2006 aw applying domain driven design and patterns wit

ApplyingDomain-DrivenDesignandPatterns:With
ExamplesinC#and.NET,1/e
ByJimmyNilsson
...............................................
Publisher:AddisonWesleyProfessional
PubDate:May08,2006
PrintISBN-10:0-321-26820-2
PrintISBN-13:978-0-321-26820-4
Pages:576

TableofContents|Index

"[This]isabookaboutdesigninthe.NETworld,driveninanagilemannerandinfused
withtheproductsoftheenterprisepatternscommunity.[It]showsyouhowtobegin
applyingsuchthingsasTDD,objectrelationalmapping,andDDDto.NET
projects...techniquesthatmanydevelopersthinkarethekeytofuturesoftware
development....Asthetechnologygetsmorecapableandsophisticated,itbecomesmore
importanttounderstandhowtouseitwell.Thisbookisavaluablesteptowardadvancing
thatunderstanding."
MartinFowler,authorofRefactoringandPatternsofEnterpriseApplicationArchitecture

Patterns,Domain-DrivenDesign(DDD),andTest-DrivenDevelopment(TDD)enable
architectsanddeveloperstocreatesystemsthatarepowerful,robust,andmaintainable.
Now,there'sacomprehensive,practicalguidetoleveragingallthesetechniquesprimarily
inMicrosoft.NETenvironments,butthediscussionsarejustasusefulforJavadevelopers.
DrawingonseminalworkbyMartinFowler(PatternsofEnterpriseApplicationArchitecture)
andEricEvans(Domain-DrivenDesign),JimmyNilssonshowshowtocreatereal-world
architecturesforany.NETapplication.Nilssonilluminateseachprinciplewithclear,wellannotatedcodeexamplesbasedonC#1.1and2.0.Hisexamplesanddiscussionswillbe
valuablebothtoC#developersandthoseworkingwithother.NETlanguagesandany
databasesevenwithotherplatforms,suchasJ2EE.Coverageincludes
·Quickprimersonpatterns,TDD,andrefactoring

·Usingarchitecturaltechniquestoimprovesoftwarequality
·Usingdomainmodelstosupportbusinessrulesandvalidation
·ApplyingenterprisepatternstoprovidepersistencesupportviaNHibernate
·PlanningeffectivelyforthepresentationlayerandUItesting


·DesigningforDependencyInjection,AspectOrientation,andothernewparadigms


ApplyingDomain-DrivenDesignandPatterns:With
ExamplesinC#and.NET,1/e
ByJimmyNilsson
...............................................
Publisher:AddisonWesleyProfessional
PubDate:May08,2006
PrintISBN-10:0-321-26820-2
PrintISBN-13:978-0-321-26820-4
Pages:576

TableofContents|Index

Copyright
PraiseforApplyingDomain-DrivenDesignandPatterns
AbouttheAuthor
Forewords
Preface:BridgingGaps
Acknowledgments
PartI:Background
Chapter1.ValuestoValue:OrEmbarrassingRamblingsWhenSelf-Reflecting
ontheLastFewYears
OverallValues
ArchitectureStylestoValue
ProcessIngredientstoValue
ContinuousIntegration
Don'tForgetAboutOperations
Summary
Chapter2.AHeadStartonPatterns
ALittleBitAboutPatterns
DesignPatterns
ArchitecturalPatterns
DesignPatternsforSpecificTypesofApplications
DomainPatterns
Summary
Chapter3.TDDandRefactoring

Test-DrivenDevelopment(TDD)
MocksandStubs
Refactoring
Summary
PartII:ApplyingDDD
Chapter4.ANewDefaultArchitecture
TheBasisoftheNewDefaultArchitecture
AFirstSketch
MakingaFirstAttemptatHookingtheUItotheDomainModel
YetAnotherDimension
Summary
Chapter5.MovingFurtherwithDomain-DrivenDesign
RefiningtheDomainModelThroughSimpleTDDExperimentation
FluentInterface
Summary
Chapter6.PreparingforInfrastructure
POCOasaLifestyle

