delaying file read/write till it's done being used? - asp.net

Question

I have a vb.net MVC3 Razor app that generates PDF files. The problem is that if 2 seperate users click the print button at the same time it throws the following exception..:
The process cannot access the file 'E:\web\xxxxxxxxxxsonl\PDF_Files\MailingLables.pdf' because it is being used by another process.
All of the controller actions to do with printing are basically like below:
Function Ind_Cert(ByVal firstName As String, ByVal lastname As String, ByVal classRef As String)
Dim _Attendance As attendance = db.attendances.Where(Function(f) f.Completed_Class = "Completed" And f.firstName = firstName And f.lastName = lastname).FirstOrDefault
If Not IsNothing(_Attendance) Then
Dim _reg_classes As List(Of reg_classes) = db.reg_classes.ToList
Dim _registrants As List(Of reg_info) = db.reg_info.ToList
Dim _courses As List(Of cours) = db.courses.ToList
Dim _Board As List(Of board_members) = db.board_members.ToList
Dim Board_Member As board_members = _Board.Where(Function(f) f.Official_Cap = "xxxxxx President").FirstOrDefault
Dim RecordId As Integer = 0
Dim conf_info As conf_info = db.conf_info.Single(Function(r) r.id = 0)
Dim conf_num As Integer = conf_info.conf_number
Dim _conf_num As String = conf_num.ToString
Dim Length As Integer
Dim _prefix As String = String.Empty
If Str(conf_num) <> "" Then
Length = Str(conf_num).Length
End If
Dim Divisor As Integer = 10 ^ (Length - 1)
Dim conf_num_start As Integer = 0
Dim Digits(Length - 1) As Integer
While (conf_num > 10)
'Extract the first digit
Digits(conf_num_start) = Int(conf_num / Divisor)
'Extract remainder number - and store it back in Num
conf_num = conf_num Mod Divisor
'Decrease Divisor's value by 1/10th units
Divisor /= 10
'Increment Index
conf_num_start += 1
End While
If conf_num = 0 Or 4 Or 5 Or 6 Or 7 Or 8 Or 9 Then _prefix = "th"
If conf_num = 1 Then _prefix = "st"
If conf_num = 2 Then _prefix = "nd"
If conf_num = 3 Then _prefix = "rd"
Dim pdfpath As String = Path.Combine(AppDomain.CurrentDomain.BaseDirectory) + "\PDF_Files\"
Dim imagepath As String = Path.Combine(AppDomain.CurrentDomain.BaseDirectory) + "\PDF_Files\"
Dim _PdfName As String = "Cert_" + lastname + ".pdf"
Dim doc As New Document
doc.SetPageSize(iTextSharp.text.PageSize.LETTER.Rotate())
doc.SetMargins(1, 1, 1, 1)
Dim _pageCounter As Integer = 0
Dim Californian As BaseFont = BaseFont.CreateFont(Path.Combine(AppDomain.CurrentDomain.BaseDirectory) + "\Fonts\" + "CALIFB.TTF", BaseFont.CP1252, BaseFont.EMBEDDED)
Dim Copper As BaseFont = BaseFont.CreateFont(Path.Combine(AppDomain.CurrentDomain.BaseDirectory) + "\Fonts\" + "COPRGTB.TTF", BaseFont.CP1252, BaseFont.EMBEDDED)
Dim Bold_Times As BaseFont = BaseFont.CreateFont(BaseFont.TIMES_BOLD, BaseFont.CP1252, False)
Dim BF_Times As BaseFont = BaseFont.CreateFont(BaseFont.TIMES_ROMAN, BaseFont.CP1252, False)
Dim F_Name As New Font(BF_Times, 16, Font.BOLD, BaseColor.BLACK)
Dim _Parking_Name As New Font(BF_Times, 18, Font.NORMAL, BaseColor.BLACK)
Dim _Parking_Date As New Font(BF_Times, 24, Font.BOLD, BaseColor.BLACK)
'**********************************Y lines for trial***********************************
Dim y_line1 As Integer = 670
Dim _Counter As Integer = 1
Dim _Page As String = 1
Dim _CertJpg As Image = Image.GetInstance(imagepath + "/cert.jpg")
Dim imageWidth As Decimal = _CertJpg.Width
Dim imageHeight As Decimal = _CertJpg.Height
Dim writer As PdfWriter = PdfWriter.GetInstance(doc, New FileStream(pdfpath + _PdfName, FileMode.Create))
doc.Open()
Dim cb As PdfContentByte = writer.DirectContent
If _Attendance.Completed_Class = "Completed" Then
Dim _confInfo As conf_info = db.conf_info.Single(Function(a) a.id = 0)
Dim year As String = Right(_confInfo.conf_start_date, 4)
Dim _reg As reg_info = db.reg_info.Single(Function(b) b.id = _Attendance.reg_id)
Dim name As String = _reg.first_name + " " + _reg.last_name
Dim _dates As String = _confInfo.conf_start_date + " - " + _confInfo.conf_end_date
Dim _course As cours = db.courses.Single(Function(c) c.course_ref = _Attendance.course_ref)
Dim _className As String = _course.course_title.ToString
Dim _hours As String = _course.course_hours
Dim _certName As String = Board_Member.First_Name + " " + Board_Member.last_name
_CertJpg.Alignment = iTextSharp.text.Image.UNDERLYING
_CertJpg.ScaleToFit(792, 611)
doc.Add(_CertJpg)
cb.BeginText()
cb.SetFontAndSize(Californian, 36)
cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, "CERTIFICATE OF COMPLETION", 396, 397.91, 0)
cb.SetFontAndSize(Bold_Times, 22)
cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, name, 396, 322.35, 0)
cb.SetFontAndSize(Bold_Times, 16)
cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, _hours + " Hours", 297.05, 285.44, 0)
cb.SetFontAndSize(Bold_Times, 16)
cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, _dates, 494.95, 285.44, 0)
cb.SetFontAndSize(Bold_Times, 16)
cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, "Class Attended: " + " " + _Attendance.course_ref + " -- " + _className, 396, 230.34, 0)
cb.SetFontAndSize(Copper, 16)
cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, _conf_num + _prefix + " Annual Conference " + _dates, 396, 193.89, 0)
cb.SetFontAndSize(Bold_Times, 13)
cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, _certName, 396, 175.69, 0)
cb.SetFontAndSize(Bold_Times, 10)
cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, "xxxxxxx President", 396, 162.64, 0)
cb.EndText()
End If
doc.Close()
Return _PdfName
Else
Return "Fail"
End If
End Function
This error happens like I said any time 2 users try to generate a PDF file at the same time. Anyone know of a fix for this issue? Google has turned up countless pages about someone can't delete a file in windows because its being used. But that isn't much help.. Any ideas???

Answer 1

You can pretty much do two things.
Add a lock on the file and block the second (and third and fourth, etc) until the lock is cleared
Create a unique file for each instance.
I'd recommend #2. You can keep the same file name, just put the file in a unique directory. A GUID is usually the easiest for me:
Dim pdfpath As String = Path.Combine(AppDomain.CurrentDomain.BaseDirectory) + "\PDF_Files\"
pdfPath = Path.Combine(pdfPath, Guid.NewGuid.ToString())
Directory.CreateDirectory(pdfPath)
Then change your return to include the path created above.

Answer 2

Can you create the PDF file with a random file name to avoid the conflict in the first place?

Answer 3

Wrap your file access code in a lock.
lock (this)
{
//Write file
}
See Lock on MSDN