Is it really downcasting not possible? It is working fine for me

问题

I know there are already some questions posted related to this same topic, but I have seen different answers so I am quite confused on which answer is correct.

On the below link, people mentioned that downcasting is not possible Convert/Cast base type to Derived type

While on this link below, people mentioned that downcasting is possible only if the derived class is an instance of the base class downcast and upcast

I did a little experiment and implemented the downcasting on a mini project(C#) using direct casting, for example: DerivedClass dc = (DerivedClass) baseClass without checking whether or not DerivedClass is an instance of BaseClass, and to my surprise, it is working fine with no errors at all, so why people said downcasting is not possible?

So my question is, is downcasting really not possible? Why people seem to have different answers on this topic? after-all which one is the REAL answer? Could someone please explain this in more detail?

Sample code:

public class Order {  }

public class PurchaseOrder : Order { // some new properties here for PurchaseOrder}

public static PurchaseOrder GetOrder()
{
    Order order = new Order();
    return (PurchaseOrder)order;
}

回答1:

Downcasting isn't possible when the object isn't actually an instance of the type you're trying to convert to - if it's just an instance of the base class.

It is possible when it's an instance of the derived class already¹.

For example, this is fine:

object x = "hello";
string y = (string) x;

Here the value of x is a reference to an instance of string, so downcasting to string is fine.

This is not fine:

object x = new object();
string y = (string) x; // Bang!

Here the value of x is a reference to an instance of object, so the cast fails.

EDIT: The code you've now included in the question is like my second example - you're creating an instance of just Order and trying to cast it to PurchaseOrder. That won't work - it isn't a PurchaseOrder. If you change it like this, however:

public static PurchaseOrder GetOrder()
{
    Order order = new PurchaseOrder();
    return (PurchaseOrder)order;
}

... it's fine. (It's odd code, but it works.)

See "Casting and type conversions" in MSDN for more information.

¹ Or if you're trying to convert to some other interface or class in the object's actual type's hierarchy. For example, this is okay too:

object x = new MemoryStream();
Stream y = (Stream) x; // Stream is in the type hierarchy of MemoryStream

回答2:

Downcasting is possible if you are sure about your type, this works fine:

class Base {}
class Derived : Base {}

Base b = new Derived();
Derived d = (Derived) b;

This does not work:

class Other : Base {}

Base b = new Other();
Derived d = (Derived) b; // InvalidCastException

来源：https://stackoverflow.com/questions/17956475/is-it-really-downcasting-not-possible-it-is-working-fine-for-me

标签

oop

inheritance

downcast