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Overriding (VB.Net and Delphi Prism Cross Reference Guide)

By Mike Prestwood

VB.Net versus Delphi Prism: A side by side comparison between VB.Net and Delphi Prism.

 
OOP Details
 

More object oriented (OO) stuff.

Overriding

[Other Languages] 

General Info: Method Overriding

Where you define or implement a virtual method in a parent class and then replace it in a descendant class.

When you decide to declare a method as virtual, you are giving permission to derived classes to extend and override the method with their own implementation. You can have the extended method call the parent method's code too.

In most OO languages you can also choose to hide a parent method. When you introduce a new implementation of the same named method with the same signature without overriding, you are hiding the parent method.

VB.Net:   Overridable, Overrides

In VB.Net, you specify a virtual method with the Overridable keyword in a parent class and extend (or replace) it in a descendant class using the Overrides keyword.

Use the base keyword in the descendant method to execute the code in the parent method, i.e. base.SomeMethod().

Syntax Example:
Public Class Robot
Public Overridable Sub Speak()
MessageBox.Show("Robot says hi")
End Sub
End Class
  
Public Class Cyborg
Inherits Robot
  
  Public Overrides Sub Speak()
MessageBox.Show("hi")
End Sub
End Class
Delphi Prism:   virtual, override

Same as Delphi. In Prism, you specify a virtual method with the virtual keyword in a parent class and extend (or replace) it in a descendant class using the override keyword. Call Inherited in the descendant method to execute the code in the parent method.

Use final to prevent further extending of a member and Sealed to prevent all members of a class from further extension.

Syntax Example:
Robot = class(System.Object)
public
method Speak; virtual;
end;
  
Cyborg = class(Robot)
public
method Speak; override;
end;












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