using System;
using System.Collections.Generic;
using System.Text;
using System.Web;
namespace Handler
{
    public class DownloadHandler : IHttpHandler
    {
        public bool IsReusable
        {
            get { return true; }
        }

        public void ProcessRequest(HttpContext context)
        {

            //context.Response.Write(context.Request.Url.ToString());
            context.Response.Write("<h1><b>您没有权限访问此文件，请联系网站管理员！</b></h1>");
            context.Response.End();
         
        }
    }
}

    <httpHandlers>
            <add verb="*" path="*.rar" type="Hander.DownloadHander, Hander"/>
    </httpHandlers>

      protected void Page_Load(object sender, EventArgs e)
        {
          //执行权限判断，如果拥有权限则执行下载

            bool b = IsCanDown();

           
            if (b)
            {
                string path = Request.QueryString["file"] != null ? Request.QueryString["file"] : "/upload/day_20140312/留言板.rar";

                get_file(path);

            }
        }

        private bool IsCanDown()
        {
            //处理逻辑
            return true;
        }

        private void get_file(string path)
        {
            //string path = Request.QueryString["file"];
            string filePath = Server.MapPath(path);//路径
            string filename = Path.GetFileName(filePath);

            FileInfo fileInfo = new FileInfo(filePath);
            Response.Clear();
            Response.ClearContent();
            Response.ClearHeaders();
            Response.AddHeader("Content-Disposition", "attachment;filename=" + filename);
            Response.AddHeader("Content-Length", fileInfo.Length.ToString());
            Response.AddHeader("Content-Transfer-Encoding", "binary");
            Response.ContentType = "application/octet-stream";
            Response.ContentEncoding = System.Text.Encoding.GetEncoding("gb2312");
            Response.WriteFile(fileInfo.FullName);
            Response.Flush();
            Response.End();
        }