IOException issue with asynchronous PDF creation - vb.net

Question

I have an asynchronous method that creates a PDF file from XML retrieved from a database. Everything works great, but occasionally I get an IOException because when I try to cleanup the temporary .fo file after creating the PDF, the file is still in use.
Public Sub FormatObjectToPdf(ByVal intRxNo As Integer, ByVal strSourceFileName As String)
Dim startInfo As New ProcessStartInfo
Dim strPdfFile As String = g_strRootPath & "Paperwork\" & intRxNo & "M.pdf"
' if the PDF file already exists, no need to re-create it
If Not File.Exists(strPdfFile) Then
Try
startInfo.Arguments = "-fo """ & strSourceFileName & """ -pdf """ & strPdfFile & """"
startInfo.FileName = g_strAppPath & "FO.NET\fonet.exe"
startInfo.UseShellExecute = True
startInfo.WindowStyle = ProcessWindowStyle.Hidden
Using proc As Process = Process.Start(startInfo)
proc.WaitForExit()
If proc.HasExited Then
proc.Dispose()
End If
End Using
Catch ex As Exception
Call InsertLog("ErrorLog", "FormatObjectToPdf: " & ex.Message, ex)
MessageBox.Show(ex.Message, "Create PDF", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
End Try
End If
' wait 3 seconds to allow file to be released
System.Threading.Thread.Sleep(3000)
' delete the source FO file when processing is complete
If File.Exists(strSourceFileName) Then
Try
File.Delete(strSourceFileName)
Catch iEx As IOException
Call InsertLog("ErrorLog", "Could not delete file '" & strSourceFileName & "': " & iEx.Message, iEx)
Catch ex As Exception
Call InsertLog("ErrorLog", "Error deleting file '" & strSourceFileName & "': " & ex.Message, ex)
End Try
End If
End Sub
The FormatObjectToPdf method is called from another method, AsyncXmlToPdf, which is actually where the IOException is thrown. I had initially thought that the exception was in FormatObjectToPdf since that is where I am deleting the .fo file, so I had added a Sleep(3000) to see if giving it a few seconds would help.
Here is the AsyncXmlToPdf:
Public Sub AsyncXmlToPdf(ByVal state As Object)
Dim intRxNo = state(0)
Dim flgPrintResult As Boolean = state(1)
Dim strFileName As String = g_strAppPath & intRxNo & ".fo"
Dim strOutput As String
Dim strPdfFile As String = g_strRootPath & "Paperwork\" & intRxNo & "M.pdf"
Try
If File.Exists(strPdfFile) Then
File.Delete(strPdfFile)
End If
If Not File.Exists(strPdfFile) AndAlso Not File.Exists(strFileName) Then
strOutput = XmlToFormatObject(intRxNo, g_strAppPath & "FO.NET\immfo.xsl")
Using writer As StreamWriter = New StreamWriter(strFileName)
writer.Write(strOutput)
End Using
Call FormatObjectToPdf(intRxNo, strFileName)
End If
Catch ex As Exception
Call InsertLog("ErrorLog", "AsyncXmlToPdf: " & ex.Message, ex)
End Try
End Sub
The only part of either method other than the declaration of strFileName that even does anything with the .fo file is in FormatObjectToPdf and that method has a Catch block for IOException. Why is the exception being caught in AsyncXmlToPdf?? Here is the actual error message:
3/25/2015 11:15 AM: [IOException] AsyncXmlToPdf: The process cannot access the file 'C:\Users\<username>\AppData\Local\Apps\2.0\1M2D4TCB.REJ\3LH3JZY2.TQC\<clickonce app>\561964.fo' because it is being used by another process.
Everything works as expected, other than the occasional orphaned .fo file when this exception occurs. Anyone have any suggestions on how I might be able to find out where the problem is?

Answer 1

The only part of either method ... that even does anything with the .fo file is in FormatObjectToPdf It appears that AsyncXmlToPdf will also try to open a streamwriter on it.
If there is a chance that some other BackGroundWorker, Thread or Task could also be working on the same set of files, it would be possible for the same file to be in use in AsyncXmlToPdf and FormatObjectToPdf. MSDN warns of this in the File.Exists entry:
Be aware that another process can potentially do something with the file in between the time you call the Exists method and perform another operation on the file, such as Delete.
In your case it looks like it might be a a coin flip which method the Exception will happen in.
If an accidental double click could start the same process twice, it is possible that the same file could be in use in both methods at the same time. You could add another test to see if you can open the file for ReadWrite. Given that the file was not supposed to exist yet, you could at least be somewhat certain as to the reason.
Some sort of flag to prevent more than one set of jobs from starting might be the final solution.