asp.net ajax实现无刷新验证码

实现ajax无刷新验证码首先需要两个aspx页面，第一个用来展示，另一个用来后台刷新验证码：

1、首先是在后台验证码的aspx文件的Page_Load中的事件代码：

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

namespace 学生在线考试系统
{
    public partial class AjaxAutoCode : System.Web.UI.Page
    {
        //验证数字
        public string authcode = string.Empty;

        protected void Page_Load(object sender, EventArgs e)
        {
            #region 第一种产生验证码的方法
            Random random = new Random();
            authcode = random.Next(1111, 9999).ToString();
            //构造图片
            Bitmap image = new Bitmap(authcode.Length * 12, 25);
            //创建画布
            Graphics g = Graphics.FromImage(image);
            try
            {
                g.Clear(Color.White);
                for (int i = 0; i < 25; i++)
                {
                    int x1 = random.Next(image.Width);
                    int x2 = random.Next(image.Width);
                    int y1 = random.Next(image.Height);
                    int y2 = random.Next(image.Height);
                    //链接两点的线条
                    g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);
                }
                Font font = new Font("Arial", 12, FontStyle.Bold | FontStyle.Italic);
                System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(
                    new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkBlue, 1.2f, true);
                g.DrawString(authcode, font, brush, 2, 2);
                //画图片的前景噪点
                for (int i = 0; i < 100; i++)
                {
                    int x = random.Next(image.Width);
                    int y = random.Next(image.Height);
                    image.SetPixel(x, y, Color.FromArgb(random.Next()));
                }
                g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);
                System.IO.MemoryStream ms = new System.IO.MemoryStream();
                image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
                ms.WriteTo(this.Response.OutputStream);
                ms.Close();
                this.Response.ContentType = "image/gif";
            }
            finally
            {
                image.Dispose();
                g.Dispose();

            }
            #endregion
        }
    }
}

　　

2、其次在显示验证码的页面定义一个JS函数

function fGetCode()
{   
    document.getElementById("getcode").src="Default2.aspx?"+Math.random();   
}

　　

3.再编辑前台页面aspx,下面是前台页面的代码片段

<label>验证码</label>
             <asp:TextBox ID="txt_checkCode" runat="server" Width="178px"></asp:TextBox> 
            <img src="Default2.aspx" alt="看不清楚？" id="getcode"/> <a href="javascript:fGetCode()">更换验证码</a>