asp.net验证码

protected void Page_Load(object sender, EventArgs e)
    {
        Response.BufferOutput = true;
        Bitmap img = new Bitmap(100, 100, PixelFormat.Format24bppRgb);
        Graphics grh = Graphics.fromImage(img);
        grh.Clear(ColorTranslator.fromHtml("#336699"));
        grh.FillRectangle(new SolidBrush(Color.Yellow), 200, 200, 200, 200);
        grh.DrawString(Show(), new Font("宋体", 20), new SolidBrush(Color.Red), new PointF(50, 50));
        //Response.ContentType = "image/Jpeg";
        img.Save(Response.OutputStream, ImageFormat.Jpeg);
        Response.ContentType = "image/Jpeg";
        grh.Dispose();
        img.Dispose();
        Response.End();
       
    }

    protected string Show()
    {
        string str = "";
        Random ran = new Random();
        for (int i = 0; i < 4; i++)
        {
            str += ran.Next(1,10);