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VB2005微软官方教程03.pdf

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C0262058x.fmPage33Wednesday,July28,20043:40PM Chapter2 LanguageEnhancementsfor VisualBasic2005 Inthischapter: Application:Generics..................................................33 Application:TheMyObject.............................................41 Application:Operators.................................................50 Application:Keywords.................................................56 VisualBasic2005introducesanumberofenhancementsdesignedtomakeyourcodingsim- plerandmoreefficient.Inthischapter,you’reintroducedtofourofthoseenhancements. Genericsprovideameansforyoutodesignclassesthatcanhandleobjectsofdifferentkinds efficientlyandinatype-safeway.TheMyObjectoffersconvenientshortcutsforaccessingBase ClassLibraryclassesthatrefertothecurrentlyrunningapplication,thelocalmachine,the application’scollectionofforms,thelogged-onuser,andWebservicesassociatedwiththe application.Byoverloadingoperatorssuchas=(equalto)and>(greaterthan),youcannow definewhathappenswhentwoinstancesofyourclassarecompared.Andfinally,you’llreview somenewkeywords. Application:Generics ThisapplicationillustratesthenewMicrosoftVisualBasic2005supportforgenerics. NewConcepts Beforegettingintotheimplementationofgenerics,it’sworthspendingaminutetoanalyze whythisfeaturehasbeenaddedtoVisualBasic2005.Genericscomefromtheneedtodeal withpotentiallyanytypeofobjectina“generic”way.Forexample,youmightwanttocreatea classthatcanacceptonlyEmployeeobjectsorCustomerobjects,andcanprocesseachonedif- ferentlybydistinguishingbetweenthem. 33 C0262058x.fmPage34Wednesday,July28,20043:40PM 34IntroducingMicrosoftVisualBasic2005forDevelopers Evenwithoutgenerics,VisualBasichasalwaysprovidedsomemechanismsfordealingwith objects,regardlessoftheirtype.Forexample,withVisualBasic6.0,youcanstoreobjectsof anytypeinaCollectionobject.Thistechniquehassomedisadvantages,whicharedescribed inthenextsection.GenericsprovideanefficientalternativetoCollections,enforcingtype safety,providingbetterperformance,andintegratingtightlywitht