验证码是一张图片。我们需要在前台代码中写一段<img>，src指向一张页面（ValidateImage.aspx）。

复制代码 代码如下:
<script language="javascript">

function changeImg() {
$("#imgCheckNo").attr("src", "ValidateImage.aspx?r=" + getRandom(999));
}
function getRandom(n) { return Math.floor(Math.random() * n + 1) }
</script>

<div>
<img id="imgCheckNo" src="ValidateImage.aspx" style="border-color: #000000; border-width: 1px;border-style: Solid">
<span onclick="changeImg();">看不清?换一张</span>
</div>
<div>
<div class="labelCss">验证码：</div>
<div>
<asp:TextBox ID="tbxCheckNo" runat="server" CssClass="tbxCss"></asp:TextBox>
</div>
<div>
<asp:Button ID="btnSubmit" runat="server" Text="立即注册" OnClick="btnSubmit_Click" />
</div>

ValidateImage.aspx 用来生产验证码图片，并将验证码的码值保存到cookie中。
代码如下：

复制代码 代码如下:
public partial class ValidateImage : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
this.CreateCheckCodeImage(RndNum());
}

/// <summary>生成验证码
/// </summary>
/// <returns></returns>
private string RndNum()
{
int number;
char code;
string checkCode = String.Empty;

System.Random random = new Random();

for (int i = 0; i < 4; i++)
{
number = random.Next();
if (number % 2 == 0)
code = (char)('0' + (char)(number % 10));
else
code = (char)('A' + (char)(number % 26));
checkCode += code.ToString();
}
Response.Cookies.Add(new HttpCookie("yzmcode", checkCode));
return checkCode;
}

/// <summary>向页面生成验证码Gif图片
/// </summary>
/// <param name="checkCode"></param>
private void CreateCheckCodeImage(string checkCode)
{
if (checkCode == null || checkCode.Trim() == String.Empty)
return;
System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length * 12.5)), 22);
Graphics g = Graphics.FromImage(image);
try
{
//生成随机生成器
Random random = new Random();
//清空图片背景色
g.Clear(Color.White);
//画图片的背景噪音线
for (int i = 0; i < 25; i++)
{
int x1 = random.Next(image.Width);
int x2 = random.Next(image.Width);
int y1 = random.Next(image.Height);
int y2 = random.Next(image.Height);
g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);
}

Font font = new System.Drawing.Font("Arial", 12, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);
g.DrawString(checkCode, font, brush, 2, 2);
//画图片的前景噪音点
for (int i = 0; i < 100; i++)
{
int x = random.Next(image.Width);
int y = random.Next(image.Height);
image.SetPixel(x, y, Color.FromArgb(random.Next()));
}
//画图片的边框线
g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);
System.IO.MemoryStream ms = new System.IO.MemoryStream();
image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
Response.ClearContent();
Response.ContentType = "image/Gif";
Response.BinaryWrite(ms.ToArray());
}
finally
{
g.Dispose();
image.Dispose();
}
}

}

最后就是后台事件，判断当前验证码是否正确。
代码如下：

复制代码 代码如下:
#region 页面控件事件
protected void btnSubmit_Click(object sender, EventArgs e)
{
if (String.Compare(Request.Cookies["yzmcode"].Value, tbxCheckNo.Text, true) != 0)
{
Response.Write("<script>alert('验证码错误!')</script>");
}
else
{
// Response.Write("<script>alert('验证吗正确!!!')</script>");
}
}
#endregion