修改web.config

读取

ConfigurationManager.AppSettings["isMail"]

修改

   public static void SetValue(string path, string KeyName, string AppValue)
   {
    XmlDocument xDoc = new XmlDocument();
    xDoc.Load(path);
    XmlNode xNode;
    XmlElement xElem1;
    xNode = xDoc.SelectSingleNode("//configuration");//固定查询 AppSettings下节点
    xElem1 = (XmlElement)xNode.SelectSingleNode("//add[@key='" + KeyName + "']");
    if (xElem1 != null) xElem1.SetAttribute("value", AppValue);
    xDoc.Save(path);
   }