从派生类型转换为基类型

Question

Given that

public abstract class AbstractOrder
{
    //some properties...
}

public class AnonymousOrder:AbstractOrder
{
    //some properties...
}


public class PartnerOrder:AbstractOrder
    {
        //some properties...
    }


public AbstractOrder FindOrderByConfirmationNumber(string confirmationNumber)
{
    ICriteria criteria =
        Session.CreateCriteria(typeof(AbstractOrder))
            .SetMaxResults(10)
            .AddOrder(Order.Desc("PurchasedDate"))
            .Add(Restrictions.Eq("ConfirmationNumber", confirmationNumber));
    var l = criteria.List<AbstractOrder>();
    AbstractOrder ao = l[0] as AbstractOrder;
    return ao as AbstractOrder;
}

Can someone please explain why

PartnerOrder order = repo.FindOrderByConfirmationNumber(confirmationNumber)

returns a type of AnonymousOrder and how I get it to return a type of PartnerOrder?

Answer 1

AbstractOrder is an Abstract class. No object can ONLY be an abstract class, it can inherit from it as a base class but it is always something else as well.

It's returning type AnonymousOrder which is the most specific class it is, but it also an AbstractOrder. You can treat it as just an AbstractOrder, nothing should be effected.

Answer 2

FindOrder is returning a type of AbstractOrder ; just look at its declaration. The object it returns may be an instance of AnonymousOrder , but all AnonymousOrder instances are also AbstractOrder instances, because AnonymousOrder inherits from AbstractOrder .

I think your real problem is trying to assign an AnonymousOrder instance to a PartnerOrder reference. This won't work, since neither type is derived from the other.

This, however, will work:

AnonymousOrder order = (AnonymousOrder)repo.FindOrderByConfirmationNumber(confirmationNumber);

Alternatively, you could test the type of the return value:

AbstractOrder order = repo.FindOrderByConfirmationNumber(confirmationNumber);
PartnerOrder partnerOrder = order as PartnerOrder;
if (partnerOrder != null)
    //...

If you really need a PartnerOrder instance for a confirmationNumber that returns an AnonymousOrder, then you'll need a method that takes an AnonymousOrder instance and returns a PartnerOrder instance. Or, perhaps, you should rethink your design.