[RESOLVED] [2005] Threading and Synchronization

[GottaGetITDone]

Top of the morning to you all!

I am just having a little trouble understanding thread synchronization. Allow me to demonstrate.

I have a class that exposes 3 methods like so:

     Class a

        Private x As Collection

        Public Sub Add()

        End Sub

        Public Sub Remove()

        End Sub

        Public Sub Print()
            For counter As Integer = 0 To x.Count-1 Step 1

            Next counter
        End Sub
    End Class

Now I have declared an instance of 'class a' and multiple threads are able to perform actions upon it.

My problem comes when one thread is running a print operation and another may be removing an item from the collection. The print operation is printing the data in a loop, when the loop began 'counter' was set to the count of the collection. The collection's size has since changed and we are headed for an "index out of bounds" error.

I need to lock the instance of the class as a whole so that only one thread can perform operations on the instance at any one time.

I know about the 'SyncLock' feature but am unsure if this is sufficient. Is the answer just to synclock the calls to the class methods? E.G...

Dim objSync as Object

SyncLock objSync
   instance.Print()
End SyncLock

Any pointers or leads would be greatly appreciated,
Have a great morning!
Matt.

[jmcilhinney]

It would be more fitting to handle the synchronisation within ClassA, e.g.:

Public Class ClassA

    Private items As ArrayList

    Public Sub Add()
        SyncLock items.SyncRoot
            'Add item here.
        End SyncLock
    End Sub

    Public Sub Remove()
        SyncLock items.SyncRoot
            'Remove item here.
        End SyncLock
    End Sub

    Public Sub Print()
        SyncLock items.SyncRoot
            For index As Integer = 0 To Me.items.Count - 1 Step 1

            Next index
        End SyncLock
    End Sub

End Class 

You now have three linked critical sections. Only one thread can enter any one of the critical sections at a time. If a thread enters the Print method and enters the SyncLock block, then another thread enters the Add or Remove method, the second thread will have to wait at the SyncLock statement until the first thread completes its job and exits the SyncLock block in the Print method. It's like having a house with multiple doors and rooms and a sensor that can tell when someone is inside. It doesn't matter which door someone enters or which room they're in, as long as someone is inside all entrances remain locked until they exit.

[GottaGetITDone]

PERFECT! Absolutely <expletive> PERFECT!!!

Damn JM your onto it mate! That is EXACTLY what I was looking for.

Using that theory, I could declare a member variable of type Object, instantiate it in the constructor and lock my critical sections using the same object each time? By using the same object instance I will create the desired effect, basically copy the SyncRoot feature. Yeah?

I was not properly conceptualizing the function of the lock object in the SyncLock statement...all clear now though.

Thanks AGAIN JM, you always seem to sort us out!
Matt.

[jmcilhinney]

Are you trying to create your own strongly-typed collection or just a class that contains a collection?