General Info: Interface

An element of coding where you define a common set of properties and methods for use with the design of two or more classes.

Both interfaces and abstract classes are types of abstraction. With interfaces, like abstract classes, you cannot provide any implementation. However, unlike abstract classes, interfaces are not based on inheritance. You can apply an Interface to any class in your class tree. In a real sense, interfaces are a technique for designing horizontally in a class hierarchy (as opposed to inheritance where you design vertically). Using interfaces in your class design allows your system to evolve without breaking existing code.

VB.Net Interfaces

With VB.Net you define an interface with the Interface keyword and use it in a class with the Implements keyword. In the resulting class, you implement each property and method and add Implements Interface.Object to each as in:

Sub Speak(ByVal pSentence As String) Implements IHuman.Speak

  MessageBox.Show(pSentence)

End Sub

Public Interface IHuman
  'Specify interface methods and properties here.

End Interface
  

Public Class Cyborg
  Inherits System.Object

End Class
  

Public Class CyborgHuman
  Inherits Cyborg
  Implements IHuman
  'Implement interface methods and properties here.

End Class

VB.Net 2008 Working WinForms Example

The following example demonstrates implementing a very simple interface. The interface is named IHuman which includes one property and one method. Our resulting class is named CyborgHuman and, for clarity, our CyborgHuman class also inherits from a class called Cyborg.

Create a form and place a button on it and alter the code as follows:

Public Class Form1
  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
  
    Dim MyRobot As New CyborgHuman
  

    MyRobot.HumanName = "Nicole"
    MyRobot.Speak("Hi, my name is " & MyRobot.HumanName & ".")
  End Sub
End Class

  

'Notice you do not specify visibility in an interface.
'Visibility is determined by the implementing class.
Public Interface IHuman
  Property HumanName()
  Sub Speak(ByVal pSentence As String)
End Interface
  

Public Class Cyborg
  Inherits System.Object
End Class
  

Public Class CyborgHuman
  Inherits Cyborg
  Implements IHuman
  

  Private FHumanName As String
  
  Public Property HumanName() Implements IHuman.HumanName
    Get
      Return FHumanName
    End Get
  

    Set(ByVal value)
      FHumanName = value
    End Set
  End Property
  

  Sub Speak(ByVal pSentence As String) Implements IHuman.Speak
    MessageBox.Show(pSentence)
  End Sub
End Class

Default Interface Visibility: Friend

If you do not specify an interfaces visibility, the default is Friend (accessible from types in the same assembly) but an interface's members are always public -- which makes sense but is noteworthy.

Interface ITalk //Default visibility is Internal.

  '...

End Interface