VB2008从入门到精通(PDF格式英文版)-第87部分
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think the animals will be？ Do you expect cow and horse； respectively？ Here’s the output：
（horse） （horse）
The generated output is not what you would expect。 This demonstrates that lambda expres
sions are stateless。 Yet in earlier explanations of lambda expressions and closures； I said that
variables can be stored in lambda expressions。 Since animal is a value type； you would expect
two copies of animal； right？ Wrong。
Closures do work； and variable state is kept across lambda expressions。 The mistake in this
code is that the same variable is referenced by two lambda expressions。 Thus； when the two
lambda expressions execute； they will reference the same animal； which was last assigned to be
a horse。
The code that solves the problem uses two unique variables； as follows：
Dim animals（1） As Func（Of String）
Dim animal1 = 〃cow〃
animals（0） = Function（） animal1
Dim animal2 = 〃horse〃
animals（1） = Function（） animal2
For Each callAnimal In animals
Console。WriteLine（callAnimal（））
Next
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304 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
This time when you run the code； you get the following output。
cow
horse
Returning to the spreadsheet example； the problem is that the behavior just described
means this code won’t work：
Dim myWorksheet As IWorksheet（Of Double）
For row As Integer = 0 To 10
myWorksheet。AssignCellCalculation（row； 1； _
Function（worksheet； cellRow； cellCol） _
myWorksheet。GetCellState（row； 0） _
myWorksheet。Calculate（10； 0））
Next
whereas this code would work：
Dim myWorksheet As IWorksheet（Of Double）
For row As Integer = 0 To 10
Dim temp = row
myWorksheet。AssignCellCalculation（row； 1； _
Function（worksheet； cellRow； cellCol） _
myWorksheet。GetCellState（temp； 0） _
myWorksheet。Calculate（10； 0））
Next
The difference between the two pieces of code is that the second one has the lambda
expression that uses the variable declared temp in the context of a loop。 For each iteration of the
loop； a new instance of temp is allocated； and hence each lambda expression has its own instance
of the row。
Assigning State Without Knowing the Type
When using generics types； one of the most mon problems occurs when you need to
work with proper types。 In the implementation of the IWorksheet interface； it is necessary to
implement the AssignCellState（） method defined in the interface IWorksheetSerialize。
The explanation of IWorksheetSerialize is slightly plicated and relates to the problem
of loading an IWorksheet without knowing the type。 Say that you are saving an IWorkbook with
multiple IWorksheet instances。 Each IWorksheet instance is a specific type。 When you want to
load an IWorkbook； how does the loader know which types there are？ The answer is that the
loader does not； and thus must first load a general type； and then make a specific cast。 Take a
look at the serialization source code in the project Devspace。Trader。mon and the namespace
Devspace。Trader。mon。ServerSpreadsheet。SerializerImpls； available as part of the down
loadable code。
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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 305
Public Interface IWorksheetSerialize
Sub AssignCellState（Of ValueType）（ByVal row As Integer； _
ByVal col As Integer； ByVal value As ValueType）
Sub AssignCellState（ByVal row As Integer； ByVal col As Integer； _
ByVal value As Object）
End Interface
The problem is that the function passes in as a parameter of one type and is assigned
to CellState； which is another undefined generics type。 The implementation of the
AssignCellState（） method is as follows：
Public Sub AssignCellState（ByVal row As Integer； ByVal col As Integer； _
ByVal value As Object） Implements IWorksheetSerialize。AssignCellState
CellState（row； col） = DirectCast（value； BaseType）
End Sub
The code looks so simple and innocent； but it is actually masking many potential problems。
Here； the value of the parameter value is of type Object。 Then to convert the value to type
BaseType—because that is what CellState is defined to be—you use a cast。 For the most part；
this code will work； if value is the correct type。
Consider the following code； which would generate an exception。
Dim worksheet As Worksheet（Of Double） = New Worksheet（Of Double）（）
worksheet。Dimension（10； 10）Dim buffer As String = 〃hello； world〃
worksheet。AssignCellState（1； 2； buffer）
The variable worksheet is declared to be of type Worksheet（Of Double）。 When the method
AssignCellState（） is called； the cell 1； 2 is assigned to be type String。 Calling AssignCellState（） is
not a problem； but in the implementation； the assignment will fail。 You can’t willy…nilly assign
a String to a Double。 Of course； this begs the question； “Why have a function of type Object？”
Sometimes you can’t get around it； and you need to write a general object method。 The proper
way to call the method would be as follows：
Dim worksheet As Worksheet（Of Double） = New Worksheet（Of Double）（）
worksheet。Dimension（10； 10）
Dim buffer As String = 〃hello； world〃
worksheet。AssignCellState（1； 2； Double。Parse（buffer））
The bolded code shows that the string buffer is parsed by the Double。Parse（） method；
converting a string buffer into a Double value。 Of course； the conversion will fail； since the string
buffer represents a string； but that is another problem。
Another way of solving the problem is to avoid the object altogether and declare the method
as being a generics method。 The advantage of the generics method is that you could
execute in a type…safe manner without explicitly forcing the user to implement parsing routines。
Consider the following modified generics method declaration of AssignCellState（）。
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306 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
Public Sub AssignCellState（Of ValueType）（ByVal row As Integer； _
ByVal col As Integer； _
ByVal value As ValueType） _
Implements IWorksheetSerialize。AssignCellState
If GetType（BaseType）。IsAssignableFrom（GetType（ValueType）） Then
CellState（row； col） = DirectCast（DirectCast（value； Object）； BaseType）
ElseIf TypeOf （value） Is String And _
GetType（Double）。IsAssignableFrom（GetType（BaseType）） Then
Dim obj As Object = DirectCast（value； Object）
Dim dValue As Double = Double。Parse（CStr（obj））
Dim objDValue As Object = CType（dValue； Object）
CellState（row； col） = DirectCast（objDValue； BaseType）
Else
Throw New InvalidCastException（〃Could not perform conversion〃）
End If
End Sub
The cell state to be assigned is a generics parameter and defined to be the type ValueType。
You can only guess what ValueType is； it is determined when the method AssignCellState（Of ）（）
is called。 For example； suppose this method call is made：
Dim buffer As String = 〃hello； world〃
worksheet。AssignCellState（1； 2； buffer）
The type for ValueType will be String； even though you have not explicitly specified it。 One
of the things that is possible with generics methods is that types can be deduced implic
itly。 The following would be an explicit usage of AssignCellState（Of ）（）。
Dim buffer As String = 〃hello； world〃
worksheet。AssignCellState（Of String）（1； 2； buffer）
Knowing that ValueType is String； AssignCellState（Of ）（） will then first check if
ValueType can be assigned to BaseType：
If GetType（BaseType）。IsAssignableFrom（GetType（ValueType）） Then
This code is rather clever； because it uses what is known as reflection to determine if one
type can be assigned to another type。 Essentially； it asks if it is OK via a cast to assign value to
CellState。 You could try to do this without the If statement； but then you risk an unnecessary
exception。 If it is permissible to assign； then the assignment is done via a two…step casting：
CellState（row； col） = DirectCast（DirectCast（value； Object）； BaseType）
Here； you first convert the type to an Object； and then convert the type to BaseType； which
happens to be the type that the spreadsheet is declared as。 It is absolutely imperative that the
cast to the object is added； otherwise； the Visual Basic piler will plain that the cast is
not possible。
But
