What should I SyncLock in this code, and where? - vb.net

Question

I have a class that has two method in it, one calls a class which creates and executes a number of threads, the other is an event handler that handles an event raised when those threads complete (and then calls the first method again).
I understand that the method that handles the event runs in the thread that raised the event. So as such, I SyncLock a member variable that says how many threads are running and subtract one from it:
SyncLock Me ' GetType(me)
_availableThreads -= 1
End SyncLock
So I have a few questions:
Main Question: Should I be SyncLock'ing _availableThreads everywhere in the class - i.e in the method that creates the threads (which adds 1 when a thread is created)
Side Questions related to this question:
I'd usually SyncLock the current instance, but I've seen code that SyncLocks the type instead, so what is the difference between sync locking Me (Current Instance) and GetType(Me)?
Would there be a performance difference between the two? and is there anything smaller I'd be able to lock for the above that doesn't affect anything else - perhaps a separate 'padlock' object created for the sole purpose of locking things within a class?
Note: The sole purpose of _availableThreads is to control how many threads can run at any given time and the threads process jobs that can take hours to run.
Code:
Public Class QManager
Private _maxThreadCount, _availableThreads As Integer
Public Sub New(ByVal maxThreadCount As Integer)
Me.MaximumThreadCount = maxThreadCount
End Sub
Public Sub WorkThroughQueue()
//get jobs from queue (priorities change, so call this every time)
Dim jobQ As Queue(Of QdJobInfo) = QueueDAO.GetJobList
//loop job queue while there are jobs and we have threads available
While jobQ.Count > 0 And _availableThreads <= _maxThreadCount
//create threads for each queued job
Dim queuedJob As New QdJob(jobQ.Dequeue)
AddHandler queuedJob.ThreadComplete, AddressOf QueuedJob_ThreadCompleted
_availableThreads += 1 //use a thread up (do we need a sync lock here?)***************************
queuedJob.Process() //go process the job
End While
//when we get here, don't do anything else - when a job completes it will call this method again
End Sub
Private Sub QueuedJob_ThreadCompleted(ByVal sender As QdJobInfo, ByVal args As EventArgs)
SyncLock Me //GetType(me)
_availableThreads -= 1
End SyncLock
//regardless of how the job ended, we want to carry on going through the rest of the jobs
WorkThroughQueue()
End Sub
#Region "Properties"
Public Property MaximumThreadCount() As Integer
Get
Return _maxThreadCount
End Get
Set(ByVal value As Integer)
If value > Environment.ProcessorCount * 2 Then
_maxThreadCount = value
Else
value = Environment.ProcessorCount
End If
LogFacade.LogInfo(_logger, "Maximum Thread Count set to " & _maxThreadCount)
End Set
End Property
#End Region
End Class

Answer 1

You shouldn't SyncLock the instance or the type. You always want to SyncLock on a variable that is fully within the control of the class, and neither of those are. You should declare a private New Object and use that for your SyncLock.
Private lockObject as New Object()
...
SyncLock lockObject
...
End SyncLock

Answer 2

Unfortunately, you need to do a few things differently here.
First off, I'd recommend avoiding SyncLock, and using Interlocked.Increment and Interlocked.Decrement to handle changing _availableThreads. This will provide thread safety for that variable without a lock.
That being said, you still will need a SyncLock around every access to your Queue - if it's being used from multiple threads. An alternative, if you're using .NET 4, would be to change over to using the new ConcurrentQueue(Of T) class instead of Queue. If you use SyncLock, you should create a private object only accessible by your class, and use it for all synchronization.

Answer 3

You should be using the Interlocked class here, the Decrement() method to decrease the count. Yes, everywhere the variable is accessed.
Using SyncLock Me is as bad as SyncLock GetType(Me). You should always use a private object to lock on so nobody can accidentally cause a deadlock. The golden rule is that you cannot lock data, you can only block code from accessing data. Since the code is your private implementation detail, the object that holds the lock state must also be a private detail. Neither your object (Me) nor the Type of that object is private. Allowing other code to lock it by accident.

Answer 4

You can substitute the thread counter with Semaphore. If you use Semaphore you do not need to exit from while loop and neither it is necessary to call WorkThroughQueue() from ThreadCompleted event handler. Semaphore is thread safe so you can use it without locking.
http://www.albahari.com/threading/part2.aspx#_Semaphore