Access to the Base Class s members in Visual Basic .NET

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Once a class is properly inherited from, any of its members (variables, properties, subs or functions) that are marked Public or Protected can be accessed by the sub class Members that are marked Private are hidden from the sub class The Friend access modi er can also be used to indicate members that can be accessed by any code in the same assembly; this is a simple way of de ning members that are accessible in the project you are coding but not from any other project The various access modi ers are there to allow you complete control over inheritance, and operate as shown in Table 71 We use the various access speci ers to create encapsulation By careful use of these, we can prevent access to sensitive or dangerous members of a class by other code that uses the class while allowing new classes based on it to control access to only those members we decide they will need access to As a class designer, it is your responsibility to use the encapsulation keywords to protect your code from unauthorized or erroneous access and yet simultaneously provide access to any class that might bene t from it by inheritance Note that the Protected and Friend keywords operate to allow us to break the strict use of encapsulation In an ideally encapsulated class, there should be no need for any of its subclasses to access Private member data; the entire range of
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Table 71
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7 n Inheritance and Polymorphism
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Access keywords
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Scope Keyword Private
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Items marked Private are visible only to the class or module they are de ned in Items marked Protected are visible to the class they are de ned in, and classes that descend from it (ie direct subclasses, and their descendants) Items marked Friend are visible to all code within the assembly (ie the current project), but hidden from code that contains a reference to it
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Use to hide member variables and internal-use only subs and functions from all other code Use to create members that are hidden from code that uses the class or its descendants, but visible to the descendants
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Protected
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Friend
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Use Friend access to create members that are freely accessible to any and all code in the current project; this allows, for example, two separate classes de ned in a project to access each others member variables Friend access is the default, so providing no access speci er for a member is equivalent to giving it Friend access Generally, use Public declarations only to de ne the Interface for a class or module Public access puts no restrictions on access from code in other classes, modules or even projects The MyBase keyword is used to indicate that the attached method call is to a method in the base class, rather than to a method with the same name in the class being implemented For example, a class can use MyBaseNew() in its constructor to indicate that it should perform the construction work of a base class object as part of its own construction work
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Public
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Items marked Public can be accessed by any code, within the current project and by any other project that has a reference to it Indicates a call to one of a base class s members The call is only allowed if appropriate access (using one of the scope keywords above) is allowed (eg we cannot use MyBase to call a Private member function)
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MyBase
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behaviour should be accessible via the Public interface However, back in the real world, there are many situations where it is either much easier or more ef cient to allow a sub class to access member data that is otherwise kept hidden Listing 77 demonstrates the use of various access speci ers
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Class Parent Public MyInt As Integer ' Available to all Protected MySingle As Single ' Available to subclasses Private MyString As String ' Not available outside ' Parent Friend MyDate As Date ' Available to code in ' project Public Sub New() ' This sub sets up a new Parent object MyInt = 0 MySingle = 314
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