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。
<asp:Image ID="Image1" runat="server" οnclick="this.src=this.src+'?'" ImageAlign="Middle" ToolTip="看不清我，就点一下我吧！" />
注意其中的onclick和TooTip属性，非常有用！或用：
<span style="cursor:pointer;text-decoration:underline;" οnclick="document.getElementById('Image1').src=document.getElementById('Image1').src+'?'">换一张</span>

输入检测，可读取输入框的文本，和设置的Session值进行比较。