<div>
 <span>Identifying Code:</span>
 <asp:TextBox ID="txtValidationCode" runat="server" Width="130px" MaxLength="4"></asp:TextBox>
 <img id="imgYZ" class="code" style=" height:23px; width:65px;" 
 src="Img.ashx" onclick="this.src=this.src+'?'"/ />
 <img src="../images/btn_change.gif" title="Change" class="btn_change" Style="cursor: hand"
 onclick="ImgChange()" />
</div>

<script language="javascript" type="text/javascript">
 function ImgChange() 
 { 
 var img=document.getElementById("imgYZ");
 img.click();
 } 
</script>

using System;
using System.Web;
using CLAIMS.BLL;
using System.Data;
using System.Configuration;
using System.Web.SessionState;
using System.Drawing;

public class Img : IHttpHandler, IRequiresSessionState
{
 
 public void ProcessRequest (HttpContext context) 
 {
 context.Response.ContentType = "image/Jpeg";
 
 string s_random = "";
 System.IO.MemoryStream ms = new System.IO.MemoryStream();
 s_random = RndNum(4);
 context.Session["random"] = s_random;
 s_random = s_random.Substring(0, 1) + " " + s_random.Substring(1, 1) + " " + s_random.Substring(2, 1) + " " + s_random.Substring(3, 1);
 
 CreateImage(s_random, ref ms);
 context.Response.ClearContent();
 context.Response.BinaryWrite(ms.ToArray());

 context.Response.Flush();
 context.Response.End();
 }

 private void CreateImage(string checkCode,ref System.IO.MemoryStream ms)
 {
 int iwidth = (int)(checkCode.Length * 18);
 System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 45);
 Graphics g = Graphics.FromImage(image);
 g.Clear(Color.White);
 //定義顏色
 Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
 //定義字體  
 //string[] font = {"Verdana","Microsoft Sans Serif","Comic Sans MS","Arial","宋體"};
 Random rand = new Random();
 //隨機輸出噪點
 for (int i = 0; i < 50; i++)
 {
  int x = rand.Next(image.Width);
  int y = rand.Next(image.Height);
  g.DrawRectangle(new Pen(Color.LightGray, 0), x, y, 1, 1);
 }

 //輸出不同字體和顏色的驗證碼字符

 for (int i = 0; i < checkCode.Length; i++)
 {
  int cindex = rand.Next(7);
  int findex = rand.Next(5);
  Font font = new System.Drawing.Font("Arial", 24, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
  Brush b = new System.Drawing.SolidBrush(c[cindex]);
  int ii = 4;
  if ((i + 1) % 2 == 0)
  {
  ii = 2;
  }
  g.DrawString(checkCode.Substring(i, 1), font, b, 3 + (i * 12), ii);
 }
 //畫一個邊框

 g.DrawRectangle(new Pen(Color.Black, 0), 0, 0, image.Width - 1, image.Height - 1);

 //輸出到瀏覽器
 image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
 
 g.Dispose();
 image.Dispose();
 }

 public static String RndNum(int VcodeNum)
 {
 String Vchar = "0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,W,X,Y,Z";
 String[] VcArray = Vchar.Split(',');
 String VNum = "";
 Random random = new Random();
 for (int i = 1; i <= VcodeNum; i++)
 {
  int iNum = 0;
  while ((iNum = Convert.ToInt32(VcArray.Length * random.NextDouble())) == VcArray.Length)
  {
  iNum = Convert.ToInt32(VcArray.Length * random.NextDouble());
  }
  VNum += VcArray[iNum];
 }
 return VNum;
 }
 
 public bool IsReusable {
 get {
  return false;
 }
 }

}