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

VB6 AI Migrator resulting VB.NET code

Private anAnimal As Animal
    Private _anAnimal2 As Animal = Nothing
    Private Property anAnimal2() As Animal
        Get
            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

VB6 AI Migrator resulting C#.NET code

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