效果

后台代码

protected override void OnPreInit(EventArgs e)
        {
            //字符 去掉了 o0 i1l
            string str = "A,B,C,D,E,F,G,H,G,K,M,N,P,Q,R,S,T,U,V,W,X,Y,Z,2,3,4,5,6,7,8,9";
            //颜色
            Color[] cs = new Color[] { Color.Red, Color.Green, Color.Black, Color.Blue, Color.Purple };
            //字体
            string[] fs = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
            //字体大小
            int[] size = new int[] { 16, 18, 20 };
            //字体样式
            FontStyle[] _fs = new FontStyle[] { FontStyle.Bold, FontStyle.Italic, FontStyle.Regular };
            Random ram = new Random();
            Bitmap img = new Bitmap(80, 25);
            Graphics g = Graphics.FromImage(img);
            //清空图片
            g.Clear(Color.White);
            //画干扰线
            for (int i = 1; i <= 5; i++)
            {
                int x1 = ram.Next(img.Width);
                int y1 = ram.Next(img.Height);
                int x2 = ram.Next(img.Width);
                int y2 = ram.Next(img.Height);
                Pen p = new Pen(cs[ram.Next(cs.Length)]);
                g.DrawLine(p, x1, y1, x2, y2);
            }
            //画字符
            string[] array = str.Split(',');
            string vcode = string.Empty;
            for (int i = 1; i <= 4; i++)
            {
                Font f = new Font(fs[ram.Next(fs.Length)], size[ram.Next(size.Length)], _fs[ram.Next(_fs.Length)]);
                Brush b = new SolidBrush(cs[ram.Next(cs.Length)]);
                string temp = array[ram.Next(array.Length)];
                vcode += temp.ToLower();
                if (ram.Next(9) % 2 == 0)
                {
                    g.DrawString(temp, f, b, new Point(20 * (i - 1) + 0, 0));
                }
                else
                {
                    g.DrawString(temp.ToLower(), f, b, new Point(20 * (i - 1) + 0, 0));
                }
            }
            //画噪点
            for (int i = 1; i <= 30; i++)
            {
                int x = ram.Next(img.Width);
                int y = ram.Next(img.Height);
                Color clr = cs[ram.Next(cs.Length)];
                img.SetPixel(x, y, clr);
            }
            //将验证码的值保存
            Response.Cookies.Add(new HttpCookie("9FDFFFADBEF1A3D9612D39754765D442", StringHelper.EncryptAES(vcode)));
            //设置页面无缓存
            Response.Buffer = true;
            Response.ExpiresAbsolute = System.DateTime.Now.AddSeconds(-1);
            Response.Expires = 0;
            Response.CacheControl = "no-cache";
            Response.AppendHeader("Pragma", "No-Cache");
            //写入文件流
            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            img.Save(ms, System.Drawing.Imaging.ImageFormat.Png);
            Response.Cache.SetNoStore();
            Response.ClearContent();
            Response.ContentType = "image/Jpeg";
            Response.BinaryWrite(ms.ToArray());
            //释放
            g.Dispose();
            img.Dispose();
            Response.End();
        }

<img src="VCode.aspx" alt="看不清，换一个" title="看不清，换一个" style="cursor:pointer;" onclick="javascript:this.src='VCode.aspx?vc='+Math.random();" />