DealingwithSaveScenarios
Let'sBuildtheFakeMechanism
DatabaseTesting
Querying
Summary
Chapter7.LettheRulesRule
CategorizationofRules
PrinciplesforRulesandTheirUsage
StartingtoCreateanAPI
RequirementsforaBasicRulesAPIRelatedtoPersistence
FocusonDomain-RelatedRules
ExtendingtheAPI
RefiningtheImplementation
BindingtothePersistenceAbstraction
GenericsandAnonymousMethodstotheRescue
WhatOthersHaveDone
Summary
PartIII:ApplyingPoEaa
Chapter8.InfrastructureforPersistence
RequirementsonthePersistenceInfrastructure
WheretoStoreData
Approach

Classification
AnotherClassification:InfrastructurePatterns
Summary
Chapter9.PuttingNHibernateintoAction
WhyNHibernate?
AShortIntroductiontoNHibernate
RequirementsofthePersistenceInfrastructure
Classification
AnotherClassification:InfrastructurePatterns
NHibernateandDDD
Summary
PartIV:What'sNext?
Chapter10.DesignTechniquestoEmbrace
ContextIsKing
AnIntroductiontoSOA
InversionofControlandDependencyInjection
Aspect-OrientedProgramming(AOP)
Summary

Chapter11.FocusontheUI
APrepilogue
TheModel-View-ControllerPattern
Test-DrivingaWebForm
MappingandWrapping
Summary
Epilogue
PartV:Appendices
AppendixA.OtherDomainModelStyles
Object-OrientedDataModel,SmartServiceLayer,andDocuments
TheDatabaseModelIstheDomainModel
PragmatismandtheNontraditionalApproach
Summary
AppendixB.CatalogofDiscussedPatterns
AbstractFactory[GoFDesignPatterns]
Aggregate[EvansDDD]
BoundedContext[EvansDDD]
ChainofResponsibility[GoFDesignPatterns]
ClassTableInheritance[FowlerPoEAA]
Coarse-GrainedLock[FowlerPoEAA]
CollectingParameterPattern[BeckSBPP]




ConcreteTableInheritance[FowlerPoEAA]

DataMapper[FowlerPoEAA]
DataTransferObjects[FowlerPoEAA]
Decorator[GoFDesignPatterns]
DependencyInjection
DomainModel[FowlerPoEAA]
EmbeddedValue[FowlerPoEAA]
Entity[EvansDDD]
Factory[EvansDDD]
FactoryMethod[GoFDesignPatterns]
ForeignKeyMapping[FowlerPoEAA]
GenerationGap[VlissidesPatternHatching]
IdentityField[FowlerPoEAA]
IdentityMap[FowlerPoEAA]
ImplicitLock[FowlerPoEAA]
LayerSupertype[FowlerPoEAA]
Layers[POSA]
LazyLoad[FowlerPoEAA]
MetadataMapping[FowlerPoEAA]
ModelViewController[FowlerPoEAA]
ModelViewPresenter[FowlerPoEAA2]
Notification[FowlerPoEAA2]
NullObject[WoolfNullObject]
OptimisticOfflineLock[FowlerPoEAA]
PartyArchetype[Arlow/NeustadtArchetypePatterns]

PessimisticOfflineLock[FowlerPoEAA]
PipesandFilters[POSA]
PresentationModel[FowlerPoEAA2]
Proxy[GoFDesignPatterns]
QueryObject[FowlerPoEAA]
Recordset[FowlerPoEAA]
Reflection[POSA]
Registry[FowlerPoEAA]
RemoteFaçade[FowlerPoEAA]
Repository[EvansDDD]
SeparatedPresentation[FowlerPoEAA2]
ServiceLayer[FowlerPoEAA]
ServiceLocator[Alur/Crupi/MalksCoreJ2EEPatterns]
Services[EvansDDD]

SingleTableInheritance[FowlerPoEAA]
Singleton[GoFDesignPatterns]
Specification[EvansDDD]
State[GoFDesignPatterns]
TableModule[FowlerPoEAA]
TemplateMethod[GoFDesignPatterns]
TransactionScript[FowlerPoEAA]
UnitofWork[FowlerPoEAA]
ValueObject[EvansDDD]
References


Index

Copyright
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Nilsson,Jimmy.
ApplyingdomaindrivendesignandpatternswithexamplesinC#and.NET/JimmyNilsson.
p.cm.
Includesbibliographicalreferences.
ISBN0-321-26820-2
1.Computersoftware-Development.2.C#(Computerprogramlanguage)3.Microsoft.NET.I.
QA76.76.D47N6452006
005.1--dc22
2006004371
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Dedication
ToLotta,Tim,andLeo:thecentersofmyuniverse.

