<asp:Label ID="Label1" runat="server" Text="請輸入驗證碼:"  ></asp:Label>
<asp:TextBox ID="tbx_check" runat="server"></asp:TextBox>
<asp:ImageButton ID="ImageButton1" runat="server" src="checknumber.ashx" alt="驗證碼"/>
<asp:Button ID="btn_check" runat="server" Text="驗證" OnClick="btn_check_Click" />

protected void btn_check_Click(object sender, EventArgs e)
{
  //不區分大小寫驗證
      if (String.Compare(tbx_check.Text.Trim(), Session["check"].ToString(), true) == 0)
      {    
          Response.Write("<script>alert('驗證碼正確') </script>");
                
      }    
      else
      {
          Response.Write("<script>alert('驗證碼錯誤') </script>");
      }
}
 
 
//點擊圖片更換驗證碼
protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
{
    ImageButton1.ImageUrl = "checknumber.ashx?id=" + new Random(100).ToString();
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
//畫板
using System.Drawing;
using System.Drawing.Imaging;
//Session傳值
using System.Web.SessionState;
 
namespace 驗證碼模塊
{
    /// <summary>
    /// Handler1 的摘要說明
    /// </summary>
    public class Handler1 : IHttpHandler, IRequiresSessionState
    {
 
        public void ProcessRequest(HttpContext context)
        {
            
            //定義位圖的寬度和高度
            int iBmpW = 200;
            int iBmpS = 50;
 
            //創建位圖對象（并初始化寬度和高度）
            Bitmap Bmp = new Bitmap(iBmpW, iBmpS);
 
            //創建畫布
            Graphics Grc = Graphics.FromImage(Bmp);//將位圖裝載到畫布里面
 
            //將畫布的背景色改為白色
            Grc.Clear(Color.White);
 
            //定義畫筆對象
            SolidBrush solidBrush = new SolidBrush(Color.Blue);
 
            //定義存放的字符串，畫布上要顯示的驗證碼,回避相似字符所以去掉I、1、o、0
            string sString = "ABCDEFGHJKLMNPQRSTUVWXYZabcdefghyzklmnopqrstuvwxtz23456789呆鍋工醬";
            //定義隨機數
            Random Rnd = new Random((int)DateTime.Now.Ticks);//強制轉換為整型
 
            //for存放隨機數的字符變量
            string scode = null;
            for (int i = 0; i < 6; i++)
            {
                string temp = sString.Substring(Rnd.Next(0, sString.Length), 1);//將隨即得到的每個字符進行字符串構造
                scode += temp;
            }
 
 
            //隨機輸出噪點
            Random rand = new Random();
            for (int i = 0; i < 10; i++)
            {
                int x = rand.Next(Bmp.Width);
                int y = rand.Next(Bmp.Height);
                Grc.DrawRectangle(new Pen(Color.LightGray, 0), x, y, 1, 1);
            }
 
 
            int z = 6;//干擾線條數
            for (int i = 0; i < z; i++)
            {
                int x1 = rand.Next(Bmp.Width);
                int x2 = rand.Next(Bmp.Width);
                int y1 = rand.Next(Bmp.Height);
                int y2 = rand.Next(Bmp.Height);
                Grc.DrawLine(new Pen(Color.LightGray, 1), x1, y1, x2, y2);//根據坐標畫線
            }
 
 
            Font ft = new Font("Arial", 25);
            Grc.DrawString(scode, ft, solidBrush, 0, 0);
            context.Response.ContentType = "image/jpg";
            Bmp.Save(context.Response.OutputStream, ImageFormat.Jpeg);
            //向外傳scode的值以驗證
            context.Session["check"] = scode;
        }
 
        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}