This document introduces aspect-oriented programming (AOP) and how it can be used to separate cross-cutting concerns from core business logic. It discusses how AOP frameworks like PostSharp allow aspects to modularize features like logging, transactions, and security. The document provides an example of how a rental booking method grows over time to include these concerns. It compares different AOP frameworks and concludes that PostSharp allows building aspects with strong robustness while minimizing code invasiveness.

1. An Introduction to Aspect-Oriented Programming in Microsoft .NET.Produce Cleaner Code with Aspect-Oriented ProgrammingGaël Fraiteurgael@sharpcrafters.comhttp://www.sharpcrafters.com/

2. AOP Facts

3. Featured PostSharp Customers

4. AgendaThe Problem with Conventional ProgrammingWhat is AOP?Why AOP?PostSharp FeaturesComparing AOP Frameworks

5. The Problem with Conventional ProgrammingPart 1

6. In the beginning there was nothing.publicclassCustomerProcesses{}

7. Customer said: let there be business value.publicclassCustomerProcesses{publicvoid RentBook( int bookId, int customerId ) {Book book = Book.GetById( bookId );Customer customer = Customer.GetById( customerId ); book.RentedTo = customer; customer.AccountLines.Add(string.Format( "Rental of book {0}.", book ), book.RentalPrice ); customer.Balance -= book.RentalPrice; }}And there was business code.

8. internalclassCustomerProcesses{privatestaticreadonlyTraceSource trace =newTraceSource( typeof (CustomerProcesses).FullName ); publicvoid RentBook( int bookId, int customerId ) { trace.TraceInformation("Entering CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} )", bookId, customerId ); try{ Book book = Book.GetById( bookId );Customer customer = Customer.GetById( customerId ); book.RentedTo = customer; customer.AccountLines.Add( string.Format( "Rental of book {0}.", book ), book.RentalPrice ); customer.Balance -= book.RentalPrice; trace.TraceInformation( "Leaving CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} )", bookId, customerId ); } catch ( Exception e ) { trace.TraceEvent( TraceEventType.Error, 0, "Exception: CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} ) failed : {2}", bookId, customerId, e.Message ); throw; } }}Testers said: Letthere be loggingAnd there was logging code.

9. internalclassCustomerProcesses{privatestaticreadonlyTraceSource trace =newTraceSource(typeof(CustomerProcesses).FullName); publicvoid RentBook(int bookId, int customerId) {if (bookId <= 0) thrownewArgumentOutOfRangeException("bookId");if (customerId <= 0) thrownewArgumentOutOfRangeException("customerId"); trace.TraceInformation("Entering CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} )", bookId, customerId); try {Book book = Book.GetById(bookId);Customer customer = Customer.GetById(customerId); book.RentedTo = customer; customer.AccountLines.Add(string.Format("Rental of book {0}.", book), book.RentalPrice); customer.Balance -= book.RentalPrice; trace.TraceInformation("Leaving CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} )“, bookId, customerId); }catch (Exception e) { trace.TraceEvent(TraceEventType.Error, 0, "Exception: CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} ) failed : {2}", bookId, customerId, e.Message);throw; } }}Devssaid: Letthere be defensive programmingThenthere was precondition checking code.

10. internalclassCustomerProcesses{privatestaticreadonlyTraceSource trace =newTraceSource(typeof(CustomerProcesses).FullName); publicvoid RentBook(int bookId, int customerId) {if (bookId <= 0) thrownewArgumentOutOfRangeException("bookId");if (customerId <= 0) thrownewArgumentOutOfRangeException("customerId"); trace.TraceInformation("Entering CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} )“, bookId, customerId); try {for (int i = 0; ; i++) {try {using (var ts = newTransactionScope()) {Book book = Book.GetById(bookId);Customer customer = Customer.GetById(customerId); book.RentedTo = customer; customer.AccountLines.Add(string.Format("Rental of book {0}.", book), book.RentalPrice); customer.Balance -= book.RentalPrice; ts.Complete(); } break; }catch (TransactionConflictException) {if (i < 3)continue;elsethrow; } } trace.TraceInformation("Leaving CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} )", bookId, customerId); }catch (Exception e) { trace.TraceEvent(TraceEventType.Error, 0,"Exception: CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} ) failed : {2}", bookId, customerId, e.Message);throw; } }}Let there be safe concurrent execution.And there was transaction handling code.

11. internalclassCustomerProcesses{privatestaticreadonlyTraceSource trace =newTraceSource(typeof(CustomerProcesses).FullName); publicvoid RentBook(int bookId, int customerId) {if (bookId <= 0) thrownewArgumentOutOfRangeException("bookId");if (customerId <= 0) thrownewArgumentOutOfRangeException("customerId"); try { trace.TraceInformation("Entering CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} )", bookId, customerId ); try {for ( int i = 0;; i++ ) {try {using ( var ts = newTransactionScope() ) {Book book = Book.GetById( bookId );Customer customer = Customer.GetById( customerId ); book.RentedTo = customer; customer.AccountLines.Add( string.Format( "Rental of book {0}.", book ), book.RentalPrice ); customer.Balance -= book.RentalPrice; ts.Complete(); } break; }catch ( TransactionConflictException ) {if ( i < 3 )continue;elsethrow; } } trace.TraceInformation("Leaving CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} )", bookId, customerId ); }catch ( Exception e ) { trace.TraceEvent( TraceEventType.Error, 0,"Exception: CustomerProcesses.CreateCustomer( bookId = {0}, customerId = {1} ) failed : {2}", bookId, customerId, e.Message );throw; } }catch ( Exception e ) {if (ExceptionManager.Handle(e)) throw; } }}Let there be user-friendly error messages.And there was exception handling code.

