Question

I have a Visual Studio 2008 C# .NET 3.5 project where a class listens for an event invocation from another class that is multithreaded. I need to ensure that my event only allows simultaneous access to a maximum of 10 threads. The 11th thread should block until one of the 10 finishes.

myobj.SomeEvent += OnSomeEvent;

private void OnSomeEvent(object sender, MyEventArgs args)
{
    // allow up to 10 threads simultaneous access. Block the 11th thread.
    using (SomeThreadLock lock = new SomeThreadLock(10))
    {         
        DoUsefulThings(args.foo);
    }
}

I do not have control over the other MyObj class, so I cannot implement a threadpool there.

What is the best way to implement this?

Thanks,
PaulH

Answer 1

You want the Semaphore class. It is, in short, a lock that only allows a specified number of callers through at any time.

Since you don’t control the creation of threads, you do need to be careful about deadlock situations. Semaphores are not reentrancy-aware- if a given thread enters a semaphore more than once, it will take more than one slot. So if each of your caller’s threads enters your semaphore more than once, there is the possibility of a deadlock.