PraiseforApplyingDomain-Driven
DesignandPatterns
"Idon'tknowwhatitwasIprofessedtodoingbeforeI
hadaddedDomain-DrivenDesignandTest-Driven
Developmenttomytoolkit,butfrommypresent
perspective,I'mreticenttocallitanythingbutchaotic
hacking.Domain-DrivenDesignandTest-Driven
Developmentaretwoapproachesthathaveconsistently
guidedmetowardapracticalapplicationofsoftware
designprinciples,andbroughtmyprojectstofruitionwith
healthy,sustainablesoftware.Thisisabookthatputsits
moneywhereitsmouthisintermsofconcretely
communicatingAgilesoftwaredevelopmentpractice.It's
potentiallyoneofthemostimpactfulguidestosoftware
developmentsuccesspracticesyetofferedtothe.NET
softwaredevelopmentcommunity."
ScottBellware,MicrosoftMVPforC#,DDDandTDD
Blogger
"JimmyNilssondoeshisreadersagreatserviceby
showingthemhowtoapplythefoundationalprinciplesof
enterpriseapplicationdesignanddevelopmenttaughtby
Evans,Fowler,andotherthoughtleaders.Thebookuses
aworkedexamplenotonlytoexplain,butalsoto
demonstrateDomain-DrivenDesign,Patternsof
EnterpriseApplicationArchitecture,andTest-Driven
Development.Jimmy'sinsightandexperiencemake
readingitapleasure,andleavethereaderwiththe
certaintythattheyhavelearnedfromamaster

practitioner.Enterprisedeveloperslookingtomasterthese
principleswillfindthebookavaluableguide."
JackGreenfield,EnterpriseToolsarchitect,VisualStudio


TeamSystem,Microsoft
"Goodsoftwarearchitectsreservetherighttoget
smarter.Beyondthegoalofshippingtheircurrentsystem,
they'reonaquesttodiscoverbetterwaystodesignand
buildsoftware.Thisbookistravelogueofsortsinwhich
Jimmydocumentshisjourneythroughawiderangeof
patterns,practices,andtechnologies,explaininghowhis
thinkingaboutenterprisesystemshasevolvedalongthe
way.Ifyou'retravelingthesameroad,thisbookisagood
companion."
TimEwald,principalarchitectatFoliageSoftwareSystems
andauthorofTransactionalCOM+:BuildingScalable
Applications
"Thisbookdoesanexcellentjobatmakingtangiblethe
largebutveryimportantideasofDomain-DrivenDesign."
FloydMarinescu,authorofEJBDesignPatternsand
creatorofInfoQ.comandTheServerSide.com
"Understandingtheconceptsanddrivingforcesinthe
problemdomainisparamounttothesuccessofsoftware
development.JimmyNilssonhasdrawninspirationfrom
thepasttenyearsofstudiesintopatternsandDomainDrivenDesignaswellasrecordedhisownexperiences
fromconcretedevelopmentprojects.Thisbookcontains
compellingexamplesofhowtheorycanbetranslatedinto
practice.Itdemonstratestheauthor'sprofound
understandingofdesignchoicesandtrade-offsinvolvedin

object-orienteddevelopment."
AndersHessellund,ITUniversityofCopenhagen,Denmark
"Thisbooktacklesanareathatisachallengeformost
developersonthe.NETplatform.Asthepatternsand


