Monitor.TryEnter Advice

Question

Hello,

We are using Parallel Extensions in one part of our vb.net app to retrieve an datatable from a dictonary(of string, datatable). In the method to retrive the table, we are using Monitor.TryEnter. Sometimes, we get the error "Object synchronization method was called from an unsynchronized block of code." Here is what our method looks like:

        Try
           if Monitor.TryEnter(_ProductCollection, 200) = true then
              _ProductCollection.TryGetValue(name, ReturnValue)
            end if
        Finally
            Monitor.Exit(_ProductCollection)
        End Try

Should I try to implement a loop to make sure we obtain the lock, before trying to exit it? I am thinking the error is being thrown because I am trying to do the monitor.exit even if the monitor.tryenter is false.

Answer 1

The error is being thrown from the Monitor.Exit call. What is happening is that TryEnter timesout occasionally and the lock is not acquired, but Monitor.Exit is always called since it is the finally block. That is the problem. Here is how you can fix it.

Dim acquired As Boolean = False
Try
  acquired = Monitor.TryEnter(_ProductionCollection, 200)
  If acquired Then
    ' Do your stuff here.
  End If
Finally
  If acquired Then
    Monitor.Exit(_ProductionCollection)
  End If
End Try