ASP.NET CAPTCHA image to achieve

Checkcode.aspx create a new file, the page do not write anything in the code, Page_Load written the following code:

        String = chkCode string.Empty;
        int ix, iy;
        ix = 80;
        iy = 24-;
        // color list, a verification code, line noise, noise  
        Color [] = {Color.Black Color, Color.Red, Color.Blue, Color.green, Color.Orange, Color.Brown, Color.Brown, Color.DarkBlue};  
        // list of fonts, codes for  
        String [] = {font "Times New Roman", "the MS Mincho", "Book Antiqua", "Gungsuh", "the MingLiU", "Arial"};  
        // verification code characters, removing some confusing character  
        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);  
        }
        // write the codes the SESSION
        the Session [ "checkcode"] = chkCode.ToLower ();
        // clear the page output cache, the cache no page provided  
        the Response.Buffer = to true;  
        Response.ExpiresAbsolute = System.DateTime.Now .AddMilliseconds (0);  
        Response.Expires = 0;  
        Response.CacheControl = "NO-Cache";  
        Response.AppendHeader ( "Pragma", "No-the Cache");  
        // memory write verification picture stream, and its output "image / Png" format  

        MS = new new the MemoryStream the MemoryStream ();  
        the try  
        {  
            bmp.Save (MS, ImageFormat.Png);  
            Response.ClearContent ();  
            Response.ContentType = "Image / Png File";  
            the Response.BinaryWrite (ms.ToArray ());  
        }  
        the finally  
        {  
            // explicitly release resources  
            bmp.Dispose ();  
            g.Dispose ();  
        }   

in login.aspx added Image control provided ImageUrl = "checkcode.aspx" can.
<asp: Image ID = "Image1 " runat = "server" οnclick = ImageAlign = "Middle" ToolTip = " see me, tap me!" / "this.src = this.src + '?'">
Note that one of the onclick attribute TooTip and very useful! Or use:
<span style = "cursor: pointer ; text-decoration: underline;" οnclick = "document.getElementById ( 'Image1') src = document.getElementById ( 'Image1') src +.. '?'"> a transducer </ span>

input detection, can read the text input box, and Session value is compared.