Simulate Windows Service with ASP.NET

Question

Hi,

I have small web app that generate PDF files as a report. I'm trying to delete those generated PDF files after 10 sec that they are generated. What I want to do is to read a folder with PDF files every 10 sec, and delete all the PDF files inside that folder.

I read this post of Easy Background Tasks in ASP.NET. The following code is the VB version.

    Protected Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
    AddTask("DoStuff", 10)
End Sub

Private Sub AddTask(ByVal name As String, ByVal seconds As Integer)
    OnCacheRemove = New CacheItemRemovedCallback(CacheItemRemoved)
    HttpRuntime.Cache.Insert(name, seconds, Nothing, DateTime.Now.AddSeconds(seconds), Cache.NoSlidingExpiration, CacheItemPriority.NotRemovable, _
     OnCacheRemove)
End Sub

Public Sub CacheItemRemoved(ByVal k As String, ByVal v As Object, ByVal r As CacheItemRemovedReason)
    ' do stuff here if it matches our taskname, like WebRequest

    DeletePDFilesInFoler()

    ' re-add our task so it recurs
AddTask(k, Convert.ToInt32(v))

End Sub

But I got this error

Delegate 'System.Web.Caching.CacheItemRemovedCallback' requires an 'AddressOf' expression or lambda expression as the only argument to its constructor.

If this code works, where I should put it. Right, now I'm putting it in the master page. How to get this error out?

Thank you

Answer 1

The error is in the error message, you're missing AddressOf. Try this:

OnCacheRemove = New CacheItemRemovedCallback(AddressOf CacheItemRemoved)

Answer 2

Rather than deleting them on a schedule, why not look for old PDFs and delete them each time a PDF is generated?

Pseudo-code:

/* Clean up old PDFs */
If Not StaticCleaningUpPDFsNow Then
   // No other reports generating simultaneously and trying to delete old PDFs
   StaticCleaningUpPDFsNow = True 
   For each f in (reportfolder\*.pdf)
      If f.DateCreated.AddSeconds(10) < Now Then f.Delete
   Next
   StaticCleaningUpPDFsNow = False
End If
/* Create PDF for the current report */
...

The overhead to look for files in a folder and delete a few is incredibly small, and doing this on demand is a much better use of resources, without the hacks around cache expirations (and the edge cases that can result).