protected void Page_Load(object sender, EventArgs e)
    {
        string imgPath = string.IsNullOrEmpty(Request["imgPath"]) ? "" : Request["imgPath"].ToString();
       
        Response.ContentType = "image/jpeg";       
        DateTime contentModified = System.IO.File.GetLastWriteTime(imgPath); 

        if (IsClientCached(contentModified))
        {
            Response.StatusCode = 304;
            
            Response.SuppressContent = true;
        }
        else
        {
            
                Thumbnail.GenerateHighThumbnail(imgPath, 80, 80); //这里是生成缩略图的代码，网上到处都是，这里就不展现了。
            
            Response.Cache.SetETagFromFileDependencies();
            Response.Cache.SetAllowResponseInBrowserHistory(true);           

            Response.Cache.SetLastModified(contentModified);
        }
       

    }

    private bool IsClientCached(DateTime contentModified)
    {
        string header = Request.Headers["If-Modified-Since"];

        if (header != null)
        {
            DateTime isModifiedSince;
            if (DateTime.TryParse(header, out isModifiedSince))
            {
                return isModifiedSince >= DateTime.Parse(contentModified.ToString());
            }
        }

        return false;
    }