practicesarchitectwhoinitiatedanddrovetheefforts
aroundenterpriseguidancefor.NET,Iknowhow
importantthisareaisforourcustomers,andampainfully
awareofthegapsthatstillexistinourguidance.
"IwasthrilledtoseeJimmywouldbesharinghisinsights
basedonhisexperiencedoingDDDandTDD.Ibelieve
thistopiccanbebesttackledatthispointintimethrough
afocusonsimplicity,patterns,andawarenessofthe
socialaspectsofbuildingapplications.
"ItrustJimmy'sexperienceandknowledgeof.NET,and
haveenjoyedhisstyleofsharingconceptsandstories.I
canhardlyimaginesomeonebettersuitedtoexplainthis
topicontheplatformIworkoneveryday.
"IwilldefinitivelyrecommendJimmy'sbooktomy
customers,myteam,andotherMicrosoftengineers.
"Ultimately,Ihopethatourindustrycangetbetterat
expressingrichconceptsincode,andthatitgetsbetterat
thesocialprocessofarticulatingandevolvingshared
knowledge.TDDandDDDlietogetherattheheartofthis
path."
EdwardJezierski,architectofMicrosoftpatternsand
practices
"Jimmyhaswoventogetherbest-of-bread
techniquesDomain-DrivenDesign,Test-Driven

Development,refactoring,anddesignpatternsintoa
convincingalternativetotheData-DrivenDevelopment
stylethathasbeenthemainstayofMicrosoftapplications.
Thecoreofthebookbreatheslifeintothetechniques
advocatedinDomain-DrivenDesign;itisrelentlessly
practical,givesthereaderinsightintothethoughtprocess
thatunderliesapplyingproventechniquesand
technologies,and,likeallofJimmy'swork,isaneasyread


welldoneJimmy!"
ChristianCrowhurst,analystdeveloper
"I'vebeenworkingonajointprojectwithJimmyfor18
monthsnow.Jimmyreallylivesashepreaches.Inthis
bookhetellsyouhowheisworkinginpractice.Heuses
O/RMappingandNHibernatedailyandhealsoreallyuses
TDDtohelpunderstandanddistillthecustomer'sbusiness
logicandtomakesurethatheonlydeliverstested,highqualitycode.Justashedeliversahigh-qualitybook
containingtestedconceptstoyou.Enjoy."
DanByström,softwaredeveloper,www.visualdesign.se/blog
"ByshowinghowtoapplyDomain-DrivenDesignand
enterpriseapplicationpatterns,Jimmymakesadifficult
topicapproachableforalargeraudience.Thisbookhas
somethingfortheexperiencedarchitectandaspiring
patternimplementeralike.Infact,anydeveloperor
architectwhowantstoimprovethewayhe/shedesigns
andimplementssoftwaresolutionsshouldreadthisbook."
Per-OlaNilsson,headofdevelopmentandsoftware
architectforLuvit
"Constantlyabreastwiththelatesttrendsinsoftware

developmentbutneversweptawaybythehype,Jimmy
expertlyextractsthevalueandleavesthefluffbehind.
Alwayshumbleandwithbothhisearswideopen,heis
uniquelywellsuitedtoguideandenlightenthereader
usingthepracticalapplicabilityasthestandard,shunning
theivorytower.Jimmydoesn'tofferhishelpfromabove
heactuallymanagestoconvinceusthatwehavejust
learnedaboutthecuttingedgefromapeer."


MatsHelander,softwaredeveloper,
www.matshelander.com
"AswithJimmy'spreviousbook,thisbookisatrue
masterpiece:awell-laidoutbalancebetweenpragmatic
softwareengineeringanddeeptheory."
FransBouma,creatorofLLBLGenPro,
weblogs.asp.net/fbouma
"Withthisbookbeingbasedonsolidground,Jimmy
tacklestherealissuesinapplyingDomain-DrivenDesign
practices.Finallywehaveamanualtokeepusfrom
hittingthewallincopingwithever-increasingcomplexity
intomorrow'sdomainmodel-basedenterprise
applications."
PaulGielens,seniorconsultantforCapgemini,
weblogs.asp.net/pgielens
"Speakingfromexperience,combiningthesubstance
behindacronymssuchasDDD,DP,andTDDin.NETcan
leadtoveryefficientandadaptiveenterprisesoftware
architectures.Thisbookoutlineshow,andwhy,these
conceptstogethermakesuchapowerfulwhole.Hence,

