mpen mpen - 24 days ago 9
C# Question

.NET Reverse Semaphore?

Perhaps it's too late at night, but I can't think of a nice way to do this.

I've started a bunch of asynchronous downloads, and I want to wait until they all complete before the program terminates. This leads me to believe I should increment something when a download starts, and decrement it when it finishes. But then how do I wait until the count is 0 again?

Semaphores sort of work in the opposite way in that you block when there are no resources available, not when they're all available (blocks when count is 0, rather than non-zero).




Based on the suggestions here, this is what I came up with. In addition to waiting until the count is 0, it will sleep if you spawn too many threads (count > max). Warning: This is not fully tested.

public class ThreadCounter
{
#region Variables
private int currentCount, maxCount;
private ManualResetEvent eqZeroEvent;
private object instanceLock = new object();
#endregion

#region Properties
public int CurrentCount
{
get
{
return currentCount;
}
set
{
lock (instanceLock)
{
currentCount = value;
AdjustZeroEvent();
AdjustMaxEvent();
}
}
}

public int MaxCount
{
get
{
return maxCount;
}
set
{
lock (instanceLock)
{
maxCount = value;
AdjustMaxEvent();
}
}
}
#endregion

#region Constructors
public ThreadCounter() : this(0) { }
public ThreadCounter(int initialCount) : this(initialCount, int.MaxValue) { }
public ThreadCounter(int initialCount, int maximumCount)
{
currentCount = initialCount;
maxCount = maximumCount;
eqZeroEvent = currentCount == 0 ? new ManualResetEvent(true) : new ManualResetEvent(false);
}
#endregion

#region Public Methods
public void Increment()
{
++CurrentCount;
}

public void Decrement()
{
--CurrentCount;
}

public void WaitUntilZero()
{
eqZeroEvent.WaitOne();
}
#endregion

#region Private Methods
private void AdjustZeroEvent()
{
if (currentCount == 0) eqZeroEvent.Set();
else eqZeroEvent.Reset();
}

private void AdjustMaxEvent()
{
if (currentCount <= maxCount) Monitor.Pulse(instanceLock);
else do { Monitor.Wait(instanceLock); } while (currentCount > maxCount);
}
#endregion
}

Answer

Check out the CountdownLatch class in this magazine article.