C#登录页面验证码刷新

验证码生成页面  png.aspx

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace Maticsoft.Web.Admin
{
    public partial class png : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            string checkCode = CreateRandomCode(4);
            Session["CheckCode"] = checkCode;
            CreateImage(checkCode);
        }
        private string CreateRandomCode(int codeCount)
        {
            // 函数功能:产生数字和字符混合的随机字符串        
            string allChar = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
            char[] allCharArray = allChar.ToCharArray();
            string randomCode = "";
            Random rand = new Random();
            for (int i = 0; i < codeCount; i++)
            {
                int r = rand.Next(61);
                randomCode += allCharArray.GetValue(r);
            }
            return randomCode;
        }
        private void CreateImage(string checkCode)
        {
            // 生成图象验证码函数   
            int iwidth = (int)(checkCode.Length * 11.5);
            System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 20);
            Graphics g = Graphics.FromImage(image);
            Font f = new System.Drawing.Font("Arial", 10, System.Drawing.FontStyle.Bold);
            Brush b = new System.Drawing.SolidBrush(Color.Azure); //字母白色        
            //g.FillRectangle(new System.Drawing.SolidBrush(Color.Blue),0,0,image.Width, image.Height);       
            g.Clear(Color.Brown);//背景灰色     
            g.DrawString(checkCode, f, b, 3, 3);
            Pen blackPen = new Pen(Color.Black, 0);
            Random rand = new Random();
            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);

            Response.ClearContent();
            Response.ContentType = "image/Jpeg";
            Response.BinaryWrite(ms.ToArray());
            g.Dispose();
            image.Dispose();
        }
    }
}

登录页面放入图片

<asp:Image ID="Image1" runat="server" ImageUrl="png.aspx" onclick="refresh(this, 'png.aspx');"/>

js脚本

        function refresh()
        {
            var url = "png.aspx?id=";
            var r = Math.random() * 1000;
            url = url + r;
            document.all("Image1").src = url;
            document.all("LoI_txtVC").value = "";
            document.all("LoI_txtVC").focus();
            return false;
        }</script>

登录判断验证码  TextBox3为验证码输入框

if (base.Session["CheckCode"] == null)
            {
                Page.ClientScript.RegisterStartupScript(Page.ClientScript.GetType(), "myscripts", "<script>alert("验证码已过期！");</script>");
                return;
            }
            if (TextBox3.Text != Session["CheckCode"].ToString())
            {
                Page.ClientScript.RegisterStartupScript(Page.ClientScript.GetType(), "myscripts", "<script>alert("验证码不正确！");</script>");
                return;
            }