asp.net 生成圖片驗證碼

identifyingcode.aspx裏沒有內容略。
identifyingcode.cs文件：
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;
using System.IO;
public partial class identifyingcode : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string tmp = RndNum(4);
        HttpCookie a = new HttpCookie("ImageV", tmp);
        Response.Cookies.Add(a);
        this.ValidateCode(tmp);