VB2008从入门到精通(PDF格式英文版)-第56部分
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
Dim ine As ITaxIne = engine。CreateIne（100）
Dim capitalGain As ITaxIne = canadaEngine。CreateCapitalGain（100）
…………………………………………………………Page 210……………………………………………………………
188 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S
account。AddIne（ine）
account。AddIne（capitalGain）
Dim deduction As ITaxDeduction = engine。CreateDeduction（20）
account。AddDeduction（deduction）
Dim taxToPay As Double = engine。CalculateTaxToPay（account）
Console。WriteLine（〃Tax to pay （〃 & taxToPay & 〃）〃）
Notice the definition of the variables engine and canadaEngine。 This is fine； because what
we are doing with the interfaces is choosing a characteristic that can be dynamically queried。
Learning More About Inheritance and
Type Casting
This chapter introduced interfaces and ponents； and delved deeper into inheritance。 Here； I’ll
provide more details about inheritance and type casting。
More Inheritance Details
In this section； I am going to clearly lay out how inheritance works in Visual Basic。 I will present
examples of usage for six scenarios。 Each example is followed by a list explaining the key points
to understand。 My goal is to illustrate all of the possible scenarios so that you have a reference
to how inheritance works。
■Note All of the examples demonstrate using methods； but it is possible to apply the same inheritance
techniques using properties。
Scenario 1： Overloading Base Class Functionality
Class Base
Public Sub Method（）
Console。WriteLine（〃Base。Method〃）
End Sub
End Class
Class Derived
Inherits Base
Public Overloads Sub Method（）
Console。WriteLine（〃Derived。Method〃）
End Sub
End Class
…………………………………………………………Page 211……………………………………………………………
CH AP T E R 7 ■ L E AR N IN G AB O U T CO M P O N E N TS AN D C L AS S H I E R AR C HI E S 189
Module Test
Public Sub Run（）
Dim derivedCls As Derived = New Derived（）
Dim baseCls As Base = derivedCls
' Calls Derived。Method
derivedCls。Method（）
' Calls Base。Method
baseCls。Method（）
End Sub
End Module
o Keywords used： Overloads in the derived class to indicate the method is being overloaded。
o Overloading a method means to change the functionality of the method in the derived class。
o Which method is called in the inheritance depends on the type of the variable on which
the method is called。 Thus； if the variable is of type Base； Base。Method（） is called； if the
variable is of type Derived； Derived。Method（） is called。
Scenario 2： Overriding Base Class Functionality
Class Base
Public Overridable Sub Method（）
Console。WriteLine（〃Base。Method〃）
End Sub
End Class
Class Derived
Inherits Base
Public Overrides Sub Method（）
Console。WriteLine（〃Derived。Method〃）
End Sub
End Class
Module Test
Public Sub Run（）
Dim derivedCls As Derived = New Derived（）
Dim baseCls As Base = derivedCls
' Calls Derived。Method
derivedCls。Method（）
' Calls Derived。Method
baseCls。Method（）
End Sub
End Module
…………………………………………………………Page 212……………………………………………………………
190 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S
o Keywords used： Overridable in the base class to indicate the method behavior can be
changed in the derived class。 Overrides is used in the derived classes to indicate a method
with the new behavior。 Multiple levels of inheritance require multiple usages of Overrides。
o Overriding a method means to change the behavior of the method in the base class to
that of the derived class。 If there are multiple levels of inheritance； then the functionality
used is the instantiated type。
o You can also define a Overridable base class method using the MustOverride keyword。
The difference between Overridable and MustOverride is that Overridable has a method
or property implementation in the base class； whereas MustOverride has no implemen
tation in the base class。
Scenario 3： Implementing an Interface
Interface IInterface
Sub Method（）
End Interface
Class Implementation
Implements IInterface
Public Sub Method（） Implements IInterface。Method
Console。WriteLine（〃Implementation。Method〃）
End Sub
End Class
Module Test
Public Sub Run（）
Dim implementation As Implementation = New Implementation（）
Dim inst As IInterface = implementation
' Calls Implementation。Method
implementation。Method（）
' Calls Implementation。Method
inst。Method（）
End Sub
End Module
o Keywords used： none。
o The class that implements the interface has a behavior； like when a class subclasses and
implements a MustOverride method。
o Whether the variable is an interface type or an implementation type； the method
Implementation。Method（） is called。
…………………………………………………………Page 213……………………………………………………………
CH AP T E R 7 ■ L E AR N IN G AB O U T CO M P O N E N TS AN D C L AS S H I E R AR C HI E S 191
Scenario 4： Implementing Two Interfaces with Same Method/Property Name
Interface IInterface1
Sub Method（）
End Interface
Interface IInterface2
Sub Method（）
End Interface
Class Implementation
Implements IInterface1； IInterface2
Public Sub Method1（） Implements IInterface1。Method
Console。WriteLine（〃Implementation。IInterface1。Method〃）
End Sub
Private Sub Method2（） Implements IInterface2。Method
Console。WriteLine（〃Implementation。IInterface2。Method〃）
End Sub
End Class
Module Test
Public Sub Run（）
Dim implementation As Implementation = New Implementation（）
Dim inst1 As IInterface1 = implementation
Dim inst2 As IInterface2 = implementation
' Calls Implementation。Method1
inst1。Method（）
' Calls Implementation。Method2
inst2。Method（）
End Sub
End Module
o Keywords used： special notation for the implementation of a particular interface method
（for example； Implements IInterface1。Method and Implements IInterface2。Method）。
o The methods that are converted to implement a particular interface can still be called。
In the example； Method1（） is declared as Public； and thus if you have a reference to
Implementation； then you can call Method1（）。
Scenario 5： Implementing an Interface That Allows Overriding
Interface IInterface
Sub Method（）
End Interface
…………………………………………………………Page 214……………………………………………………………
192 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S
Class Implementation
Implements IInterface
Public Overridable Sub Method（） Implements IInterface。Method
Console。WriteLine（〃Implementation。Method〃）
End Sub
End Class
Class ImplementationDerived
Inherits Implementation
Public Overrides Sub Method（）
Console。WriteLine（〃ImplementationDerived。Method〃）
End Sub
End Class
Module Test
Public Sub Run（）
Dim implementation As ImplementationDerived = _
New ImplementationDerived（）
Dim inst As IInterface = implementation
' Calls ImplementationDerived。Method
implementation。Method（）
' Calls ImplementationDerived。Method
inst。Method（）
End Sub
End Module
o Keywords used： Overridable and Overrides。
o By default； when you implement an interface without using the Overridable keyword；
you are saying that the derived class can overload only the method。 In many cases； this
is appropriate； as you will want to override the method and thus will need to use the
Overridable and Overrides keywords。
o This is a mon problem； and most beginning Visual Basic programmers are puzzled
by the overloading behavior。
