《VB2008从入门到精通(PDF格式英文版)》第132章

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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278 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　
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。
Separating　Data　Objects　from　Action　Objects　
Another　approach　is　to　develop　a　number　of　data　objects　whose　only　role　is　to　be　used　in　seri
alization　and　data　referencing。　Such　an　approach　is　useful　when　using　binary　serialization；
because　you　are　then　able　to　more　effectively　manage　the　version　problem。　The　following　is　an
example　of　how　such　an　architecture　would　be　realized。　
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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　279　
Class　MyObject2　
End　Class　
Class　Doer　
Private　_object　As　MyObject2　
Private　_networkIdentifier　As　Integer　
End　Class　
The　class　Doer　has　no　serialization　attribute　and　will　not　be　serialized；　but　it　references
MyObject2。　The　network　identifier　data　member　has　been　moved　from　MyObject2　to　Doer。　The
result　is　that　MyObject2　contains　nothing　that　is　transient。　
pleting　Custom　Types　
When　writing　custom　types；　two　methods　are　often　implemented；　especially　where　objects　are
to　be　pared　a　lot： Equals（）　and　GetHashCode（）。　These　two　methods　are　used　by　the
library　API　when　paring　and　manipulating　instances　in　a　list　or　a　collection。　It　just　happened
in　this　chapter’s　example　that　a　list　of　string　types　did　the　right　thing。　If　TextProcessor　had　used
the　Ticket　type；　then　the　Equals（）　method　of　the　list　used　to　find　date　duplicates　would　not　have
worked。　The　default　implementations　of Equals（）　and　GetHashCode（）　are　not　implemented
properly。　This　is　not　the API’s　fault。　Rather；　it　is　a　recognition　that　Microsoft　cannot　know
the　structure　of　an　object　and　what　is　considered　as　making　a　type　unique。　
Implementing　GetHashCode（）　
The　MSDN　documentation　for　Object。GetHashCode　defines　the　GetHashCode（）　method　as　follows
（http：//msdn2。microsoft。/en…us/library/system。object。gethashc