XmlDocument xmlDoc = new XmlDocument();

                xmlDoc.LoadXml(xmlText);

            //plu.xml 编码是ANSI的。否则称上品名是乱码

                XmlElement xeRoot = xmlDoc.DocumentElement;

                XmlDeclaration xdl = xmlDoc.CreateXmlDeclaration("1.0", "gb2312", "yes");

                xmlDoc.InsertBefore(xdl, xeRoot);

                xmlDoc.Save(fileFullName);