App_Code/common2/common/WebConfig.cs

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using System;
using System.Configuration;
using System.Collections;
using System.Collections.Specialized;
using System.Xml;

namespace RM.Common
{
    public class WebConfig
    {
        public static NameValueCollection GetAppSettings()
        {
            return ConfigurationManager.AppSettings;
        }

        /// <summary>
        /// 修改web.config文件appSettings配置节中的Add里的value属性
        /// </summary>
        /// <remarks>
        /// 注意，调用该函数后，会使整个Web Application重启，导致当前所有的会话丢失
        /// </remarks>
        public static void SetAppSettings(NameValueCollection Settings)
        {
            string XPath = "/configuration/appSettings/add[@key='?']";
            XmlDocument domWebConfig = new XmlDocument();

            domWebConfig.Load((System.Web.HttpContext.Current.Server.MapPath("/web.config")));
            foreach (string key in Settings)
            {
                XmlNode addKey = domWebConfig.SelectSingleNode((XPath.Replace("?", key)));
                if (addKey == null)
                {
                    continue;
                }
                addKey.Attributes["value"].InnerText = Settings[key];
            }
            domWebConfig.Save((System.Web.HttpContext.Current.Server.MapPath("/web.config")));
        }
    }    
}