Semaphore Counts

Question

In .NET, is there a way to get the count on a semaphore. I don't need a threadsafe count, just a reasonable approximation so report the status on a GUI thread.

Currently I'm using a dual-counter. The real semaphore and another variable that is incremented and decremented in sync, but this is annoying and error prone.

Answer 1

No. To do this you would have to wrap (subclass) Semaphore into something with a counter. Since you don't care about thread safety it won't be that hard.

Answer 2

Instead of the Semaphore, you could use Interlocked.Increment and Interlocked.Decrement. This will allow you to read the current value easily, since it's a regular variable.

If you use it in conjunction with some kind of Mutex for the situation where the count reaches 0, that should get the job done. Inheriting from your chosen mutex or WaitHandle will get you a nice reusable component.

See the Interlocked class.

Answer 3

Process Explorer does it, somehow. It probably involves deep windows API magic, because it extracts the maximum count as well as the current count.

You can also try the moral equivalent of

int old_count=0;
if (AcquireSemaphore(timeout=0)) {
  ReleaseSemaphore(&old_count);
  old_count += 1;
}

but this has the caveat that it is lightly intrusive to code that uses the semaphore.