Redefining Shared Members in .NET framework

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Redefining Shared Members
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You can use neither the Overridable nor the Overrides keyword with shared members because shared members can t be overridden. Either they re inherited as they are or they must be shadowed and redefined from scratch in the derived class. You cannot use the MyBase variable to invoke shared methods defined in the base class if you re redefining them in the derived class because MyBase is forbidden in shared methods. For example, say that you have a Person class with the following shared method:
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...(In the Person (base) class)... Shared Function AreBrothers(ByVal p1 As Person, ByVal p2 As Person) As Boolean Return (p1.Father Is p2.Father) Or (p1.Mother Is p2.Mother) End Function
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In addition, you have an Employee class that inherits from Person and redefines the AreBrothers shared method so that two Employee objects can be considered brothers if they have one parent in common and the same family name. The following code builds on the AreBrother shared method in the Person class so that if you later change the def inition in the Person class, the Employee class automatically uses the new definition:
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In the Employee (derived) class
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Shared Shadows Function AreBrothers(ByVal e1 As Employee, _
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ByVal e2 As Employee) As Boolean Return Person.AreBrothers(e1, e2) And (e1.LastName = e2.LastName) End Function
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Unfortunately, no keyword lets you reference shared members in the base class in a generic way (similar to what the MyBase keyword does with instance members of the base class). You have to hard-code the name of the base class inside the source code of the derived class when calling a shared method of the base class.
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Sealed and Virtual Classes
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Visual Basic .NET provides a few additional keywords that let you decide whether other developers can or must inherit from your class and whether they have to override some of its members.
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Part II:
Object-Oriented Programming
The NotInheritable Keyword
For security (or other) reasons, you might want to ensure that no one extends a class you created. You can achieve this by simply marking the class with the NotInheritable keyword:
Ensure that no one can inherit from the Employee class. NotInheritable Class Employee End Class
Classes that can t be inherited from are called sealed classes. In general, you rarely need to seal a class, but good candidates for the NotInheritable keyword are utility classes that expose only shared members. As you might expect, you can t use the Overridable keyword inside a sealed class.
The MustInherit Keyword
A situation that arises more frequently is that you want to prevent users from using your class as is and instead force them to inherit from it. In this case, the class is called a virtual or abstract class. You can use it only to derive new classes and can t instantiate it directly. To prevent direct usage of a class, you must flag it with the MustInherit keyword. You typically use this keyword when a class is meant to define a behavior or an archetypal object that never concretely exists. A typical example is the Animal class, which should be defined as virtual because you never instantiate a generic animal; rather, you create a specific animal a cat, a dog, and so on, which derives some of its properties from the abstract Animal class. Here s a more business-oriented example: your application deals with different types of documents invoices, orders, payrolls, and so on and all of them have some behaviors in common in that they can be stored, printed, displayed, or attached to an e-mail message. It makes sense to gather this common behavior in a Document class, but at the same time you want to be sure that no one mistakenly creates a generic Doc ument object. After all, you never say I am creating a document. Rather, you say I am creating an invoice, an order, and so on.
MustInherit Class Document Contents in RTF format Private m_RTFText As String Overridable Property RTFText() As String Get Return m_RTFText End Get Set(ByVal Value As String) m_RTFText = Value End Set End Property
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