c# webform网站图片另存代码

好辛苦生成了二维码，然后要实现点击“保存图片”，弹出选择路径进行保存的效果。

look：

    string qrcodeurl = "";
    string username = "";
    protected void btnSave_Click(object sender, EventArgs e)
    {
        //生成图片
        Bitmap image = Create_ImgCode(qrcodeurl, );
        //保存图片
        string server_saveurl = Server.MapPath("~/") + "Upload\\qrcode";
        SaveImg(server_saveurl, image);
        FileDownload(server_saveurl + "\\" + username + ".png");
    }

    /// <summary>
    /// 生成二维码图片
    /// </summary>
    /// <param name="codeNumber">要生成二维码的字符串</param>
    /// <param name="size">大小尺寸</param>
    /// <returns>二维码图片</returns>
    public Bitmap Create_ImgCode(string codeNumber, int size)
    {
        //创建二维码生成类
        QRCodeEncoder qrCodeEncoder = new QRCodeEncoder();
        //设置编码模式
        qrCodeEncoder.QRCodeEncodeMode = QRCodeEncoder.ENCODE_MODE.BYTE;
        //设置编码测量度
        qrCodeEncoder.QRCodeScale = size;
        //设置编码版本
        qrCodeEncoder.QRCodeVersion = ;
        //设置编码错误纠正
        qrCodeEncoder.QRCodeErrorCorrect = QRCodeEncoder.ERROR_CORRECTION.M;
        //生成二维码图片
        System.Drawing.Bitmap image = qrCodeEncoder.Encode(codeNumber);
        return image;
    } 

    /// <summary>
    /// 保存图片,在服务端
    /// </summary>
    /// <param name="strPath">保存路径</param>
    /// <param name="img">图片</param>
    public string SaveImg(string strPath, Bitmap img)
    {
        //保存图片到目录
        if (!Directory.Exists(strPath))
        {
            //当前目录不存在，则创建
            Directory.CreateDirectory(strPath);
        }
        //文件名称
        string filename = strPath + "/" + username + ".png";
        if (File.Exists(filename))
        {
            return filename;
        }
        img.Save(filename, System.Drawing.Imaging.ImageFormat.Png);
        return filename;
    }

    /// <summary>
    /// webform另存图片,在客户端
    /// </summary>
    /// <param name="FullFileName"></param>
    private void FileDownload(string FullFileName)
    {
        FileInfo DownloadFile = new FileInfo(FullFileName);
        Response.Clear();
        Response.ClearHeaders();
        Response.Buffer = false;
        Response.ContentType = "application/octet-stream";
        Response.AppendHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(DownloadFile.FullName, System.Text.Encoding.UTF8));
        Response.AppendHeader("Content-Length", DownloadFile.Length.ToString());
        Response.WriteFile(DownloadFile.FullName);
        Response.Flush();
        Response.End();
    }

都是网上找的，找了好久在想到先把图片保存到服务端，然后再从服务端下载到客服端。