ASP.NET后台生成随机验证码

/// <summary>
        /// 改变验证码
        /// 编写人：
        /// 编写日期：2017-02-28
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        protected void btnChangeImg_Click(object sender, EventArgs e)
        {
            CreatImg();
        }
        /// <summary>
        /// 取随机生成的图片
        /// 编写人：
        /// 编写日期：2017-02-28
        /// </summary>
        private void CreatImg()
        {
            string strRnd = CreateRnd(5);
            ViewState["rnd"] = strRnd;
            TD_YZM.InnerHtml = "<img src="Pages/ReadTempImg.aspx?Uid=" + Epoint.Frame.Bizlogic.common.Encrypt(strRnd, "EpointPW") + "">";//登陆           
        }
        public string CreateRnd(int Leng)
        {//产生随机数除Ii0O
            string Ar1 = "A,B,C,D,E,F,G,H,J,K,L,M,N,P,Q,R,S,T,U,V,W,X,Y,Z,2,3,4,5,6,7,8,9";
            //登录页面验证是否简单验证，即验证码为数字组成，否则由数字+字母组成。如果简单=1,否则=0
            string LoginSimpleVlidate = ApplicationOperate.GetConfigValueByName("LoginSimpleVlidate");
            if (LoginSimpleVlidate == "1")
                Ar1 = "0,1,2,3,4,5,6,7,8,9";

            string[] ListAr = Ar1.Split(',');
            Random RD = new Random();
            int rnd = 0;
            string strRnd = "";
            for (int j = 1; j < Leng; j++)
            {
                rnd = RD.Next(0, ListAr.Length);
                strRnd += ListAr[rnd];
            }
            return strRnd;
        }