namespace WebApplication1
{
 /// summary>
 /// $codebehindclassname$ 的摘要说明
 /// /summary>
 [WebService(Namespace = "http://tempuri.org/")]
 [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
 public class Handler1 : IHttpHandler, IRequiresSessionState
 {
  public void ProcessRequest(HttpContext context)
  {
   context.Response.ContentType = "image/gif";
   //建立Bitmap对象，绘图
   Bitmap basemap = new Bitmap(100, 30);
   Graphics graph = Graphics.FromImage(basemap);
   graph.FillRectangle(new SolidBrush(Color.White), 0, 0, 100, 30);
   Font font = new Font(FontFamily.GenericSerif, 24, FontStyle.Bold, GraphicsUnit.Pixel);
   Random r = new Random();
   string letters = "ABCDEFGHIJKLMNPQRSTUVWXYZ";
   string letter;
   StringBuilder s = new StringBuilder();
   //添加随机的五个字母
   for (int x = 0; x  5; x++)
   {
    letter = letters.Substring(r.Next(0, letters.Length - 1), 1);
    s.Append(letter);
    graph.DrawString(letter, font, new SolidBrush(Color.Black), x * 19, r.Next(0, 8));
   }
   //混淆背景
   Pen linePen = new Pen(new SolidBrush(Color.Black), 2);
   for (int x = 0; x  6; x++)
    graph.DrawLine(linePen, new Point(r.Next(0, 99), r.Next(0, 29)), new Point(r.Next(0, 99), r.Next(0, 29)));
   //将图片保存到输出流中  
   basemap.Save(context.Response.OutputStream, ImageFormat.Gif);
   context.Session["CheckCode"] = s.ToString(); //如果没有实现IRequiresSessionState，则这里会出错，也无法生成图片
   context.Response.End();
  }
  public bool IsReusable
  {
   get
   {
    return false;
   }
  }
 }
}

html xmlns="http://www.w3.org/1999/xhtml" >
head runat="server">
 title>/title>
/head>
body>
 form id="form1" runat="server">
 div>
  asp:TextBox ID="txtCode" runat="server">/asp:TextBox>
  img id="imgCode" alt="看不清？点击换一张" src="Handler1.ashx" style="cursor:pointer" onclick="this.src=this.src+'?'" />br />
  asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
 /div>
 /form>
/body>
/html>