12. AKSEL BACHMEIER architectYOHJI YAMAMOTOcustomer

13. We want a nice separation of concerns (assembly > namespace > class > method)OOP forces us to write crap!Code ScatteringCode TanglingCode CouplingLayer 1Layer 2Why do we write ugly code?

14. SecurityException HandlingTracingMonitoringTransactionData BindingThread SyncCachingValidationNon-Functional RequirementsCross-Cutting Concerns

15. We have patterns. How to implement them?Template-Based Code GeneratorsAnonymous Methods (Functional Programming)Object-Oriented Alternatives

16. Encapsulating Infrastructure Concerns?

17. Aspects!

18. Strengthen Applications With AspectsShow Me!

19. Show Me!1. Add a reference to PostSharp.dll

20. Show Me!2. Write an aspect

21. Show Me!3. Apply the aspect

22. Show Me!How does it work?1. Source2. Compiler3. PostSharp4. Run Time

23. The Idea Behind AOPPart 3

24. Problem DomainCross-Cutting ConcernsSolution DomainSeparation of Concerns

25. What is AOP?An extension of (not an alternative to) OOP that addresses the issue of cross-cutting concerns by providing a mean to:Encapsulate cross-cutting concerns into Aspects = collection of transformations of codeApplyaspects to elements of code

26. 15 Years of AOP HistoryHype YearsProductivity YearsResearch Years

27. Why You Should CareThe benefits of aspect-oriented programming

28. The benefits of aspect-oriented programmingDecrease Development CostsWriteFewer lines of codeReadFewer lines of code Concise, clear, understandable codeSize-Cost relationship is superlinear

29. The benefits of aspect-oriented programmingImprove QualityFewer lines of code -> Fewer DefectsMore automation -> Fewer DefectsLess boiler-plate code -> More interesting work -> Increased attention -> Fewer Defects

30. The benefits of aspect-oriented programmingDecrease Maintenance CostsRemember: Fewer DefectsMaintenance = 75% Reading CodeHow do you change a pattern once it’s implemented?Better architecture metrics:Decreased component couplingIncreased component cohesion

31. The benefits of aspect-oriented programmingDecrease Maintenance Costs

32. The benefits of aspect-oriented programmingOptimize SkillsetsI understand provisioning processes better than anyone in this company,I master multithreading better than anyone on earthPicture © Darren Rogers and chatwoodsfp (Flicker)

33. Features

34. FeaturesCode Transformation PrimitivesModificationsIntroductionsAround MethodsMethod InterceptionProperty InterceptionField InterceptionEvent InterceptionInterface IntroductionMethod IntroductionProperty IntroductionEvent IntroductionMember ImportCustom Attribute IntroManaged Resource Intro

35. FeaturesComposite AspectsAspects composed of multiple primitive transformationsAdvice = Additional Behavior ≈ TransformationPointcut = Expression selecting target elements of codeDeclarativeLINQ over System.ReflectionAdding aspects dynamically: IAspectProvider

36. FeaturesAspect MulticastingUsing a single line of code, apply an aspects to multiple elements of code based on:Attributes (public/private, virtual/sealed, …)Naming conventions

37. FeaturesAttribute InheritanceInterfacesClassesVirtual MethodsAssemblies (!)- or -

38. Robust Aspect CompositionMultiple aspects on the same element of codeAspect dependency frameworkOrderingRequirementConflictStrong ordering and commutativity Deterministic BehaviorDCBA

39. Visual Studio Extension1. Code Adornment + Clickable Tooltips“What aspects are applied to a given element of code?”1. Adornment + TooltipAspectsBase CodeBi-Directional Navigation2. Aspect Browser“Which elements of code is a given a given aspect applied to?”2. Aspect Browser

40. Comparing Aspect FrameworksPart 4

41. Comparing Aspect FrameworksWhat to compare?ExpressiveNon-InvasiveSupportedFrameworkRobustProductiveDynamic

42. Build-Time MSIL TransformationConsumer ObjectEnhanced ObjectAspects

43. AO InfrastructureTransparent ProxiesEnhanced ObjectConsumer ObjectTransparent ProxyRealProxyAspectsThe transparent proxy is type-compatible with the enhanced object(CLR-provided magic)

44. JIT-Emitted ProxyAO InfrastructureConsumer ObjectProxyEnhanced ObjectAspectsThe proxy implements an interface of the enhanced object.

45. JIT-Emitted SubclassAO InfrastructureConsumer ObjectProxyEnhanced ObjectAspectsThe proxy extends (inherits from) the enhanced object.

46. Comparing Aspect FrameworksStatic vs Dynamic AOPSpring.NETCastleMS Unity/PIABPostSharpLinFuBuild-Time:Very ExpressiveRobust ModelNot InvasiveStaticRun-Time:Less ExpressiveBrittle ModelInvasiveDynamicHybrid

47. Comparing Aspect FrameworksExpressiveness (1/2)What can you do with the framework?

48. Comparing Aspect FrameworksExpressiveness (2/2)How do you apply aspects to code?

49. Comparing Aspect FrameworksNon-InvasivenessCan you use aspects without deep refactoring?Require the use of factory methods

50. Comparing Aspect FrameworksRobustnessCan you prevent aspects from being improperly used?

51. Comparing Aspect FrameworksMisc.Other points that matter

52. Comparing Aspect FrameworksMy Own SummaryAspects on Service Boundaries: use your favorite application framework.Aspects on Ordinary and GUI Objects: use PostSharp.You can mix PostSharp with your favorite application framework!

53. Summary

54. We need Aspects!

55. We have great frameworks!and PostSharp happens to be the best in .NET :).

56. http://www.sharpcrafters.com/gael@sharpcrafters.com