Object-Oriented Programming in .NET framework

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Object-Oriented Programming
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The Protected Scope Qualifier
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Protected is a new scope qualifier that makes a member or a nested class visible inside the current class, as well to all classes derived by the current class. Put another way, Protected members are private members that are also inherited by derived classes. In general, a class author defines one or more members as Protected to provide inheritors with a way to modify the usual behavior of the class, but without letting regular clients do the same. For example, consider this Report class that prints a document with a header and a footer:
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Class Report Public Sub Print() Print the header. Print the body of the document. Print the footer. End Sub End Class
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This is an example of how you should never write a class that is meant to be inherited from. Here s a better version, which splits the three basic steps into their own Protected methods:
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Class Report Public Sub Print() OnPrintHeader() OnPrintBody() OnPrintFooter() End Sub Protected Overridable Sub OnPrintHeader() End Sub Protected Overridable Sub OnPrintBody() End Sub Protected Overridable Sub OnPrintFooter() End Sub End Class
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Because you usually provide Protected members so that inheritors can customize the base class behavior, these members are often marked with the Overridable keyword as well. For this reason, you should rarely use the Protected keyword with fields (which can t be overridden). Thanks to the three protected and overridable members, you can easily create new classes that reuse most of the code in the base Report class. For example, here s a Report2 class that displays no header and adds totals before the standard footer text:
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Class Report2 Inherits Report Protected Overrides Sub OnPrintHeader() Print no header. End Sub Protected Overrides Sub OnPrintFooter() Print all totals here. Print the standard footer. MyBase.OnPrintFooter() End Sub End Class
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Inheritance
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This pattern based on protected and overridable members is applied extensively anywhere in the .NET Framework. For example, the System.Windows.Forms.Form class exposes tons of Onxxxx methods, so all form classes you define in your program (which derive from the base Form class) can control every minor detail of the form s appearance and behavior.
The Protected Friend Scope Qualifier
The fifth scope qualifier available in Visual Basic .NET is Protected Friend, which com bines the features of the Friend and Protected keywords and therefore defines a mem ber or a nested class that s visible to the entire assembly and to all inherited classes. This keyword seems to be redundant you might think that Friend also comprises inherited classes until you consider that Visual Basic .NET allows you to inherit classes from other assemblies. In the Report sample class introduced in the previous section, you might flag the three Onxxxx methods with the Protected Friend Overridable keywords. This change in source code would enable other classes in the same assembly to individually print the header, the body, or the footer of the document, without providing clients outside the assembly with this capability.
Using Scope Qualifiers with Constructors
You might find it interesting to see what happens when you apply a scope qualifier other than Public to a constructor in a Public class. Using a Friend constructor makes the class creatable from inside the assembly but not from outside it, while leaving the class usable outside the current assembly. This is the closest equivalent of PublicNotCreatable classes in Visual Basic 6:
Public Class Widget This class can be created only from inside the current assembly. Friend Sub New() End Sub End Class
Part II:
Object-Oriented Programming
You can see that clients outside the current assembly must have a way to receive an instance of the Widget class somehow for example, by calling a Public method of another class. A class whose methods are used to create instances of another class is often called factory class. You can define a Private Sub New method if you want to prevent clients inside and outside the assembly from instancing the class. This approach can be useful when the class contains only shared members, so there s no point in creating an instance of it:
Class Triangle This private constructor prevents clients from instancing this class. Private Sub New() No implementation code here. End Sub Add here all the shared members for this class. Shared Function GetArea( ... ) As Double End Function End Class
Another use for Private constructors arises when you want clients to create instances through a shared member rather than with the usual New keyword, as in the following example:
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