《VB2008从入门到精通(PDF格式英文版)》第160章

version；　minor　version；　build　number；　and　revision。　The　build　number　can　represent　a　daily
build　number；　but　this　is　not　required。　The　revision　identifier　can　represent　a　random　number；
but　this　is　not　required　either。　For　my　assemblies；　I　use　a　revision　number　of　0；　and　consider　the
build　number　as　a　patch　number。　
Visual　Basic　Express　has　a　built…in　mechanism　that　automatically　updates　the　build　and
revision　numbers。　Here；　the　asterisk　represents　the　auto…increment：　
Alternatively；　you　can　use　a　versioning　tool；　or　you　can　increment　the　numbers　manually。
The　gacutil　tool　can　be　executed　multiple　times　with　multiple　versions；　as　shown　in　Figure　12…4。　
■Note For　more　information　about　using　versioning　tools；　see　this　blog　entry　about　auto…incrementing
assembly　versions：　http：//weblogs。asp/bradleyb/archive/2005/12/02/432150。aspx。　
Figure　12…4。　An　assembly　added　three　times　to　the　GAC　with　three　different　versions　
In　Figure　12…4；　the　assembly　VersioningAssembly　has　been　added　three　times　to　the　GAC
with　three　different　versions　（1。0。0。0；　1。1。0。0；　an　d　1。2。0。0）。　With　the　GAC　in　this　state；　an　appli
cation　or　another　assembly　has　the　option　to　reference　three　different　versions　of　the　same
assembly。　
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336 CH　AP　T　E　R 1　2 ■ L　E　A　R　N　I　N　G A　B　OU　T A　PP　L　I　CA　TI　O　N CO　N　F　I　G　U　R　AT　IO　N AN　D D　Y　N　A　M　I　C L　O　AD　I　N　G　
For　an　application　or　assembly　to　use　another　assembly；　you　create　a　reference。　When　the
application　or　assembly　is　piled；　a　specific　version　number　of　the　assembly　is　referenced。
For　example；　if　a　reference　to　the　version　1。1。0。0　of　VersioningAssembly　is　defined；　then　version
1。1。0。0　of　the　assembly　is　loaded。　
Adding　an　Assembly　Redirection　to　a　Configuration　File　
Let’s　say　an　application　or　assembly　needs　to　use　a　new　version　of　the　VersioningAssembly
assembly。　To　make　the　application　or　assembly　aware　of　the　new　assembly；　you　update　the
application　or　assembly　configuration　file　that　references　the　old　assembly。　The　configuration
file　update　includes　an　assembly　redirection。　Essentially；　what　the　redirection　says　is　that　if　a
certain　version　of　an　assembly　is　requested；　the　new　version　should　be　loaded。　The　following　is
an　example　of　an　assembly　redirection。　
This　configuration　file　includes　an　assemblyBinding　XML　element　that　defines　a　collection
of　assemblies　that　will　be　affected。　The　collection　of　assemblies　is　embedded　within　the
dependentAssembly　element。　Within　the　dependentAssembly　element　are　two　child　elements：
assemblyIdentity　and bindingRedirect。　The　assemblyIdentity　element　is　used　to　identity　the
assembly　for　which　a　reference　will　be　redirected。
The　bindingRedirect　element　contains　two　attributes：　oldVersion　and newVersion。　The
oldVersion　attribute　identifies　the　version　of　the　old　assembly　in　the　calling　assembly　or　appli
cation。　If　the　specified　version　of　the　old　assembly　is　found；　the　newVersion　attribute　is　used　to
identify　which　assembly　version　should　be　used　instead。　In　the　example；　the　old　version　is　1。1。0。0；
and　the　new　version　is　1。2。0。0。　The　new　version　has　an　incremented　minor　number；　indicating
a　new　version　of　an　assembly。　However；　the　binding　redirection　does　not　care　whether　the
newVersion　attribute　references　a　newer　version　or　an　older　version。　The　version　identifiers
identified　by　the　attributes　newVersion　and　oldVersion　are　just　that：　identifiers。　
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CH　AP　T　E　R 1　2 ■ L　E　AR　N　IN　G AB　O　U　T AP　P　L　I　CAT　I　ON CO　N　F　IG　U　R　AT　IO　N A　N　D D　Y　N　A　M　IC L　O　AD　IN　G　337　
Implementing　a　Shared　Typed　Convention…Based
Architecture　
There　has　been　quite　a　bit　of　talk　in　the　software　munity　about　convention　over　configu
ration。　Most　of　this　talk　began　in　earnest　with　the　development　of　Ruby　on　Rails　（http：//　
rubyonrails。org/）。　Ruby　on　Rails　（Rails　for　short）　is　a　tool　that　allows　people　to　very
quickly　create　web　sites　that　provide　useful　functionality。
Many　attribute　the　success　of　Rails　to　its　use　of　convention　over　configuration。　Some　say
it　is　the　Ruby　language。　Others　say　it　is　because　Rails　is　a　professional　product。　I　believe　it’s　a
bination　of　factors；　but　the　convention　over　configuration　angle　does　play　an　important　role。　
Let’s　go　back　to　the　problem　of　loading　code　dynamically；　or　for　that　matter；　executing
code　dynamically。　How　much　do　you　expect　the　programmer　to　know；　and　how　much　do　you
expect　the　programmer　to　guess？　Consider　this　code：　
Interface　IDefinition　
End　Interface
Sub　DoIt（ByVal　def　As　IDefinition）　
"　Do　Something　with　def　
End　Sub　
In　the　code；　you　can　see　an　interface　IDefinition　and　a　method　DoIt（）；　with　a　parameter
of　type　IDefinition。　This　creates　a　contract　where　to　call　DoIt（）；　you　need　to　pass　an　instance
of　type　IDefinition。　
Is　it　correct　to　assume　that　the　dynamic　loading　of　a　type　can　fulfill　the　cont