xnode =  xdoc.selectsinglenode("http://appsettings");

   xelem1 = (xmlelement)xnode.selectsinglenode("http://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)//讀xmlpath是文件路徑+文件名，appkey是 key name，appvalue是value
  {
   xmldocument xdoc = new xmldocument();
   xdoc.load(xmlpath);
   xmlnode xnode;
   xmlelement xelem1;

   xnode =  xdoc.selectsinglenode("http://appsettings");

   xelem1 = (xmlelement)xnode.selectsinglenode("http://add[@key='" + appkey + "']");
   if ( xelem1 != null )
   {
    appvalue=xelem1.getattribute ("value");
   }
   else
   {
//    messagebox.show ("there is not any information!");
   }

  }

  #endregion