先建一张生成验证码的网页CheckCode.aspx:
CheckCode.aspx.cs代码如下
添加引用:
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;

public partial class Default2 : System.Web.UI.Page
{
    private Bitmap validateimage;
    private Graphics g;
    protected void Page_Load(object sender, EventArgs e)
    {
        Response.BufferOutput = true;  //特别注意
        Response.Cache.SetExpires(DateTime.Now.AddMilliseconds(-1));//特别注意
        Response.Cache.SetCacheability(HttpCacheability.NoCache);//特别注意
        Response.AppendHeader("  Pragma", "No-Cache"); //特别注意
        string VNum = MakeValidateCode();
        Session["VNum"] = VNum;//取得验证码，以便后来验证
        ValidateCode(VNum);
    }
    public void ValidateCode(string VNum)
    {
        validateimage = new Bitmap(60, 20, PixelFormat.Format24bppRgb);
        g = Graphics.FromImage(validateimage);
        g.FillRectangle(new LinearGradientBrush(new Point(0, 0), new Point(110, 20), Color.FromArgb(240, 255, 255, 255), Color.FromArgb(240, 255, 255, 255)), 0, 0, 200, 200); //矩形框
        g.DrawString(VNum, new Font("arial", 11), new SolidBrush(Color.Red), new PointF(6, 0));//字体/颜色
        g.Save();
        MemoryStream ms = new MemoryStream();
        validateimage.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
        Response.ClearContent();
        Response.C;
        Response.BinaryWrite(ms.ToArray());
        Response.End();
    }
    string MakeValidateCode()
    {
        char[] s = new char[] { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z','A','B', 'C','D','E','F','G','H','I','J','K','L','M','N','P','Q','I','S','T','U','V','W','X','Y','Z'};//枚举数组
        string num = "";
        Random r = new Random();
        for (int i = 0; i < 5; i++)
        {
            num += s[r.Next(0, s.Length)].ToString();
        }
        return num;
    }
}

然后是显示验证码的页面 的html中加入<img src="Default2.aspx"  id="yzm" alt="点击刷新"  />
在</head>之前一行加入javascript脚本语言如下(当然写在JS文件里面更好):
<script type="text/javascript">
function reloadcode(){
document.getElementById("yzm").src ="CheckCode.aspx";
}
</script>