Programming Visual Basic 2005

The Root of All Classes: Object

All Visual Basic 2005 classes, of any type, ultimately derive from System.Object. Interestingly, this includes value types!

Versioning with New and Override

In Visual Basic 2005, the programmer's decision to override an overridable method is made explicit with the overrides keyword. This helps with versioning .

Here's how: assume for a moment that the Window base class of the previous example was written by Company A. Suppose also that the ListBox and RadioButton classes were written by programmers from Company B, using a purchased copy of the Company A Window class as a base. The programmers in Company B have little or no control over the design of the Window class, including future changes that Company A might choose to make.

Now suppose that one of the programmers for Company B decides to add a Sort method to ListBox:

Public Class ListBox
  Inherits Window
   Public Overridable Sub Sort()
     '...
   End Sub

This presents no problems until Company A, the author of Window, releases Version 2 of its Window class, and it turns out that the programmers in Company A have also added a Sort method to their Public Class Window:

Public Class Window
   Public Overridable Sub Sort()
     '...
   End Sub

In other object-oriented languages (such as C++), the new overridable Sort method in Window would now act as a base method for the Sort method in ListBox. The compiler would call the Sort method in ListBox when you intend to call the Sort in Window. In Java, if the Sort in Window had a ...