static void Main(string[] args){  XmlDocument xDoc = new XmlDocument();  xDoc.Load("../../App.config");//加載xml文件  XmlNode xNode;  XmlElement xElem1;  XmlElement xElem2;  xNode = xDoc.SelectSingleNode("//appSettings");//獲取指定的xml子節(jié)點(diǎn)  xElem1 = (XmlElement)xNode.SelectSingleNode("//add[@key='type']");//獲取子節(jié)點(diǎn)中指定的子節(jié)點(diǎn)  //如果能獲取到節(jié)點(diǎn)，就修改節(jié)點(diǎn)的value值  if (xElem1 != null)  {    xElem1.SetAttribute("value", "driver");//給節(jié)點(diǎn)中的value屬性賦值(修改操作)  }  //如果不能獲取到節(jié)點(diǎn)，就創(chuàng)建節(jié)點(diǎn)  else  {    xElem2 = xDoc.CreateElement("add");    xElem2.SetAttribute("key", "type");    xElem2.SetAttribute("value","teacher");    xNode.AppendChild(xElem2);  }  xDoc.Save("../../App.config");//保存xml文檔  Console.WriteLine("保存成功！");}