Unlike abstract classes, you don't implement the members (properties and procedures) of the interface. The general idea of an interface is only to lay a foundation, as creating a structural base, that the new classes would follow, although they can customize the behavior(s) of the parent interface but the parent interface does not "decide" what the behavior of a member would be.

To create an interface, you start with the Interface keyword followed by the name of the interface. You end the interface definition with an End Interface line. By tradition or good habit, the name of an interface starts with I. Here is an example of a starting interface:

File: RegularTriangle.vb

Public Interface ITriangle

End Interface

After creating an interface, you can derive a class from it. When deriving from an interface, instead of the Inherits keyword, you use Implements followed by the name of the interface. Here is an example of a new class named Regular and that is based on the above ITriangle interface:

File: RegularTriangle.vb

Public Interface ITriangle

End Interface

Public Class RegularTriangle
    Implements ITriangle

End Class

As with other classes, once you have derived the class, you can create objects from it and instantiate it using the New operator. Here is an example:

File: Exercise.vb

Module Exercise

    Public Function Main() As Integer
        Dim reg As RegularTriangle
        reg = New RegularTriangle

        Return 0
    End Function

End Module

As mentioned earlier, the purpose of having an interface is to create a skeleton that derived classes would follow. To do this, in the body of the interface, you can create the necessary members that you want to make available to new classes. The members can be the same types of methods or properties as those we have used in classes so far. Here are examples:

File: RegularTriangle.vb

Public Interface ITriangle
    ReadOnly Property Name() As String
    Property Base() As Double
    Property Height() As Double

    Function CalculatePerimeter() As Double
    Function CalculateArea() As Double
End Interface

The primary rule you must observe when deriving a class from an interface is that you must implement each member of the interface in the derived class. If you omit implementing a member of the parent interface, you would receive an error. When implementing a member of the interface, it must be followed by the Implements keyword, the name of the interface, the period operator, and the name of the member that it is implementing. Based on this, our Regular class can implement the ITriangle interface as follows:

File: RegularTriangle.vb

Public Interface ITriangle
    ReadOnly Property Name() As String
    Property Base() As Double
    Property Height() As Double

    Function CalculatePerimeter() As Double
    Function CalculateArea() As Double
End Interface

Public Class RegularTriangle
    Implements ITriangle

    Public bas As Double
    Public hgt As Double
    Public sd1 As Double
    Public sd2 As Double

    ' Default constructor: the user will specify the dimensions
    Public Sub New()
        bas = 0
        hgt = 0
        sd1 = 0
        sd2 = 0
    End Sub

    ' A triangle based on known base and height
    Public Sub New(ByVal b As Double, ByVal h As Double)
        bas = b
        hgt = h
    End Sub

    ' A triangle based on the measurements of the sides
    Public Sub New(ByVal b As Double, ByVal side1 As Double, ByVal side2 As Double)
        bas = b
        sd1 = side1
        sd2 = side2
    End Sub

    ' A triangle whose all sides and the height are known
    Public Sub New(ByVal b As Double, ByVal h As Double, _
                   ByVal side1 As Double, ByVal side2 As Double)
        bas = b
        hgt = h
        sd1 = side1
        sd2 = side2
    End Sub

    Public Property Base() As Double Implements ITriangle.Base
        Get
            Return bas
        End Get
        Set(ByVal Value As Double)
            If bas < 0 Then
                bas = 0
            Else
                bas = Value
            End If
        End Set
    End Property

    Public Function CalculateArea() As Double Implements ITriangle.CalculateArea
        Return bas * hgt / 2
    End Function

    Public Function CalculatePerimeter() As Double Implements _
   ITriangle.CalculatePerimeter
        Return bas + sd1 + sd2
    End Function

    Public Property Height() As Double Implements ITriangle.Height
        Get
            Return hgt
        End Get
        Set(ByVal Value As Double)
            If hgt < 0 Then
                hgt = 0
            Else
                hgt = Value
            End If
        End Set
    End Property

    Public ReadOnly Property Name() As String Implements ITriangle.Name
        Get
            Return "Regular Triangle"
        End Get
    End Property

End Class

Once the class has been defined like this, you can then instantiate and use it. Here is an example:

File: Exercise.vb

Module Exercise

    Public Function Main() As Integer
        Dim reg As RegularTriangle = New RegularTriangle(35.28, 26.44)

        Console.WriteLine("Triangle Type: {0}", reg.Name)
        Console.WriteLine("=-= Characteristics =-=")
        Console.WriteLine("Base:   {0}", reg.Base)
        Console.WriteLine("Height: {0}", reg.Height)
        Console.WriteLine("Area:   {0}", reg.CalculateArea)

        Return 0
    End Function

End Module

This would produce:

Triangle Type: Regular Triangle
=-= Characteristics =-=
Base:   35.28
Height: 26.44
Area:   466.4016

In the same way, you can derive other classes from an interface. For example, from our ITriangle class, you can derive an isosceles, a right, or an equilateral triangle. Always remember that when you derive a class, you must implement all of the members of the interface. You can also add new members as you see fit.

Public Class Employee
    Private fn As String
    Private ln As String

    Public Property FirstName() As String
        Get
            Return fn
        End Get
        Set(ByVal value As String)
            fn = value
        End Set
    End Property

    Public Property LastName() As String
        Get
            Return ln
        End Get
        Set(ByVal value As String)
            ln = value
        End Set
    End Property

    Public Overrides Function ToString() As String
        Return LastName & ", " & FirstName
    End Function

    Public Overrides Function Equals(ByVal obj As Object) As Boolean
        Dim empl As Employee
        empl = TryCast(obj, Employee)
        If Me.FirstName = empl.FirstName And Me.LastName = empl.LastName Then
            Return True
        Else
            Return False
        End If
    End Function
End Class

Public Class Exercise
    Private Function AreEqual(ByVal x As Integer, _
                              ByVal y As Integer) As Boolean
        Return x = y
    End Function

    Private Function AreEqual(ByVal x As Object, ByVal y As Object) As Boolean
        If TypeOf x Is Employee And TypeOf y Is Employee Then
            Return x.Equals(y)
        End If
    End Function

    Private Sub BtnProcess_Click(ByVal sender As System.Object, _
                                 ByVal e As System.EventArgs) _
                                 Handles BtnProcess.Click
        Dim empl1 As Employee

        empl1 = New Employee
        empl1.FirstName = "Patricia"
        empl1.LastName = "Katts"

        Dim empl2 As New Employee
        With empl2
            .FirstName = "Raymond" REM = "Patricia"
            .LastName = "Kouma" REM = "Katts"
        End With

        MsgBox(empl1.ToString & " = " & empl2.ToString & ": " & AreEqual(empl1, empl2))

    End Sub
End Class