在程序运行时更改config文件内容

/// <summary>
        /// 更新app.config的函数
        /// </summary>
        /// <param name="strFile">config文件名称</param>
        /// <param name="key">键</param>
        /// <param name="Xvalue">键值</param>
        public static void UpdateConfig(string strFile, string key, string Xvalue)
        {

            ////配置文件的源文件
            //<?xml   version="1.0"   encoding="utf-8"   ?>   
            //<configuration>   
            //<appSettings>   
            //<add   key="ServerName"   value=""/>   
            //</appSettings>   
            //</configuration>   

            XmlDocument doc = new XmlDocument();
            doc.Load(strFile);
            XmlNode node = doc.SelectSingleNode(@"//add[@key='" + key + "']");
            XmlElement ele = (XmlElement)node;
            ele.SetAttribute("value", Xvalue);
            doc.Save(strFile);
        }