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copied!<p>There would be three things I, as an interviewer, would look for in the answer.</p> <ul> <li>Knowledge of how ORM frameworks handle lazy loading</li> <li>Acknowledgement that you wouldn't implement this in production without considering an existing framework.</li> <li>The actual design of a "home-grown" solution.</li> </ul> <p>This is how I would answer this question.</p> <p>Lazy loading a common piece of functionality implemented by ORM frameworks like NHibernate or Entity Framework. My first attempt would be use one of these frameworks(point 1)</p> <p>If an ORM framework is out of the question I would implement lazy loading using an interception framework like Castle Interception or Unity Interception.(point 2).</p> <p>To implement lazy loading from scratch I would use a Decorator pattern (bonus point), subclass the Order class and define lazy loading behaviour there. Here is design showing relevant properties:</p> <pre><code>class Order { public int ID {get;set;} public virtual IList<OrderLine> Lines{get;set;} } interface IOrderRepository { Order GetOrder(id); IList<OrderLine> GetOrderLines(Order order); } class OrderService { IOrderRepository _repository; public OrderService(IOrderRepository repository) { _repository = repository; } public Order GetOrder(int id) { return new OrderDecorator(_repository.GetOrder(id)); } } public class OrderDecorator : Order { public OrderDecorator (IOrderRepository repository) { _OrderLines = new Lazy<IList<OrderLine>>(()=>repository.GetOrderLines(this)); } Lazy<IList<OrderLine>> _OrderLines; public override IList<OrderLine> Lines { get { if (base.OrderLines == null) base.OrderLines = _OrderLines.Value; return base.OrderLines; } set {base.OrderLines=value;} } } </code></pre> <p>And here is the version without the <code>Lazy<></code> fanciness. After editing to make sure the code strictly conforms to the decorator pattern, the lazy version doesn't actually add anything, so I'd just go with the below version.</p> <pre><code>public class OrderDecorator : Order { public OrderDecorator (IOrderRepository repository) { _Repo = repository; } IOrderRepository _Repo; public override IList<OrderLine> Lines { get { if (base.OrderLines == null) base.OrderLines = repository.GetOrderLines(this); return base.OrderLines; } set {base.OrderLines=value;} } } </code></pre> <p><strong>UPDATE</strong> Here is the project layout. There was a comment about coupling the Order and the repository. It actually isn't the case as the repository is coupled to the decorator, not not the order.</p> <ul> <li>object collection project <ul> <li>Order</li> <li>LineItem</li> </ul></li> <li>Service Layer <ul> <li>OrderService</li> </ul></li> <li>Repository Layer <ul> <li>OrderRepository</li> <li><strong>OrderDecorator</strong></li> </ul></li> </ul>

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