How to send multiple files with Response. ASP.NET - asp.net

Question

I'm trying to call below code in a loop hundreds of times:
Sub ExportReport(ByVal en As MyReport)
Dim warnings As Warning() = Nothing
Dim streamids As String() = Nothing
Dim mimeType As String = Nothing
Dim encoding As String = Nothing
Dim extension As String = Nothing
Dim bytes As Byte()
bytes = aReport.ServerReport.Render("WORD", Nothing, mimeType, encoding, extension, streamids, warnings)
Response.Buffer = True
Response.Clear()
Response.ContentType = mimeType
Response.AddHeader("content-disposition", "attachment; filename=" & en.ToString() & "." + extension)
Response.BinaryWrite(bytes)
Response.Flush()
Response.End()
End Sub
And I'm getting this error :
Server cannot append header after HTTP headers have been sent.
How can I change the code so that I can loop this piece of code? Thanks.
EDIT :
I added this line after Response.End()
Response.Redirect(Request.Url.AbsoluteUri)
And I get this error :
Cannot redirect after HTTP headers have been sent.

Answer 1

WWW works on a request / response mechanism. For every request there is only 1 response. You cannot change that basic mechanism. When browser sends a request it is expecting one and only one response. So if it receives more than 1 response, it either issues a warning to the user to block this behaviour or may choose to ignore the extra responses by itself. Thus these extra responses may be lost.
Having said that you have 2 options with you:
Zip all the files that you want to download and download as a single file.
You can use Popular framework Ionic.Zip.
First, keep all your files in a local directory on the server.
Then use this library to zip the entire folder.
Pseudo code:
Imports (var zip = New Ionic.Zip.ZipFile())
{
zip.AddDirectory("DirectoryOnDisk", "rootInZipFile")
Response.Clear()
Response.AddHeader("Content-Disposition", "attachment; filename=DownloadedFile.zip")
Response.ContentType = "application/zip"
zip.Save(Response.OutputStream)
Response.End()
}
Add a mechanism to issue multiple request using Javascript to get multiple responses, so browser still treats this behaviour as normal.

Answer 2

A normal web page will have a load of (headers) stuff set up for you already, but you don't want any of that: you want complete control over what is sent to the browser. If you cause a redirect to something which sends the headers shown in code later here, the browser will (normally) download the data.
In the code-behind you can have something like
Protected Sub btn_click(ByVal sender As Object, ByVal e As EventArgs) Handles btn.Click
Response.Redirect("~/sendfile.ashx?ref=" & enReference, False)
Context.ApplicationInstance.CompleteRequest()
End Sub
You will also need to add a generic handler (right-click on the project in Solution Explorer, Add->New Item... -> Visual Basic--Web--General choose "Generic Handler"; give it a name like sendfile.ashx) which is somewhat like
Imports System.IO
Public Class sendfile
Implements System.Web.IHttpHandler, IReadOnlySessionState
Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
Dim enReference = context.Request.QueryString("ref")
' do whatever is needed to get the report from enReference '
Dim bytes As Byte() = aReport.ServerReport.Render("WORD", Nothing, mimeType, encoding, extension, streamids, warnings)
Dim downloadName = yourfilename & "." & yourextension
context.Response.ContentType = "application/octet-stream"
context.Response.AddHeader("content-disposition", "attachment; filename=""" & downloadName & """" )
context.Response.BinaryWrite(bytes)
End Sub
ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
Get
Return False
End Get
End Property
End Class
and you will need to work out the code to create the data to be sent.
If you don't need to use session state then you can remove the , IReadOnlySessionState part on the Implements line.
You might need to add context.Response.Flush(). If you find that the response does not have a Content-Length header, then you ought to add one so that the browser can show a meaningful download progress.