protected void Page_Load(object sender, EventArgs e)
        {
            // 创建一个包含随机内容的验证码文本
            System.Random rand = new Random();
            int len = rand.Next(4, 6);
            char[] chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();
            System.Text.StringBuilder myStr = new System.Text.StringBuilder();
            for (int iCount = 0; iCount < len; iCount++)
            {
                myStr.Append(chars[rand.Next(chars.Length)]);
            }

string text = myStr.ToString();
            // 保存验证码到 session 中以便其他模块使用
            this.Session["checkcode"] = text;

Size ImageSize = Size.Empty;
            Font myFont = new Font("MS Sans Serif", 20);

// 计算验证码图片大小
            using (Bitmap bmp = new Bitmap(10, 10))
            {
                using (Graphics g = Graphics.FromImage(bmp))
                {
                    SizeF size = g.MeasureString(text, myFont, 10000);
                    ImageSize.Width = (int)size.Width + 8;
                    ImageSize.Height = (int)size.Height + 8;
                }
            }

// 创建验证码图片
            using (Bitmap bmp = new Bitmap(ImageSize.Width, ImageSize.Height))
            {
                // 绘制验证码文本
                using (Graphics g = Graphics.FromImage(bmp))
                {
                    g.Clear(Color.White);
                    using (StringFormat f = new StringFormat())
                    {
                        f.Alignment = StringAlignment.Near;
                        f.LineAlignment = StringAlignment.Center;
                        f.FormatFlags = StringFormatFlags.NoWrap;
                        g.DrawString(
                            text,
                            myFont,
                            Brushes.Black,
                            new RectangleF(
                            0,
                            0,
                            ImageSize.Width,
                            ImageSize.Height),
                            f);
                    }//using
                }//using

// 制造噪声 杂点面积占图片面积的 30%
                int num = ImageSize.Width * ImageSize.Height * 30 / 100;

for (int iCount = 0; iCount < num; iCount++)
                {
                    // 在随机的位置使用随机的颜色设置图片的像素
                    int x = rand.Next(ImageSize.Width);
                    int y = rand.Next(ImageSize.Height);
                    int r = rand.Next(255);
                    int g = rand.Next(255);
                    int b = rand.Next(255);
                    Color c = Color.FromArgb(r, g, b);
                    bmp.SetPixel(x, y, c);
                }//for                // 输出图片
                System.IO.MemoryStream ms = new System.IO.MemoryStream();
                bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Png);
                this.Response.ContentType = "image/png";
                ms.WriteTo(this.Response.OutputStream);
                ms.Close();

}//using            
              myFont.Dispose();

}//private void Page_Load(object sender, System.EventArgs e)

//别忘了 Systm.Drawing;