《VB2008从入门到精通(PDF格式英文版)》第136章

Managed；　which　references　the　variable　_managed。　The　idea　behind　this　class　is　to　assign　an
object　in　the　constructor　that　can　be　referenced　using　the　property。　The　class　Container　does
not　know　what　the　variable　_managed　does　or　its　capabilities。　Container　does　not　care；　because
Container　is　acting　like　a　basket　that　holds　an　instance　of　whatever　is　given　to　it。　
The　Container　class　could　be　used　as　follows：　
Dim　container　As　Container　=　New　Container（New　MyType（））　
TryCast（container。Managed；　MyType）。Method（）　
When　Container　is　instantiated；　the　_managed　data　member　is　assigned　an　instance　of
MyType。　MyType　is　a　type　that　is　used　for　illustrative　purposes　and　has　a　single　method；　Method（）。
To　retrieve　and　use　the　managed　type；　the　property　Managed　is　referenced。　However；　the
method　Method（）　cannot　be　called　directly　because　the　property　Managed　is　of　type　Object；　and
thus　you　need　to　cast　the　property　to　type　MyType　so　that　the　call　to　Method（）　is　legal。　
The　cast　using　TryCast　will　result　in　a　valid　instance　of　MyType　or　a　Nothing　value；　resulting
in　a　null　object　reference　exception。　Here’s　a　safe　way　of　using　the　Managed　property：　
Dim　container　As　Container　=　New　Container（New　MyType（））　
If　TypeOf　container。Managed　Is　MyType　Then　
TryCast（container。Managed；　MyType）。Method（）　
End　If　
The　bolded　code　is　the　addition　of　the　If　block　to　test　whether　the　container　references　the
type　MyType。　Not　shown　is　what　the　code　should　do　if　the　property　Managed　is　not　MyType。　The
fact　that　you　need　to　verify　that　the　container　references　the　correct　type；　and　you　also　need　to
think　of　an　alternative　plan　if　the　type　is　incorrect；　adds　quite　a　bit　of　code。　It’s　like　that　sentence
about　the　duck—sure；　you　get　the　general　idea　of　what　is　being　said；　but　are　you　100％　sure　of
the　meaning？　
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Now　look　at　the　following　code；　which　implements　a　container　using generics。　
Public　Class　GenericsContainer（Of　ManagedType）　
Private　_managed　As　ManagedType　
Public　Sub　New（ByVal　toManage　As　ManagedType）　
_managed　=　toManage　
End　Sub　
Public　ReadOnly　Property　Managed（）　As　ManagedType　
Get　
Return　_managed　
End　Get　
End　Property　
End　Class　
You　can　write　code　that　uses generics　and　code　that　provides　types　based　on
generics。　The　definition　of　GenericsContainer　demonstrates　code　that　provides　types　based　on
generics。　You’ll　see　the　code　that　uses generics　next。
generics　parameters　are　associated　with　types；　such　as　classes　and　interfaces；　or　with
methods。　In　the　case　of　GenericsContainer；　the generics　parameter　ManagedType　is　a
placeholder　for　an　actual　type。　
■Note monly；　developers　use　the　notation　of　a　single　letter　when　defining　a generics　parameter。
I　am　not　a　fan　of　that　notation；　because　it　tells　me　nothing；　especially　when　there　are　multiple　parameters。　I
remend　using　an　identifier　that　describes　what　the　parameter　does；　appended　with　the　word　Type；　to
indicate　a generics　parameter　is　being　defined。　
With　GenericsContainer；　ManagedType　is　used　as　an　identifier　in　the　place　of　the　identifier
object　in　the　type　Container。　This　is　a　rule　of　thumb。　Whenever　you　find　yourself　using　an　object
generically　to　define　a　type；　you　probably　could　use generics。　Think　of generics
types　as　general　things。　
The　following　code　demonstrates　how　to　use　GenericsContainer　with　MyType。　
Dim　container　As　GenericsContainer（Of　MyType）　=　_　
New　GenericsContainer（Of　MyType）（New　MyType（））　
container。Managed。Method（）　
When　instantiating　GenericsContainer；　notice　how　the　identifier　ManagedType　must　be
replaced　with　an　identifier　that　represents　an　already　existing　type。　This　identifier　replacement
results　in　a　new　and　unique　type　（also　called　a　concrete　type）。　The　advantage　of generics　is
that　when　you　provide　a　concrete　type；　you　don’t　need　to　check　to　make　sure　that　everything　is
correct。　
The runtime　will　generate　a　type　that　has　an　intent　similar　to　the　following　code。　
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288 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　
Public　Class　GenericsContainerMyType
Dim　_managed　As　MyType　
Public　Sub　New（ByVal　toManage　As　MyType）
_managed　=　toManage　
End　Sub　
Public　ReadOnly　Property　Managed（）　As　MyType
Get
Return　_managed　
End　Get　
End　Property　
End　Class　
Visual　Basic　piles　a generics　type　as　an　inplete　type。　When　the　inplete
type　is　concretized； creates　a　brand…new　type；　and　does　this　without　requiring　the　devel
oper　to　do　anything　in　particular。　This　means　that　if　you　use　GenericsContainer　with　15　different
types； will　generate　15　definitions　of　GenericsContainer　while　the　program　is　executing。　
ABSTRACTION　AND GENERICS　
With generics；　you　can　verify　and　ensure　that　everything　is　being　said　properly；　with　the　cost　being
plexity。　Consider　the　sentence　that　clearly　described　the　duck。　To　produce　it；　you　need　to　have　learned
additional　rules　regarding　grammar。　Whe