string filePath = context.Server.MapPath("~/" + uploadFolder+"/"+file_name);//路径
System.IO.FileInfo fileInfo = new System.IO.FileInfo(filePath);
if (fileInfo.Exists == true)
{
//以字符流的形式下载文件
FileStream fs = new FileStream(filePath, FileMode.Open);
byte[] bytes = new byte[(int)fs.Length];
fs.Read(bytes, 0, bytes.Length);
fs.Close();
Response.ContentType = "application/octet-stream";
//通知浏览器下载文件而不是打开
Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(file_name, System.Text.Encoding.UTF8));
Response.BinaryWrite(bytes);
Response.Flush();
Response.End();
}
else
{
//context.Response.Write("the download files is not exists");
}

asp.net已流的方式下载文件