MyClass - Implementing Polymorphism in VB.Net - Code

(Page 2 of 2 )

I will replicate the exact same code, as first published in the original tip, and I will place the keyword in the appropriate place.

Lets see some code below:

Class Parent
Public Overridable Sub p1()
Console.Write("Parent.p1")
End Sub
Public Sub p2()
MyClass.p1()
'Implementing keyword MyClass here tells all derived classes to refer to the base / abstract class wherein the keyword MyClass appears
End Sub
End Class

Class Child
Inherits Parent
Public Overrides Sub p1()
Console.Write("Child.p1")
MyBase.p2()
End Sub
End Class

Sub Main()
Dim p As Parent
Dim c As Child
p = New Parent()
p.p1() 'OK
p.p2() 'OK
p = New Child()
p.p1() 'stack overflow error is prevented

If MyClass is not implemented above in the base class, you will get a stack overflow error. So why does the stack overflow occur?

• Parent.p2() calls Parent.p1()
• Parent.p1() is polymorphic because it can be overridden
• Child.p1() overrides Parent.p1() so any calls to Parent.p1() will actually call Child.p1() if you have an instance of class Child
• Child.p1() calls MyBase.p2()
• MyBase.p2() is actually Parent.p2()

Now if you are still with me, the pitfall comes when you have an instance of class Child and call procedures p1 and p2. Calling p1 produces the following execution flow:

Child.p1 -> Parent.p2 -> Child.p1 -> Parent.p2 -> Child.p1 -> etc.

And the cycle repeats until we have no stack space left. Calling p2 produces the following execution flow:

Child.p2 -> Child.p1 -> Parent.p2 -> Child.p1 -> Parent.p2 -> Child.p1 -> etc.

Implementing the keyword MyClass in the base class tells all derived classes to use the method in the base class itself and, therefore, a stack overflow error is prevented.

p.p2() 'stack overflow error is prevented
c = New Child()
c.p1() 'stack overflow error is prevented
c.p2() 'stack overflow error is prevented
End Sub

I hope this will help VB.Net developers along the way to develop software the object-oriented way.

DISCLAIMER: The content provided in this article is not warranted or guaranteed by Developer Shed, Inc. The content provided is intended for entertainment and/or educational purposes in order to introduce to the reader key ideas, concepts, and/or product reviews. As such it is incumbent upon the reader to employ real-world tactics for security and implementation of best practices. We are not liable for any negative consequences that may result from implementing any information covered in our articles or tutorials. If this is a hardware review, it is not recommended to open and/or modify your hardware.

Comments On: MyClass - Implementing Polymorphism in VB.Net

>>> Be the FIRST to comment on this article!