thereaderisgivenaninvaluableshortcuttosuccessful
enterprisesoftwarearchitectures,makingitamustread
foranydeveloperofsuch."
MartinRosén-Lidholm,softwarearchitectforExense
Healthcare
"Doyouwanttobuildahigh-qualityenterprisesystem
withobjectorientationandarelationaldatabase?Don't
missthisone.JimmyshowsyouhowTest-Driven
Developmentcanbethedrivingforcethroughoutthe
wholeproject.HeshowshowtomaptheOOdesigntothe


database,astepoftentreatedverybrieflyinliterature,
whichalonemakesthebookworththemoney.Thereare
alsoplentyofdesigntipsanddiscussionssoyoucan
followhisreasoning."
IngemarLundberg,passionatesoftwaredeveloper,
www.ingolundberg.com
"Thisimportantandtimelybookisamustreadforanyone
wantingtogetintoDomain-DrivenDesigninC#."
GregoryYoung,MicrosoftMVPforC#,independent
consultant
"Thisbookdealswithseveralimportantconceptsusedin
modernsoftwaredevelopment,suchasTest-Driven
Development,refactoring,patterns,andofcourse,
Domain-DrivenDesign.JimmyNilssonwritesaboutthese
thingsinaveryconversationaltone,andyoualmostget
thefeelingthatheisactuallysittingnexttoyou,showing
youhisexamplesandhavingaconversationaboutthe
prosandconsofdifferentsolutionswithyoupersonally."

NiclasNilsson,softwaredeveloperandeducatorfor
Activa;notabrothertoJimmy,notevenrelated
"ThisbookdoesanexcellentjobofbringingDomainDrivenDesign(DDD)intopracticalcontextbybridgingthe
gapsbetweendifferentabstractionlevelsstartingfromthe
businessperspectivedowntothesystemlevel.Thebook
isidealforsoftwarearchitectsanddevelopersbecauseit
iswritteninawaythatallowsittobeusedasaprimeror
reference.Theauthorexcelsinpresentingallaspectsof
DDDinawaythatisnotonlyeducationalbutalsofunto
read."
GuntherLenz,programmanagerandauthor


"Notsoverylongago,Ibloggedaboutmyoverallfeeling
thatasarchitectswemaybefailingtoprovideenough
guidancefordeveloperstosortthroughthehundredsof
differentwaystobuildsoftware.Therearemanyideasout
therethatareexcellentbythemselvesandcomewith
perfectlywell-reasonedjustifications,workingexamples,
andanimpassionedcommunityofbelievers.Veryfewof
themcanbeusedbythemselvestobuildapplications,
muchlesssystemsofapplicationsandservices.This
leavesituptodeveloperstofindwaystoputthe
individualideastogetherintoapplications.Thisisalways
muchharderinrealitythantheproponentsofany
particularapproach,liketothinkabout.Developers,on
theotherhand,needtogetsomeworkdoneandoften
don'tcaretowadethroughallthiseverytimetheystarta
newapplication.Thisistrueevenwhentheyaretruly
interestedinthepotentialgainsofnewapproachesor

ideas.Jimmy'sbookispointeddirectlyatthisproblem.
Howdoyouapplysomeofthegreatideasthathavecome
forwardinthelastfewyearstoreal,livedevelopment?I
thinkthisbookdoesagreatjobofbothexplainingthe
underlyingideaswithenoughdetailtomakethem
practicalandputtingthegroupofideastogetherinaway
thatwillhelpdevelopersseeapathfromstarttoend.
Domain-DrivenDesign,Test-DrivenDevelopment,
DependencyInjection,persistence,andmanyother
practicalissuesarecovered.Afterreadingthisbook,I
thinkareaderwillreallybeabletotaketheseimportant
ideasandusethemtogethereffectively.Consideringthe
greatvalueoftheindividualideaspresented,butthe
limitednumbersofexamplesofpeoplereallyusingthem,
thisworkrepresentsagreatwinforusall.Ihighly
recommendit."
PhilipNelson,chiefscientistforPAi,
xcskiwinn.org/community/blogs/panmanphil/default.aspx


