Type Casting

[René Nordby]

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.Invalid CastException: Specified
cast is not valid.', so want am I missing.



Please give a hint.



René

[Jeff Johnson [MVP: VB]]

Re: Type Casting


"René Nordby" <rene@bluedot.d k> wrote in message
news:%23qOvulZd EHA.1356@TK2MSF TNGP09.phx.gbl. ..
[color=blue]
> I have a class A and a class B, that 100% inherits from class A (this[/color]
means[color=blue]
> 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[/color]
object[color=blue]
> become a B type.
>
>
>
> I have tryied it, but gets the error 'System.Invalid CastException:[/color]
Specified[color=blue]
> cast is not valid.', so want am I missing.[/color]

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.

[René Nordby]

Re: Type Casting

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]" <i.get@enough.s pam> wrote in message
news:%23FoWosZd EHA.2752@TK2MSF TNGP12.phx.gbl. ..[color=blue]
>
> "René Nordby" <rene@bluedot.d k> wrote in message
> news:%23qOvulZd EHA.1356@TK2MSF TNGP09.phx.gbl. ..
>[color=green]
> > I have a class A and a class B, that 100% inherits from class A (this[/color]
> means[color=green]
> > 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[/color]
> object[color=green]
> > become a B type.
> >
> >
> >
> > I have tryied it, but gets the error 'System.Invalid CastException:[/color]
> Specified[color=green]
> > cast is not valid.', so want am I missing.[/color]
>
> You're missing the fact that it can't be done. You can cast a derived[/color]
class[color=blue]
> into its base class, but you can't cast a base class to a derived class.
> Someone correct me if I'm wrong.
>
>[/color]

[Nice Chap]

Re: Type Casting

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

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 Nordby]

Re: Type Casting

Could you please spell it out a little more?

René

"Nice Chap" <NiceChap@Plasm aDyne.com> wrote in message
news:uk8cA4ZdEH A.4048@TK2MSFTN GP12.phx.gbl...[color=blue]
> Hi Jeff,
>[color=green]
> > You're missing the fact that it can't be done. You can cast a derived[/color]
> class[color=green]
> > into its base class, but you can't cast a base class to a derived class.
> > Someone correct me if I'm wrong.[/color]
>
> You can indeed cast a base class reference to a sub-class reference and[/color]
the[color=blue]
> operation would result in a 'narrowing' conversion. BUT the conversion[/color]
will[color=blue]
> 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.
>
>[/color]

[Cor Ligthert]

Re: Type Casting

René

Have a look at this page.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vblr7/html/vakeydirectcast.asp

Maybe it makes some things clear.

Cor

[Anon-E-Moose]

Re: Type Casting

"Nice Chap" <NiceChap@Plasm aDyne.com> wrote in
news:uk8cA4ZdEH A.4048@TK2MSFTN GP12.phx.gbl:
[color=blue]
> 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.[/color]

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

Thanks.

[Tom Shelton]

Re: Type Casting

In article <Xns9535AB438E3 6Danonemooseyah oocom@140.99.99 .130>, Anon-E-Moose wrote:[color=blue]
> "Nice Chap" <NiceChap@Plasm aDyne.com> wrote in
> news:uk8cA4ZdEH A.4048@TK2MSFTN GP12.phx.gbl:
>[color=green]
>> 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.[/color]
>
> ??? Can someone re-write this in clearer terms?
>
> Thanks.
>[/color]

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]

[Luhar]

