An asynchronous counter which can be awaited on

Question

I have a connection class which has several async methods such as SendText, SendImage etc.

The connection class has a Disconnect method, and when it is called I have to be careful not to start changing the inner state of the class before all async methods have completed execution.

I believe a good way to achieve this is to simply keep a running total of the number of operations in execution, and then when I want to Disconnect I can simply set Disconnecting = true and then wait for the count to reach 0

I'm thinking of something along the lines of this

class ReferenceCounter
{
    void Increment();

    void Decrement();

    async Task WaitForCounterToReachZero();
}

Then when an async operation starts I could do

refCounter.Increment();

When it ends

refCounter.Decrement();

and inside the Disconnect method

disconnecting = true;
taskCancellationSource.Cancel();
await refCounter.WaitForCounterToReachZero();
Cleanup();

Are there any built in .NET classes like this?

Or more importantly for me, is there a better way of doing this?

If it was synchronous code it would be as simple as

lock (thisLock)
{
    while (counter > 0)
        Monitor.Wait(thisLock);
}

I just found the built in CountdownEvent class which does the same thing, but it has no async Wait method nor does it have any events, so I'd have to block.

Answer 1

Well, assuming you'll never increment again after you make it 0, you could do something like this:

public class Latch
{
    private int count = 0;
    private readonly TaskCompletionSource<object> tcs =
        new TaskCompletionSource<object>();

    public void Increment()
    {
        Interlocked.Increment(ref count);
    }

    public void Decrement()
    {
        if (Interlocked.Decrement(ref count) == 0)
        {
            tcs.TrySetValue(null);
        }
    }

    public Task Task { get { return tcs.Task; } }
}

Then you can await someLatch.Task. Alternatively, you could make the latch itself awaitable:

public TaskAwaiter GetAwaiter()
{
    return tcs.Task.GetAwaiter();
}

You should probably consider how you want to guard against the "count rises after getting down to 0" aspect thuogh - think about what you'd want it to do. (In the code above, once the TCS's value has been set, further awaits will complete immediately.)