Hi there,



Is there anyone that knows how to do the following?



I have a class A and a class B, that 100% inherits from class A (this means
that I don't have other code in class B, than the Inherit statement).



Now I want to make a Type Casting on an object of the type A, so that object
become a B type.



I have tryied it, but gets the error 'System.InvalidCastException: Specified
cast is not valid.', so want am I missing.



Please give a hint.



René

"René Nordby" <(E-Mail Removed)> wrote in message
news:%(E-Mail Removed)...

> I have a class A and a class B, that 100% inherits from class A (this
means
> that I don't have other code in class B, than the Inherit statement).
>
>
>
> Now I want to make a Type Casting on an object of the type A, so that
object
> become a B type.
>
>
>
> I have tryied it, but gets the error 'System.InvalidCastException:
Specified
> cast is not valid.', so want am I missing.

You're missing the fact that it can't be done. You can cast a derived class
into its base class, but you can't cast a base class to a derived class.
Someone correct me if I'm wrong.

Hi Jeff,

Thanks for your quick answer.

When you say so, then I thnink of String and Integer datatypes, where you
can cast from String to Integer and the other way around, so how is that
done?

I know that both String and Integer is derived from Object, so I have also
tryied to let my two classes inherit from a base class, but it give me the
same result.

"Jeff Johnson [MVP: VB]" <(E-Mail Removed)> wrote in message
news:%(E-Mail Removed)...
>
> "René Nordby" <(E-Mail Removed)> wrote in message
> news:%(E-Mail Removed)...
>
> > I have a class A and a class B, that 100% inherits from class A (this
> means
> > that I don't have other code in class B, than the Inherit statement).
> >
> >
> >
> > Now I want to make a Type Casting on an object of the type A, so that
> object
> > become a B type.
> >
> >
> >
> > I have tryied it, but gets the error 'System.InvalidCastException:
> Specified
> > cast is not valid.', so want am I missing.
>
> You're missing the fact that it can't be done. You can cast a derived
class
> into its base class, but you can't cast a base class to a derived class.
> Someone correct me if I'm wrong.
>
>

Hi Jeff,

> You're missing the fact that it can't be done. You can cast a derived
class
> into its base class, but you can't cast a base class to a derived class.
> Someone correct me if I'm wrong.

You can indeed cast a base class reference to a sub-class reference and the
operation would result in a 'narrowing' conversion. BUT the conversion will
succeed ONLY if the base class reference you are trying to cast was
originally obtained from an earlier widening conversion from sub-class to
base-class.

Could you please spell it out a little more?

René

"Nice Chap" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Hi Jeff,
>
> > You're missing the fact that it can't be done. You can cast a derived
> class
> > into its base class, but you can't cast a base class to a derived class.
> > Someone correct me if I'm wrong.
>
> You can indeed cast a base class reference to a sub-class reference and
the
> operation would result in a 'narrowing' conversion. BUT the conversion
will
> succeed ONLY if the base class reference you are trying to cast was
> originally obtained from an earlier widening conversion from sub-class to
> base-class.
>
>

René

Have a look at this page.

http://msdn.microsoft.com/library/de...directcast.asp

Maybe it makes some things clear.

Cor

"Nice Chap" <(E-Mail Removed)> wrote in
news:(E-Mail Removed):

> You can indeed cast a base class reference to a sub-class reference
> and the operation would result in a 'narrowing' conversion. BUT the
> conversion will succeed ONLY if the base class reference you are
> trying to cast was originally obtained from an earlier widening
> conversion from sub-class to base-class.

??? Can someone re-write this in clearer terms?

Thanks.

