VB2008从入门到精通(PDF格式英文版)-第71章
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o This is a mon problem； and most beginning Visual Basic programmers are puzzled
by the overloading behavior。
Scenario 6： An Inheritance Tree That Overrides and Overloads
Class Base
Public Overridable Sub Method（）
Console。WriteLine（〃Base。Method〃）
End Sub
End Class
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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 193
Class Derived1
Inherits Base
Public Overrides Sub Method（）
Console。WriteLine（〃Derived1。Method〃）
End Sub
End Class
Class Derived2
Inherits Derived1
Public Overloads Overridable Sub Method（）
Console。WriteLine（〃Derived2。Method〃）
End Sub
End Class
Class Derived3
Inherits Derived2
Public Overrides Sub Method（）
Console。WriteLine（〃Derived3。Method〃）
End Sub
End Class
Module Test
Public Sub Run（）
Dim derivedCls As Derived3 = new Derived3（）
Dim baseCls As Base = derivedCls
Dim derived2cls As Derived2 = derivedCls
' Calls Derived3。Method
derivedCls。Method（）
' Calls Derived1。Method
baseCls。Method（）
' Calls Derived3。Method
derived2cls。Method（）
End Sub
End Module
o Keywords used： Overridable； Overrides； and Overloads。
o The inheritance hierarchy is saying that Derived1。Method（） overrides the behavior of
Base。Method（）。 Derived2。Method（） overloads the behavior of Derived1。Method（）； while
establishing a new overriding base method。 Derived3。Method（） overrides the behavior
Derived2。Method（）； and not Base。Method（） （very important）。
o When confronted with a plex inheritance hierarchy as illustrated by scenario 6； it is
important that you start at the base class and work up the inheritance hierarchy。
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194 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
More Type…Casting Details
This chapter illustrated some type…casting examples。 In Visual Basic； you have two ways to
type cast：
o A forced type cast； which can also be used on value types
o A type cast that queries if a type cast is possible
Consider this hierarchy：
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
The next step is to instantiate the type Derived and cast the instance to the base type：
Dim derivedCls As Derived = New Derived（）
Dim baseCls As Base = derivedCls
When casting from a derived type to a base class type； a type cast is not necessary and you
can assume it is implied。
A type cast is necessary when you want to cast a base class instance to a derived class instance。
Following is the source code for a forced type cast； assuming the inheritance hierarchy from
the previous cast。
Dim backToDerived As Derived = DirectCast（baseCls； Derived）
The forced cast is the use of the method DirectCase。 The cast is forced because a conver
sion to the desired type will occur； regardless if it is possible or not。 If the cast is not possible； a
cast exception is thrown。
Another way to perform a type cast is to use a query cast； as illustrated by the following
code； again assuming the inheritance hierarchy of this section。
Dim backToDerived As Derived = TryCast（baseCls； Derived）
In the code； the cast involves using the TryCast method。 This cast is a query because a cast
will be attempted。 If the cast is successful； then an instance of the type is assigned to the vari
able backToDerived。 If the cast is not possible； then backToDerived is assigned a Nothing value。
No exception is thrown。 This casting technique is possible only for reference types。
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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 195
The Important Stuff to Remember
In this chapter； you learned about interfaces and implementations。 Here are the key points
to remember：
o Using interfaces is not like using inheritance。 They are two separate designs； even though
interfaces will use inheritance。
o Interfaces at an abstract level imply ideas of how you would like your application to work。
o Ideas when implemented as interfaces should be general and applicable to multiple
application implementations for the domain。
o Ideas are represented using Visual Basic interfaces。 And interfaces are implemented
using classes or structures。 But note that interfaces are reference types。
o Factories are used to instantiate implementations and return an instance that imple
ments an interface。 Using a factory means that the user of an interface does not need to
know which type of implementation object is instantiated。
o Interfaces can be considered as facets that might be targeting a specific characteristic of
an implementation。 However； as illustrated in the previous chapter； interfaces do not
expose Friend state or the Friend workings of the implementations。
o ponents are a fundamental way of developing code and should be your primary
way of writing code。 For the remainder of this book； interfaces will be used whenever
possible。 Take notice and try to understand what the idea behind the interface is。
Some Things for You to Do
The following are a couple things for you to do to apply what you’ve learned in this chapter。
1。 Implement your own tax system using the predefined base classes。
■Note Because of the number of possible tax systems； I do not provide an answer for exercise 1。 If you
want me to review your answer； please send it to me at christianhgross@gmail。。
2。 Add functionality to the tax engine’s base classes that implement the behavior of a
minimum tax…free amount。 This means if your ine is not above the minimum tax
free amount； you do not pay taxes。
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C H A P T E R 8
■ ■ ■
Learning About ponent
Oriented Architecture
So far； you have learned the essentials of Visual Basic。 With the essentials； you can write a
functional application that uses classes； objects； interfaces； and inheritance。 In this chapter；
you’ll learn about a Visual Basic programming technique that some developers define as structural。
A structural programming technique is when the code does not directly serve to solve a business
problem； but solves a problem relating to building the application。
Another purpose of this chapt