"Takingtheleapfromadata-centricmindsettothe
deeperrealizationofOOPfoundinDomain-DrivenDesign,
Test-DrivenDevelopment,object/relationalpersistence,
andotherAgilemethodsandpatternscanbeanarduous
anddisorientingundertakingfortheuninitiated.
"Withapatient,pragmatic,andmentoringstyle,Jimmy
takesthereaderalongfortheleap,exploringtheissues
andtheoptions,givingsoundadvicealongtheway.This
bookshowsyouhowtointegratethevariousmethodsand
patternsintoafullycoherentapproachtodesigningand

creatingsuperblymaintainable.Netsoftware."
GeorgeHicks,seniordeveloperforPropertyWorks
"Ifyouhaveeverreadhisblog,youalreadyknowthat
JimmyNilssonlikestochallengeestablished'truths'inthe
.NETcommunity,lookingforbetterwaystodesign
software.Heacknowledgesthatsoftwaredesignishard,
andthatone-size-fits-allsolutionsdonotwork;prosand
conshavetobebalancedincontextoftheproblemat
handbeforerulingawinner.Forbuildingtestableand
maintainablesoftwarewithcomplexbusiness
requirements,JimmychoosestouseDomain-Driven
Design,andhebringsalongatoolboxofprovenand
establishedprinciples,patterns,andpracticestocarryit
out.Jimmy'sinformalwritingstyleanduseofexamples
andTest-DrivenDevelopmentmakethisbookvery
approachable,especiallyconsideringallthegroundthatis
covered.BecauseheisontopoftheDDDstuff,Jimmy
(andfriends)bringsyoudistilledknowledgeon,and
referencesto,manyofthemostvaluabletechniquesand
resourceswithinsoftwaredevelopmenttoday.Ibelieve
thisbooktobeavaluableguidetoapplyingDDD,andfor
developerswhowanttoimprovetheirgeneraldesign
skills."


AndreasBrink,softwaredeveloper
"InApplyingDomain-DrivenDesignandPatterns,Jimmy
Nilssonstrengthenshispositionasanauthorityonapplied
softwarearchitecture.Withaniceblendofhispersonal
thoughtsonthesubjects,Jimmytakesthereaderona

tourthroughmostofthemodernmust-knowdesign
techniques,leavingnothinguntouched.Jimmyshowshow
toimplementthethoughtsofother,moretheoretical
thoughtleadersinanappealingandeasytofollow
structure.IamcertainthatApplyingDomain-Driven
DesignandPatternswillbecomeamandatorytitleinthe
enterprisebookshelf."
MikaelFreidlitz,vicepresidentofContentandKnowledge
ProgramsatIASA
"Domain-DrivenDesignisanimportanttechniquethatcan
helpproducequalitybusinessapplicationsthatevolvewith
theneedsofthebusiness.Inanidealworld,practicing
DDDwouldbeaboutOOdesignbutinreality,the
technologiesweworkwithimposenumerousconstraints.
"Thisbooktacklesthatchallengeheadon,bridgingthe
gapbetweenDDDconceptsandtheactionoftranslating
themintopracticeonthe.NETplatform.Jimmynotonly
hasadeepunderstandingofDDDandenterprise
technologies,buthaslearnedmanylessonsfromhis
extensiveindustryexperience,andtakesarefreshingly
pragmaticapproach.Thisisavaluablebook."
RodJohnson,founderofSpringFramework,CEOof
Interface21
"Thisisagreatbook.Ittakesyouonahands-on
informativetravelintheworldofDomain-DrivenDesign.
Agreatnumberofimportantissuesarediscussed,


explained,andshowninrelevantcontexts.Thesewillgive
youagreatfoundationwhenyouareworkingwithyour

ownDomainModel-basedsystem.Boththeactual
developmenttechniquesaswellaspracticalworkmethods
(asincorporationofTest-DrivenDevelopment)are
discussed."
Trond-EirikKolloen,softwarearchitectanddeveloper
"Canwefocusonthehouseprojectsnow?"
Lotta,wife
"Aaa,dee,dee,dee,pee..."(Swedishpronunciationofthe
bookabbreviation)
Leo,son,fouryearsold
"Dad,doyoureallythinksomeonewillreadit?"
Tim,son,eightyearsold


AbouttheAuthor
JimmyNilssonownsandrunstheSwedishconsulting
companyJNSKAB.Hehaswrittennumeroustechnicalarticles
andtwobooks.Hehasalsobeentrainingandspeakingat
conferences,butaboveeverythingelse,heisadeveloperwith
almost20yearsofexperience(www.jnsk.se/weblog/).




