Object-Oriented Programming in .NET

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Object-Oriented Programming
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base class, you must dutifully apply these changes to all the classes that inherit from Person. The MyBase keyword lets you implement a better solution:
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Overrides Function CompleteName() As String If Title <> Then CompleteName = Title & CompleteName &= MyBase.CompleteName End Function
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Constructors in Derived Classes
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Even though you declare constructor procedures with the Sub keyword, they aren t ordinary methods and aren t inherited from the base class in the way all other methods are. It s up to you to provide the derived class with one or more constructors if you want the derived class to be creatable using the same syntax as the base class. If the base class has a Sub New procedure that takes no arguments, you don t strictly need to define an explicit constructor for the derived class. The same happens if you have a class with no explicit constructor because in that case the Visual Basic compiler creates a hidden constructor for you behind the scenes. As a matter of fact, all the preceding examples show that you can create an instance of the Employee class without defining a constructor for it:
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Dim e As Employee = New Employee()
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Things are different when the base class doesn t include either an explicit or an implicit parameterless constructor. In this case, the derived class must contain a constructor method, and the first executable line of this method must be a call to the base class s constructor. Say that the Person2 class has the following constructor method:
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Class Person2 Sub New(ByVal firstName As String, ByVal lastName As String) Me.FirstName = firstName Me.LastName = lastName End Sub
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...(other properties and methods as in Person class) ...
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End Class
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The derived Employee2 class must therefore contain the following code:
Class Employee2 Inherits Person2 Sub New(ByVal firstName As String, ByVal lastName As String) The first executable statement *must* be a call to the constructor in the base class. MyBase.New(firstName, lastName) You can continue with the initialization step here. End Sub End Class
5:
Inheritance
The constructor in the derived class can have a different argument signature from the constructor in the base class, but in this case the first executable statement must be a call to the base class s constructor:
Public Title As String A new field
Sub New(ByVal firstName As String, ByVal lastName As String, _ ByVal title As String) MyBase.New(firstName, lastName) Me.Title = title End Sub
The MyClass Keyword
You can easily miss a subtle but important detail of inheritance: when a client calls a nonoverridden method of an inherited class, the called method is the one defined in the base class, but it runs in the context of the derived class. The simplest way to explain this concept is through an example, once again based on the Person-Employee pair. Let s define a Person3 base class exposing a TitledName method that returns the complete name of the person, prefixed with his or her title if one has been specified:
Enum Gender NotSpecified Male Female End Enum Class Person3 (In a real-world class, these would be properties.) Public FirstName As String Public LastName As String Public Gender As Gender = Gender.NotSpecified ...(other members omitted for brevity) ... Dim m_Title As String Overridable Property Title() As String Get Return m_Title End Get Set(ByVal Value As String) m_Title = Value End Set End Property Prefix the name with a Function TitledName() As If Title <> Then Return Title & Else Return FirstName End If title if one has been specified. String & FirstName & & LastName & & LastName
Part II:
Object-Oriented Programming
End Function End Class
The derived Employee3 class doesn t override the TitledName method, but it does override the Title property, so it s never an empty string:
Class Employee3 Inherits Person3 Override Title to provide a title if no one has been assigned. Overrides Property Title() As String Get If MyBase.Title <> Then Return MyBase.Title ElseIf Gender = Gender.Male Then Return Mr." ElseIf Gender = Gender.Female Then Return Mrs." End If End Get Set(ByVal Value As String) MyBase.Title = Value End Set End Property End Class
Because the derived class doesn t override the TitledName property, the version in the base class is used. However, that code runs in the context of the derived class. Therefore, it uses the overridden version of the Title property, the one defined in Employee3 instead of the one defined in Person3:
Dim e As New Employee3( Joe", Doe )
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