1.
An Introduction to Aspect-Oriented Programming in Microsoft .NET.<br />Produce Cleaner Code with Aspect-Oriented Programming<br />Gaël Fraiteur<br />gael@sharpcrafters.comhttp://www.sharpcrafters.com/<br />

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AOP Facts<br />

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Featured PostSharp Customers<br />

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Agenda<br />The Problem with Conventional Programming<br />What is AOP?<br />Why AOP?<br />PostSharp Features<br />Comparing AOP Frameworks<br />

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The Problem with Conventional Programming<br />Part 1<br />

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In the beginning there was nothing.<br />publicclassCustomerProcesses<br />{<br />}<br />

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

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

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

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

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

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AKSEL BACHMEIER architect<br />YOHJI YAMAMOTOcustomer<br />

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We want a nice separation of concerns (assembly > namespace > class > method)<br />OOP forces us to write crap!<br />Code Scattering<br />Code Tangling<br />Code Coupling<br />Layer 1<br />Layer 2<br />Why do we write ugly code?<br />

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Security<br />Exception Handling<br />Tracing<br />Monitoring<br />Transaction<br />Data Binding<br />Thread Sync<br />Caching<br />Validation<br />Non-Functional Requirements<br />Cross-Cutting Concerns<br />

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We have patterns. How to implement them?<br />Template-Based Code Generators<br />Anonymous Methods (Functional Programming)<br />Object-Oriented Alternatives<br />

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Encapsulating Infrastructure Concerns?<br />

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Aspects!<br />

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Strengthen Applications With Aspects<br />Show Me!<br />

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Show Me!<br />1. Add a reference to PostSharp.dll<br />

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Show Me!<br />2. Write an aspect<br />

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Show Me!<br />3. Apply the aspect<br />

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Show Me!<br />How does it work?<br />1. Source<br />2. Compiler<br />3. PostSharp<br />4. Run Time<br />

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The Idea Behind AOP<br />Part 3<br />

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Problem Domain<br />Cross-Cutting Concerns<br />Solution Domain<br />Separation of Concerns<br />

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

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15 Years of AOP History<br />Hype Years<br />Productivity Years<br />Research Years<br />

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Why You Should Care<br />The benefits of aspect-oriented programming<br />

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The benefits of aspect-oriented programming<br />Decrease Development Costs<br />WriteFewer lines of code<br />ReadFewer lines of code <br />Concise, clear, understandable code<br />Size-Cost relationship is superlinear<br />

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The benefits of aspect-oriented programming<br />Improve Quality<br />Fewer lines of code -> Fewer Defects<br />More automation -> Fewer Defects<br />Less boiler-plate code -> More interesting work -> Increased attention -> Fewer Defects<br />

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The benefits of aspect-oriented programming<br />Decrease Maintenance Costs<br />Remember: <br />Fewer Defects<br />Maintenance = 75% Reading Code<br />How do you change a pattern once it’s implemented?<br />Better architecture metrics:<br />Decreased component coupling<br />Increased component cohesion<br />

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The benefits of aspect-oriented programming<br />Decrease Maintenance Costs<br />

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The benefits of aspect-oriented programming<br />Optimize Skillsets<br />I understand provisioning processes better than anyone in this company,<br />I master multithreading better than anyone on earth<br />Picture © Darren Rogers and chatwoodsfp (Flicker)<br />

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Features<br />

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Features<br />Code Transformation Primitives<br />Modifications<br />Introductions<br />Around Methods<br />Method Interception<br />Property Interception<br />Field Interception<br />Event Interception<br />Interface Introduction<br />Method Introduction<br />Property Introduction<br />Event Introduction<br />Member Import<br />Custom Attribute Intro<br />Managed Resource Intro<br />

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Features<br />Composite Aspects<br />Aspects composed of multiple primitive transformations<br />Advice = Additional Behavior ≈ Transformation<br />Pointcut = Expression selecting target elements of code<br />Declarative<br />LINQ over System.Reflection<br />Adding aspects dynamically: IAspectProvider<br />

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Features<br />Aspect Multicasting<br />Using a single line of code, apply an aspects to multiple elements of code based on:<br />Attributes (public/private, virtual/sealed, …)<br />Naming conventions<br />

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Features<br />Attribute Inheritance<br />Interfaces<br />Classes<br />Virtual Methods<br />Assemblies (!)<br />- or -<br />

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Robust Aspect Composition<br />Multiple aspects on the same element of code<br />Aspect dependency framework<br />Ordering<br />Requirement<br />Conflict<br />Strong ordering and commutativity<br /> Deterministic Behavior<br />D<br />C<br />B<br />A<br />

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Visual Studio Extension<br />1. Code Adornment + Clickable Tooltips<br />“What aspects are applied to a given element of code?”<br />1. Adornment + Tooltip<br />Aspects<br />Base Code<br />Bi-Directional Navigation<br />2. Aspect Browser<br />“Which elements of code is a given a given aspect applied to?”<br />2. Aspect Browser<br />

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Comparing Aspect Frameworks<br />Part 4<br />

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Comparing Aspect Frameworks<br />What to compare?<br />Expressive<br />Non-Invasive<br />Supported<br />Framework<br />Robust<br />Productive<br />Dynamic<br />

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Build-Time MSIL Transformation<br />Consumer Object<br />Enhanced Object<br />Aspects<br />

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AO Infrastructure<br />Transparent Proxies<br />Enhanced Object<br />Consumer Object<br />Transparent Proxy<br />RealProxy<br />Aspects<br />The transparent proxy is type-compatible with the enhanced object(CLR-provided magic)<br />

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JIT-Emitted Proxy<br />AO Infrastructure<br />Consumer Object<br />Proxy<br />Enhanced Object<br />Aspects<br />The proxy implements an interface of the enhanced object.<br />

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JIT-Emitted Subclass<br />AO Infrastructure<br />Consumer Object<br />Proxy<br />Enhanced Object<br />Aspects<br />The proxy extends (inherits from) the enhanced object.<br />

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Comparing Aspect Frameworks<br />Static vs Dynamic AOP<br />Spring.NET<br />Castle<br />MS Unity/PIAB<br />PostSharp<br />LinFu<br />Build-Time:<br />Very Expressive<br />Robust Model<br />Not InvasiveStatic<br />Run-Time:Less ExpressiveBrittle Model<br />InvasiveDynamic<br />Hybrid<br />

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Comparing Aspect Frameworks<br />Expressiveness (1/2)<br />What can you do with the framework?<br />

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Comparing Aspect Frameworks<br />Expressiveness (2/2)<br />How do you apply aspects to code?<br />

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Comparing Aspect Frameworks<br />Non-Invasiveness<br />Can you use aspects without deep refactoring?<br />Require the use of factory methods<br />

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Comparing Aspect Frameworks<br />Robustness<br />Can you prevent aspects from being improperly used?<br />

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Comparing Aspect Frameworks<br />Misc.<br />Other points that matter<br />

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Comparing Aspect Frameworks<br />My Own Summary<br />Aspects on Service Boundaries: use your favorite application framework.<br />Aspects on Ordinary and GUI Objects: use PostSharp.<br />You can mix PostSharp with your favorite application framework!<br />

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Summary<br />

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We need Aspects!<br />

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We have great frameworks!<br />and PostSharp happens to be the best in .NET :).<br />

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http://www.sharpcrafters.com/gael@sharpcrafters.com<br />