Common Service Locator: Separate hard reference of your IoC/DI container from your solution

Service Locator

Dependency Injection is an way to separate dependency. Someone can use Constructor Injection, Setter Injection etc for injecting dependency. Service Locator solves the same problem as Dependency Injector but it uses different approach. Service Locator holds reference of all types/objects an application needs. So whenever you need to use an instance of a type/class, ask service locator to provide the instance. Simply, the Service Locator uses a container to hold all references to class/type to instance/object mapping. You can follow the links for details:

• Dependency Injection
• Service Locator

Why Common Service Locator?

Nowadays as a .NET developer, we have a set of DI/IoC containers to choose from. We have option for Spring.NET, Unity Application Block, Castle Windsor etc. Whenever you use any of them in your project, the project is tightly coupled with that container. So if you start your project with Spring.Net Container then your project will be tightly coupled with the Spring.Net container. But later if your decide to use Unity Application Block then can you imagine how much hassles are waiting for you? You need to remove all reference of Spring.Net container from your code. Another example might be, you are developing a library that will be used by other users. Now if you use a IoC Container (say Spring.Net) then you are forcing users to use Spring.Net whenever they want to use the library. So if it would be possible to loose the coupling between your code and the IoC container then you can easily remove dependencies on IoC container.The Common Service Locator project at codeplex is aimed to attain the goal.

Use Common Service Locator

1. Get an understanding of IServiceLocator interface

The Start with an Interface IServiceLocator which has few methods which is generic to any IoC container.The IServiceLocator inherits from IServiceProvider.

public interface IServiceProvider

{

    object GetService(Type serviceType);

}

public interface IServiceLocator : IServiceProvider

{

    object GetInstance(Type serviceType);

    object GetInstance(Type serviceType, string key);

    IEnumerable<object> GetAllInstances(Type serviceType);

    TService GetInstance<TService>();

    TService GetInstance<TService>(string key);

    IEnumerable<TService> GetAllInstances<TService>();

}

The Common service Locator will implement the interface IServiceLocator. The Common Service Locator framework has provided an abstract implementation of IServiceLocator called ServiceLocatorImplBase. The ServiceLocatorImplBase has two abstract methods which actually depends on IoC Container.

• DoGetAllInstances: get all instance of a particular type from Container.
• DoGetInstance: Get a particular instance from Container

For getting instance from container, ServiceLocatorImplBase uses above two methods. And those two methods are implemented by derived class. And Container is actually needed by those two methods. So Container is specific to individual implementation.

2. Develop your custom Service Locator

I have developed two service Locators for Unity and Spring.Net. UnityServiceLocator can use Unity container whereas Spring.Net can uses Object Factory. The following class diagram shows two implementations that I have developed in the project attached with this post.

Figure: Two sample implementations of Common Service Locator

3. Develop BootStrapper

So I have developed two service locators till now. Now I need to develop two bootstrappers for Spring.Net and Unity as type registration are different for two.

public abstract class CommonBootStrapper

{

    public static IServiceLocator Locator;

    protected CommonBootStrapper()

    {

        Locator = CreateServiceLocator();            

    }

    protected abstract IServiceLocator CreateServiceLocator();

}

The common bootstrapper has a instance of IServiceLocator which means it can hold reference to any class’s instance inheriting from ServiceLocatorImplBase (as ServiceLocatorImplBase implements IServiceLocator). So we can put the UnityServiceLocator or SpringServiceLocator in the CommonBootStrapper’s Locator variable. Now we need to two different bootstrappers for Unity and Spring.Net. The Unity implementation is shown below.

public class UnityBootStrapper : CommonBootStrapper

{

    protected override IServiceLocator CreateServiceLocator()

    {

        UnityContainer container = new UnityContainer();

        RegisterTypes(container);

        return new UnityServiceLocator(container);

    }

    private static void RegisterTypes(IUnityContainer container)

    {

        container.RegisterType<ICustomer, GoldenCustomer>(typeof(ICustomer).FullName);

    }

}

As shown in code above, The UnityBootStrapper needs to override the CreateServiceLocator and in this method I have created an UnityContainer and packed it within UnityServiceLocator. For Spring.Net you can create an SpringServiceLocator with ObjectFactory packed in it.

4. Develop a BootStrapper Manager to provide a single point of access to Service Locator

Finally you need to develop a bootstrapper manager to hide the concrete implementation of CommonBootStrapper. The following code shows the BootStrapperManager

public class BootStrapperManager

{

    private static CommonBootStrapper _bootStrapper;

    public static void Initialize(CommonBootStrapper bootStrapper)

    {

        _bootStrapper = bootStrapper;

    }

    public static CommonBootStrapper BootStrapper

    {

        get { return _bootStrapper; }

    }

}

5. Access the IServiceLocator through BootStrapperManager

Now you need to register your BootStrapper to BootStrapperManager at some entry point to your application (like Main method). I have register as shown below:

BootStrapperManager.Initialize(new UnityBootStrapper());

Now from the application I only use CommonBootStrapper.Locator to get the IServiceLocator instance. So to get any instance from container I can write code like below:

ICustomer customer = CommonBootStrapper.Locator.GetInstance<ICustomer>(typeof(ICustomer).FullName);

customer.SendMessage("This is me.");

Conclusion

I have just gone through some basic ideas on how you can use Common Service Locator to decouple dependency to IoC Container from your production code. You can get more details on this visiting CodePlex link. For making my implementation I have included the Source Code from Common Service Locator in my Visual Studio Solution.
codeproject