abstract class Gateway
    {
        Gateway[] required;

        bool IsOpen;

        public async Task<bool> Open()
        {
            var tasks = new List<Task>();
            foreach (var n in required)
            {
                if (!n.IsOpen)
                {
                    tasks.Add(n.Open());
                }
            }

            await Task.WhenAll(tasks);
            await SelfOpen();

            return IsOpen;
        }

        protected abstract Task SelfOpen();
    }