验证的代码

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;
using System.Drawing.Imaging;

public partial class Validata : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Session["code"] = RandNum(4);
        ValidateCode(Session["code"].ToString(), 40, 20, "黑体", 10, "#FFFFFF");       
    }

    /// <summary>
    /// 该方法用于生成指定位数的随机数
    /// </summary>
    /// <param name="VcodeNum">参数是随机数的位数</param>
    /// <returns>返回一个随机数字符串</returns>
    private string RandNum(int VcodeNum)
    {
        string Vchar = "0,1,2,3,4,5,6,7,8,9";
        string[] VcArray = Vchar.Split(',');//拆分成数组
        string VNum = "";
        int temp = -1;//记录上次随机数值，尽量避避免生产几个一样的随机数

        Random rand = new Random();
        //采用一个简单的算法以保证生成随机数的不同
        for (int i = 0; i < VcodeNum; i++)
        {
            if (temp != -1)
            {
                rand = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));
            }

            int t = rand.Next(VcArray.Length - 1);
            if (temp != -1 && temp == t)
            {
                return RandNum(VcodeNum);

            }
            temp = t;
            VNum += VcArray[t];
        }
        return VNum;
    }

    /// <summary>
    /// 生成图片并写入字符
    /// </summary>
    /// <param name="VNum">目标字符</param>
    /// <param name="w">宽</param>
    /// <param name="h">高</param>
    /// <param name="font">字体文件</param>
    /// <param name="fontSize">字体大小</param>
    /// <param name="bgColor">图片背景颜色</param>
    private void ValidateCode(string VNum, int w, int h, string font, int fontSize, string bgColor)
    {
        Bitmap Img = new Bitmap(w, h);//生成图像的实例
        Graphics g = Graphics.FromImage(Img);//从Img对象生成新的Graphics对象
        g.Clear(ColorTranslator.FromHtml(bgColor));//背景颜色
        Font f = new Font(font, fontSize);//生成Font类的实例
        SolidBrush s = new SolidBrush(Color.Black);//生成笔刷类的实例
        g.DrawString(VNum, f, s, 3, 3);//将VNum写入图片中
        Img.Save(Response.OutputStream, ImageFormat.Jpeg);//将此图像以Jpeg图像文件的格式保存到流中
        Response.ContentType = "image/Jpeg";
        //回收资源
        g.Dispose();
        Img.Dispose();
        Response.End();
    }

}
**********************************
上面的是产生验证码的　　命名为 xxx.aspx.cs
**********************************

然后在登陆页面 [ login.aspx ] 中加一个Image控件　把地址指向它的前台就行。。
如下：

验证码:<asp:TextBox ID="Chkcode" runat="server" Columns="4" MaxLength="4"/>
      &nbsp;　<img src="Validata.aspx">

下一步，在login.aspx.cs　的确定按扭里加下面代码　用于比较：

if ( Chkcode.Text == Session["Code"].ToString() )
                {
                    Session["Admin"] = "Admin";
                    Response.Write("<script>alert('登陆成功！');
　　　　}