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:
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.
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