asp.net 自动检测缓存内容是否变化

1 使用cache.Insert方法时，新建一个System.Web.Caching.CacheDependency对象，告诉缓存，当缓存的内容发生变化时，将删除缓存，并重新缓存

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;

namespace Cachting
{
    public class CityListInfo
    {
        public string Timestamp { get; set; }
        public string Html { get; set; }
    }

    public partial class CitiesControl : System.Web.UI.UserControl
    {
        private static readonly string fileName = "/CitiesList.html";
        private static readonly string CACHE_KEY = "cities_html";
        private CityListInfo cityInfo;
        private bool cached = false;
        protected void Page_Load(object sender, EventArgs e)
        {
            cityInfo = Cache[CACHE_KEY] as CityListInfo;
            if (cityInfo==null)
            {
                cityInfo = new CityListInfo { Timestamp=DateTime.Now.ToLongTimeString(),
                    Html =File.ReadAllText(MapPath(fileName))};
                //只要未修改文件就应该缓存数据
                Cache.Insert(CACHE_KEY, cityInfo,new System.Web.Caching.CacheDependency(MapPath(fileName)));
            }
            else
            {
                cached = true;
            }
        }

        public string GetCities()
        {
            //return File.ReadAllText(MapPath(fileName));
            return cityInfo.Html;

        }

        public string GetTimeStamp()
        {
            // return DateTime.Now.ToLongTimeString();
            return cityInfo.Timestamp + (cached ? "<b>Cached</b>" : "");
        }
    }
}