Forewords
Buildingenterprisesoftwareisrarelyeasy.Althoughwehavea
plethoraoftoolsandframeworkstomakeiteasier,westillhave
tofigureouthowtousethesetoolswell.Therearelotsof
approachesyoucantake,butthetrickisknowingwhichoneto
useinspecificsituationshardlyeverdoesoneapproachworkin

allcases.Overthelastfewyearsthere'sgrownupacommunity
ofpeoplelookingtocaptureapproachestodesignenterprise
applicationsanddocumentthemintheformofpatterns(Ikeep
anoverviewwithlinksat
/>Peopleinvolvedinthiseffort,suchasme,trytofindcommon
approachesanddescribehowtodothemwellandwhenthey
areapplicable.Theresultingworkisprettywideranging,and
thatcanleadtotoomuchchoiceforthereader.
WhenIstartedwritingPatternsofEnterpriseApplication
Architecture(Addison-Wesley,2002),Ilookedforthiskindof
designadviceintheMicrosoftworld.Istruggledtofindmuchof
anything,butonerarebookthattackledtheterritorywas
Jimmy'searlierbook.Ilikedhisinformalwritingstyleand
eagernesstodigintoconceptsthatmanyothersskimmedover.
Soit'sfittingthatJimmydecidedtotakemanyoftheideas
frommeandtheothersintheenterprisepatternscommunity
andshowhowyoucanapplytheminwriting.NETapplications.
Thefocusofthisenterprisepatternscommunityisdocumenting
gooddesigns,butanotherthreadrunsthroughus.Wearealso
bigfansofagilemethods,embracingtechniquessuchasTestDrivenDevelopment(TDD)andrefactoring.SoJimmyalso
broughttheseideasintothisbook.Manypeoplethinkthat
pattern-people'sfocusondesignandTDD'sfocusonevolution
areatodds.Thehugeoverlapbetweenpattern-peopleand
TDDersshowsthisisn'ttrue,andJimmyhasweavedbothof


thesethreadsintothisbook.
Theresultisabookaboutdesigninthe.NETworld,driveninan
agilemannerandinfusedwiththeproductsoftheenterprise
patternscommunity.It'sabookthatshowsyouhowtobegin

applyingsuchthingsasTDD,object-relationalmapping,and
domain-drivendesignto.NETprojects.Ifyouhaven'tyetcome
acrosstheseconcepts,you'llfindthatthisbookisan
introductiontotechniquesthatmanydevelopersthinkarethe
keyforfuturesoftwaredevelopment.Ifyouarefamiliarwith
theseideas,thebookwillhelpyoupassthoseideasontoyour
colleagues.
ManypeoplefeeltheMicrosoftcommunityhasnotbeenasgood
asothersinpropagatinggooddesignadviceforenterprise
applications.Asthetechnologybecomesmorecapableand
sophisticated,itbecomesmoreimportanttounderstandhowto
useitwell.Thisbookisavaluablestepinadvancingthat
understanding.
MartinFowler

ThebestwaytolearnhowtodoDomain-DrivenDesign(DDD)
istositdownnexttoafriendly,patient,experienced
practitionerandworkthroughproblemstogether,step-by-step.
Thatiswhatreadingthisbookislike.
Thisbookdoesnotpushanewgrandscheme.Itunaffectedly
reportsononeexpertpractitioner'suseofandcombinationof
thecurrentpracticeshehasbeendrawnto.
JimmyNilssonreiterateswhatmanyofushavebeensaying:
thatseveralcurrentlytrendytopicsspecifically,DDD,Patternsof
EnterpriseApplicationArchitecture(PoEAA),andTest-Driven
Development(TDD)arenotalternativestoeachother,butare
mutuallyreinforcingelementsofsuccessfuldevelopment.


