Overloading, Overriding, and Shadowing

Overloading, overriding, and shadowing are also important concepts in Visual Basic OOP. These techniques allow you to create multiple members with the same name. We've already discussed them in the previous chapter; here's how they work in overview:

• Overloaded—Overloaded members provide different versions of a property or method that have the same name, but that accept different number of parameters (or parameters of different types).

• Overridden—Overridden properties and methods are used to replace an inherited property or method. When you override a member from a base class, you replace it. Overridden members must accept the same data type and number of arguments.

• Shadowed—Shadowed members are used to create a local version of a member that has broader scope. You also can shadow a type with any other type. For example, you can declare a property that shadows an inherited method with the same name.

We saw overloading in the previous chapter, although we'll have a little more to say about it here. In particular, there is an Overloads keyword that you can use to indicate that you're overloading a method or property. You don't need to use Overloads when you're overloading members of the same class, but it becomes important when you're working with inheritance—see "Overloading Base Class Members" in this chapter.

What's overriding? If an inherited property or method needs to behave differently in the derived class, it can be overridden; that is, you can define a new implementation of the method in the derived class. The following modifiers are used to control how properties and methods are overridden:

• Overridable— Allows a property or method in a class to be overridden.

• Overrides— Overrides an Overridable property or method.

• NotOverridable— Prevents a property or method from being overridden. Note that public methods are NotOverridable by default.

• MustOverride— Requires that a derived class override the property or method. MustOverride methods must be declared in MustInherit classes.

Let's see an example of overriding at work. In the Inheritance example on the CD-ROM, the base class is Animal, and when you call its Breathing method, it displays "Breathing…". However, if you were to derive a class named Fish from Animal, that wouldn't be quite appropriate—you might want this method to display something like "Bubbling…" instead. To do that, you can override the Animal class's Breathing method in the Fish class. All that takes is to mark the Animal class's Breathing method as Overridable, and to use the Overrides keyword when defining the Breathing method in the Fish class:

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

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

Public Class Fish
    Inherits Animal

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

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

Now I can declare a Fish object named jaws and when I use its overriden Breathing method, you'll see "Bubbling…", not "Breathing…":

    Dim jaws As Fish

    Private Sub Button2_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button2.Click
        jaws = New Fish(Me)
        jaws.Breathing()
    End Sub

You can see this code at work in the Inheritance example on the CD-ROM in Figure 12.2.

Figure 12.2: Overriding a base class method in the Inheritance example.

    Dim obj As Animal

    Private Sub Button2_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button2.Click
        obj = New Dog(Me)
        ⋮
    End Sub

   Dim obj As Animal

   Private Sub Button2_Click(ByVal sender As System.Object, _
       ByVal e As System.EventArgs) Handles Button2.Click
       obj = New Fish(Me)
       obj.Breathing()
   End Sub