ASP.NET Cache缓存管理基于文件的依赖

public class CacheCD
{
    public static string GetMessageByCache//使用方法和属性都可以被HtmlEncode（）传回去
    {
        //HttpContext context = HttpContext.Current;

        get
        {
            //string message = context.Cache[ "Message" ] as string;
            string message = HttpRuntime.Cache[ "Message" ] as string;
            if ( message == null )
            {
                string path = HttpContext.Current.Server.MapPath( "~/App_Data/TextFile.txt" );

                message = System.IO.File.ReadAllText( path );//

                //为了以后访问，将信息保存在缓存中，设置相对过期的时间为 1 小时
                //context.Cache.Add(
                HttpRuntime.Cache.Add(
                    "Message",
                    message,
                    new System.Web.Caching.CacheDependency( path ),
                    System.Web.Caching.Cache.NoAbsoluteExpiration,
                    new TimeSpan( 1, 0, 0 ),
                    System.Web.Caching.CacheItemPriority.Normal,
                    //System.Web.Caching.CacheItemPriority.Default,//ok
                    //new System.Web.Caching.CacheItemRemovedCallback( Callback ) //ok
                   Callback );

            }
            return message;
        }
    }

    public static void Callback( string key, object value, System.Web.Caching.CacheItemRemovedReason reason )
    {

    }
}