实现验证码的功能

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;

public partial class validimage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        HttpContext.Current.Session["CheckCode"] = RndNum(4);
        CreateImages(HttpContext.Current.Session["CheckCode"].ToString());
    }

    /// <summary>
    /// 生成验证图片
    /// </summary>
    /// <param name="checkCode">验证字符</param>
    private void CreateImages(string checkCode)
    {
        int iwidth = (int)(checkCode.Length * 13);
        System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 23);
        Graphics g = Graphics.FromImage(image);
        g.Clear(Color.White);