《VB2008从入门到精通(PDF格式英文版)》第135章

with　two implementations。　If　not；　use　text　streams。　
o When　streaming　data；　it　is　best　to　customize　as　little　as　possible。　Doing　so　will　pli
cate　your　program；　and　potentially　introduce　errors　where　none　should　exist。　
o It　is　important　that　you　understand　the　concept　of　marshaling　and　the　fact　that　each
medium　will　have　a　different　representation　of　the　type。　A　large　part　of　your　programming
day　will　involve　moving　data　from　one　stream　to　another。
Some　Things　for　You　to　Do　
The　following　are　some　exercises　to　apply　what　you　learned　in　this　chapter。　
1。 In　the　implementation　of　TextProcessor；　the　display　help　routine　was　not　very　helpful。
Fix　the　implementation。　
2。 There　were　no　testing　routines　for　TextProcessor。　Devise　some　realistic　tests。　By　realistic
tests；　I　mean　tests　that　don’t　just　focus　on　the　class　library　and　consider　the　application
tested。　Focus　on　plete　application　tests。　
3。 Having　implemented　the　display　help　routine；　think　about　whether　or　not　the　imple
mentation　is　correct。　The　class　Bootstrap　is　a　general　class　that　uses　an IProcessor
instance；　which　means　that　different　console　applications　will　process　different　data。
Thus；　writing　a　general　help　output　might　work；　but　it　will　not　help　in　resolving　problems。
Fix　the　console　application　TextProcessor　and ReaderWriter　project　so　that　the　help
message　is　both　specific　and　general。　
4。 In　the　Bootstrap　class；　when　the　output　was　redirected　to　a　file　（as　indicated　by　the …out
argument）；　there　was　no　check　on　whether　or　not　the　file　exists。　Extend　the　Bootstrap
class　to　include　an　additional　mand…line　argument　that　verifies　if　it　is　fine　to　overwrite
the　output　file　if　it　exists。　If　an　output　file　does　exist　and　there　is　no　explicit　overwriting；
generate　an　error　and　stop　processing。　
5。 The　code　in　the　final　solution　for　IProcessor。Process（）　has　been　identified　as　being　hard
to　maintain　because　the　code　to　check　for　duplicate　dates　is　scattered　throughout　the
method。　Rewrite　the　method　implementation　so　that　the　code　is　logical　and　maintainable。　
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C H A P T E R 1 1　
■　■　■　
Learning　About Generics　
Chapter　9　explained　how　to　use　lists；　delegates；　and　lambda　expressions。　In　that　chapter；　you
also　saw　an　example　of generics　when　using　lists　to　manage　a　collection　of　object　instances。　
The　main　focus　of　this　chapter　is generics　and　how　to　use　them　in　a　black　box　context
（the　code　doesn’t　know　the　specifics　of　the generics　parameter　types）。　The　secondary
focus　is　an　implementation　of　lambda　expressions　using　a　spreadsheet。　The　idea　is　to　get　you
well　versed　in generics　and　lambda　expressions；　which　you　will　likely　use　in　your　own
production　code；　so　that　that　there　will　be　no　surprises　in　your　projects。
Why　Use Generics？　
Here’s　a　surprise　for　you：　there　is　no　imperative　need　for generics；　so　you　could　skip　this
chapter　and　read　the　next　one；　right？　Wrong。　I　could　just　as　easily　have　said　there　is　no　need　for
Visual　Basic　properties；　nor　any　other　Visual　Basic　construct　that　enriches　your　programming
abilities。　The　reasons　we　have　Visual　Basic　properties　and generics　are　programming
elegance　and　expressiveness。　
To　understand　what　I　am　trying　to　get　at；　consider　this　sentence：　
Ducks　walk　flat　feet　quack　loud　
Reading　the　sentence；　you　get　an　idea　of　what　is　being　said；　but　you　are　not　pletely
sure。　Visual　Basic　without generics　is　like　this　sentence；　in　that　you　express　your　ideas　in
code；　but　some　things　are　not　as　clear　as　you　would　like。　
Visual　Basic　with generics　is　like　this　sentence：　
Ducks　walk　in　a　funny　manner　due　to　their　flat　feet；　and　when　they　quack；　it　is　very　
loud。　
The　sentence　is　clearer　and　uses　more　words　to　describe　the　same　thing。　The　reason　we
talk　using　a　more　sophisticated　language　is　that　we　want　to　explain　concepts　and　be　under
stood。　If　you　accept　that；　then　you　can　accept　why　there　is　a　need　and　context　for generics。
And　if　not；　feel　free　to　skip　this　chapter　and　read　the　next　one。
An　example　that　illustrates　how　using generics　makes　your　code　clearer；　as　well　as
more　concise；　than　code　that　does　not　use generics　is　a　container。　A container　is　a　type
that　is　used　to　manage　other　types；　lists　and　collections　are　examples　of　containers。　To　keep
285　
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286 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　
things　simple；　let’s　look　at　a　container　that　manages　a　single　reference。　The　following　is　the　less
concise　version　that　uses　the　Object　type。　
Public　Class　Container
Private　_managed　As　Object　
Public　Sub　New（ByVal　toManage　As　Object）　
_managed　=　toManage　
End　Sub　
Public　ReadOnly　Property　Managed（）　As　Object　
Get　
Return　_managed　
End　Get　
End　Property　
End　Class　
In　the　code；　the　class　Container　has　a　constructor　with　a　parameter　and　a　single　property；
Managed；　which　references　the　variable　_managed。　The　idea　beh