VB2008从入门到精通(PDF格式英文版)-第107章
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have a lowercase c。 Here’s the example：
Dim workbook As IWorkbook
Dim worksheet1 As IWorksheetBase = workbook（〃configuration〃）
Dim worksheet2 As IWorksheetBase = workbook（〃Configuration〃）
This approach uses a buffer that is typed in by hand and is considered hard…coded。 The
preferred alternative is to hard…code a structure that is then referenced throughout the source
code； like this：
Public Module WorksheetIdentifiers
Public Const Configuration As String = 〃configuration〃
End Module
Dim workbook As IWorkbook
Dim worksheet1 As IWorksheetBase = workbook（WorksheetIdentifiers。Configuration）
Dim worksheet2 As IWorksheetBase = workbook（WorksheetIdentifiers。Configuration）
The module WorksheetIdentifiers still contains a hard…coded string buffer； but this buffer
has been centralized to a single location。 The workbook default property references the identifier
within the class WorksheetIdentifiers。 Thus； if you change the class WorksheetIdentifiers； you
also change the identifiers used by the default property。 That way； the chances of having a typo
break an application are reduced。
Let’s get back to the IWorkbook interface and； in particular； the default property。 The default
property is of the type IWorksheetBase； which is a bit tedious because the IWorksheetBase inter
face is a rudimentary interface and most likely not the interface that you will use； because we
have IWorksheet（Of BaseType）； and therein lies the problem。 To get an IWorksheet instance；
you would need a cast； as follows：
Dim workbook As IWorkbook
Dim worksheet As IWorksheet（Of String） = _
TryCast（workbook（WorksheetIdentifiers。Configuration）； IWorksheet（Of String））
The bolded code is the cast you need each and every time you want to reference an IWorksheet
instance。 The cast is not a big deal； but it is tedious。 I personally would prefer being able to call
a property； method； or default property that returns the type I want。
Therein lies the problem； because the way I want to use a default property； property； or
method is as a mixed type； and you can’t define a default property with mixed types。 You can
define a default property that is a fixed type。 To understand this problem； look at the following
declaration of a workbook； which does pile。
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300 CH AP T E R 1 1 ■ L E A R N I N G A B OU T 。 N E T G E N E R I CS
Imports System。Reflection
Imports Devspace。Trader。mon
_
Public Interface IWorkbook（Of BaseType）
Inherits IDebug
Default Property Item（ByVal identifier As String） As IWorksheet（Of BaseType）
ReadOnly Property Identifier（） As String
End Interface
In this declaration of IWorkbook； a generics type is used； but then the problem is that
the default property can return only IWorksheet instances of a single type； such as Double or
String。 Remember that there are multiple worksheet types （as illustrated in Figure 11…2）。
What we want to do is use method…level generics declarations； like this：
_
Public Interface IMixedType
Default Property Item（Of BaseType）（ByVal identifier As String） _
As IWorksheet（Of BaseType）
End Interface
The problem with the method…level declaration is that it does not pile。 There are two
ways to declare a generics parameter。 The first is what you have seen the most often； and
that is at the type level：
Class MyType（Of GenericType）
End Class
Declaring at the type level means that whenever you use the type and specify a type for the
generics parameter； MyType bees fixed to a certain type。 So say you declared MyType as
follows：
Dim cls As MyType = New MyType（Of Integer） （）
Now any references to GenericType within MyType will bee Integer。 This form of
generics solves many problems； as illustrated in Chapter 9。
But in the case of the IWorkbook； we don’t want a fixed type。 We want the ability to have a
collection type contain mixed types of IWorksheet。 The way to achieve that is to use generics
methods； like this：
Function Method（Of GenericType）（） As MyType（Of GenericType）
End Function
Now the generics parameter is associated with the method； rather than the type。 And
that means MyType can mix types。 So we could have different IWorksheet types in a single work
book。 And wouldn’t it be great if there were a default property with mixed types？ But you can’t
have generics default properties and properties that are not declared at the type level。
Thus； the use of a generics parameter with a default property or property will not work。 In
my opinion； that is a real design flaw in Visual Basic； because it means we need to write code
like this：
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CH AP T E R 1 1 ■ L E A R N IN G AB O U T 。 N E T G E N E R I CS 301
Imports Devspace。Trader。mon
Imports System。Reflection
_
Public Interface IWorkbook
Inherits IDebug
Function GetSheet（Of BaseType）（ByVal identifier As String） _
As IWorksheet（Of BaseType）
ReadOnly Property Identifier（） As String
Default Property Item（ByVal identifier As String） As IWorksheetBase
End Interface
In this modified declaration； the method GetSheet（） acts like the Get part of the default
property； but notice where the generics parameter BaseType is declared。 The declaration
is after the method identifier and before the first bracket。 In the case of IWorkbook； we use the
method…level generics parameter to allow the caller to determine the type of the work
sheet instances。 The implementation of IWorkbook has to do nothing other than perform the
appropriate cast。 Method…level generics parameters are great when you are dealing with
mixed types； as in the case of IWorkbook。
The code to retrieve a worksheet that previously needed a cast is rewritten as follows：
Dim workbook As IWorkbook
Dim worksheet As IWorksheet（Of String） = _
workbook。GetSheet（Of String）（WorksheetIdentifiers。Configuration）
The cast has not disappeared pletely。 It is done for us in the implementation of the
GetSheet（Of BaseType）（） method； as demonstrated by the following code。
Friend Class Workbook
Implements IWorkbook； IDebug
Private _worksheets As IDictionary（Of String； IWorksheetBase） = _
New Dictionary（Of String； IWorksheetBase）（）
。 。 。
Public Function GetSheet（Of BaseType）（ByVal identifier As String） _
As IWorksheet（Of BaseType） Implements IWorkbook。GetSheet
SyncLock _worksheets
Dim retval As IWorksheet（Of BaseType） = Nothing
If _worksheets。ContainsKey（identifier） Then
retval = TryCast（_worksheets。Item（identifier）； _
IWorksheet（Of BaseType））
Else
retval = New Worksheet（Of BaseType）（identifier）
_worksheets。Add（identifier； retval）
End If
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302 CH AP T E R 1 1 ■ L E A R N I N G A B OU T 。 N E T G E N E R I CS
Return retval
End SyncLock
End Functi
