public Task <bool> Delete(int orderId)
        {
            //soft delete by setting cancelled property
            bool found = OrderList.Any(o => (o.Id == orderId) && (!o.Cancelled));

            if (found)
            {
                OrderList.First(o => o.Id == orderId).Cancelled = true;
            }
            return(Task <bool> .FromResult(found));
        }