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。