Inheriting Constructors

By default, constructors are not inherited in Visual Basic. The idea here is that initializing a derived class's object will be different from initializing an object of the base class, so you're responsible for creating your own constructor in derived classes, if you want to give them one.

We saw an example of using constructors and inheritance in the Inheritance example on the CD-ROM, as discussed in the In Depth section of this chapter. In that case, I'm deriving the Dog class from the Animal class. The Animal class has a constructor that accepts a form, the main form for the program, and stores it in a protected member named MainForm. To pass that form from the Dog class's constructor back to the Animal class's constructor, I called the Animal class's constructor as MyBase.New (see the In Depth section of this chapter for more information):

Public Class Form1
    Inherits System.Windows.Forms.Form

    'Windows Form Designer generated code

   Dim spot As Dog

    Private Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click
        spot = New Dog(Me)
        spot.Breathing()
    End Sub
End Class

Public Class Animal
    Protected MainForm As Form1

    Public Sub New(ByVal form1 As Form1)
        MainForm = form1
    End Sub

    Public Sub Breathing()
        MainForm.TextBox1.Text = "Breathing..."
    End Sub
End Class

Public Class Dog
    Inherits Animal

    Public Sub New(ByVal form1 As Form1)
        MyBase.New(form1)
    End Sub

    Public Sub Barking()
        MyBase.MainForm.TextBox1.Text = "Barking..."
    End Sub
End Class

Note that if you do call a base class's constructor from a derived class's constructor, that call must be the first line in the derived class's constructor.