怎么用c#做随机数字图片验证码

using System;
using System.Data;
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 check : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            //  首先调用RndNum函数生成4位数字的验证码
            string str_ValidateCode = RndNum(4);
            //  然后把它保存在用于验证的Session里边
            Session["ValidateCode"] = str_ValidateCode;
            //调用CreateImage函数，把验证码绘制到图片中
            CreateImage(str_ValidateCode);
        }