Awaitable AutoResetEvent

If you want to build your own, Stephen Toub has the definitive blog post on the subject.

If you want to use one that's already written, I have one in my AsyncEx library. AFAIK, there's no other option as of the time of this writing.

I think there is good example on MSDN: https://msdn.microsoft.com/en-us/library/hh873178%28v=vs.110%29.aspx#WHToTap

public static Task WaitOneAsync(this WaitHandle waitHandle)
{
    if (waitHandle == null) 
        throw new ArgumentNullException("waitHandle");

    var tcs = new TaskCompletionSource<bool>();
    var rwh = ThreadPool.RegisterWaitForSingleObject(waitHandle, 
        delegate { tcs.TrySetResult(true); }, null, -1, true);
    var t = tcs.Task;
    t.ContinueWith( (antecedent) => rwh.Unregister(null));
    return t;
}

Here's the source for Stephen Toub's AsyncAutoResetEvent, in case his blog goes offline.

public class AsyncAutoResetEvent
{
    private static readonly Task s_completed = Task.FromResult(true);
    private readonly Queue<TaskCompletionSource<bool>> m_waits = new Queue<TaskCompletionSource<bool>>();
    private bool m_signaled;

    public Task WaitAsync()
    {
        lock (m_waits)
        {
            if (m_signaled)
            {
                m_signaled = false;
                return s_completed;
            }
            else
            {
                var tcs = new TaskCompletionSource<bool>();
                m_waits.Enqueue(tcs);
                return tcs.Task;
            }
        }
    }

    public void Set()
    {
        TaskCompletionSource<bool> toRelease = null;

        lock (m_waits)
        {
            if (m_waits.Count > 0)
                toRelease = m_waits.Dequeue();
            else if (!m_signaled)
                m_signaled = true;
        }

        toRelease?.SetResult(true);
    }
}