/// <summary>
    /// 指定要下载文件的虚拟路径及文件名
    /// </summary>
    /// <param name="FileName"></param>
    public void downloadfile(string FileName)
    {

          //打开要下载的文件 
          System.IO.FileStream r = new System.IO.FileStream(Server.MapPath(FileName), System.IO.FileMode.Open); 
          //设置基本信息 
          Response.Buffer = false; 
          Response.AddHeader("Connection", "Keep-Alive"); 
          Response.ContentType = "application/octet-stream"; 
          Response.AddHeader("Content-Disposition", "attachment;filename=" + System.IO.Path.GetFileName(FileName)); 
          Response.AddHeader("Content-Length", r.Length.ToString()); 


          while (true) 
          { 
              //开辟缓冲区空间 
              byte[] buffer = new byte[1024]; 
              //读取文件的数据 
              int leng = r.Read(buffer, 0, 1024); 
              if (leng == 0)//到文件尾，结束 
                  break; 
              if (leng == 1024)//读出的文件数据长度等于缓冲区长度，直接将缓冲区数据写入 
                  Response.BinaryWrite(buffer); 
              else 
              { 
                  //读出文件数据比缓冲区小，重新定义缓冲区大小，只用于读取文件的最后一个数据块 
                  byte[] b = new byte[leng]; 
                  for (int i = 0; i < leng; i++) 
                  b[i] = buffer[i]; 
                  Response.BinaryWrite(b); 
              } 
          } 
          r.Close();//关闭下载文件 
          Response.End();//结束文件下载 
    }