asp.net上传图片并同时生成缩略图

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<%@ Page Language="C#" ResponseEncoding="gb2312" %>

<%@ Import Namespace="System" %>
<%@ Import Namespace="System.IO" %>
<%@ Import Namespace="System.Drawing" %>
<%@ Import Namespace="System.Drawing.Imaging" %>

    void Page_Load(Object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            ImgPreview.Visible = false;
        }
    }
    void GetThumbnailImage(int width, int height, string strInfo, int left, int right)
    {
        string file = "Uploads/" + uploadFile.PostedFile.FileName.Substring(uploadFile.PostedFile.FileName.LastIndexOf('\\') + 1);
        string newfile = "Uploads/" + uploadFile.PostedFile.FileName.Substring(uploadFile.PostedFile.FileName.LastIndexOf('\\') + 1) + ".jpg";
        string strAdd = strInfo;
        System.Drawing.Image oldimage = System.Drawing.Image.FromFile(Server.MapPath(file));
        System.Drawing.Image thumbnailImage =
        oldimage.GetThumbnailImage(width, height, new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback), IntPtr.Zero);
        Response.Clear();
        Bitmap output = new Bitmap(thumbnailImage);
        Graphics g = Graphics.FromImage(output);
        g.DrawString(strAdd, new Font("Courier New", 14), new SolidBrush(Color.Red), left, right);
        output.Save(Server.MapPath(newfile), System.Drawing.Imaging.ImageFormat.Jpeg);
        Response.ContentType = "image/gif";
        ImgPreview.Visible = true;
        ImgPreview.ImageUrl = newfile;
    }
    bool ThumbnailCallback()
    {
        return true;
    }

    void Button_Click(object sender, EventArgs e)
    {
        int width, height, left, right;
        string strAddInfo = txtAddInfo.Text;
        width = Int32.Parse(txtWidth.Text);
        height = Int32.Parse(txtHeight.Text);
        left = Int32.Parse(txtLeft.Text);
        right = Int32.Parse(txtRight.Text);
        if (!(uploadFile.PostedFile.ContentLength > 0))
        {
            lblErrInfo.Text = "没有选择文件";
        }
        else
        {

            string path = Server.MapPath("./Uploads/" + uploadFile.PostedFile.FileName.Substring(uploadFile.PostedFile.FileName.LastIndexOf('\\') + 1));
            if (File.Exists(path))
            {
                lblErrInfo.Text = "已经有同名文件";
            }
            else
            {
                uploadFile.PostedFile.SaveAs(path);
                GetThumbnailImage(width, height, strAddInfo, left, right);
            }
        }
    }
</script>