VB.Net – 类和对象
定义类时，可以为数据类型定义蓝图.这实际上并没有定义任何数据，但它确实定义了类名的含义，即类的对象将包含什么以及可以对这样的对象执行什么操作.
对象是类的实例.构成类的方法和变量称为类的成员.
类定义
类定义以关键字类开头后跟班级名称;和类主体，以End Class语句结束.以下是类定义的一般形式 :
[ <attributelist> ] [ accessmodifier ] [ Shadows ] [ MustInherit | NotInheritable ] [ Partial ] _ Class name [ ( Of typelist ) ] [ Inherits classname ] [ Implements interfacenames ] [ statements ] End Class
其中，
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属性列表 是适用于该类的属性列表.可选.
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accessmodifier 定义类的访问级别，其值为 - Public，受保护，朋友，受保护的朋友和私人.可选.
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阴影 表示该变量重新声明并隐藏了一个名称相同的元素，或者基类中重载元素的集合.可选.
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MustInherit 指定该类只能用作基类，而您不能直接从它创建一个对象，即一个抽象类.可选.
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NotInheritable 指定该类不能用作基类.
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部分 表示该类的部分定义.
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继承 指定它继承的基类.
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Implements 指定类继承的接口.
以下示例演示了Box类，有三个数据成员，长度，宽度和高度:
Module mybox
Class Box
Public length As Double ' Length of a box
Public breadth As Double ' Breadth of a box
Public height As Double ' Height of a box
End Class
Sub Main()
Dim Box1 As Box = New Box() ' Declare Box1 of type Box
Dim Box2 As Box = New Box() ' Declare Box2 of type Box
Dim volume As Double = 0.0 ' Store the volume of a box here
' box 1 specification
Box1.height = 5.0
Box1.length = 6.0
Box1.breadth = 7.0
' box 2 specification
Box2.height = 10.0
Box2.length = 12.0
Box2.breadth = 13.0
'volume of box 1
volume = Box1.height * Box1.length * Box1.breadth
Console.WriteLine("Volume of Box1 : {0}", volume)
'volume of box 2
volume = Box2.height * Box2.length * Box2.breadth
Console.WriteLine("Volume of Box2 : {0}", volume)
Console.ReadKey()
End Sub
End Module
编译并执行上述代码时，会产生以下结果 :
Volume of Box1 : 210 Volume of Box2 : 1560
成员函数和封装
类的成员函数是一个函数，它在类定义中的定义或原型与任何其他变量一样.它对该类所属的任何对象进行操作，并且可以访问该对象的类的所有成员.
成员变量是对象的属性(从设计角度来看)并且它们被保密以实现封装.这些变量只能使用公共成员函数访问.
让我们在上面设置概念并获取类中不同类成员的值 :
Module mybox
Class Box
Public length As Double ' Length of a box
Public breadth As Double ' Breadth of a box
Public height As Double ' Height of a box
Public Sub setLength(ByVal len As Double)
length = len
End Sub
Public Sub setBreadth(ByVal bre As Double)
breadth = bre
End Sub
Public Sub setHeight(ByVal hei As Double)
height = hei
End Sub
Public Function getVolume() As Double
Return length * breadth * height
End Function
End Class
Sub Main()
Dim Box1 As Box = New Box() ' Declare Box1 of type Box
Dim Box2 As Box = New Box() ' Declare Box2 of type Box
Dim volume As Double = 0.0 ' Store the volume of a box here
' box 1 specification
Box1.setLength(6.0)
Box1.setBreadth(7.0)
Box1.setHeight(5.0)
'box 2 specification
Box2.setLength(12.0)
Box2.setBreadth(13.0)
Box2.setHeight(10.0)
' volume of box 1
volume = Box1.getVolume()
Console.WriteLine("Volume of Box1 : {0}", volume)
'volume of box 2
volume = Box2.getVolume()
Console.WriteLine("Volume of Box2 : {0}", volume)
Console.ReadKey()
End Sub
End Module
编译执行上述代码时，产生t他跟随结果:
Volume of Box1 : 210 Volume of Box2 : 1560
构造函数和析构函数
类构造函数是类的一个特殊成员Sub，只要我们创建新的对象就会执行它类.构造函数的名称为 New ，它没有任何返回类型.
以下程序解释了构造函数的概念 :
Class Line
Private length As Double ' Length of a line
Public Sub New() 'constructor
Console.WriteLine("Object is being created")
End Sub
Public Sub setLength(ByVal len As Double)
length = len
End Sub
Public Function getLength() As Double
Return length
End Function
Shared Sub Main()
Dim line As Line = New Line()
'set line length
line.setLength(6.0)
Console.WriteLine("Length of line : {0}", line.getLength())
Console.ReadKey()
End Sub
End Class
当编译并执行上面的代码时，它产生以下结果 :
Object is being created Length of line : 6
默认构造函数没有任何参数，但如果需要，构造函数可以有参数.这种构造函数称为参数化构造函数.此技术可帮助您在创建对象时为其分配初始值，如以下示例所示 :
Class Line
Private length As Double ' Length of a line
Public Sub New(ByVal len As Double) 'parameterised constructor
Console.WriteLine("Object is being created, length = {0}", len)
length = len
End Sub
Public Sub setLength(ByVal len As Double)
length = len
End Sub
Public Function getLength() As Double
Return length
End Function
Shared Sub Main()
Dim line As Line = New Line(10.0)
Console.WriteLine("Length of line set by constructor : {0}", line.getLength())
'set line length
line.setLength(6.0)
Console.WriteLine("Length of line set by setLength : {0}", line.getLength())
Console.ReadKey()
End Sub
End Class
编译并执行上述代码时，会产生以下结果 :
Object is being created, length = 10 Length of line set by constructor : 10 Length of line set by setLength : 6
析构函数是类的特殊成员Sub，只要其类的对象超出范围，就会执行.
