Inheritance and late binding

[Bob]

Hi,
I have a class (ClassA) that passes a Widget to a public function
(FixWarts) in a member variable class (Doctor).
Doctor.FixWarts checks one of the Widgets public properties (Wartcount)
and then takes some action.
Works fine.

Then along came another class ( ClassB) with a need for a DeluxWidget. The
underlying behaviour of the DeluxWidget Wartcount property is incompatible
with its base class Widget, so it has a shadows property Wartcount.
(DeluxWidget gets / sets , Widget is readonly)

In ClassB I declared myWidget as a deluxWidget and passed it into member
instance of Doctor.FixWarts.

FixWarts tries to use the Widget Wartcount property even though it has been
passed a DeluxWidget. The IDE confirms that a DeluxWidget has been passed
in.

I can fix this using latebinding and declaring an object in Wartcount,
newing it to the appropriate class (Widget or DeluxWidget) and assigning
the WartCount input parameter to the object. Then I ask the object for its
wartcount property and it uses the correct one.

Have I missed something?
Should I be able to feed a Widget or DeluxWidget into FixWarts and get
FixWarts to correctly choose which version of the property to use?
thanks
Bob

 

[Scott Swigart]

In the base class, make Widget.WartCount "Overridable"

Overridable Public Property WartCount as Integer

In deluxWidget, declare the property as follows:

Overrides Public Property WartCount as Integer

Then, you will get the behavior you want. The term for this kind of
behavior is polymorphism.