js刷新验证码（.net）

<a href="javascript:changeImg();"  title="看不清？点击换一张">
 <img id="imgcode" src="Handler/CheckCodeHandler.ashx?type=login&stamp=1" width="80" height="30" onclick="changeUrl()"/>
</a>

function changeImg() {
  document.getElementById("imgcode").src = "Handler/CheckCodeHandler.ashx?type=login&stamp=" + Math.random();
 }

using System;
using System.Web;
namespace YK.WebSite.Handler
{
    /// <summary>
    /// 用于验证码的处理程序
    /// 调用该处理程序时，要加参数type，表明是哪一个功能需要的验证码
    /// 获取验证码的SessionKey是YK.Website.CheckCode.[type]
    /// </summary>
    public class CheckCodeHandler : IHttpHandler
    {
        public HttpResponse Response
        {
            get
            {
                return HttpContext.Current.Response;
            }
        }
        public HttpRequest Request
        {
            get
            {
                return HttpContext.Current.Request;
            }
        }
        public void Proce***equest(HttpContext context)
        {
            string type = Request.QueryString["type"];
            if (String.IsNullOrWhiteSpace(type)) return;
            Response.Buffer = true;
            Response.ExpiresAbsolute = System.DateTime.Now.AddMilliseconds(0);
            Response.Expires = 0;
            Response.CacheControl = "no-cache";
            Response.AppendHeader("Pragma", "No-Cache");
            string chkCode = String.Empty;
            byte[] bytes = YK.Common.Util.CheckCodeKit.GetChkCodeBytes(ref chkCode);
            YK.Core.Context.Context.SetDataBySession("YK.Website.CheckCode." + type, chkCode);
            Response.ClearContent();
            Response.ContentType = "p_w_picpath/Png";
            Response.BinaryWrite(bytes);
        }
        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}