ashx 指定客户端缓存

方式一：（比较简陋的）

                context.Response.Cache.SetCacheability(HttpCacheability.Public); 
                context.Response.Expires = 100; //Minutes

方式二：（比较全的设置）

                TimeSpan refresh = new TimeSpan(0, 15, 0);//时，分，秒
                context.Response.Cache.SetExpires(DateTime.Now.Add(refresh));
                context.Response.Cache.SetMaxAge(refresh);
                context.Response.Cache.SetCacheability(HttpCacheability.Public);
                context.Response.CacheControl = HttpCacheability.Public.ToString();
                context.Response.Cache.SetValidUntilExpires(true);