AsyncCancelEventArgs

public class AsyncCancelEventArgs : CancelEventArgs
    {
        public AsyncCancelEventDeferral Deferral;

        public AsyncCancelEventDeferral GetDeferral()
        {
            Deferral = new AsyncCancelEventDeferral();

            return Deferral;
        }
    }

    public class AsyncCancelEventDeferral : TaskCompletionSource<bool>
    {
        public void Complete()
        {
            SetResult(true);
        }
    }

    public static class AsyncCancelExtensions
    {
        public static async Task<bool> InvokeAsync<T>(this EventHandler<T> @event, object sender, T args) where T : AsyncCancelEventArgs
        {
            if (@event != null)
            {
                @event.DynamicInvoke(new object[] { sender, args });

                if (args.Deferral != null)
                {
                    await args.Deferral.Task;
                }

                return args.Cancel;
            }

            return false;
        }
    }