In article <Xns9535AB438E36Danonemooseyahoocom@140.99.99.130>, Anon-E-Moose wrote:
> "Nice Chap" <(E-Mail Removed)> wrote in
> news:(E-Mail Removed):
>
>> You can indeed cast a base class reference to a sub-class reference
>> and the operation would result in a 'narrowing' conversion. BUT the
>> conversion will succeed ONLY if the base class reference you are
>> trying to cast was originally obtained from an earlier widening
>> conversion from sub-class to base-class.
>
> ??? Can someone re-write this in clearer terms?
>
> Thanks.
>

What the op was saying was that, you can only cast a base class
reference to a subclass if that base class reference points to a
subclass instance.

In other words, casting a subclass to a base class is a narrowing
conversion, in that you loose access to some methods/properties of the
subclass since you'll be accessing it through the base class variable.
Casting a baseclass reference to a subclass would be a widening
conversion, since you would be picking up methods/properties. This is
not a safe operation, unless the base class reference actually points
to a subclass instance... Boy, this is confusing isn't it Here is
some psuedo code that may help...

class Base
end class

class SubClass
inherits base
end class

' this works
dim b as base = new subclass
dim s as subclass = ctype(b, subclass)

' and this works
dim s as subclass = new subclass
dim b as base = ctype(s, base)

' this doesn't
dim b as base = new base
dim s as subclass = ctype(b, subclass)


--
Tom Shelton [MVP]

I believe the framework uses TypeConverters to cast between Strings &
Integer and vice versa. Google TypeConverter to get more info.

hth,

Luhar

"René Nordby" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Hi Jeff,
>
> Thanks for your quick answer.
>
> When you say so, then I thnink of String and Integer datatypes, where you
> can cast from String to Integer and the other way around, so how is that
> done?
>
> I know that both String and Integer is derived from Object, so I have also
> tryied to let my two classes inherit from a base class, but it give me the
> same result.
>
> "Jeff Johnson [MVP: VB]" <(E-Mail Removed)> wrote in message
> news:%(E-Mail Removed)...
> >
> > "René Nordby" <(E-Mail Removed)> wrote in message
> > news:%(E-Mail Removed)...
> >
> > > I have a class A and a class B, that 100% inherits from class A (this
> > means
> > > that I don't have other code in class B, than the Inherit statement).
> > >
> > >
> > >
> > > Now I want to make a Type Casting on an object of the type A, so that
> > object
> > > become a B type.
> > >
> > >
> > >
> > > I have tryied it, but gets the error 'System.InvalidCastException:
> > Specified
> > > cast is not valid.', so want am I missing.
> >
> > You're missing the fact that it can't be done. You can cast a derived
> class
> > into its base class, but you can't cast a base class to a derived class.
> > Someone correct me if I'm wrong.
> >
> >
>
>

Rene, I think you should be able to do this. I think there is something
else going on. I have a project right now for example with a base class
LineItem and two classes deriving (inheriting) from it OrderItem and
ShipmentItem. I can use CTYPE() to perform any of the following
conversions:

OrderItem to LineItem
LineItem to OrderItem
ShipmentItem to LineItem
LineItem to ShipmentItem

The only thing I can not do is convert OrderItem to ShipmentItem, which
makes sense because they are not the same "thing". If you think about it,
OrderItem is a LineItem and ShipmentItem is a LineItem but OrderItem is not
a ShipmentItem.. You need to ask yourself what kind of real world objects
your Classes A and B represent and whether the conversion makes sense. Or
if you want to post you code we could look at it.

Good luck!
-Carl

"René Nordby" <(E-Mail Removed)> wrote in message
news:%(E-Mail Removed)...
> Hi there,
>
>
>
> Is there anyone that knows how to do the following?
>
>
>
> I have a class A and a class B, that 100% inherits from class A (this
means
> that I don't have other code in class B, than the Inherit statement).
>
>
>
> Now I want to make a Type Casting on an object of the type A, so that
object
> become a B type.
>
>
>
> I have tryied it, but gets the error 'System.InvalidCastException:
Specified
> cast is not valid.', so want am I missing.
>
>
>
> Please give a hint.
>
>
>
> René
>
>
>
>