读取，设置指定目录下的配置文件的节点key value。config或XML

public static string GetValue(string key)//读取指定节点
        {
            XmlDocument doc = new XmlDocument();
            string nodeVal = string.Empty;
            try
            {
                doc.Load(HttpContext.Current.Server.MapPath("/Config/AppSetings.config"));//路径
                XmlNode node;
                XmlElement element;
                node = doc.SelectSingleNode("//" + "appSettings");//父节点
                element = (XmlElement)node.SelectSingleNode("//add[@key='" + key + "']");//add 节点的 key名称
                if (element != null)
                {
                    nodeVal = element.GetAttribute("value");
                }
                return nodeVal;
            }
            catch (Exception)
            {
                return "";
                throw;
            }
        }

/// <summary>
        /// 根据Key修改Value
        /// </summary>
        /// <param name="key">要修改的Key</param>
        /// <param name="value">要修改为的值</param>
        public static void SetValue(string key, string value)
        {
            FileInfo fi = new FileInfo(HttpContext.Current.Server.MapPath("/Config/AppSetings.config"));
            //判断文件属性是否只读?是则修改为一般属性
            if (fi.IsReadOnly==true)
            {
                fi.IsReadOnly = false;
            }
            System.Xml.XmlDocument xDoc = new System.Xml.XmlDocument();
            xDoc.Load(HttpContext.Current.Server.MapPath("/Config/AppSetings.config"));
            System.Xml.XmlNode xNode;
            System.Xml.XmlElement xElem1;
            System.Xml.XmlElement xElem2;
            xNode = xDoc.SelectSingleNode("//appSettings");

            xElem1 = (System.Xml.XmlElement)xNode.SelectSingleNode("//add[@key='" + key + "']");
            if (xElem1 != null) xElem1.SetAttribute("value", value);
            else
            {
                xElem2 = xDoc.CreateElement("add");
                xElem2.SetAttribute("key", key);
                xElem2.SetAttribute("value", value);
                xNode.AppendChild(xElem2);
            }
            xDoc.Save(HttpContext.Current.Server.MapPath("/Config/AppSetings.config"));
        }

第二种方法修改

/// <summary>

        /// 修改web.config中appSettings键的值
        /// </summary>
        /// <param name="configPath">web.config路径</param>
        /// <param name="keyName">键的名称</param>
        /// <param name="keyValue">键的值</param>
        public void UpdAppSettings(string configPath,string keyName,string keyValue)
        {
            XmlDocument doc = new XmlDocument();
            try
            {
                doc.Load(configPath);
                XmlNode node;
                XmlElement element;
                node = doc.SelectSingleNode("//appSettings");
                element = (XmlElement)node.SelectSingleNode("//add[@key='" + keyName + "']");
                if (element != null)
                {
                    element.SetAttribute("value", keyValue);
                    doc.Save(configPath);
                }
            }
            catch (Exception)
            {
                throw;
            }
        }

Config.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <appSettings>
    <add key="IntervalTime" value="12"/><!--扫码间隔时间-->
    <add key="FCKeditor" value="Files"/>
  </appSettings>
  <system.web>

  </system.web>
</configuration>