The Zen of Inversion of Control

The Zen of Inversion of Control Christopher Fairbairn ARANZ Medical Ltd christopher@christec.co.nz www.christec.co.nz/blog/

A definition? “Inversion of Control, or IoC, is an abstract principle describing an aspect of some software architecture designs in which the flow of control of a system is inverted in comparison to the traditional architecture of software libraries.” www.christec.co.nz/blog/

Complicating a simple idea www.christec.co.nz/blog/

An improved definition? “Inversion of Control is a style of software construction where reusable, generic code controls the execution of problem-specific code with the strong connotation that the reusable code and the problem- specific code are developed independently” www.christec.co.nz/blog/

An improved definition? “High-level modules should not depend on low-level modules. Both should depend on abstractions.” Robert C. Martin . www.christec.co.nz/blog/

A worked example http://www.code-magazine.com/Article.aspx?quickid=0903101 Jon Stonecash CODE magazine March/April issue www.christec.co.nz/blog/

Code Demo Developing throw away code “It’s ok, this will never go into production use…”. www.christec.co.nz/blog/

Code Demo Making it more flexible “It’s only one additional parameter. No harm done…”. www.christec.co.nz/blog/

A scalability problem? •Can we only process files older than 5 days? •Can we delete files rather than listing them? •I want PDF files as well as XLS files. “I can’t keep doing this for every change in requirements…”. www.christec.co.nz/blog/

The Core Logic Enumerate over each file Compare to filter condition Process files that match www.christec.co.nz/blog/

The Project Specific Logic What is the What to do “filter” with files condition? that match? www.christec.co.nz/blog/

Delegates “A delegate is a type defining a method signature, so that delegate instances can hold and invoke a method or list of methods that match its signature.” C# Language Pocket Reference www.christec.co.nz/blog/

Delegates An original C# 1.0 feature Define delegate int CalculationHandler(int a, int b); Construct CalculationHandler ch = new CalculationHandler(AddNumbers); Invoke int result = ch(1, 2); www.christec.co.nz/blog/

Delegates An original C# 1.0 feature Define delegate int CalculationHandler(int a, int b); Construct CalculationHandler ch = AddNumbers; Invoke int result = ch(1, 2); www.christec.co.nz/blog/

Delegates C# 2.0 introduced Anonymous Delegates Define delegate int CalculationHandler(int a, int b); Construct CalculationHandler ch = delegate(int a, int b){ return a + b; }; Invoke int result = ch(1, 2); www.christec.co.nz/blog/

Code Demo Inverting Control “My secret plan to allow my workmates to do the hard work”  www.christec.co.nz/blog/

Delegates C# 3.0 introduced Lambda Expressions Define delegate int CalculationHandler(int a, int b); Construct CalculationHandler ch = (a, b) => a + b; Invoke int result = ch(1, 2); www.christec.co.nz/blog/

.NET 3.5 BCL The .NET 3.5 BCL introduces some common delegate types namespace System { delegate void Action<T>(T obj); delegate TResult Func<T, TResult>(T arg); } www.christec.co.nz/blog/

Code Demo Reducing the amount of code “The less I need to type the happier I’ll be…”. www.christec.co.nz/blog/

What have we achieved? •Separation of Concerns •Inversion of Control “But I’ve also made it harder for my clients to use…”. www.christec.co.nz/blog/

IoC Containers “Dependency Injection refers to the process of automatically supplying an external dependency to a software component. It is a specific form of inversion of control where the concept being inverted is the process of obtaining the needed dependency.” www.christec.co.nz/blog/

IoC Containers •Hashtable of interface vs implementing type •In its simplest form basically Dictionary<Type, Object> •Lifecycle management etc www.christec.co.nz/blog/

Using Ninject •Adding objects to the container •XML, Fluent, Database, Code… • Bind<IFileFilter>().To<SomeClass>(); “IoC containers perform two basic functions: putting dependencies in and getting fully constructed objects out…”. www.christec.co.nz/blog/

Using Ninject •Instantiating objects via the Container •Constructor or Property Injection •IFileFilter filter = kernel.Get<IFileFilter>(); “IoC containers perform two basic functions: putting dependencies in and getting fully constructed objects out…”. www.christec.co.nz/blog/

Code Demo Dependency Injection with Ninject “Hiding the complexity with another layer of indirection…”. www.christec.co.nz/blog/

IoC Advantages •Ability to quickly reconfigure / repurpose •Reduces hard dependencies between modules •Centralises configuration www.christec.co.nz/blog/

A bridge too far? •As with any technique this can be taken to ill-logical extremes and beyond •There are trade offs and gains with •Flexability •Maintainability •Understandability www.christec.co.nz/blog/

Find out more Inversion of Control Containers and the Dependency Injection Pattern - Martin Fowler http://martinfowler.com/articles/injection.html Loosen Up: Tame Your Dependencies for More Flexible Apps – James Kovacs http://msdn.microsoft.com/en-us/magazine/cc337885.aspx www.christec.co.nz/blog/

Find out more www.christec.co.nz/blog/

Contact Information Christopher Fairbairn ARANZ Medical Ltd Questions? Email: christopher@christec.co.nz Website: www.christec.co.nz/blog/ www.christec.co.nz/blog/

The Zen of Inversion of Control

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