AOP via Policy Injection & Unity Application BlocksDmitri Nesterukdmitri@activemesa.comhttp://activemesa.com http://spbalt.net

AgendaAOP and Policy Injection

AOP overviewThe world of proxiesPolicy Injection AB

Unity interceptionInterceptor extensionCustom attributes

AOP and Policy InjectionPart I

Aspect-Oriented Programming

Separation of functional and non-functional requirements

Without mixing code

→

Ways of adding AOPPost-build assembly modification

PostSharpMetaprogramming

BooNemerle

ProxiesSpring.netCastlePolicy Injection AB

compilerextension

ProxyA proxy is a substitute object

It has both the original functionalityAnd the AOP-added features

Can take an existing object and intercept property access, method calls, etc.Must (typically) use an interface

That way, implementation can be transparently substituted

What is PIAB?Part of Enterprise Libraryhttp://entlib.codeplex.comAllows you to intercept calls to an object

MethodProperty Get/Set

Lets you set up a handler pipeline

Is PIAB an AOP framework?[…] However, the Policy Injection Application Block is not an AOP framework implementation for the following reasons:

It uses interception to enable only pre-processing handlers and post-processing handlers.It does not insert code into methods.It does not provide interception for class constructors.

Policy Injection Application Block, MSDNhttp://msdn.microsoft.com/en-us/library/cc511729.aspx

How does it work?By default, PIAB uses transparent proxies

Used in remotingTo create a proxy for your class, it must

Either implement a well-known interface; orDerive from MarshalByRefObject

PIAB uses a factory to create objectsJust like Unity!

Scenariopublic interface IPerson{ int Age { get; set; } string FirstName { get; set; } string LastName { get; set; } string GetInfo();}

public class Person : IPerson{ public Person(int age, string firstName, string lastName) { Age = age; FirstName = firstName; LastName = lastName; } public string GetInfo() { return FirstName + " " + LastName + " is " + Age + " years old."; }}

Object creation (no aspects yet!)

Concrete typesOnly if object derived from MarshalByRefObjectvar p = PolicyInjection.Create<Person>(…);Will fail otherwise

Interface typesvar p = PolicyInjection.Create<Person, IPerson>( 27, "Dmitri", "Nesteruk");

Can apply to existing objectPerson p = new Person(27, "Dmitri", "Nesteruk");IPerson wp = PolicyInjection.Wrap<IPerson>(p);

Applying policiesA policy needs to specify

Where it needs to be applied (matching rules)What needs to be done (handler)

EntLib provides several ‘stock’ handlers

ExampleIn each property assignmentCheck that the value is not null

And throw ArgumentException if it is

Typically edited in designer

Stock handlersPIAB comes pre-packaged with several ‘stock’ handlers

These handlers integrate with other application blocks

Can always write your own

Including a designer

AuthorizationCachingCustomException handling

LoggingPerformanceValidation

Custom validation policyDemo

Custom handler creationImplement ICallHandlerImplement Invoke() to

Do your own pre/postprocessingTo invoke next item in chain, usereturn getNext()(input, getNext);

Make sure to tag handler class with[ConfigurationElementType( typeof(CustomCallHandlerData))]Create your custom policy

Warning: sign the assembly; orEdit config manually

ICallHandler is defined in Unity

AB

Unity InterceptionPart II

Unity Object ResolutionUnity is capable of giving us predefined object instances

IPerson p = uc.Resolve<IPerson>();IPerson can resolve to a predefined type, but it doesn’t have to!

It can resolve instead to a type constructed at runtimeThis type can use the functionality of Person and enhance it

Unity & PIABBoth Unity and PIAB give out objects

Unity resolves dependenciesPIAB creates proxies

Synergy? Absolutely.In Unity, we can decide whether or not a proxy needs to be createdThen we can selectively apply PIABAnd yield an object conforming to an interface

Interoperation between Unity and PIAB is possible, but…

There is an alternative

Unity Interception ExtensionSince v1.2, Unity has a…

Microsoft.Practices.Unity.InterceptionExtension

Works just like PIAB, butEasy to configure in codeIntegrates with Unity container

ArtefactsCall handler (as per PIAB)Handler attribute – used to decorate elements it affectsUnity extension configuration

Interceptor implementationDerive from HandlerAttribute

Override CreateHandler()

Make a handler that implements ICallHandler

Define Invoke()Use getNext().Invoke() to call decorated methodAdd code around it as necessary

Add and configure interceptionuc.AddNewExtension<Interception>();uc.Configure<Interception>() .SetInterceptorFor<IPerson>( new TransparentProxyInterceptor());

Unity InterceptionDemo

ConclusionsPIAB is useful for ‘stock’ interceptionUnity is useful for general-purpose interceptionUnity is positioned to supersede PIAB for AOPBest for dynamic reconfiguration

Use precompilation for speed

LinksAOP frameworks to try

PostSharp http://postsharp.org Our webcast at http://www.techdays.ru/videos/1320.html Spring.net http://www.springframework.net/Castle http://www.castleproject.org Enterprise library http://entlib.codeplex.com

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