Question

VB.NET 2010, .NET 4

Hello all,

My question is, imagine I have two List(Of T) objects and a subroutine in a multithreaded environment that modifies both of these objects. I don’t understand locks very well, so I’m unsure whether I can just do:

SyncLock CType(List1, IList).SyncRoot

  List1.Clear()
  List2.Clear()

End SyncLock

or must I:

SyncLock CType(List1, IList).SyncRoot
  SyncLock CType(List2, IList).SyncRoot

    List1.Clear()
    List2.Clear()

  End SyncLock
End SyncLock

? Any insight? Am I even on the right track? Any comments will be appreciated.

Thanks a lot in advance, Brian

Answer 1

First, it is bad practice to lock on a non-private object, as something else might lock on it, and then things go downhill from there, instead lock on a private member object, something like

Class Class1
    Private lockObject as New Object

    Public Sub Clear()
        SyncLock lockObject
           ...
        End Synclock
    End Sub
End Class

Now, for the actual question: Unless every operation you do modifies both lists (doubtful), you should have one lock per list. While possible to use one lock object to mean “I’m doing something with a list”, it doesn’t make much sense to block a method in another thread that isn’t operating on the same list as yours, and will just slow down everything.

So, in short: Use one lock object per set of locks (Locks for operating on list1, locks for list 2, etc). It’s slightly more code for sections that operate on both lists, but the code will be more performant.

Also, as a general note: Hold the lock for as little time as possible. The less time you spent in a lock, the less chance another thread will come along and be blocked until you’re done.

The MSDN Page on SyncLock might also be worth a read, it contains this information and a few examples. And @BasicLife is correct, always make sure to take the locks in the same order everywhere.

And a general rule throughout the framework, unless you are accessing a static member on a class, unless otherwise stated, it is not thread safe. So when operating on lists, you will want to lock when you add, remove, clear or enumerate over the list, I’m sure there’s others, but those are the most common.