The ReadWriteLock allows multiple readers and a single writer. You are
obtaining a read lock which does not block other readers. A reader
holding a lock does not block another reader obtaining a lock. A reader
holding a lock will block a writer from obtaining a lock until the reader
is finished. A writer holding a lock will block both readers and writers
from obtaining a lock until the writer has released. Couple of other
things:
1) After calling one of the acquire methods, always check if you have
obtained the lock before proceeding since you are specifying a timeout.
2) After calling an acquire method you have to be certain to call the
release if you aquired the lock.
Here is a code sample of a safe way to aquire and release a writer lock:
try
{
readWriteLock.AcquireWriterLock(lockTimeout);
if (readWriteLock.IsWriterLockHeld)
{
// do your thing
}
else
{
// handle the timeout senario
}
}
finally
{
if (readWriteLock.IsWriterLockHeld)
readWriteLock.ReleaseWriterLock();
}
/Chad
On Wed, 29 Sep 2004 15:11:55 -0400, Jekke Bladt <[EMAIL PROTECTED]>
wrote:
>All--
>
>I'm having a problem with the ReadWriteLock object in .net and wondering
>if I'm misunderstanding the concept:
>
>The class I've written manages a log that stores messages in a queue and
>periodically flushes the queue to save the messages to a text file.
>
>In order to minimize blocking, I've written the class to copy the queue
>into an array, reset the queue, and release the reader lock. There is no
>place anywhere else that the queue is reinitialized or anything is
>dequeued.
>
>And yet, I am still having a problem where, during amsg = mQueue.ToArray
>is failing because the queue is empty. FileSave is never called with the
>queue empty.
>
>I suspect there's some conceptual error here, but I'll be damned if I
>can figure it out on my own. Can anyone help?
>
>TIA
>
>--Jekke
>
>
> Protected Sub FileSave()
> 'Lock all threads out of the queue while we copy it.
> mWRLQueue.AcquireReaderLock(miQueueReadWriteTimeout)
> 'Copy the message queue to an array.
> Dim amsg() As Object
> If mQueue.Count > 0 Then
> amsg = mQueue.ToArray()
> 'Clear the queue
> mQueue = New LogMessageQueue
> Else
> mWRLQueue.ReleaseReaderLock()
> Exit Sub
> End If
> mWRLQueue.ReleaseReaderLock()
>
> 'The queue is not touched after this point. Nothing to see below
>here...probably
>
>
>
>
>
> 'Saving the file is a critical section. Disc read/write must
> 'be done by exactly one thread at a time.
> SyncLock FileSLMessage
> 'Open or create the logfile
> Dim sr As StreamWriter = File.AppendText(msFileName)
>
> 'Write each message to the end of the logfile.
> Dim thisObj As Object
> Dim thisMsg As LogMessage
> For Each thisObj In amsg
> thisMsg = CType(thisObj, LogMessage)
> 'Only write messages that reach the required level
> 'of severity. This way, developers can change
>SeverityTolerance
> 'on later builds to get less verbose information.
> If thisMsg.Severity >= miSeverityTolerance Then
> sr.WriteLine(thisMsg.ToString())
> End If
> Next
>
> 'Save and close the file.
> sr.Flush()
> sr.Close()
> End SyncLock
>
> End Sub
>
>
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http://www.develop.com/courses/edotnet
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