VB2008从入门到精通(PDF格式英文版)-第79部分
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ticket。
Converting a Binary Stream into a Text Stream
Converting a binary stream into a text stream involves using the …provided formatter to
create a Ticket instance that is then converted into text。 The following is the plete source
code （you’ll need a reference to LottoLibrary in Binary2Text）。
Imports System。IO
Imports System。Runtime。Serialization。Formatters。Binary
Public Class LottoTicketProcessor ： Implements IBinary2TextProcessor
Public Sub Process（ByVal input As Stream； ByVal output As TextWriter） _
Implements IBinary2TextProcessor。Process
Dim builder As StringBuilder = New StringBuilder（）
Try
Do While True
Dim formatter As BinaryFormatter = New BinaryFormatter（）
Dim ticket As LottoLibrary。Ticket = _
DirectCast（formatter。Deserialize（input）；
LottoLibrary。Ticket）
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builder。AppendFormat（ _
〃｛0｝。｛1｝。｛2｝ ｛3｝ ｛4｝ ｛5｝ ｛6｝ ｛7｝ ｛8｝ ｛9｝ ｛10｝〃；
ticket。DrawDate。Year；
ticket。DrawDate。Month；
ticket。DrawDate。Day；
ticket。Numbers（0）；
ticket。Numbers（1）；
ticket。Numbers（2）；
ticket。Numbers（3）；
ticket。Numbers（4）；
ticket。Numbers（5）；
ticket。Bonus；
ControlChars。NewLine）
Loop
Catch e As Exception
End Try
output。Write（builder。ToString（））
End Sub
End Class
In the code； a binary stream is read using the BinaryFormatter class； which reads the data
for an object from the data stream。 Notice how the method Deserialize（） does not ask which
type to read。 This is because all of that information is saved in the stream。 Deserialize（） will
read data from the stream； associate the data with a type； instantiate it； and populate the data
members。
The bolded code in the preceding listing points out where deserialization bees tricky；
which centers around when to read what type。 A binary stream when processed by the
BinaryFormatter will read and write objects。 BinaryFormatter will instantiate whatever it encoun
ters and assumes that the caller of BinaryFormatter knows which type is being manipulated。 If
the caller does not know this； an exception will occur because the type cast to the specific type
will fail。
The exception block is necessary because you don’t know how many Ticket objects have
been saved； as the count has not been saved to the stream。 provides the Position and
Length properties to determine if any instances are left to be read； but those properties work
only with files。 If the binary stream being read is a console data stream； there is no length or
position。 Thus； the only real solution is to keep reading until you can’t do so anymore and
assume processing is plete。
INTENTION AND IMPLEMENTATION
The fact that Position and Length have different behaviors depending on the implementation might seem
to break the contract of being able to separate intention from implementation。 It seems to break the po
nent software paradigm because； as a developer； you do need to know about the stream implementation。 The
rule has not been broken； but been put on hold； because sometimes you are left with no other option。 A file
stream and console stream share many characteristics； but knowing the length is not one of them。 A programmer
could force the console stream to give back some value； but that would be incorrect。 The smarter approach；
and the one chosen by the implementers； is to generate an exception。
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The binary stream formatter can bee even pickier。 Look at Figure 10…8 again and notice how the
type information with version identifiers is stored in the stream。 Imagine the situation where you create an
assembly that saves some objects。 Then x years and n versions of the program later； you try to load the file。
You can’t； because the version of the type does not exist。 It is smart for the binary formatter to not instantiate
a version of a type that does not exist； because otherwise there could be serialization failures。
Tweaking Serialization
When dealing with serialization； you may e across a particular text or binary format that
will require some extra work。 Also； you may have some objects that you want to exclude from
serialization。
Performing Custom Serialization
Sometimes I think there are as many file formats as there are grains of sand on a beach。 As just
two examples； specializations of text formats are XML and JSON （for JavaScript Object Notation；
used in JavaScript for your web browser）。 In most cases； the default serializations will work。
However； you may need to tweak a particular serialization。
Many serialization techniques allow custom serializations of a particular object。 The default
serialization implies a certain marshaling。 For example； it might mean an integer will be marshaled
as an integer in another representation （in SQL Server； for example）。 Sometimes； however； you
might want different representations in different streams。 In that case； you need to implement
the marshaling of the data member yourself。 With most serialization platforms； it means imple
menting a particular interface。
Following is an example that performs a custom serialization for a binary stream。
Imports System。Runtime。Serialization
_
Class MyObject
Implements ISerializable
Private value As Integer
Public Sub New（）
End Sub
Public Sub New（ByVal info As SerializationInfo；
ByVal context As StreamingContext）
value = Integer。Parse（CStr（info。GetValue（〃value〃； GetType（String））））
End Sub
Public Sub GetObjectData（ByVal info As SerializationInfo；
ByVal context As StreamingContext）
Implements ISerializable。GetObjectData
info。AddValue（〃value〃； value。ToString（））
End Sub
End Class
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In the example； the interface System。Runtime。Serialization。ISerializable is implemented。
This means when BinaryFormatter serializes or deserializes MyObject； BinaryFormatter will not
manipulate the binary stream； but delegate the manipulation to MyObject。 With many serializa
tion platforms； there is an explicit method； property； or flag to indicate whether MyObject is
being written to the stream or read from the stream。 In the case of binary serialization； when an
object is written to the stream； the GetObjectData（） method is called； and when the object is
being read from the stream； the constructor is called。
Serialization involves two directions； and a developer must implement both； in the same
way。 In the example； the AddValue（） method is called； indicating that the data member is written as
a string； and when reading； the value data member must be read as a string。
■Note One of the biggest challenges with serialization is that each and every serialization platform seems
to have its own way of doing things。 Sometimes there will be mon methods and attributes； but other times
they will not exist。 A universal approach to serialization won’t work。 You should avoid performing custom seri
alization whenever possible。 Most serialization platforms are smart enough to know what to do with each data
member。 So the best approach is to let the platform figure things out。
Declaring a Data Member As Nonserializable
In the example of the Ticket type； all of the data members were serialized。 However； some
times that is not desired。 Suppose an object that you want to serialize has a network connection
or some other data value that will not make any sense if the object is serialized and then dese
rialized some time later。 When the object is serialized； the network connection will also be
serialized； which is not appropriate。 It is not appropriate to serialize a network connection
because that object is transient and applies only to the context of the object instance。
To mark a data member as nonserializable； attributes are often used； as in the following
example。
Class MyObject2
Private _networkIdentifier As Integer
End Class
In the example； _networkIdentifier will not be written or read from the data stream。
