Using Polymorphism

    Polymorphism is a feature of object-oriented languages that allows values of different data types to be handled using a uniform interface. Visual Basic 6 provides this feature of object-oriented languages by implementing interfaces.

    By the way, an interface can be instantiated with several different classes that implement the interface, and it can polymorphically access the members of the class instances.

    The classes and interfaces in this case are upgraded as indicated in the sections above. The issue in this case is the creation process for instances. If you create an instance of the target interface, the variable must be declared as the interface type, but the instance must be of CoClass class, which has the original implementation. In case of a creation of an implementing class, the variable must be declared and instanced in the same class type. An important observation is that a variable of the interface type could be changed to any of the implementing class types.

    In the following example, there is an interface (Animal) and two implementing classes (Dog and Bird).

    Original VB6 code

    Private anAnimal As Animal
    Private anAnimal2 As New Animal
    Private Function BirdName() As String
        Set anAnimal = New Bird
        BirdName = anAnimal.Name
    End Sub

    VBUC resulting VB.NET code

    Private anAnimal As Animal
        Private _anAnimal2 As Animal = Nothing
        Private Property anAnimal2() As Animal
                If _anAnimal2 Is Nothing Then
                    _anAnimal2 = New Animal_CoClass
                End If
                Return _anAnimal2
            End Get
            Set(ByVal Value As Animal)
                _anAnimal2 = value
            End Set
        End Property
        Private Function BirdName () As String
            anAnimal = New Bird
            Return anAnimal.Name
        End Sub

    VBUC resulting C#.NET code

    private Animal anAnimal = null;
        private Animal _anAnimal2 = null;
        private Animal anAnimal2
                if (_anAnimal2 == null)
                    _anAnimal2 = new Animal_CoClass();
                return _anAnimal2;
                _anAnimal2 = value;
        private string  BirdName()
                anAnimal = new Bird();
                return anAnimal.Name;

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