Writing To Text File, File In Use By Another Process, Multithreaded Application .NET - vb.net

Question

Afternoon,
I have a program whereby I really need to be keeping a log of some kind to ascertain what the program is doing. Essentially the software monitors for a window on the desktop to live pause a call recording on our Call Recorder Server.
Lets on argument say a Call Recording itself was pulled up by a monitoring agent and they state that a certain sensitive part of the conversation has been recorded when really it should have been silenced, if they were to say the agent hadn't done their job and clicked the pause recording button OR the onfocus action hadn't occurred I would have a situation whereby I would need to prove what the software was doing at the same.
I decided that I would write the actions of the software to a .txt file stored in the users app data.
This works for the most part, however every now and then even though the .txt is never accessed by any other program I get 'This file is in use by another process'.
This application is multithreaded and does make very frequent calls to write to the log, I am using the below code:
Private Sub WriteToLog(ByVal strSubTitle As String, ByVal strLogInfo As String)
Try
If My.Computer.FileSystem.FileExists(strLogFilePath) = True Then
'Delete yesterdays log file
Dim strFileDate As Date = File.GetCreationTime(strLogFilePath)
strFileDate = FormatDateTime(strFileDate, DateFormat.ShortDate)
'If strFileDate < Date.Today Then
' My.Computer.FileSystem.DeleteFile(strLogFilePath)
'End If
Using outFile As IO.StreamWriter = My.Computer.FileSystem.OpenTextFileWriter(strLogFilePath, True)
outFile.WriteLine("" & Date.Today & "," & Date.Now.ToLongTimeString & ", " & strUsername & ", " & strSubTitle & ", " & Replace(strLogInfo, ",", "|") & "")
outFile.Close()
End Using
''CSV File
'outFile.WriteLine("" & Date.Today & "," & Date.Now.ToLongTimeString & ", " & strUsername & ", " & strSubTitle & ", " & Replace(strLogInfo, ",", "|") & "")
'outFile.Close()
Else
Using outFile As IO.StreamWriter = My.Computer.FileSystem.OpenTextFileWriter(strLogFilePath, False)
outFile.WriteLine("" & Date.Today & "," & Date.Now.ToLongTimeString & ", " & strUsername & ", " & strSubTitle & ", " & Replace(strLogInfo, ",", "|") & "")
outFile.Close()
End Using
'CSV File
'outFile.WriteLine("Date, Time, Username, Sub(Process), Information")
'outFile.WriteLine("" & Date.Today & "," & Date.Now.ToLongTimeString & ", " & strUsername & ", " & strSubTitle & ", " & Replace(strLogInfo, ",", "|") & "")
'outFile.Close()
End If
Catch ex As Exception
CreateErrorFile(ex.Message, ex.StackTrace, "Log Write Failure!")
End Try
End Sub
Is there any advice/pointers someone could state as to why this would be saying another process is using the file.
I'm guessing the situation would occur when two separate threads try to do the 'WriteToLog' Sub while one or the other is writing to the file.
Am I on the right tracks? If so how could I rectify this?
Cheers,
James

Answer 1

You will either want to make it so that only one thread has the ability to write to the log file by using the Invoke method from the secondary thread(s) to call the write functionality on the main thread, or you can use one of .NET's various synchronization mechanisms.
Here is a simple example of the first approach:
Private Sub BackgroundMethod()
'do stuff
Me.Invoke(New Action(Of String)(AddressOf WriteToLog), "write a line blah blah")
'do more stuff
End Sub
Private Sub WriteToLog(valueToWrite As String)
System.IO.File.AppendAllLines(MyLogFilePath, {valueToWrite})
End Sub
Here's an example using a SyncLock block:
Private lock As New Object()
Private Sub BackgroundMethod()
'do stuff
WriteToLog("write a line blah blah")
'do more stuff
End Sub
Private Sub WriteToLog(valueToWrite As String)
SyncLock (lock)
System.IO.File.AppendAllLines(MyLogFilePath, {valueToWrite})
End SyncLock
End Sub
MSDN has good information on synchronization mechanisms: http://msdn.microsoft.com/en-us/library/ms228964%28v=vs.110%29.aspx

Answer 2

I would use a global Queue where new entries get added to and if the log writer is not busy then start it to write down all available lines.
sth like:
Private LogList As New Queue(Of String)
Private WriterBusy As Boolean = False
Private Sub WriteToLog(ByVal strSubTitle As String, ByVal strLogInfo As String)
LogList.Enqueue("" & Date.Today & "," & Date.Now.ToLongTimeString & ", " & strUsername & ", " & strSubTitle & ", " & Replace(strLogInfo, ",", "|") & "")
If WriterBusy = True Then Exit Sub
WriterBusy = True
Try
If My.Computer.FileSystem.FileExists(strLogFilePath) = True Then
...
Using outFile As IO.StreamWriter = My.Computer.FileSystem.OpenTextFileWriter(strLogFilePath, True)
While LogList.Count > 0
outFile.WriteLine(LogList.Dequeue)
End While
outFile.Close()
End Using
...
End Try
WriterBusy = False
End Sub