Casting Issue Correct Question! Sorry

[Daniel]

Hi guys who just answered me.....it really would have helped if i had
written it right.

Ok i will use better names to explain my problem.

I have this:

InterFaceClass
^
ClassA
^
^
ClassB
ClassC
So i have an interface class that ClassA inherits from. ClassB and C then
inherit from classA.

i then want to do this:

ClassB myObj = (ClassB)some_Cl assA_Instance;

And i get a invalid cast. Why?

[Peter Duniho]

"Daniel" <Da*****@vestry online.comwrote in message
news:%2******** ********@TK2MSF TNGP04.phx.gbl. ..

Then i am confused again.

My class that was i was casting down to inherited and did not have any new
methods, just one inherited method that was overidden by implementation?
Why couldnt i cast?

Did you read the other thread to which I pointed you? If you haven't, go
back and do that now. I don't know if it will answer your question, but if
you haven't tried, there's no point in anyone making a further effort to
help.

I don't think you understand the difference between the class itself and an
instance of that class. As I posted earlier, the question of whether an
inherited class has new methods, new fields, overridden methods, etc. is
entirely irrelevant to the question of casting. Once you create an instance
of a class, its type is determined. No amount of casting will change that.
If the instance was not originally created as the more-derived type, you
cannot later cast it to that more-derived type.

Pete

[Peter Duniho]

"Daniel" <Da*****@vestry online.comwrote in message
news:%2******** ********@TK2MSF TNGP04.phx.gbl. ..

Ok i understand now.

What i dont get is this.

GenericAnimal g = (GenericAnimal) Dog;

yet if you look at g's type it remains Dog?? Why has it not become a
generic type?

Did you read the other thread I pointed you to?

The instance doesn't change type. Only the type of the reference *to* that
instance has changed. Casting it to GenericAnimal doesn't change the
instance...it just restricts your view of the instance to those things
within a GenericAnimal.

Sometimes I wonder if teaching people OOP with C# is such a great idea.
Ironically, because things are more abstracted in comparison to C++, the
difference between an object and a reference to that object are lost. I
suspect that if people had to distinguish between an instance and a pointer
to that instance (as they do in C++), these questions wouldn't come up
nearly as often as they do.

Anyway, back to the question: "g" is not the instance. It's a reference to
the instance. Likewise, "Dog" is not the instance, it's a reference to the
instance (that is, let's ignore that you made the same error you made
previously, using the name of a class rather than the name of an instance of
that class :) ). The type of the reference affects what of the instance you
can look at, but it doesn't change the instance itself. When you create a
new Dog, you are creating an instance. The variables that reference that
instance define the type used to view the instance, but not the type of the
instance itself.

Thus, the instance always keeps its original type. You can always cast
references to that type back and forth, as long as they remain consistent
with the type of the actual instance. IMHO, it's a mistake to think of
casting as being "up-cast" or "down-cast". In all cases, the casting is
just providing a new type through which to view the instance. The type cast
to must always be a type within the inheritance tree of the instance, and
will always either be the type of the instance itself, or an ancestor of
that instance's type (that is, a base class). But no matter what happens,
the validity of the cast is relative to the original instance, not the type
being cast from.

Pete

[Tom Porterfield]

Daniel wrote:

Ok i understand now.

What i dont get is this.

GenericAnimal g = (GenericAnimal) Dog;

yet if you look at g's type it remains Dog?? Why has it not become a
generic type?

It is its generic, but it is also a Dog. So you could use g wherever a
GenericAnimal is called for and wherever a Dog is called for. But just
because you are treating it as a GenericAnimal doesn't mean it isn't still a
Dog. A Dog is a GenericAnimal, and this specific instance of a
GenericAnimal is a Dog, but not all GenericAnimals are Dogs.
--
Tom Porterfield

[Jon Skeet [C# MVP]]

Daniel <Da*****@vestry online.comwrote :

Then i am confused again.

My class that was i was casting down to inherited and did not have any new
methods, just one inherited method that was overidden by implementation? Why
couldnt i cast?

Just because it doesn't have any new methods or fields doesn't mean you
can cast from one form to the other. It's not the right type, that's
all there is to it.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Daniel]

Ok great

Thanks guys i fully understand now.

i am one of these people that doesn't like a 'vague' understanding, i keep
asking questions until i feel i understand 100% which i do now. I think it
is my c++ background haunting me and it has taken a while to adjust to the
no pointer scenario.

Thanks for all the replies and patience.
"Tom Porterfield" <tp******@mvps. orgwrote in message
news:%2******** ********@TK2MSF TNGP03.phx.gbl. ..

Daniel wrote:

>Ok i understand now.

What i dont get is this.

GenericAnima l g = (GenericAnimal) Dog;

yet if you look at g's type it remains Dog?? Why has it not become a
generic type?

It is its generic, but it is also a Dog. So you could use g wherever a
GenericAnimal is called for and wherever a Dog is called for. But just
because you are treating it as a GenericAnimal doesn't mean it isn't still
a Dog. A Dog is a GenericAnimal, and this specific instance of a
GenericAnimal is a Dog, but not all GenericAnimals are Dogs.
--
Tom Porterfield