Re: Type Casting

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

hth,

Luhar

"René Nordby" <rene@bluedot.d k> wrote in message
news:OFjuJzZdEH A.1048@tk2msftn gp13.phx.gbl...[color=blue]
> 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]" <i.get@enough.s pam> wrote in message
> news:%23FoWosZd EHA.2752@TK2MSF TNGP12.phx.gbl. ..[color=green]
> >
> > "René Nordby" <rene@bluedot.d k> wrote in message
> > news:%23qOvulZd EHA.1356@TK2MSF TNGP09.phx.gbl. ..
> >[color=darkred]
> > > I have a class A and a class B, that 100% inherits from class A (this[/color]
> > means[color=darkred]
> > > 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[/color]
> > object[color=darkred]
> > > become a B type.
> > >
> > >
> > >
> > > I have tryied it, but gets the error 'System.Invalid CastException:[/color]
> > Specified[color=darkred]
> > > cast is not valid.', so want am I missing.[/color]
> >
> > You're missing the fact that it can't be done. You can cast a derived[/color]
> class[color=green]
> > into its base class, but you can't cast a base class to a derived class.
> > Someone correct me if I'm wrong.
> >
> >[/color]
>
>[/color]

[Carl Tribble]

Re: Type Casting

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" <rene@bluedot.d k> wrote in message
news:%23qOvulZd EHA.1356@TK2MSF TNGP09.phx.gbl. ..[color=blue]
> 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[/color]
means[color=blue]
> 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[/color]
object[color=blue]
> become a B type.
>
>
>
> I have tryied it, but gets the error 'System.Invalid CastException:[/color]
Specified[color=blue]
> cast is not valid.', so want am I missing.
>
>
>
> Please give a hint.
>
>
>
> René
>
>
>
>[/color]

[Nice Chap]

Re: Type Casting

Hi Tom,
[color=blue]
>Casting a baseclass reference to a subclass would be a widening[/color]
conversion, since you would be picking up methods/properties.<

Sorry, it is a 'Narrowing Conversion' ....

[René Nordby]

Re: Type Casting

OK, thanks everyone for your replies.

I will try to explain what I'm trying to do, and hopefully some one knows
how to do it.

I want a Function to return a Class. With that Class in hand, I should be
able to Cast/Convert it, to two other Classes.

In pseudo code it would look like this

Sub GetClassA
myClassA = CType(GetClass, ClassA)
End Sub

Sub GetClassB
myClassB = CType(GetClass, ClassB)
End Sub

Function GetClass As ClassC
Return ClassC
End Function

Is that possible, and how?

Keep up the good work
René

"René Nordby" <rene@bluedot.d k> wrote in message
news:%23qOvulZd EHA.1356@TK2MSF TNGP09.phx.gbl. ..[color=blue]
> 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[/color]
means[color=blue]
> 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[/color]
object[color=blue]
> become a B type.
>
>
>
> I have tryied it, but gets the error 'System.Invalid CastException:[/color]
Specified[color=blue]
> cast is not valid.', so want am I missing.
>
>
>
> Please give a hint.
>
>
>
> René
>
>
>
>[/color]

[Jay B. Harlow [MVP - Outlook]]

Re: Type Casting

Nice Chap & Tom,
Casting a class from a base class or to a derived class is NOT a conversion!
So it can be neither Widening nor Narrowing!

It is a Cast.

A Conversion is physically changing the type of the object for example from
Integer to String, or String to Double.

A Cast the object stays the same type, however the variable it is placed
into changes. For example casting the sender parameter to the specific type
the object is. The object itself stays the same type however how you program
uses it changes.

You should use DirectCast for casting operations and use CType (or their
short cuts) for Conversions, for in VS.NET 2005 (aka Whidbey, due out
sometime in 2005) we can overload the CType operator to define how to
convert our class from one type to another (for example a Complex number
class can define how to convert itself to & from a Double).

Hope this helps
Jay


"Nice Chap" <NiceChap@Plasm aDyne.com> wrote in message
news:uTW0mFkdEH A.3988@tk2msftn gp13.phx.gbl...[color=blue]
> Hi Tom,
>[color=green]
> >Casting a baseclass reference to a subclass would be a widening[/color]
> conversion, since you would be picking up methods/properties.<
>
> Sorry, it is a 'Narrowing Conversion' ....
>
>[/color]

[Carl Tribble]

Re: Type Casting

I just tried this and it works. Is this what you are looking for? -Carl.

