CODEWITHSUNDEEP
vb.netinheritance
MattH
MattH

Reputation: 1975

From Child instance call base class method that was overridden

Consider the following code:

Public Class Animal

Public Overridable Function Speak() As String
    Return "Hello"
End Function

End Class

Public Class Dog
    Inherits Animal

    Public Overrides Function Speak() As String
        Return "Ruff"
    End Function

End Class

Dim dog As New Dog
Dim animal As Animal
animal = CType(dog, Animal)
// Want "Hello", getting "Ruff"
animal.Speak()

How can I convert/ctype the instance of Dog to Animal and have Animal.Speak get called?

Upvotes: 7

Views: 36695

Answers (5)

Steven A. Lowe
Steven A. Lowe

Reputation: 61262

You don't; the subclass's method overrides the superclass's method, by definition of inheritance.

If you want the overridden method to be available, expose it in the subclass, e.g.

Public Class Dog 
    Inherits Animal
    Public Overrides Function Speak() As String
        Return "Ruff"
    End Function
    Public Function SpeakAsAnimal() As String
        Return MyBase.Speak()
    End Function
End Class

Upvotes: 13

tfrascaroli
tfrascaroli

Reputation: 1219

I know this has been posted a few months back, but I'm still going to try and reply, maybe just for the sake of completeness.

You've been told that you can access the overriden method from within the dog class, and that you can then expose it with a different name. But what about using a conditional?

You can just do:

Public Class Animal

Public Overridable Function Speak(Optional ByVal speakNormal as Boolean = False) As String
    Return "Hello"
End Function

End Class

Public Class Dog
    Inherits Animal

    Public Overrides Function Speak(Optional ByVal speakNormal as Boolean = False) As String
        If speakNormal then
            return MyBase.Speak()
        Else
            Return "Ruff"
        End If
    End Function

End Class

And then call them like:

Dim dog As New Dog
Dim animal As new Animal
animal.Speak() //"Hello"
dog.Speak()//"Ruff"
dog.Speak(true)//"Hello"

Alternatively, you can getTheAnimalInTheDog and make it Speak() :

You can just do:

Public Class Animal

Public Overridable Function Speak() As String
    Return "Hello"
End Function

Public MustOverride Function GetTheAnimalInMe() As Animal

End Class

Public Class Dog
    Inherits Animal

    Public Overrides Function Speak() As String
        Return "Ruff"
    End Function

    Public Overrides Function GetTheAnimalInMe() As Animal
        Dim a As New Animal
        //Load a with the necessary custom parameters (if any)
        Return a
    End Function
End Class

And then again:

Dim dog As New Dog
Dim animal As new Animal
animal.Speak() //"Hello"
dog.Speak()//"Ruff"
dog.GetTheAnimalInMe().Speak()//"Hello"

Hope it helps ;)

Upvotes: 0

Matt Burke
Matt Burke

Reputation: 3326

I think if you drop "Overridable" and change "Overrides" to "New" you'll get what you want.

Public Class Animal

Public Function Speak() As String
    Return "Hello"
End Function

End Class

Public Class Dog
    Inherits Animal

    Public New Function Speak() As String
        Return "Ruff"
    End Function

End Class

Dim dog As New Dog
Dim animal As Animal
dog.Speak() ' should be "Ruff"
animal = CType(dog, Animal)
animal.Speak() ' should be "Hello"

Upvotes: 0

Stephen Wrighton
Stephen Wrighton

Reputation: 37880

I don't think you can.

The thing is that the object is still a dog. the behavior you're describing (getting "ruff" from the casted object rather than "hello") is standard because you want to be able to use the animal class to let a bunch of different type of animals speak.

For example, if you had a third class as thus:

Public Class Cat
    Inherits Animal

    Public Overrides Function Speak() As String
        Return "Meow"
    End Function
End Class

Then you'd be able to access them like thus:

protected sub Something
    Dim oCat as New Cat
    Dim oDog as New Dog

    MakeSpeak(oCat)
    MakeSpeak(oDog)
End sub

protected sub MakeSpeak(ani as animal)
    Console.WriteLine(ani.Speak())
end sub

What you're talking about doing basically breaks the inheritance chain. Now, this can be done, by setting up the Speak function to accept a parameter which tells it to return it's base value or not or a separate SPEAK function for the base value, but out of the box, you're not going to get things that behave this way.

Upvotes: 0

Mike Deck
Mike Deck

Reputation: 18397

I would ask why you are trying to get this type of behavior. It seems to me that the fact you need to invoke the parent class' implementation of a method is an indication that you have a design flaw somewhere else in the system.

Bottom line though, as others have stated there is no way to invoke the parent class' implementation given the way you've structured your classes. Now within the Dog class you could call

MyBase.Speak()

which would invoke the parent class' implementation, but from outside the Dog class there's no way to do it.

Upvotes: 1

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