《VB2008从入门到精通(PDF格式英文版)》第142章

is　not　the　responsibility　of　the　IWorkbook　interface；　but　of　the　IWorkbook　interface　implementation。
■Note The　IWorkbook　interface　does　not　provide　a　Clear（）　method　to　reset　the　workbook　and　delete　all
of　the　referenced　worksheets。　It　would　seem　logical　to　have　a　Clear（）　method；　but　in　a　garbage…collected
environment；　that’s　pletely　unnecessary。　If　you　don’t　want　to　use　a　workbook　anymore；　just　don’t　refer
ence　it；　and　the　garbage　collector　will　take　care　of　the　rest。　Think　of　it　as　having　the　option　of　serving　dinner
to　your　guests　on　real　plates　or　paper　plates。　Real　plates　might　seem　better；　but　they　break　and　you　need　to
wash　them。　Paper　plates　are　used　once　and　thrown　away。　Of　course；　with　paper　plates　you　have　recycling
issues　that　you　don’t　have　in　；　because　the　memory　is　recycled　for　you。　
The　property Identifier　identifies　the　workbook　represented　by　the　current IWorkbook
object。　The　identifier　might　be　a　path　or　file　name　and　is　pletely　dependent　on　the　imple
mentation　of　IWorkbook。　
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CH　AP　T　E　R 1　1 ■ L　E　A　R　N　IN　G AB　O　U　T 。　N　E　T G　E　N　E　R　I　CS　299　
The　default　property；　Item；　is　the　primary　way　of　getting　and　retrieving　worksheets；　where
each　worksheet　is　referenced　using　a　string　identifier。　The　identifier　does　not　need　to　be　a　string—　
it　could　have　been　a　custom　type；　enumeration；　or　interface　that　is　implemented。　Using　a　string
keeps　things　simple；　but　there　are　maintenance　issues。　
Let’s　say　all　workbooks　have　a　configuration　worksheet。　So　for　most　of　the　code；　the　string
identifier 〃configuration〃　is　used。　However；　a　new　programmer　decides　to　use 〃Configuration〃
（with　a　capital　C）。　This　slight　change　will　cause　problems　because 〃configuration〃　is　meant　to
have　a　lowercase　c。　Here’s　the　example：　
Dim　workbook　As　IWorkbook　
Dim　worksheet1　As　IWorksheetBase　=　workbook（〃configuration〃）　
Dim　worksheet2　As　IWorksheetBase　=　workbook（〃Configuration〃）　
This　approach　uses　a　buffer　that　is　typed　in　by　hand　and　is　considered　hard…coded。　The
preferred　alternative　is　to　hard…code　a　structure　that　is　then　referenced　throughout　the　source
code；　like　this：　
Public　Module　WorksheetIdentifiers
Public　Const　Configuration　As　String　=　〃configuration〃　
End　Module　
Dim　workbook　As　IWorkbook　
Dim　worksheet1　As　IWorksheetBase　=　workbook（WorksheetIdentifiers。Configuration）　
Dim　worksheet2　As　IWorksheetBase　=　workbook（WorksheetIdentifiers。Configuration）　
The　module　WorksheetIdentifiers　still　contains　a　hard…coded　string　buffer；　but　this　buffer
has　been　centralized　to　a　single　location。　The　workbook　default　property　references　the　identifier
within　the　class　WorksheetIdentifiers。　Thus；　if　you　change　the　class　WorksheetIdentifiers；　you
also　change　the　identifiers　used　by　the　default　property。　That　way；　the　chances　of　having　a　typo
break　an　application　are　reduced。　
Let’s　get　back　to　the　IWorkbook　interface　and；　in　particular；　the　default　property。　The　default
property　is　of　the　type　IWorksheetBase；　which　is　a　bit　tedious　because　the　IWorksheetBase　inter
face　is　a　rudimentary　interface　and　most　likely　not　the　interface　that　you　will　use；　because　we
have IWorksheet（Of　BaseType）；　and　therein　lies　the　problem。　To　get　an　IWorksheet　instance；
you　would　need　a　cast；　as　follows：　
Dim　workbook　As　IWorkbook　
Dim　worksheet　As　IWorksheet（Of　String）　=　_
TryCast（workbook（WorksheetIdentifiers。Configuration）；　IWorksheet（Of　String））　
The　bolded　code　is　the　cast　you　need　each　and　every　time　you　want　to　reference　an　IWorksheet
instance。　The　cast　is　not　a　big　deal；　but　it　is　tedious。　I　personally　would　prefer　being　able　to　call
a　property；　method；　or　default　property　that　returns　the　type　I　want。　
Therein　lies　the　problem；　because　the　way　I　want　to　use　a　default　property；　property；　or
method　is　as　a　mixed　type；　and　you　can’t　define　a　default　property　with　mixed　types。　You　can
define　a　default　property　that　is　a　fixed　type。　To　understand　this　problem；　look　at　the　following
declaration　of　a　workbook；　which　does　pile。　
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300 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　
Imports　System。Reflection　
Imports　Devspace。Trader。mon　
_　
Public　Interface　IWorkbook（Of　BaseType）　
Inherits　IDebug　
Default　Property　Item（ByVal　identifier　As　String）　As　IWorksheet（Of　BaseType）　
ReadOnly　Property　Identifier（）　As　String　
End　Interface　
In　this　declaration　of　IWorkbook；　a generics　type　is　used；　but　then　the　problem　is　that
the　default　property　can　return　only　IWorksheet　instances　of　a　single　type；　such　as　Double　or
String。　Remember　that　there　are　multiple　worksheet　types　（as　illustrated　in　Figure　11…2）。　
What　we　want　to　do　is　use　method…level generics　declarations；　like　this：　
_　
Public　Interface　IMixedType
Default　Property　Item（Of　BaseType）（ByVal　identifier　As　String）　_　
As　IWorksheet（Of　BaseType）
End　Interface　
The　problem　with　the　method…level　declaration　is　that　it　does　not　pile。　There　are　two
ways　to　declare　a generics　parameter。　The　first　is　what　you　have　seen　the　most　often；　and
that　is　at　the　type　level：　
Class　MyType（Of　GenericType）　
End　Class　
Declaring　at　the　type　level　means　that　whenever　you　use　the　type　and　specify　a　type　for　the
generics　parameter；　MyTyp