Overloading and Overriding in Visual Basic.NET 2005 - Method overriding in Visual Basic 2005 (Page 4 of 5 )
In all of the previous sections, we worked with only "overloading." This section focuses on "overriding."
Before proceeding further, please note that there is no relationship or similarity between "overloading" and "overriding." They are completely different. To demonstrate "overriding," one needs to work with inheritance. If you are new to inheritance in Visual Basic.NET, I strongly suggest that you go through the following article:
Inheritance with VB.NET 2005
Let us start with defining two simple classes as follows:
Public Class Parent
Public Sub DispMsg()
MessageBox.Show("From Parent")
End Sub
End Class
Public Class Child
Inherits Parent
Public Sub DispMsg()
MessageBox.Show("From child")
End Sub
End Class
From the above, we have two classes, "Parent" and "Child." The "Child" class gets inherited from the "Parent" class. The parent class has a method named "DispMsg," and the same method is redefined in the child with the same name.
If we create a parent object (instance), we will have only one method and we can call it without any problem. But if we create a child object, we will have two methods -- one inherited from the parent -- with the same name and signature. Thus the program gives out a warning and defaults to the child method whenever "DispMsg" is called from the child object.
To eliminate this warning, we should either "override" or "shadow" the parent (or inherited) method. I shall cover the "shadow" technique in my upcoming articles. For now, let us concentrate on the "override" technique.
The following are the modifications to the previous classes which are needed to implement overriding:
Public Class Parent
Public Overridable Sub DispMsg()
MessageBox.Show("From Parent")
End Sub
End Class
Public Class Child
Inherits Parent
Public Overrides Sub DispMsg()
MessageBox.Show("From child")
End Sub
End Class
To override (or redefine) an inherited method in the child class, the same method must be declared with "overridable" in the parent and with "overrides" in the child. To test the above classes, you can use the following code:
Dim objP As New Parent
objP.DispMsg() 'displays "From Parent"
Dim obj As New Child
obj.DispMsg() 'displayes "From Child"
Please note that even if you use a parent reference with the child object, the result would be the same as accessing the child member. Try testing it out using the following:
CType(obj, Parent).DispMsg()'displayes "From Child"
DirectCast(obj, Parent).DispMsg()'displayes "From Child"
Dim RefParent As Parent = obj
RefParent.DispMsg()'displayes "From Child"
Next: Multi-level method overriding in Visual Basic 2005 inheritance >>
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