使用ASP.Net Framework2.0实现大文件的异步流式传输

Question

我正在开发一个ASP.NET Framework2.0应用程序。在一个特定的页面上，我提供了一个链接到用户。单击此链接将打开一个窗口，其中包含另一个aspx页面。该页面实际上将http请求发送到指向文件的第三方url (类似于从云中下载文件的镜像url)。使用response.write将http响应发送回第一个页面上的用户，用户从该页面单击链接。

现在，我面临的问题是，如果文件大小很低，那么它就可以正常工作。但是，如果文件很大(即超过1 GB)，那么我的应用程序将等待，直到从URL下载完整个文件。我曾尝试使用response.flush()将块数据逐个发送给用户，但用户仍然无法使用应用程序，因为工作进程正忙于从第三方URL获取数据流。

有没有什么方法可以异步下载大文件，以便我的弹出窗口完成执行(下载将正在进行中)，并且用户可以在应用程序上并行执行其他活动。

谢谢，苏沃迪普

Answer 1

使用WebClient读取远程文件。您可以从WebClient获取流，而不是下载。将其放入while()循环中，并将来自WebClient流的字节推送到响应流中。通过这种方式，您将同时进行异步下载和上传。

HttpRequest示例：

private void WriteFileInDownloadDirectly()
{
    //Create a stream for the file
    Stream stream = null;

    //This controls how many bytes to read at a time and send to the client
    int bytesToRead = 10000;

    // Buffer to read bytes in chunk size specified above
    byte[] buffer = new byte[bytesToRead];

    // The number of bytes read
    try
    {
        //Create a WebRequest to get the file
        HttpWebRequest fileReq = (HttpWebRequest)HttpWebRequest.Create("Remote File URL");

        //Create a response for this request
        HttpWebResponse fileResp = (HttpWebResponse)fileReq.GetResponse();

        if (fileReq.ContentLength > 0)
            fileResp.ContentLength = fileReq.ContentLength;

        //Get the Stream returned from the response
        stream = fileResp.GetResponseStream();

        // prepare the response to the client. resp is the client Response
        var resp = HttpContext.Current.Response;

        //Indicate the type of data being sent
        resp.ContentType = "application/octet-stream";

        //Name the file 
        resp.AddHeader("Content-Disposition", $"attachment; filename=\"{ Path.GetFileName("Local File Path - can be fake") }\"");
        resp.AddHeader("Content-Length", fileResp.ContentLength.ToString());

        int length;
        do
        {
            // Verify that the client is connected.
            if (resp.IsClientConnected)
            {
                // Read data into the buffer.
                length = stream.Read(buffer, 0, bytesToRead);

                // and write it out to the response's output stream
                resp.OutputStream.Write(buffer, 0, length);

                // Flush the data
                resp.Flush();

                //Clear the buffer
                buffer = new byte[bytesToRead];
            }
            else
            {
                // cancel the download if client has disconnected
                length = -1;
            }
        } while (length > 0); //Repeat until no data is read
    }
    finally
    {
        if (stream != null)
        {
            //Close the input stream
            stream.Close();
        }
    }
}

WebClient流读取：

using (WebClient client = new WebClient())
{
     Stream largeFileStream = client.OpenRead("My Address");
}