public void SetXmlFileValue(string xmlPath,string AppKey,string AppValue)//写xmlPath是文件路径+文件名，AppKey是 Key Name，AppValue是Value
  {
   XmlDocument xDoc = new XmlDocument();
   xDoc.Load(xmlPath);
   XmlNode xNode;
   XmlElement xElem1;
   XmlElement xElem2;

   xNode =  xDoc.SelectSingleNode("//appSettings");

   xElem1 = (XmlElement)xNode.SelectSingleNode("//add[key='" + AppKey + "']");
   if ( xElem1 != null )
   {
    xElem1.SetAttribute("value",AppValue);
   }
   else
   {
    xElem2 = xDoc.CreateElement("add");
    xElem2.SetAttribute("key",AppKey);
    xElem2.SetAttribute("value",AppValue);
    xNode.AppendChild(xElem2);
   }
   xDoc.Save(xmlPath);
  }

  public void GetXmlFileValue(string xmlPath,string AppKey,ref string AppValue)//读xmlPath是文件路径+文件名，AppKey是 Key Name，AppValue是Value
  {
   XmlDocument xDoc = new XmlDocument();
   xDoc.Load(xmlPath);
   XmlNode xNode;
   XmlElement xElem1;

   xNode =  xDoc.SelectSingleNode("//appSettings");

   xElem1 = (XmlElement)xNode.SelectSingleNode("//add[key='" + AppKey + "']");
   if ( xElem1 != null )
   {
    AppValue=xElem1.GetAttribute ("value");
   }
   else
   {
//    MessageBox.Show ("There is not any information!");
   }

  }

  #endregion