<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <system.web>
    ...
  </system.web>
  <appSettings>
    <add key="ConnectionString" value="server=(local);database=dbname;uid=username;pwd=password"/>
  </appSettings>
</configuration>

string connString = ConfigurationSettings.AppSettings["ConnectionString"];

[XmlElement]
public string ConnectionString
{
   get { return connectionString; }
   set { connectionString = value; }
}

<appSettings>
  <add key="AppConfigPath" value="/FilePath/file.config"/>
</appSettings>

public class AppConfigSetting
{
   //获取配置对象
   public static AppConfig Get()
   {
      //尝试获取缓存中的对象
      AppConfig config = (AppConfig)HttpContext.Current.Cache["AppConfig"];
      //如果缓存中没有该配置对象，则直接获取对象
      if (config == null)
      {
         //新建序列化对象并指定其类型
         XmlSerializer serial = new XmlSerializer(typeof(AppConfig));
    
         try
         {
            string file = HttpContext.Current.Server.MapPath(GetFile());
      
            //读取文件流
            FileStream fs = new FileStream(file, FileMode.Open);
      
            //文件流反序列化为对象
            config = (AppConfig)serial.Deserialize(fs);
            fs.Close();
      
            //将对象加入到缓存中
            HttpContext.Current.Cache.Insert("AppConfig", config, new CacheDependency(file));
         }
         catch (System.IO.FileNotFoundException)
         {
            config = new AppConfig();
         }
      }
  
      return config;
   }
   //保存配置对象
   public static void Save(AppConfig config)
   {
      string file = HttpContext.Current.Server.MapPath(GetFile());
      XmlSerializer serial = new XmlSerializer (typeof(AppConfig));
      FileStream fs = new FileStream(file, FileMode.Create);
      //对象序列化为文件
      serial.Serialize(fs, config);
      fs.Close();
   }
   //获取配置文件路径
   private static string GetFile()
   {
      string path = (string)HttpContext.Current.Cache["FilePath"];
      if (path == null)
      {
         path=ConfigurationSettings.AppSettings["AppConfigPath"];
         HttpContext.Current.Cache["FilePath"] = path;
      }
      return path;
   }
}

//代码仅为使用演示
AppConfig config = AppConfigSetting.Get();
string connString = config.ConnectionString;
...
config.ConnectionString = connString;
AppConfigSetting.Save(config);