asp.net验证码图片生成示例

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）。

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：

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