Introductionto AOP

Programming paradigm

which aims to increase modularity

by allowing the separation of

cross-cutting concerns

en.wikipedia.org/wiki/aspect-oriented-programming

Programming paradigm

which aims to increase modularity

by allowing the separation of

cross-cutting concerns

en.wikipedia.org/wiki/aspect-oriented-programming

Programming Paradigm

It is *NOT*- a programming language

- solution to all your problems

- a first class citizen in Java or .NET

- a replacement for OOP

It is

- a programming pattern

- a tool in your toolbox

- an easily added language

feature

- complimentary to OOP

Modularity

Aspects can…• Exist in isolated and encapsulated constructs

• Be attached to almost any standard code construct

• Be transported between projects

Cross-Cutting Concerns

…aspects of a program that affect other concerns. These concerns often cannot be cleanly decomposed from the rest of the system in both the design and

implementation, and can result in either scattering (code duplication), tangling (significant dependencies between systems),

or both.

http://en.wikipedia.org/wiki/Cross-cutting_concern

…more simply

Any code functionality that repeats itself in a regular pattern across many unrelated tiers/layers within a

codebase.

Cross-Cutting Concerns

Services

Domain Model

Repositories

Model

ViewModel

View

Logg

ing Se

curi

ty

Au

dit

ing

Val

idat

ion

Undo/Redo

Thread Management

INotifyPropertyChanged

Unit of Work Management

Circuit Breaker

AOP Styles

1. InterceptionInterjecting the execution of cross-cutting concerns at run time.Non-invasive approach to modifying the code execution path.

2. IL WeavingInterjecting the planned execution of cross-cutting concerns during the compilation process.Invasive approach to modifying the code execution path.

Interception

• Existing in IoC container implementations

• Follows a very strict decorator pattern

• Likely implemented using weak typing

• Run-time operation

• Can be applied en-masse

• Easy to implement if you’re already using IoC

Decorator Pattern

//new functionality before

//the code that already exists

//new functionality after

//exception handling

Weak Typing

En-masse Attachment

IL Weaving

• Post compilation process

• Alters the assembly/executable at an IL level

• Scary to people

• Can be attached to almost every code construct

• Very rare that it required changes to existing code

• Run and compile time operations

• Can be much easier to add to legacy code

The Process

Write CodeCompile

Application exe/dll

AOP post

compiler/weaver

Throw

compile-time

error

exe/dllDeploy

Constructs

• Methods (private/public/internal/etc)

• Properties

• Field level variables

• Events

• Event registration/de-registration

• Code generation

Run Time vs Compile Time

Run Time

• All aspect code executes in application context

Compile Time

• Code can be designated to run during post-compilation

• Compilation can fail for correct syntaxes but incorrect business logic

Tooling

• Interception• Castle Winsdor

• StructureMap

• Ninject (with Ninject.Extensions)

• AutoFac (with Autofac.Extras)

• IL Weaving• PostSharp (current)

• LinFu

The Traditional Arguments Against AOP

• It is “magic”

• We won’t be able to debug properly

• How do we know where aspects will be executed?

• It’s another thing we have to learn

• Changing the aspect will break the entire application

“Its magic!” & “We can’t debug it!”

• Both interception and IL weaving provide full line-by-line debugging

• Attachment of aspects is explicit as you want it to be

“But which aspect(s) will run?”

• This is a tooling/discoverability problem, not a problem with AOP itself

• PostSharp has very good VS integration showing what aspects are attached to a piece of code

• Interceptor discoverability is a bigger problem

“Its another thing we have to learn”

Is it worse if we train them and they leave or if we don’t train

them and they stay?

“Changing the aspect can break the entire application!”

• You can probably do that in an number of different ways with any codebase that doesn’t use AOP

• This is a problem caused by tight coupling and poor separation of concerns

Your Project is Already Established…

You use IoC

• For logging, caching and possibly transaction management use interception

• If the cross cutting concerns are more complex (Undo/Redo, thread management) use IL Weaving

You don’t use IoC

• The only real option is to use IL Weaving

You’re Starting a New Project…

You have AOP buy-in from senior developers

• Start with IL Weaving, but start with simple, pre-built aspects

• Write custom aspects as experience and need grows

AOP is still unknown to the senior people on the team

• Start with IL Weaving, but start in a very isolated manner

• Use this as a proof of concept until you have senior level buy-in

gracias

Donald Belcham@dbelcham

donald.belcham@igloocoder.com