Overriding in VB.NET

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723 Overriding
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Overriding a member of a class is replacing it with a new de nition Only properties and methods can be overridden, which is sensible since ideally member variables should all be marked Private to protect them from being misused The use of the word method to describe a sub or function becomes clearer once we consider the
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7 n Inheritance and Polymorphism
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use of overriding, since we can now think of a particular class s method of doing some job For example, the BankAccount s method of doing a withdrawal was overridden by the SpecialInvestmentAccount class, so while both classes have a Withdraw function, each had a different method of doing it By default, a method in a class can not be overridden This is a sensible approach, since it means that we, as designers of a base class, get to say whether any of its methods can be replaced by alternative code Since as the designer of a base class we are aware of its implementation details, there is no-one better placed to decide whether a particular method can be overridden or not If we wish to de ne a method of a class so that it can be overridden in descendant classes, we use the keyword Overridable in its de nition (Listing 79)
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Class SomeClass Public Overridable Sub SomeMethod() 'This method can be redefined in a descendant class End Sub Public Sub SomeOtherMethod() 'This method can NOT be redefined in a descendant 'class End Sub End Class Listing 79: Making a method overridable
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Using the code in Listing 79 as a base class, we would nd that VB would allow us to override the SomeMethod() sub but would prevent us from overriding the SomeOtherMethod() sub Marking a method as overridable is not all that is necessary to allow us to override it in another class The method s signature, that is the combination of its name and list of parameters, must be the same in the original method and the new method that overrides it For example, Visual Basic will not allow this (Listing 710)
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Class OtherClass Inherits SomeClass Public Overrides Sub SomeMethod( _ ByVal SomeValue As Integer) 'This method is not signature-compatible with the one 'it is overriding, so VB does not allow the override End Sub End Class Listing 710: Overriding a method requires total compatibility (this code does not work)
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Once we can override a method, we can provide class-speci c ways of doing some job In our banking example classes, the plain BankAccount s Withdraw() method was replaced with a more complex method for a SpecialInvestmentAccount To allow that to happen, we had to mark the plain
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72 Code Inheritance
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Withdraw() method as Overridable As a result, each class had the ability to
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work out for itself how a withdrawal would be made This is a goal in inheritance; delegate work to classes in such a way that they do the work without us worrying about the details of how they do it
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724 Abstract Classes
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As base class designers, we get to be even more speci c about the way a class is inherited from than simply indicating which methods can be overridden and which classes or code can access them We can de ne a method in a class so that it must be overridden in a class that inherits from it Of course, by doing this, we would also be saying that the class we are de ning is not a complete class, since we have deliberately left a hole where the method that must be overridden is Why would we do this One good reason could be that we wanted to create a base class that de ned the core behaviour of a family of classes This would be a class created only for inheritance purposes, and to create it we would also use the MustInherit keyword as part of the class de nition For example, let s say we wanted to create a new version of the calculator program developed in the activities at the end of 4 The new version is to use inheritance to allow a range of different types of calculation on pairs of numbers Every calculation would involve Number1 and Number2, both of which could be de ned as properties or public member variables of every calculation class Every calculation would also have a Calculate() method to make it do its job and return a result However, at this stage, we re more interested in de ning what a calculation does than how it works We can de ne the core behaviour of a calculation class as shown in Listing 711
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MustInherit Class Calculation Public Number1 As Double Public Number2 As Double 'Here is the function that must be overridden Protected MustOverride Function Calculate() As Double Public Overridable Sub DisplayResult() ConsoleWriteLine("The result is {0}", Calculate) End Sub End Class Listing 711: A class that must be inherited from
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The Calculation class shown above cannot be instantiated We can create a reference variable for the class, such as:
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