SyncLock on List in Multithread still throws ArgumentException

Question

I need to read from a list on a (background) thread while it may be updated from another (main thread). So I try to just make a temporary list not to access the original object. As updates may occur on multiple places, it would be convenient to place SyncLock on the read logic. Is that inherently wrong? What are my options for locking this correctly, or other ways to acquire an accessible copy of the list in a multithread condition?

 ' In Main thread: 
 Public SomeList = New List(Of SomeClass) 
 ' ..edit list


 ' In other thread: 
  Dim tempList As List(Of SomeClass)
  SyncLock SomeList
      tempList = SomeList.ToList
  End SyncLock

SomeList.ToList throws:

ArgumentException, Destination array was not long enough. Check destIndex and length, and the array's lower bounds.

score 1 · Accepted Answer · answered Jul 28 '12 at 13:25

1

After reviewing .ToList and thus New List(Of SomeClass)(.), in Reflector, the exception must be coming from is2.CopyTo(Me._items, 0) where Me._items has just been set to New T(count - 1) {}.

This means the number of items in the input collection (cast to ICollection(Of T) in is2) must have increased after is2.Count was retrieved.

As such, I re-ask the assumption in my now deleted answer: Do all places in the Main thread, in ' ..edit list, also use SyncLock SomeList when modifying the list?

answered Jul 28 '12 at 13:25

Mark Hurd

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You are correct, I was not properly educated on the use of SynckLock when asking this question, so it's now clear the list was modified by other thread without a lock, as you said. Thanks for the update! – bretddog Jul 28 '12 at 20:37

score 0 · Answer 2 · answered Jul 25 '12 at 08:33

0

Not sure what the equivalent is in VB, but can't you do something like this?

IList<string> roDinosaurs = dinosaurs.AsReadOnly();

Or

tempList.AddRange(SomeList);

answered Jul 25 '12 at 08:33

M Afifi

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Thanks! Maybe this works, at least so far, but was looking to understand also why my use of synclock is faulty. – bretddog Jul 25 '12 at 10:31
@bretddog I don't believe it is though. Its the method that's throwing the error rather than the SyncLock, I don't think they are related. (It certainly wouldn't be with C# and lock). – M Afifi Jul 25 '12 at 10:43

SyncLock on List in Multithread still throws ArgumentException

2 Answers2