The .NET Framework and Visual Basic .NET Object Programming in .NET

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The .NET Framework and Visual Basic .NET Object Programming
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Notice that the DrawingShape class contains concrete and abstract code. A class does not need to be completely abstract when it is labeled as MustInherit.
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Polymorphism
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From Greek roots, polymorphism means many forms or many faces. Polymorphism is often required when a general routine, such as a Print method, needs to be executed across many objects, but each object implements its print method differently. Polymorphism can be accomplished via the following methods: 1. Overriding methods that are labeled as overridable. 2. Overriding methods that are labeled as abstract. 3. Implementing interfaces (discussed in the Interfaces section later in this chapter).
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Modules
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A module is a class that only contains shared members. Members do not need the shared keyword, as they are implicitly shared. Public module members are essentially global members. Modules are not inheritable, and instances of modules cannot be created. Interfaces, which are covered later in this chapter, cannot be implemented on modules. The following is an example of a module:
Public Module Utilities Public Sub CopyFile(ByVal Src As String, ByVal Dest As String) copy code End Sub Public Function ReadKeys() As String Read keystrokes from keyboard End Function End Module
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Notice that the Shared keyword is not used, although both methods are implicitly shared. These methods can be executed from another part of the application by simply using the method name. The following code will work:
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CopyFile( C:\Test.txt , D:\abc.txt )
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Structures
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Structures are light classes. All structures are derived from System.ValueType. When an instance of a structure is created, memory is allocated onto the stack. When making an assignment, a deep copy is done, which means that all data is copied; not just the reference. Structures support properties, fields, methods, and interfaces. Structures cannot be inherited and cannot have events. The no parameter constructor for a structure is automatically created by the Visual Basic .NET compiler and cannot be overridden, which allows the following syntax:
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Dim z as myStructure automatically creates instance
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Structures can have parameterized constructors, as shown in the following code:
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Public Structure myStructure Public x As Integer Public y As Integer Public Sub New(ByVal x As Integer, ByVal y As Integer) Me.x = x Me.y = y End Sub End Structure
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This example shows a structure called myStructure and its parameterized constructor. An instance of this structure may be created by issuing any of the following commands:
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Dim z as myStructure use the default constructor Dim z as new myStructure also uses the default constructor Dim z as new myStructure(5,9) use parameterized constructor
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Since structure assignment is done by performing a deep copy, keep a structure limited in size. Depending on how the structure is used, there could be a performance gain to converting structures over 50 bytes in size to classes.
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The .NET Framework and Visual Basic .NET Object Programming
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Interfaces
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Interfaces can be created and used when it is necessary to separate specification from implementation, which is the basis for polymorphism. In many respects, an interface is similar to an abstract class (a class labeled as MustInherit) that has no concrete members (all methods are labeled as MustOverride). The differences between interfaces and abstract classes are shown in Table 4.3. The last item in Table 4.3 is probably the most compelling reason to use an interface. Consider the scenario in which an application is being written to maintain a list of cars, and two vendors have written car classes for this application. VendorA supplied a class called GeneralCar and VendorB supplied a class called SportsCar. At some point, a complete list of cars must be printed, based on properties that are unique in each of the car classes. Assumptions are that the source code is unavailable for GeneralCar and SportsCar, and these classes don t derive from a common class that has its source code available. In this scenario, a new class is created for each of the car classes, called PrintableGeneralCar, which derives from GeneralCar, and PrintableSportsCar, which derives from SportsCar. In addition, an interface has been created called Ireport. (See Figure 4.12).
Table 4.3 Differences between Interfaces and Abstract Classes ABSTRACT CLASS Can contain data members, such as variables. Only supports single inheritance. Can provide concrete methods along with abstract methods. Requires a common base class to separate specification from implementation.
INTERFACE Cannot contain data Supports multiple inheritance Cannot provide concrete methods Does not require a common base class to separate specification from implementation