如何把数据存进XML的数据库

写入XML文件出错,原因:System.Xml.XmlException: 缺少根元素。 at System.Xml.XmlTextReader.Read() at System.Xml.XmlReader.MoveToContent() at System.Data.DataSet.ReadXmlSchema(XmlReader reader, Boolean denyResolving) at System.Data.DataSet.ReadXmlSchema(Stream stream) at ASP.WebForm1_aspx.Submit_Click(Object sender, EventArgs e) in D:\messagebook\WebForm1.aspx:line 24

存放原代码的语句 string datafile="db/guest.xml";
路径要怎么改？