Furthermore,allthreeoftheseareharderthantheylookat

first.Theyrequireextensiveknowledgeoverawiderange.This
bookdoesspendsometimeadvocatingtheseapproaches,but
mostlyitfocusesonthedetailsofhowtomakethemwork.
Effectivedesignisnotjustabunchoftechniquestobelearned
byrote;itisawayofthinking.AsJimmydivesintoanexample
hegivesusalittlewindowintohismind.Henotonlyshowshis
solutionandexplainsit,heletsusseehowhegotthere.
WhenIamdesigningsomething,dozensofconsiderationsflit
throughmymind.IftheyarefactorsI'vedealtwithoften,they
passsoquicklyIambarelyconsciousofthem.Iftheyarein
areaswhereIhavelessconfidence,Idwellonthemmore.I
presumethisistypicalofdesigners,butitisdifficultto
communicatetoanotherperson.AsJimmywalksthroughhis
examples,itisasifhewereslowingthisprocessdowntoan
observablepace.Ateverylittlejuncture,threeorfour
alternativespresentthemselvesandgetweighedandrejected
infavoroftheoneheeventuallychooses.
Forexample,wewantmodelobjectsthatareimplementedfree
ofentanglementwiththepersistencetechnology.Sowhatare
eightways(eight!)thatapersistenceframeworkcanforceyou
tocontaminatetheimplementationofadomainobject?What
considerationswouldleadyoutocompromiseonsomeofthese
points?Whatdothecurrentlypopularframeworks,including
the.NETplatform,impose?
Jimmythinkspragmatically.Hedrawsonhisexperienceto
makeadesignchoicethatwilllikelytakehimtowardthegoal,
adheringtothedeeperdesignprinciple,ratherthanthechoice
thatlooksthemostlikeatextbookexample.Andallofhis
decisionsareprovisional.
ThefirstdesignprincipleJimmyholdsinfrontofhimselfisthe

fundamentalgoalofDDD:adesignthatreflectsdeep
understandingofthebusinessproblemathandinaformthat


allowsadaptationtonewwrinkles.Sowhysomuchdiscussion
oftechnicalframeworkandarchitecture?
Itisacommonmisperception,perhapsanaturalone,thatsuch
apriorityonthedomaindemandslesstechnicaltalentandskill.
Wouldthatthisweretrue.Itwouldnotbequitesodifficultto
becomeacompetentdomaindesigner.Ironically,torenderclear
andusefuldomainconceptsinsoftware,tokeepthemfrom
beingsuffocatedundertechnicalclutter,requiresparticularly
deftuseoftechnology.Myobservationisthatthosewiththe
greatestmasteryoftechnologyandarchitecturalprinciples
oftenknowhowtokeeptechnologyinitsplaceandareamong
themosteffectivedomainmodelers.
Idonotrefertotheknowledgeofeveryquirkofcomplextools,
buttothemasteryofthesortofknowledgelaidoutinMartin
Fowler'sPoEAA,becausenaïveapplicationoftechnology
paradoxicallymakesthattechnologymoreintrusiveintothe
application.
Formanypeoplethisbookwillfillingapsofhowtoimplement
expressiveobjectmodelsinpractice.Ipickedupanumberof
usefulwaysofthinkingthroughtheapplicationoftechnical
frameworks,andIespeciallyfirmedupmyunderstandingof
someparticularsofdoingDDDina.NETsetting.
Inadditiontotechnicalarchitecture,Jimmyspendsagreatdeal
oftimeonhowtowritetests.TDDcomplementsDDDina
differentway.Intheabsenceofafocusonrefininganever
moreusefulmodel,TDDispronetofragmentedapplications,

whereasingle-mindedattackononefeatureatatimeleadsto
anunextendablesystem.Acomprehensivetestsuiteactually
allowssuchateamtocontinuemakingprogresslongerthan
wouldbepossiblewithoutit,butthisisjustthebasestvalueof
TDD.
Atitsbest,thetestsuiteisthelaboratoryforthedomainmodel
andatechnicalexpressionoftheubiquitouslanguage.Testsof