Do I have to double check before and after blocking a list?

I have a service in the application that allows me to send her messages from different sources, which will be placed in a simple list. A service running in its stream will periodically process all messages in the list into different files; one file for each source, which is then managed for size.

My question is about the correct way to check messages and execute code blocking that accesses the list. Access to the list is possible only in two places; one is where the message is added to the list, and the other is where the messages are downloaded from the list to the processing list.

Adding a message to the list:

Public Sub WriteMessage(ByVal messageProvider As IEventLogMessageProvider, ByVal logLevel As EventLogLevel, ByVal message As String)
    SyncLock _SyncLockObject
        _LogMessages.Add(New EventLogMessage(messageProvider, logLevel, Now, message))
    End SyncLock
End Sub

List Processing:

Dim localList As New List(Of EventLogMessage)
SyncLock _SyncLockObject
    If (_LogMessages.Count > 0) Then
        localList.AddRange(_LogMessages)
        _LogMessages.Clear()
    End If
End SyncLock

' process list into files...

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Dim localList As New List(Of EventLogMessage)
If (_LogMessages.Count > 0) Then
    SyncLock _SyncLockObject
        If (_LogMessages.Count > 0) Then
            localList.AddRange(_LogMessages)
            _LogMessages.Clear()
        End If
    End SyncLock
End If

' process list into files...

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Private _ReadWriteLock As New Threading.ReaderWriterLockSlim()

Private Sub Process()
    ' create local processing list
    Dim processList As New List(Of String)
    Try
        ' enter read lock mode
        _ReadWriteLock.EnterUpgradeableReadLock()
        ' if there are any messages in the 'global' list
        ' then dump them into the local processing list
        If (_Messages.Count > 0) Then
            Try
                ' upgrade to a write lock to prevent others from writing to
                ' the 'global' list while this reads and clears the 'global' list
                _ReadWriteLock.EnterWriteLock()
                processList.AddRange(_Messages)
                _Messages.Clear()
            Finally
                ' alway release the write lock
                _ReadWriteLock.ExitWriteLock()
            End Try
        End If
    Finally
        ' always release the read lock
        _ReadWriteLock.ExitUpgradeableReadLock()
    End Try
    ' if any messages were dumped into the local processing list, process them
    If (processList.Count > 0) Then
        ProcessMessages(processList)
    End If
End Sub

Private Sub AddMessage(ByVal message As String)
    Try
        ' enter write lock mode
        _ReadWriteLock.EnterWriteLock()
        _Messages.Add(message)
    Finally
        ' always release the write lock
        _ReadWriteLock.ExitWriteLock()
    End Try
End Sub

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Private _Messages As New System.Collections.Concurrent.BlockingCollection(Of String)

Private Sub Process()
    ' process each message in the list
    For Each item In _Messages
        ProcessMessage(_Messages.Take())
    Next
End Sub

Private Sub AddMessage(ByVal message As String)
    ' add a message to the 'global' list
    _Messages.Add(message)
End Sub

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