Cross-process read-write synchronization primative in .NET?

Question

Is there a read/write locking mechanism that works across processes (similar to Mutex, but read/write instead exclusive locking)? I would like to allow concurrent read access, but exclusive write access.

Answer 1

No. As Richard noted above, there is no such out of the box mechanism in .NET. This is how to implement it using a mutex and a semaphore.

Method #1 is described in http://www.joecheng.com/blog/entries/Writinganinter-processRea.html, quoting:

// create or open global mutex
GlobalMutex mutex = new GlobalMutex("IdOfProtectedResource.Mutex");
// create or open global semaphore
int MoreThanMaxNumberOfReadersEver = 100;

GlobalSemaphore semaphore = new GlobalSemaphore("IdOfProtectedResource.Semaphore", MoreThanMaxNumberOfReadersEver);

public void AcquireReadLock()
{
  mutex.Acquire();
  semaphore.Acquire();
  mutex.Release();
}

public void ReleaseReadLock()
{
  semaphore.Release();
}

public void AcquireWriteLock()
{
  mutex.Acquire();
  for (int i = 0; i < MoreThanMaxNumberOfReadersEver; i++)
    semaphore.Acquire(); // drain out all readers-in-progress
  mutex.Release();
}

public void ReleaseWriteLock()
{
  for (int i = 0; i < MoreThanMaxNumberOfReadersEver; i++)
    semaphore.Release();
}

An alternative would be:

Read locking - as above. Write locking as follows (pseudocode):

- Lock mutex
- Busy loop until the samaphore is not taken AT ALL:
-- wait, release.
-- Release returns value; 
-- if value N-1 then break loop.
-- yield (give up CPU cycle) by using Sleep(1) or alternative
- Do write
- Release mutex

It must be noted that more efficient approach is possible, as here: http://en.wikipedia.org/wiki/Readers-writers_problem#The_second_readers-writers_problem Look for the words "This solution is suboptimal" in the article above.