Overriding

  • Thread starter Thread starter Roshawn
  • Start date Start date
R

Roshawn

Hi,

Is it possible to override a property, function, or sub in a base class if the Overridable
keyword isn't specified?

For example:

Public Class ExampleA
Sub DoSomething()
Dim str as String = "Do something already"
End Sub
End Class

Public Class ExampleB
Inherits ExampleA

Overrides Sub DoSomething()
Dim str as String = "Already doing something"
End Sub
End Class


Would this work?

Thanks,
Roshawn
 
In addition, can I override a property that's already been overridden?

For example:

Public Class One
Overridable Sub DoSomething()
Dim str as String = "Do something already"
End Sub
End Class

Public Class Two
Inherits One
Overrides Sub DoSomething()
Dim str as String = "Already doing something"
End Sub
End Class

Public Class Three
Inherits Two
Overrides Sub DoSomething()
Dim str as String = "Won't stop until I get enough!"
End Sub
End Class


Would that work?
 
Roshawn said:
In addition, can I override a property that's already been overridden?

For example:

Public Class One
Overridable Sub DoSomething()
Dim str as String = "Do something already"
End Sub
End Class

Public Class Two
Inherits One
Overrides Sub DoSomething()
Dim str as String = "Already doing something"
End Sub
End Class

Public Class Three
Inherits Two
Overrides Sub DoSomething()
Dim str as String = "Won't stop until I get enough!"
End Sub
End Class


Would that work?
Click to expand...

Yes, as the overrides are in different subclasses.
 
Hi,

Is it possible to override a property, function, or sub in a base class if the Overridable
keyword isn't specified?

For example:

Public Class ExampleA
Sub DoSomething()
Dim str as String = "Do something already"
End Sub
End Class

Public Class ExampleB
Inherits ExampleA

Overrides Sub DoSomething()
Dim str as String = "Already doing something"
End Sub
End Class

Would this work?

Thanks,
Roshawn
Click to expand...

Here is a quick sample:

Option Strict On
Option Explicit On

Module Module1

Sub Main()
Dim one As A = New A
' call an A through an A reference
one.DoStuff()
' Output = "In A.DoStuff"

' call an B through an A reference
Dim two As A = New B()
two.DoStuff()
' Output = "In A.DoStuff (not what you would expect?)"

' call an C through an A reference
Dim three As A = New C
three.DoStuff()
' Output = "In A.DoStuff (not what you would expect?)"

' Call an B through an B reference
Dim four As B = New B
four.DoStuff()
' Output = "In B.DoStuff"

' call an C through an C reference
Dim five As B = New C
five.DoStuff()
' Output = "In C.DoStuff"
End Sub

Class A
Public Sub DoStuff()
Console.WriteLine("In A.DoStuff")
End Sub
End Class

Class B
Inherits A
Public Overridable Shadows Sub DoStuff()
Console.WriteLine("In B.DoStuff")
End Sub
End Class

Class C
Inherits B

Public Overrides Sub DoStuff()
Console.WriteLine("In C.DoStuff")
End Sub
End Class

End Module

If you do not declare a sub/function/property overrideable, then sure
you can override it in the subclass, but you will not get polymorphic
behavior either. This is shadowing. You can find documentation on
it.
 
Roshawn,
As Tom suggests you can shadows a base method that was not marked
overridable.

HOWEVER!! Avoid creating a design that requires the use of shadows, as are
"anti-polymorphic", while Overrides are polymorphic.
Public Class ExampleB
Inherits ExampleA

Shadows Sub DoSomething()
Dim str as String = "Already doing something"
End Sub
End Class
Click to expand...

For example:

Dim a As ExampleA = New ExampleA
a.DoSomething()

' Now create an ExampleA variable containing a derived object
Dim b As ExampleA = New ExampleB
b.DoSomething()

Notice that str is still "Do something already" as DoSomething is not
polymorphic, b is defined as an ExampleA type, so you get ExampleA behavior.
Overridable indicates you want polymorphism.


To continue your second example, Two can override a method such that derived
classes can no longer override them.
Public Class Two
Inherits One
Overrides NotOverridable Sub DoSomething()
Dim str as String = "Already doing something"
End Sub
End Class
Click to expand...

This is useful when Two needs to dictate what the expected behavior will be,
without allowing derived classes to modify it.
 
Back
Top