Thread Sync Queue Problem

[shipcreak@gmail.com]

I have an interesting problem with a sort of producer-consumer system
for error logging. Consider the following code:

<code>
SyncLock _eventList.Sync Root

Dim item As ExceptionLogEnt ry

' If there are items, get the top one
If _eventList.Coun t > 0 Then

' Get the item from the queue
item = CType(_eventLis t.Dequeue, _
ExceptionLogEnt ry)

' If we got an item to log, log it
If Not item Is Nothing Then

WriteToLog(Buil dExceptionRepor t(item))

End If

End If

End SyncLock
</code>

This method is running as a new thread, created thus:

<code>
_thread = New Thread(AddressO f StartWatching)
_thread.Start()
</code>

FACTS:
1) The _thread is System.Threadin g.Thread, the _eventList is
System.Collecti ons.Queue, both are member variables of the class.

2) The class this code appears in could have more than one instances
(hence using threading).

3) I'm using .NET 1.1.

4) The _eventList object is referenced by the producer class (not
listed here), where Enqueue is called inside a SyncLock block.

MY PROBLEM:
Thing is, it seems that "_eventList.cou nt > 0" returns true, and still
is just before the dequeue call, but after the dequeue call it goes to
0, and "item" is NOTHING!!

I have no idea why this could be. Can anyone help!?!

Cheers,

<Shipcreak />



Full class follows:

Imports System.Text
Imports System.Threadin g

Public MustInherit Class LoggerBase

Private _name As String
Private _eventList As Queue
Private _thread As Thread
Private _stopRequested As Boolean

Public Sub New(ByVal pName As String)
_name = pName
End Sub

Public Sub AssignEventList (ByVal eventList As Queue)
If Not _eventList Is Nothing Then
SyncLock _eventList
_eventList = eventList
End SyncLock
Else
_eventList = eventList
End If
End Sub

Public Sub Start()

_stopRequested = False

_thread = New Thread(AddressO f StartWatching)

If Not _name Is Nothing AndAlso _name.Length > 0 Then
_thread.Name = _name
Else
_thread.Name = "Event Logging Thread"
End If

_thread.Start()

End Sub

Public Sub RequestStop()
_stopRequested = True
End Sub

Private Sub StartWatching()

While Not _stopRequested

' Lock the list
SyncLock _eventList.Sync Root

Dim item As ExceptionLogEnt ry

' If there are items, get the top one
If _eventList.Coun t > 0 Then

' Get the item from the queue
item = CType(_eventLis t.Dequeue, ExceptionLogEnt ry)

' If we got an item to log, log it
If Not item Is Nothing Then

WriteToLog(Buil dExceptionRepor t(item))

End If

End If

End SyncLock


' Wait for 1 sec to allow other threads to take their turn
Thread.CurrentT hread.Sleep(100 )

End While

End Sub

Private Function BuildExceptionR eport( _
ByVal ex As ExceptionLogEnt ry) As String

Dim ret As New StringBuilder
Dim e As ExceptionLogEnt ry = ex

If Not e Is Nothing Then

ret.AppendForma t("OS: {0}" & vbCrLf, ex.OSInfo)
ret.AppendForma t("Processor usage: {0}" & vbCrLf, _
ex.ProcessorUsa ge)
ret.AppendForma t("Working Set: {0}" & vbCrLf, _
ex.WorkingSet)
ret.AppendForma t("Free Memory: {0}" & vbCrLf, _
ex.FreeMemory)
ret.AppendForma t("OS UserName: {0}" & vbCrLf, _
ex.OSUserName)
ret.AppendForma t("Machine Name: {0}" & vbCrLf, _
ex.MachineName)
'ret.AppendForm at("Total HDD: {0}" & vbCrLf, _
ex.HDDTotal)
'ret.AppendForm at("Free HDD: {0}" & vbCrLf, _
ex.FreeHDD)
ret.AppendForma t("Occurred At: {0}" & vbCrLf, _
ex.Occurred)
ret.Append("Exc eption details:" & vbCrLf)

AppendException Details(ex.Caus ingException, ret)

WriteToLog(ret. ToString)

Else

WriteToLog("No exception data provided")

End If

End Function

Private Sub AppendException Details(ByVal ex As Exception, _
ByVal report As StringBuilder)

Dim e As Exception = e

Do While Not e Is Nothing

report.AppendFo rmat(" Message: {0}" & vbCrLf, _
e.Message)
report.AppendFo rmat(" Source: {0}" & vbCrLf, _
e.Source)
report.AppendFo rmat(" Method: {0}" & vbCrLf, _
e.TargetSite.Na me)
report.AppendFo rmat(" Help Link: {0}" & vbCrLf, _
e.HelpLink)
report.AppendFo rmat(" Stack Trace: {0}" & vbCrLf, _
e.StackTrace)
report.Append("------------------" & vbCrLf)

e = e.InnerExceptio n

Loop

End Sub

Protected MustOverride Sub WriteToLog(ByVa l item As String)
Protected MustOverride Sub WriteToLog(ByVa l item As String, _
ByVal type As EventLogEntryTy pe)

End Class

[Larry Lard]

Re: Thread Sync Queue Problem


shipcreak@gmail .com wrote:[color=blue]
> I have an interesting problem with a sort of producer-consumer system
> for error logging. Consider the following code:
>
> <code>
> SyncLock _eventList.Sync Root
>
> Dim item As ExceptionLogEnt ry
>
> ' If there are items, get the top one
> If _eventList.Coun t > 0 Then
>
> ' Get the item from the queue
> item = CType(_eventLis t.Dequeue, _
> ExceptionLogEnt ry)
>
> ' If we got an item to log, log it
> If Not item Is Nothing Then
>
> WriteToLog(Buil dExceptionRepor t(item))
>
> End If
>
> End If
>
> End SyncLock
> </code>[/color]

In my not-particularly-expert opinion, this looks OK.

[snip][color=blue]
> 4) The _eventList object is referenced by the producer class (not
> listed here), where Enqueue is called inside a SyncLock block.
>
> MY PROBLEM:
> Thing is, it seems that "_eventList.cou nt > 0" returns true, and still
> is just before the dequeue call, but after the dequeue call it goes to
> 0, and "item" is NOTHING!!
>
> I have no idea why this could be. Can anyone help!?![/color]

I think you're going to have to have a look at the enqueueing code, and
if you can't find the problem, post some of it. The key point which you
may need reminding of (from the docs, my emphasis):


Queue.Enqueue Method

Adds an object to the end of the Queue.

Public Overridable Sub Enqueue( _
ByVal obj As Object _
)

Parameters
obj
The object to add to the Queue. ****The value can be a null reference
(Nothing in Visual Basic). ****



My example code:

Sub Main()
Dim q As New Queue

q.Enqueue("appl e")
q.Enqueue("bana na")
q.Enqueue(Nothi ng)
q.Enqueue("pear ")

Dim o As Object
Do While q.Count > 0
o = q.Dequeue
' everything implements ToString, right?
Console.WriteLi ne(o.ToString)
' OOPS
Loop

Console.ReadLin e()
End Sub

--
Larry Lard
Replies to group please

[Barney]

Re: Thread Sync Queue Problem

Yeah, you're right. I worked out what it was yesterday afternoon
eventually.

I was adding the result of a call to a factory class method to the
queue, but the factory was returning Nothing.

Put null in, get null out. Simple.

Thanks for the reply, one's never guaranteed one in here, so many
people with so many problems an' all that. :-)

<Shipcreak />