ASP.NET实现验证码图片



新建一个checkcode.aspx文件，页面中不用写任何东西，在代码中，Page_Load中写入如下代码：

        string chkCode = string.Empty;
        int ix, iy;
        ix = 80;
        iy = 24;
        //颜色列表，用于验证码、噪线、噪点 
        Color[] color ={ Color.Black, Color.Red, Color.Blue, Color.Green, Color.Orange, Color.Brown, Color.Brown, Color.DarkBlue }; 
        //字体列表，用于验证码 
        string[] font ={ "Times New Roman", "MS Mincho", "Book Antiqua", "Gungsuh", "MingLiU", "Arial" }; 
        //验证码的字符集，去掉了一些容易混淆的字符 
        char[] character ={ '2', '3', '4', '5', '6', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J', 'K', 'L', 'M', 'N', 'P', 'R', 'S', 'T', 'W', 'X', 'Y' }; 
        Random rnd = new Random(); 
        //生成验证码字符串 
        for (int i = 0; i < 4; i++) 
        { 
            chkCode += character[rnd.Next(character.Length)]; 
        } 
        Bitmap bmp = new Bitmap(ix, iy); 
        Graphics g = Graphics.FromImage(bmp); 
        g.Clear(Color.White); 
        //画噪线 
        for (int i = 0; i < 10; i++) 
        { 
            int x1 = rnd.Next(ix); 
            int y1 = rnd.Next(iy); 
            int x2 = rnd.Next(ix); 
            int y2 = rnd.Next(iy); 
            Color clr = color[rnd.Next(color.Length)]; 
            g.DrawLine(new Pen(clr), x1, y1, x2, y2); 
        } 
        //画验证码字符串 
        for (int i = 0; i < chkCode.Length; i++) 
        { 
            string fnt = font[rnd.Next(font.Length)]; 
            Font ft = new Font(fnt, 14, FontStyle.Bold); 
            Color clr = color[rnd.Next(color.Length)]; 
            g.DrawString(chkCode[i].ToString(), ft, new SolidBrush(clr), (float)i * 16 + 2, (float)2); 
        } 
        //画噪点 
        for (int i = 0; i < 50; i++) 
        { 
            int x = rnd.Next(bmp.Width); 
            int y = rnd.Next(bmp.Height); 
            Color clr = color[rnd.Next(color.Length)]; 
            bmp.SetPixel(x, y, clr); 
        }
        //将验证码写入SESSION
        Session["checkcode"] = chkCode.ToLower();
        //清除该页输出缓存，设置该页无缓存 
        Response.Buffer = true; 
        Response.ExpiresAbsolute = System.DateTime.Now.AddMilliseconds(0); 
        Response.Expires = 0; 
        Response.CacheControl = "no-cache"; 
        Response.AppendHeader("Pragma", "No-Cache"); 
        //将验证码图片写入内存流，并将其以 "image/Png" 格式输出  

        MemoryStream ms = new MemoryStream(); 
        try 
        { 
            bmp.Save(ms, ImageFormat.Png); 
            Response.ClearContent(); 
            Response.ContentType = "image/Png"; 
            Response.BinaryWrite(ms.ToArray()); 
        } 
        finally 
        { 
            //显式释放资源 
            bmp.Dispose(); 
            g.Dispose(); 
        }   

在login.aspx中，添加Image控件，设置ImageUrl=“checkcode.aspx" 即可。

注意其中的onclick和TooTip属性，非常有用！或用：
换一张

输入检测，可读取输入框的文本，和设置的Session值进行比较。