一个生成Validatecode 的代码。

Response.Clear();
            Response.ContentType = "image/jpeg";
            int width = 50;
            int height = 25;
            Bitmap bmp = new Bitmap(width, height, PixelFormat.Format24bppRgb);
            Graphics g = Graphics.FromImage(bmp);
            HatchBrush b = new HatchBrush(HatchStyle.Cross, Color.LightGray, Color.WhiteSmoke);
            g.FillRectangle(b, 0, 0, width, height);
            Random r = new Random(Convert.ToInt32(DateTime.Now.Millisecond));
            string Code = r.Next(1000, 9999).ToString();
            Session["ValidateCode"] = Code;
            g.DrawString(Code, new Font("Arial", 10, FontStyle.Bold), SystemBrushes.WindowText, 10.0F, 5.0F);
            bmp.Save(Response.OutputStream, ImageFormat.Jpeg);
            b.Dispose();
            g.Dispose();
            bmp.Dispose();
            Response.End();