Function.cs

/// <summary>
        /// 生成随机数
        /// </summary>
        /// <param name="codeCount">要生成的随机数位数</param>
        /// <returns></returns>
        public static string CreateRandomCode(int codeCount)
        {
            string allChar = "0,1,2,3,4,5,6,7,8,9";
            string[] allCharArray = allChar.Split(',');
            string randomCode = "";
            int temp = -1;

            Random rand = new Random();
            for(int i = 0; i < codeCount; i++)
            {
                if(temp != -1)
                {
                    rand = new Random(i*temp*((int)DateTime.Now.Ticks));
                }
                int t = rand.Next(9);
                if(temp == t)
                {
                    return CreateRandomCode(codeCount);
                }
                temp = t;
                randomCode += allCharArray[t];
            }
            return randomCode;
        }
        /// <summary>
        /// 生成验证码图片
        /// </summary>
        /// <param name="checkCode"></param>
        public static void CreateImage(string checkCode)
        {
            int iwidth = (int)(checkCode.Length * 11.5);
            System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 20);
            Graphics g = Graphics.FromImage(image);
            Font f = new System.Drawing.Font("Arial", 10, System.Drawing.FontStyle.Bold);
            Brush b = new System.Drawing.SolidBrush(Color.White);
            //g.FillRectangle(new System.Drawing.SolidBrush(Color.Blue),0,0,image.Width, image.Height);
            g.Clear(Color.Blue);
            g.DrawString(checkCode, f, b, 3, 3);

            Pen blackPen = new Pen(Color.Brown,0);
            Random rand = new Random();
            for (int i=0;i<5;i++)
            {
                int y = rand.Next(image.Height);

                //g.DrawLine(blackPen,0,y,image.Width,y);
            }
            
            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            for(int i=0; i<90; i++)
            {
                int x = rand.Next(image.Width);
                int y = rand.Next(image.Height);
                image.SetPixel(x, y, Color.FromArgb(rand.Next()));
            }
            image.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);
            System.Web.HttpContext.Current.Response.ClearContent();
            System.Web.HttpContext.Current.Response.ContentType = "image/Jpeg";
            System.Web.HttpContext.Current.Response.BinaryWrite(ms.ToArray());
            g.Dispose();
            image.Dispose();
        }

ValidateCode.aspx.cs

private void Page_Load(object sender, System.EventArgs e)
        {
            // 在此处放置用户代码以初始化页面
            string checkCode = LemongTree.BLL.Function.CreateRandomCode(4);
            Session["CheckCode"] = checkCode;
            LemongTree.BLL.Function.CreateImage(checkCode);
        }

.aspx文件的调用