缓存使用：记录缓存何时被移除

有些时候我们需要知道缓存被更新的频率，为此探究一下缓存的相关事件（CacheItemRemoveCallback）。下面是一个使用示例：

        const string Key = "anystringtest";
        const string path = @"/csdata/siteuser.xml";
        private void Bind() {
            System.Web.Caching.Cache ocache = HttpRuntime.Cache;

            SiteUser user = ocache.Get(Key) as SiteUser;
            if (user == null)
            {
                user = new SiteUser();
                user = user.DeSerializeFromXML(typeof(SiteUser), Server.MapPath(path)) as SiteUser;

                CacheDependency depend = new CacheDependency(
                    Server.MapPath(path));
                TimeSpan ts = TimeSpan.Zero;
                ocache.Insert(Key, user, depend,
                        DateTime.Now.AddMinutes(3), ts, CacheItemPriority.Default,
                        /***** 使用匿名委托 *****/
                        (string key, object obj, CacheItemRemovedReason reason) => {
                            string content = string.Format("{0} -  缓存移除. key={1}\r\n",
                                DateTime.Now, key);
                            file.Write(Server.MapPath("/csdata/log.txt"), content);
                        }
                        );
                lbldata.Text = "from cache";

            }
            lbluser.Text = user.NickName;
        }