Reading from a MemoryStream using async - asp.net

Question

Im using iTextSharp (not that this is relative to iTextSharp) to generate PDF bytes using code similar to the following:
Function GetFileBytes() as Byte()
Dim result() As Byte
Using ms As New MemoryStream
Dim reader As New PdfReader(pdfForm)
Dim writer As New PdfStamper(reader, ms)
Dim fields As AcroFields = writer.AcroFields
fields.SetField("Customer Name", custInfo.FirstName + " " + custInfo.LastName)
result = ms.GetBuffer
End Using
End Function
I want to make this an asynchronous task instead, returning Task(Of Byte()) instead, but I'm having problems figuring out how to make my memorystream write in an asynchronous mode, and how to return a Task(Of Byte()) in the process. Any tips, suggestions? Thanks!
EDIT 1
I wanted to clarify the need for using a Task and using this code, as it seems some comments so far are along the lines of "why are you doing it this way" and "no just do this instead, and I want to politely say "thanks, but I want to do it this way" :)
Here's my scenario, imagine a website that collects data on a few pages, then presents a review screen, and then gives the user the ability to view a PDF containing that data. On the review screen I need to generate a 1 MB PDF file, upload it to BLOB storage, and have it available when the user chooses to download it.
I want to generate and upload the PDF while the user is reviewing the review screen and not when they choose the "view PDF" button. That code will be done using my example, which is why I'm curious how to create a Task of type Byte(). Any help is always appreciated, thanks again!

Answer 1

if you really want to make is async a simpler approach would be to create a private thread in the class, that thread on page_init will start the process then on Page_LoadComplete the thread would wait using thread.join() and then you would get the function result, but as mentioned the relative speed gain is minimal, consider that threads(or tasks) are really expensive
Option 2
create an ajax request to a generic handler that returns the pdf file data, this way no thread involved and better user experience
<%# WebHandler Language="VB" Class="GetFile" %>
Imports System
Imports System.Web
Imports System.IO
Public Class GetFile : Implements IHttpHandler
Public Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
context.Response.Clear()
If File.Exists(filePath & docName) Then
context.Response.AddHeader("content-disposition", "attachment; filename=" & HttpContext.Current.Server.UrlEncode(docName))
context.Response.ContentType = "text/pdf"
context.Response.WriteFile(filelocation & docName) 'in your case this writes the bites of the generated file, so instead of the path just pass in the data
End If
context.Response.Flush()
context.Response.End()
End Sub
Public ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
Get
Return False
End Get
End Property
End Class

Answer 2

Responding to your edit: this scenario makes sense. Wrap everything in a Task.Run call. Be sure to be thread-safe (don't use the HttpContext in the task for example). Also log errors because they might be bugs.
Note, though, that with ASP.NET background work can be aborted at any time (e.g. when the worker process recycles). You have to tolerate suddenly disappearing background work. In your case the PDF might be missing when the user tries to access it. Also, what if it is still being generated when the user tries to download it. Must solve those issues.

Answer 3

Here's my scenario, imagine a website that collects data on a few pages, then presents a review screen, and then gives the user the ability to view a PDF containing that data. On the review screen I need to generate a 1 MB PDF file, upload it to BLOB storage, and have it available when the user chooses to download it.
Since you refer to "blob storage", I'll assume you're using Azure.
You need a worker role or service to do this reliably. The worker role should just be reading from an Azure queue and generating PDFs.
So, when your web role generates the review screen, before returning the response, it will also place that data into the Azure queue. When your web role is notified that the user accepts the review, then it should (asynchronously) wait until the PDF file is generated.

Answer 4

(Don't use this code below.. see end of "answer" for reasoning)..
Thanks to everyone for the help. In the end I was approaching this wrong. All I needed was to create an Async wrapper function to return my Task(Of Byte())
So if my initial code was this:
Function GetFileBytes() as Byte()
Dim result() As Byte
Using ms As New MemoryStream
Dim reader As New PdfReader(pdfForm)
Dim writer As New PdfStamper(reader, ms)
Dim fields As AcroFields = writer.AcroFields
fields.SetField("Customer Name", custInfo.FirstName + " " + custInfo.LastName)
result = ms.GetBuffer
End Using
End Function
Now, I created a new method called GetFileBytesAsync to return Task(Of Byte()) calling the same GetFileBytes method described above, it's just called using a Task.Factory.StartNew, like so:
Public Shared Async Function GetFileBytesAsync() As Task(Of Byte())
Dim t As Task(Of Byte()) = Task.Factory.StartNew(Function()
Return GetFileBytes()
End Function)
Return Await t
End Function
And in my calling function, I now just use the Result property of my task object to access the byte array. HTH!
EDIT: OK, so as Stephen Cleary aptly pointed out, I'm doing this all wrong, the above code is not recommended and it isn't doing anything asynchronously.. as a matter of fact its creating new threads to create some blocking code. Sorry...
I came across this TechEd talk: http://channel9.msdn.com/Events/TechEd/NorthAmerica/2013/DEV-B337 - it's How to Build ASP.NET Web Applications Using Async. At the 54 Minute Mark it actually indicates that trying to use a "background thread" on an ASP.NET Web Server is wrong, as there are no background threads for use in IIS, and in essence you are taking up threads that could\should be used to respond to web requests.