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

Windows does not include a cross process Reader-Writer lock. A combination of Semaphore and Mutex could be used to construct ones (the Mutex is held by a writer for exclusive access or by a Reader which then uses the Semaphore to release other readers—i.e. writers would wait on just the mutex and readers for either).

However, if contention is expected to be low (i.e. no thread holds a lock for long) then mutual exclusion may still be faster: the additional complexity of the reader-writer lock overwhelms any benefit of allowing multiple readers in. (A reader-writer lock will only be faster if there are many more readers and locks are held for significant time—but only your profiling can confirm this.)

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.