General Info: Abstract Class / Abstract Member

An abstract class member is a member that is specified in a class but not implemented. Classes that inherit from the class will have to implement the abstract member. Abstract members are a technique for ensuring a common interface with descendant classes. An abstract class is a class you cannot instantiate. A pure abstract class is a class with only abstract members.

Languages Focus: Abstraction

Abstraction is supported at various levels with each language. A language could enforce abstraction at the class level (either enforcing a no-instantiation rule or a only abstract members rule), and with class members (member methods and/or properties).

Public MustInherit Class Cyborg

  Public MustOverride Sub Speak(ByVal pMessage As String)

End Class

Public Class Series600

  Inherits Cyborg

  Public Overrides Sub Speak(ByVal pMessage As String)

    MessageBox.Show(pMessage)

  End Sub

End Class

An Abstract Example

The following demonstrates the abstract method above. The following assumes a form with a button.

Public Class Form1
  Private Sub Button1_Click(ByVal sender As System.Object,_
      ByVal e As System.EventArgs) Handles Button1.Click
 
    Dim MyKiller As New Series600
 
    MyKiller.Greet()
    MyKiller.Speak("I am alive.")
  End Sub
End Class
 

Public MustInherit Class Cyborg
  Public MustOverride Sub Speak(ByVal pMessage As String)
  Public MustOverride Sub Walk()
 

  'You can implement methods in an abstract class!
  Public Overridable Sub Greet()
    MessageBox.Show("Hello human")
  End Sub
End Class

 
Public Class Series600
  Inherits Cyborg
 

  Public Overrides Sub Greet()
    MessageBox.Show("Hello")
  End Sub

  Public Overrides Sub Speak(ByVal pMessage As String)
    MessageBox.Show(pMessage)
  End Sub
 

  Public Overrides Sub Walk()
    'Implement walk here.
  End Sub
End Class

Abstract Properties

VB.Net does support setting a property to abstract with the MustOverride keyword. A single property line with the MustOverride keyword is all that's required. In most cases, you'll probably also want to...