.connection_string")

                {

                    nl.InnerXml = @" 数据库连接字符串;";

break;

                }

            }

            XmlTextWriter writer = new XmlTextWriter(FileName, Encoding.Default);

            writer.Formatting = Formatting.Indented;

            xmldoc.Save(writer);

 configuration = new Configuration().Configure("hibernate.cfg.xml")

                .SetProperty("connection.connection_string", " 数据库连接字符串");