生成验证码图片

在yzm.aspx页面中：

　　　　 Bitmap img = new Bitmap(100, 50);
        Graphics g = Graphics.FromImage(img);
        List<Color> clist = new List<Color>();
        clist.Add(Color.Red);
        clist.Add(Color.Yellow);
        clist.Add(Color.Blue);
        clist.Add(Color.Green);
        clist.Add(Color.Aqua);
        clist.Add(Color.Orange);
        clist.Add(Color.Pink);

        //验证码内容
        Random r = new Random();
        string ss = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
        string s = "";

        Color bkcolor = clist[r.Next(0, clist.Count)];
        clist.Remove(bkcolor);
        g.FillRectangle(new SolidBrush(bkcolor), 0, 0, 100, 50);

        for (int i = 0; i < 4; i++)
        {
            s += ss[r.Next(0, ss.Length)];
        }

        Font f = new Font("微软雅黑", 20);
        Brush b = new SolidBrush(clist[r.Next(0, clist.Count)]);
        g.DrawString(s, f, b, 5, 5);

        Session["YZM"] = s;

        for (var i = 0; i < 5; i++)
        {
            Pen pp = new Pen(new SolidBrush(clist[r.Next(0, clist.Count)]), r.Next(1, 3));
            Point p1 = new Point(r.Next(0, 100), r.Next(0, 50));
            Point p2 = new Point(r.Next(0, 100), r.Next(0, 50));
            g.DrawLine(pp, p1, p2);
        }
        img.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);

在另一个aspx页面中：

<img id="yzm" src="YZM.aspx" />

JS端点击更换图片：

a = 0;
document.getElementById("yzm").onclick = function () {
  a++;
  this.src = "YZM.aspx?a="+a;
}

 验证验证码正误时，用session：

if (TextBox1.Text.ToUpper() == Session["YZM"].ToString().ToUpper()) 正确
else  错误