Option Explicit On
Option Strict On
Public Class A
Inherits C
End Class
Public Class B
Inherits C
End Class
Public Class C
End Class
Public Class Test
Public Function GetClassA() As A
Return CType(GetClassC (), A)
End Function
Public Function GEtClassB() As B
Return CType(GetClassC (), B)
End Function
Public Function GetClassC() As C
Return New C
End Function
End Class




"René Nordby" <rene@bluedot.d k> wrote in message
news:%23gDRMild EHA.212@TK2MSFT NGP12.phx.gbl.. .[color=blue]
> OK, thanks everyone for your replies.
>
> I will try to explain what I'm trying to do, and hopefully some one knows
> how to do it.
>
> I want a Function to return a Class. With that Class in hand, I should be
> able to Cast/Convert it, to two other Classes.
>
> In pseudo code it would look like this
>
> Sub GetClassA
> myClassA = CType(GetClass, ClassA)
> End Sub
>
> Sub GetClassB
> myClassB = CType(GetClass, ClassB)
> End Sub
>
> Function GetClass As ClassC
> Return ClassC
> End Function
>
> Is that possible, and how?
>
> Keep up the good work
> René
>
> "René Nordby" <rene@bluedot.d k> wrote in message
> news:%23qOvulZd EHA.1356@TK2MSF TNGP09.phx.gbl. ..[color=green]
> > 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[/color]
> means[color=green]
> > 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[/color]
> object[color=green]
> > become a B type.
> >
> >
> >
> > I have tryied it, but gets the error 'System.Invalid CastException:[/color]
> Specified[color=green]
> > cast is not valid.', so want am I missing.
> >
> >
> >
> > Please give a hint.
> >
> >
> >
> > René
> >
> >
> >
> >[/color]
>
>[/color]

[Jay B. Harlow [MVP - Outlook]]

Re: Type Casting

René,
I would have GetClassA & GetClassB create the correct type of object, and
not include GetClass.
[color=blue]
> Sub GetClassA[/color]
myClassA = New ClassA[color=blue]
> End Sub
>
> Sub GetClassB[/color]
myClassB = New ClassB[color=blue]
> End Sub[/color]

Alternatively you would need to pass a parameter to GetClass that indicates
what kind of object to actually create. I would either use an Enum or the
Type & Activator.Creat eInstance.

Hope this helps
Jay



"René Nordby" <rene@bluedot.d k> wrote in message
news:%23gDRMild EHA.212@TK2MSFT NGP12.phx.gbl.. .[color=blue]
> OK, thanks everyone for your replies.
>
> I will try to explain what I'm trying to do, and hopefully some one knows
> how to do it.
>
> I want a Function to return a Class. With that Class in hand, I should be
> able to Cast/Convert it, to two other Classes.
>
> In pseudo code it would look like this
>
> Sub GetClassA
> myClassA = CType(GetClass, ClassA)
> End Sub
>
> Sub GetClassB
> myClassB = CType(GetClass, ClassB)
> End Sub
>
> Function GetClass As ClassC
> Return ClassC
> End Function
>
> Is that possible, and how?
>
> Keep up the good work
> René
>
> "René Nordby" <rene@bluedot.d k> wrote in message
> news:%23qOvulZd EHA.1356@TK2MSF TNGP09.phx.gbl. ..[color=green]
> > 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[/color]
> means[color=green]
> > 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[/color]
> object[color=green]
> > become a B type.
> >
> >
> >
> > I have tryied it, but gets the error 'System.Invalid CastException:[/color]
> Specified[color=green]
> > cast is not valid.', so want am I missing.
> >
> >
> >
> > Please give a hint.
> >
> >
> >
> > René
> >
> >
> >
> >[/color]
>
>[/color]