Disadvantages of Inheritance in Visual Basic .NET

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714 Disadvantages of Inheritance
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The use of inheritance can also have a down-side, since a change made to an ancestor class (either to alter its behaviour or to x a bug) could cause problems in any descendent classes If we use encapsulation rigorously, this should not happen; a class interface should not change just because we have made alterations to the implementation However, as we ll see shortly, we do not always base a Sub class on just the public interface of its parents; sometimes the sub class can have special access to otherwise hidden class members, and so we could make a change to a base class that did not affect the interface but did affect members that a sub class had direct access to The result could be that the modi ed internal behaviour of the base class caused problems with the sub class Another problem with inheritance (admittedly, not a very profound problem) is simply that over time we can create inheritance relationships to such a depth (ie sub-sub-sub-sub -classes) that we could lose sight of the capabilities of the ultimate ancestor class, and therefore lose control of the code This is a problem that also exists in structured programming, since we can have a sub that calls a sub that calls, etc to a level of nesting that makes it dif cult to follow all of the possible consequences In structured programming the cure is to be careful not to create callstructures of arbitrary depth; the onus is on the person writing a sub to be aware of the effects of other subs he or she is calling In inheritance relationships in object-oriented programming, good use of encapsulation and careful consideration of the level of access a class has to its base class is important, and the onus is on the designer of the Base class to provide this As a developer of a base class, you have a responsibility to make sure inheritors can only access members it is safe for them to access
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Exercise 72
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Microsoft Visual Studio NET provides programmers with sources of information and operational cues to ensure that errors are less likely when using inheritance Start up VS NET with a new or existing project and examine the following features a) Dynamic Help: display the Dynamic Help window (click on the Dynamic Help tab at the bottom of the Properties window if it is not visible) and then enter a private, public or Dim statement that creates a reference variable for an existing class (eg DivideByZeroException) Note that a list of help topics appears in the Dynamic Help window click on one of the top entries (eg the DivideByZeroException members entry) and examine the consequent help page Class overview page note that this help page for any class displays an ancestry chart for that class (always starting from SystemObject) and
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72 Code Inheritance
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indicating the class, its ancestors and its descendents (note also the Implements list, indicating interface classes this class implements) Class Members page (always at the top of the class overview) go to this and examine the format that the list of member properties and methods is shown in Note that only public and protected (explained later) members are listed, and make yourself familiar with the various icons used (hovering the mouse cursor over an icon will reveal its description) Click on a speci c class member to go to its de nition page Note that keywords such as Overridable, MustOverride (described later), NotOverridable and others are used to describe the possibilities for using this member in code inheritance
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Code Inheritance
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Code inheritance is invoked when a class uses the keyword Inherits at the start of its de nition The Inherits keyword must be followed by the name of a class that exists in the scope of the module being coded; either a class de ned within the same project or within a project to which a reference has been created Visual Basic NET will not compile a class that tries to inherit from another class that is not in scope, and Visual Studio will mark the faulty Inherits statement with a blue wavy underline
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