A 析构函数的名称为 Finalize ，它既不能返回值也不能接受任何参数.析构函数对于在退出程序之前释放资源非常有用，例如关闭文件，释放内存等.
析构函数不能被继承或重载.
下面的例子解释了析构函数的概念 :
Class Line
Private length As Double ' Length of a line
Public Sub New() 'parameterised constructor
Console.WriteLine("Object is being created")
End Sub
Protected Overrides Sub Finalize() ' destructor
Console.WriteLine("Object is being deleted")
End Sub
Public Sub setLength(ByVal len As Double)
length = len
End Sub
Public Function getLength() As Double
Return length
End Function
Shared Sub Main()
Dim line As Line = New Line()
'set line length
line.setLength(6.0)
Console.WriteLine("Length of line : {0}", line.getLength())
Console.ReadKey()
End Sub
End Class
当编译并执行上面的代码时，它产生以下结果 :
Object is being created Length of line : 6 Object is being deleted
VB.Net类的共享成员
我们可以使用Shared关键字将类成员定义为static.当我们将一个类的成员声明为Shared时，这意味着无论创建了多少个类的对象，该成员只有一个副本.
关键字共享意味着一个类只存在一个成员实例.共享变量用于定义常量，因为可以通过调用类来检索它们的值，而无需创建它的实例.
可以在成员函数或类定义之外初始化共享变量.您还可以在类定义中初始化共享变量.
您还可以将成员函数声明为Shared.此类函数只能访问共享变量.甚至在创建对象之前就存在共享函数.
以下示例演示共享成员的使用 :
Class StaticVar
Public Shared num As Integer
Public Sub count()
num = num + 1
End Sub
Public Shared Function getNum() As Integer
Return num
End Function
Shared Sub Main()
Dim s As StaticVar = New StaticVar()
s.count()
s.count()
s.count()
Console.WriteLine("Value of variable num: {0}", StaticVar.getNum())
Console.ReadKey()
End Sub
End Class
编译并执行上述代码时，它会产生以下代码结果 :
Value of variable num: 3
继承
面向对象编程中最重要的概念之一是继承.继承允许我们根据另一个类定义一个类，这样可以更容易地创建和维护一个应用程序.这也提供了重用代码功能和快速实现时间的机会.
创建类时，程序员可以指定新类，而不是编写全新的数据成员和成员函数.应该继承现有类的成员.此现有类称为 base 类，新类称为派生类.
Base&派生类
一个类可以从多个类或接口派生，这意味着它可以从多个基类或接口继承数据和函数.
VB.Net中用于创建派生类的语法如下 :
<access-specifier> Class <base_class> ... End Class Class <derived_class>: Inherits <base_class> ... End Class
考虑一个基类Shape及其派生类Rectangle :
' Base class
Class Shape
Protected width As Integer
Protected height As Integer
Public Sub setWidth(ByVal w As Integer)
width = w
End Sub
Public Sub setHeight(ByVal h As Integer)
height = h
End Sub
End Class
' Derived class
Class Rectangle : Inherits Shape
Public Function getArea() As Integer
Return (width * height)
End Function
End Class
Class RectangleTester
Shared Sub Main()
Dim rect As Rectangle = New Rectangle()
rect.setWidth(5)
rect.setHeight(7)
' Print the area of the object.
Console.WriteLine("Total area: {0}", rect.getArea())
Console.ReadKey()
End Sub
End Class
编译并执行上述代码时，会产生以下结果 :
Total area: 35
基类初始化
派生类继承基类成员变量和成员方法.因此，应在创建子类之前创建超类对象.超类或基类在VB.Net中隐含地称为 MyBase
以下程序演示了这个 :
' Base class
Class Rectangle
Protected width As Double
Protected length As Double
Public Sub New(ByVal l As Double, ByVal w As Double)
length = l
width = w
End Sub
Public Function GetArea() As Double
Return (width * length)
End Function
Public Overridable Sub Display()
Console.WriteLine("Length: {0}", length)
Console.WriteLine("Width: {0}", width)
Console.WriteLine("Area: {0}", GetArea())
End Sub
'end class Rectangle
End Class
'Derived class
Class Tabletop : Inherits Rectangle
Private cost As Double
Public Sub New(ByVal l As Double, ByVal w As Double)
MyBase.New(l, w)
End Sub
Public Function GetCost() As Double
Dim cost As Double
cost = GetArea() * 70
Return cost
End Function
Public Overrides Sub Display()
MyBase.Display()
Console.WriteLine("Cost: {0}", GetCost())
End Sub
'end class Tabletop
End Class
Class RectangleTester
Shared Sub Main()
Dim t As Tabletop = New Tabletop(4.5, 7.5)
t.Display()
Console.ReadKey()
End Sub
End Class
当编译并执行上述代码时，它会产生以下结果 :
Length: 4.5 Width: 7.5 Area: 33.75 Cost: 2362.5
VB .Net支持多重继承.