强制下载文件对话框不起作用 - ASP.NET C#

Question

I am trying to implement force download file dialog in my ASP.NET C# application. The files I'd like to force download are media files not locally available available on the web server but are being served from a different location.

I am getting an error 'http://remote-site-to-webserver/somefile.asf' is not a valid virtual path.

I have searched the web for solutions but all examples point to relative path on the server using Server.MapPath

In the example below I created a webhandler.ashx page and send the download request to this page.

<%@ WebHandler Language="C#" Class="DownloadHandler" %>

using System;
using System.Web;

public class DownloadHandler : IHttpHandler {
public void ProcessRequest(HttpContext context) {
var fileName = "http://remote-site-to-webserver/somefile.asf";
var r = context.Response;
r.AddHeader("Content-Disposition", "attachment; filename=" + fileName);
r.WriteFile(context.Server.MapPath(fileName));
}
public bool IsReusable { get { return false; } }
}

Answer 1

The Content-Disposition header looks wrong to me. I think it should be:

r.AddHeader("Content-Disposition", 
    "attachment; filename=DefaultNewFilename.ext");

the filename is the default name given to the downloaded file... Or in otherwords it's what is shown in the browsers save dialog.

You may also want:

r.AddHeader("Content-Type", "application/octetstream");

I'm not sure that's required.... But I've always included it for video files and so on.

Answer 2

Server.MapPath() 

is not used for remote http files. it is just a tool for converting virtual addresses to physical addresses, ie you can retrieve "C:\inetpub\wwwroot\MyWebSite\Files\blah.txt" by giving "~/Files/blah.txt" to Server.MapPath method.

if you are interested in downloading a file from another web server you will have to use HttpWebRequest class.

this is a sample code:

                HttpWebRequest httpRequest = (HttpWebRequest)WebRequest.Create("http://remote-site-to-webserver/somefile.asf");
                httpRequest.Credentials = CredentialCache.DefaultCredentials; //or a NetworkCredential if needed

                HttpWebResponse httpResponse = (HttpWebResponse)httpRequest.GetResponse();

                Stream dataStream = httpResponse.GetResponseStream();

now you can output the dataStream into your response.

Answer 3

In order for the download to start from a different server, you need to send a redirect answer to the client ( Response.Redirect(mediaURL) ).

As a consequence, you cannot force the download dialog from your web server because the browser will send a separate request to the other server. This must be solved on the server where the media is served from.

The only alternative is that you act as an intermediate, ie you download the media file to your server and send it as the response to the client. This shouldn't be too difficult if it's a small file that easily fits into memory. However, if it's a large file it might involve some tricky coding so you can receive and send it piecewise.