<a href="download.ashx?url=<%=Server.UrlEncode("111.txt")%>">下载</a>

public void ProcessRequest(HttpContext context)
        {
            string url = HttpContext.Current.Server.UrlDecode(context.Request.QueryString["url"]);
            downloadfile(url);
        }

public bool IsReusable
        {
            get
            {
                return false;
            }
        }
        public void downloadfile(string s_fileName)
        {
            HttpContext.Current.Response.ContentType = "application/ms-download";
            string s_path = HttpContext.Current.Server.MapPath(".") + s_fileName;
            System.IO.FileInfo file = new System.IO.FileInfo(s_path);
            HttpContext.Current.Response.Clear();
            HttpContext.Current.Response.AddHeader("Content-Type", "application/octet-stream");
            HttpContext.Current.Response.Charset = "utf-8";
            HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment;filename=" + System.Web.HttpUtility.UrlEncode(file.Name, System.Text.Encoding.UTF8));
            HttpContext.Current.Response.AddHeader("Content-Length", file.Length.ToString());
            HttpContext.Current.Response.WriteFile(file.FullName);
            HttpContext.Current.Response.Flush();
            HttpContext.Current.Response.Clear();
            HttpContext.Current.Response.End();

}