asp.net 驗證碼

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.IO;

/// <summary>
/// 產生隨機驗證碼
/// </summary>
public partial class ProductInfo_Login_Range : System.Web.UI.Page
{
    Random random = new Random();
    protected void Page_Load(object sender, EventArgs e)
    {
        string str = getRandomValidate(4);
        Session.Add("RangeCode",str); //這一步是為了將驗證碼寫入Session，進行驗證，不能預設，也可一使用cookie
        //HttpCookie cook = new HttpCookie("code");
        //cook.Value = str;
        getImageValidate(str);
    }
     //得到隨機字串,長度自己定義
    private string getRandomValidate(int len)
    {
        int num;
        int tem;
        string rtuStr = "";
        for (int i = 0; i < len; i++)
        {
            num = random.Next();
            if (num % 2 == 0)
                tem = num % 10 + '0';//產生數字
            else
                tem = num % 26 + 'A';//產生字元
            rtuStr += Convert.ToChar(tem).ToString();
        }
        return rtuStr;
    }
    //產生映像
    private void getImageValidate(string strValue)
    {
        //string str = "OO00"; //前兩個為字母O，後兩個為數字0
        int width = Convert.ToInt32(strValue.Length * 12);    //計算映像寬度
        Bitmap img = new Bitmap(width, 23);
        Graphics gfc = Graphics.FromImage(img);           //產生Graphics對象，進行畫圖
        gfc.Clear(Color.White);