Tutor.com Classroom How-To #5: providing automatic reader-to-writer lock upgrade

These How-To tips are taken from The Tutor.com Classroom: Architecture and techniques using Silverlight, WPF, and the Microsoft .NET Framework.

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Create a class that contains a module-level System.Threading.ReaderWriterLock (e.g. m_Lock) and wrap the AcquireReaderLock() function. Then create an AcquireWriterLock() function that returns a LockCookie wrapper (SafeLockCookie), checks to see if the read lock is held, and upgrades if necessary, and a ReleaseWriterLock()function that processes the SafeLockCookie and releases the lock or downgrades back to read:

public SafeLockCookie AcquireWriterLock()
{
  //if we have a read lock, upgrade
  if (m_Lock.IsReaderLockHeld)
  {
    return new SafeLockCookie(m_Lock.UpgradeToWriterLock(TIMEOUT_MS));
  }
  else
  {
    m_Lock.AcquireWriterLock(TIMEOUT_MS);
  return null;
  }
}

public void ReleaseWriterLock(SafeLockCookie SafeLockCookie)
{
  //do we need to downgrade?
  if (SafeLockCookie != null)
  {
    m_Lock.DowngradeFromWriterLock(ref SafeLockCookie.LockCookie);
  }
  else
  {
    m_Lock.ReleaseWriterLock();
  }
}

Then have calling functions request write locks by storing a SafeLockCookie when requesting the lock and returning it when releasing:

SafeLockCookie lc = m_Lock.AcquireWriterLock();
try
{
  ...
}
finally
{
  m_Lock.ReleaseWriterLock(lc);
}