Niraj Bhatt – Architect's Blog

Ruminations on .NET, Architecture & Design

Dependency Inversion, Dependency Injection, DI Containers and Service Locator

Here is a post I have been planning to do for years :) . Yes, I am talking about Dependency Injection, which by this time, has most likely made its way into your code base. I got to recollect few thoughts around it in a recent discussion and hence writing them down.

Dependencies are the common norm of object oriented programming, helping us to adhere to software principles like Single Responsibility, Encapsulation, etc. Instead of establishing dependencies using direct references between classes or components, they are better managed through an abstraction. Most of the GOF patterns are based around this principle which is commonly referred as ‘Dependency Inversion’. Using Dependency inversion principle we can create flexible Object Oriented designs, making our code base reusable and maintainable.

To further enhance value proposition of the dependency inversion, you can pass the dependencies via a constructor or a property from the root of your application, instead of instantiating them within your class. This would allow you to mock / stub your dependency and make your code easily testable (read unit testing).

E.g.

IA ob = new A(); //Direct instantiation
public D (IA ob) { … } //constructor injection
public IA ob { get; set } //property injection – create object and set the property

Entire software industry sees value in above approach and most people refer to this entire structure (pattern) as Dependency Injection. But beyond this things get little murkier.

Confusion starts for programmers who see above as a standard way of programming. To them Dependency Injection (DI) is the automated way of injecting dependencies using a DI container (DI container at times is also referred as IoC (Inversion of Control) container. As Martin Fowler points out in his classic article, IoC is a generic term used in quite a few cases – e.g. the callback approach of Win32 programming model. In order to avoid confusion and keep this approach discernible, the term DI was coined. In this case, IoC refers to inversion of dependencies creation, which are created outside of the dependent class and then injected.) If you are surprised by this statement, let me tell you 90% of people who have walked to me to ask if I was using DI, wanted to know the about the DI container and my experience with it.

So what the heck is this ‘DI container’? A DI container is a framework which can identify constructor arguments or properties on the objects being created and automatically inject them as part of object creation. Coming from the .NET world, the containers I use include StructureMap, Autofac and Unity. DI containers can be wired up using few lines of code at the starting of your program or you can even specify configuration in a XML file. Beyond that, containers are transparent to the rest of your code base. Most containers also provide AOP (Aspect Oriented Programming) functionality and its variants. This allows you to bundle cross cutting concerns like database transactions, logging, caching, etc. as aspects and avoid boilerplate code throughout the system (I have written a CodeProject article on those lines). Before you feel I am over simplifying things, let me state if you haven’t worked with DI containers in past you are likely to be faced with a learning curve. As is the case with most of the other frameworks, a pilot is strongly recommended. As a side note, the preferred injection rule is – unless your constructor requires too many parameters (ensure you haven’t violated SRP), you should resort to constructor injection and avoid property injection (see fowler’s article for a detailed comparison between the two injection types).

Finally, let’s talk about Service Locator an alternative to DI. A Service Locator holds all the services (dependencies) required by your system (in code or using a configuration file) and returns a specific service instance on request. Service Locator can come in handy for scenarios where DI container is not compatible with a given framework (e.g. WCF, ASP.NET Web APIs) or you want more control over the object creation (e.g. create the object late in the cycle). While you can mock service locator, mocking it would be little cumbersome when compared to DI. Service Locator is generally seen as an anti-pattern in DI world. Interestingly, most DI containers offer APIs which can allow us to use them as Service Locator (lookup for Resolve / GetInstance methods on the container).

Below sample shows you StructureMap – a DI container, in action (use NuGet to add StructureMap dependency).

class Program
{
static void Main(string[] args)
{
var container = new Container(registry =>
{
registry.Scan(x =>
{
x.TheCallingAssembly();
x.WithDefaultConventions();
});
});
var ob = container.GetInstance<DependentClass>();
ob.CallDummy();
}
}

public interface IDependency
{
void Dummy();
}

public class Dependency : IDependency
{
public void Dummy()
{
Console.WriteLine("Hi There!");
}
}

public class DependentClass
{
private readonly IDependency _dep;
public DependentClass(IDependency ob)
{
_dep = ob;
}
public void CallDummy()
{
_dep.Dummy();
}
}

I will try to post some subtle issues around DI containers in future. I hope above helps anyone looking for a quick start on DI and associated terms :) .

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2 responses to “Dependency Inversion, Dependency Injection, DI Containers and Service Locator

  1. Jay December 1, 2012 at 7:11 pm

    Am really impressed with the article… very well narrated and makes a quick , simple and easy to understand…. Conveying/documenting our understanding clearly in an article is a skill… I think this article justifies. Am looking forward for more articles on one such topic.

  2. Pingback: Blog Posts of the Week (25th Nov'12 - 01st Dec'12) - The South Asia MVP Blog - Site Home - TechNet Blogs

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