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



Please give a hint.



René

 

[Jeff Johnson [MVP: VB]]

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.

 

[René Nordby]

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.

 

[Nice Chap]

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 Nordby]

Could you please spell it out a little more?

René

 

[Cor Ligthert]

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]

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.

 

[Tom Shelton]

??? 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)

 

[Luhar]

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

hth,

Luhar

 

[Carl Tribble]

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

 

[Nice Chap]

Hi Tom,

Casting a baseclass reference to a subclass would be a widening

conversion, since you would be picking up methods/properties.<

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

 

[René Nordby]

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

 

[Jay B. Harlow [MVP - Outlook]]

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

 

[Carl Tribble]

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

 

[Jay B. Harlow [MVP - Outlook]]

René,
I would have GetClassA & GetClassB create the correct type of object, and
not include GetClass.

Sub GetClassA myClassA = New ClassA
End Sub

Sub GetClassB myClassB = New ClassB
End Sub

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.CreateInstance.

Hope this helps
Jay

 

[Jay B. Harlow [MVP - Outlook]]

Carl,
No! it doesn't work, as soon as you attempt to run either GetClassA or
GetClassB, you get InvalidCastException!

The other posts in this thread attempt to explain why your code fails.

Hope this helps
Jay

 

[Cor Ligthert]

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.

You can not return a class, you can return an object.

You convert no class, you cast or convert an object.

Cor

 

[René Nordby]

Absolutely, this is what I'm looking for, but as Jay writes, it doesn't work
when it comes to runtime, so what should be done, to get it to work.

I see what Jay means by returning the right object from start, but that's
not the solution, sorry.

The above code is what I want, but how should it be implemented, so it
works?

René

 

[René Nordby]

Hi All,

Now I have solved it, but I'm not 100% sure that it is the rigt OO way, so
if anyone have any comments you are mostly welcome.

Option Explicit On
Option Strict On

Public Class A
Inherits B
End Class

Public Class B
End Class

Public Class C
Inherits A
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

Private Function GetClassC() As C
Return New C
End Function
End Class

 

[Jay B. Harlow [MVP - Outlook]]

René,

Now I have solved it, but I'm not 100% sure that it is the rigt OO way, so
if anyone have any comments you are mostly welcome.

No you didn't, as your GetClassB method will fail with a
InvalidCastException!
It "cannot" be implemented in VS.NET 2003, as CType cannot change the type
of a C object into a B or A object. It can however Cast a B or A object in a
C variable to a B or A variable. Only predefined CTYPE conversions are
defined in VS.NET 2003, C to A & C to B are not predefined. Also see my
message to Nice chap about the difference between CType the conversion &
DirectCast the casting.

VS.NET 2005 (aka Whidbey, due out in 2005) you can overload CType such that
it can create a B or A object given a C object, HOWEVER!!! your sample seems
to be an extreme abuse of this ability! If you provide better details on
what you are really trying to accomplish we may be able to offer other
solutions then the two I offered earlier.

The correct way (only way really) to implement this is:

I would have GetClassA & GetClassB create the correct type of object, and
not include GetClass.

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.CreateInstance.

Public Function GetClassA() As A
Return DirectCast(GetClassC(gettype(A)), A)
End Function

Public Function GetClassB() As B
Return DirectCast(GetClassC(GetType(B)), B)
End Function

Private Function GetClassC(type As Type) As C
Return DirectCast(Activator.CreateInstance(type), C)
End Function

Hope this helps
Jay