VB2008从入门到精通(PDF格式英文版)-第78部分
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Next
#End If
Dim reader As TextReader = Nothing
Dim writer As Stream = Nothing
If args。Length = 0 Then
reader = Console。In
writer = Console。OpenStandardOutput（）
ElseIf args。Length = 1 Then
If args（0） = 〃…help〃 Then
Text2BinaryBootstrap。DisplayHelp（）
Return
End If
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272 CH AP T E R 1 0 ■ L E A R N I N G A B OU T P E R S IS TE N CE
reader = File。OpenText（args（0））
writer = Console。OpenStandardOutput（）
ElseIf args。Length = 2 Then
If args（0） 〃…out〃 Then
Text2BinaryBootstrap。DisplayHelp（）
Return
End If
reader = Console。In
writer = File。Open（args（1）； FileMode。Create）
ElseIf args。Length = 3 Then
If args（0） 〃…out〃 Then
Text2BinaryBootstrap。DisplayHelp（）
Return
End If
reader = File。OpenText（args（2））
writer = File。Open（args（1）； FileMode。Create）
Else
Text2BinaryBootstrap。DisplayHelp（）
Return
End If
processor。Process（reader； writer）
writer。Close（）
End Sub
End Module
The implementation is nearly identical to the Bootstrap class； the differences are bolded。
Notice that instead of using the predefined Console。Out data member； the OpenStandardOutput（）
method is called； and instead of File。CreateText（）； the File。Open（） method is called。
The implementation of the binary…to…text stream shell is nearly identical to the one for the
text…to…binary stream shell。 You can find it in this book’s downloadable source code。
Defining the Type
The central piece when converting from a text stream to a binary stream or vice versa is the
definition of a type。 The type might be a class or a struct； and it is identified as being a type that
can be read and written。 Identifying a type to be read and written is necessary so that when
reads or writes the type； the underlying infrastructure knows what to do。 Remember that
a binary data stream is a data stream where only the program knows what it means。 To help
read and write those types； you add identifiers that tell what to do。 Fortunately； you
don’t need to know the inner details of binary serialization； but just give an indication of
how to manage it。
For the lottery…prediction example； we will define a type that represents a lottery ticket。
The lottery ticket will contain the draw date； numbers； and bonus number。 Following is the
plete declaration （in the LottoLibrary project）。
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CH A PT E R 1 0 ■ L E A R N I N G A B O U T P E R S IS T E N CE 273
_
Public Class Ticket
Public Sub New（）
End Sub
Public Sub New（ByVal drawDate As DateTime； ByVal numbers As Integer（）； _
ByVal bonus As Integer）
_drawDate = drawDate
_numbers = numbers
_bonus = bonus
End Sub
Public Property Bonus（） As Integer
Get
Return _bonus
End Get
Set（ByVal value As Integer）
_bonus = value
End Set
End Property
Public Property DrawDate（） As DateTime
Get
Return _drawDate
End Get
Set（ByVal value As DateTime）
_drawDate = value
End Set
End Property
Public Property Numbers（） As Integer（）
Get
Return _numbers
End Get
Set（ByVal value As Integer（））
_numbers = value
End Set
End Property
Private _bonus As Integer
Private _drawDate As DateTime
Private _numbers As Integer（）
End Class
Ticket is plain…vanilla data class。 However； the bolded parts deserve an explanation。 The
first bolded part is a set of angled brackets surrounding the identifier Serializable； which
represents a attribute。 In ； you have the ability to describe types； methods； and so on。
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274 CH AP T E R 1 0 ■ L E A R N I N G A B OU T P E R S IS TE N CE
Attributes are descriptions that are used in a particular context。 As opposed to the identifiers
Public and MustInherit； which are descriptions that fundamentally describe how a type will
behave； the Serializable attribute describes the behavior of a type in a specific context—when
the object is to be converted from memory to a data stream and vice versa。 Fundamental descrip
tions are important to the runtime； whereas attributes are generally not important
to the runtime； but important to libraries that the runtime will execute。
■Note Serialization is the process of turning an object into a format that can be saved or sent over the
network。 Deserialization is the process of retrieving the serialized object。
The Serializable attribute describes the ability to serialize the type as it is declared。 When
Ticket is converted into a binary stream； the programmer does not need to do anything other
than pass an instance to the data stream。 The data stream libraries handle all of the other details。
In the declaration of Ticket； the parameterless constructor has been bolded to emphasize
that this type of constructor is necessary when converting a data stream into an object instance。
When binary streams restore types； they instantiate an empty object and then assign the data
members。 Thus； when an object is created； a parameterless constructor is needed。
Converting a Text Stream into a Binary Stream
To convert the stream from text to binary involves breaking apart the text stream； instantiating
a Ticket instance； assigning the data members； and saving the instance to the binary stream。
All of these steps are performed in the following source code （you’ll need a reference to
LottoLibrary in Text2Binary）。
Imports System。IO
Imports System。Runtime。Serialization。Formatters。Binary
Imports System。Text
Public Class LottoTicketProcessor ： Implements IText2BinaryProcessor
Public Sub Process（ByVal reader As TextReader； ByVal writer As Stream） _
Implements IText2BinaryProcessor。Process
Dim retval As StringBuilder = New StringBuilder（）
Do While reader。Peek（） …1
Dim lineOfText As String = reader。ReadLine（）
Dim splitUpText As String（）= lineOfText。Split（New Char（） ｛〃 〃c｝）
Dim dateSplit As String（） = splitUpText（0）。Split（New Char（） ｛〃。〃c｝）
Dim ticket As LottoLibrary。Ticket = _
New LottoLibrary。Ticket（ _
New DateTime（ _
Integer。Parse（dateSplit（0））； _
Integer。Parse（dateSplit（1））； _
Integer。Parse（dateSplit（2）））； _
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CH A PT E R 1 0 ■ L E A R N I N G A B O U T P E R S IS T E N CE 275
New Integer（） ｛ _
Integer。Parse（splitUpText（1））； _
Integer。Parse（splitUpText（2））； _
Integer。Parse（splitUpText（3））； _
Integer。Parse（splitUpText（4））； _
Integer。Parse（splitUpText（5））； _
Integer。Parse（splitUpText（6）） ｝； _
Integer。Parse（splitUpText（7）））
Dim formatter As BinaryFormatter = New BinaryFormatter（）
formatter。Serialize（writer； ticket）
Loop
End Sub
End Class
The code splits the text stream by reading a line of text and then splitting apart the fields。
The split…apart fields are then converted into numbers by using the Integer。Parse（） method。
This process of splitting and conversion is called marshaling data。 Marshaling is a technical
term that means to convert a type from one medium to another。
We manage the text marshaling； but manages the binary marshaling； which is still there
behind the scenes。 The System。Runtime。Serialization。Formatters。Binary。BinaryFormatter type
manages marshaling of the Ticket instance to the binary stream。 The Serializable attribute
is used by the BinaryFormatter as an indicator that the Ticket type is allowed to be serialized and
deserialized。 In essence； converting from a text stream to a binary stream means to marshal a
text…defined ticket into a …defined ticket； and that is then marshaled into a binary…defined
ticket。
Converting a Binary Stream into a Text Stream
Conv
