Chapter 44: Dependency Injection

Section 44.1: Dependency Injection C# and ASP.NET with Unity

First why we should use depedency injection in our code ? We want to decouple other components from other classes in our program. For example we have class AnimalController which have code like this :

public class AnimalController()

{

private SantaAndHisReindeer _SantaAndHisReindeer = new SantaAndHisReindeer();

public AnimalController(){ Console.WriteLine("");

}

}

We look at this code and we think everything is ok but now our AnimalController is reliant on object _SantaAndHisReindeer. Automatically my Controller is bad to testing and reusability of my code will be very hard.

Very good explanation why we should use Depedency Injection and interfaces here.

If we want Unity to handle DI, the road to achieve this is very simple :) With NuGet( package manager) we can easily import unity to our code.

in Visual Studio Tools -> NuGet Package Manager -> Manage Packages for Solution -> in search input write unity -> choose our project-> click install

Now two files with nice comments will be created.

in App-Data folder UnityConfig.cs and UnityMvcActivator.cs

UnityConfig - in RegisterTypes method, we can see type that will be injection in our constructors.

namespace Vegan.WebUi.App_Start

{

public class UnityConfig

{

#region Unity Container

private static Lazy<IUnityContainer> container = new Lazy<IUnityContainer>(() =>

{

var container = new UnityContainer(); RegisterTypes(container);

return container;

});

///<summary>

///Gets the configured Unity container.

///</summary>

public static IUnityContainer GetConfiguredContainer()

{

return container.Value;

}

#endregion

///<summary>Registers the type mappings with the Unity container.</summary>

///<param name="container">The unity container to configure.</param>

///<remarks>There is no need to register concrete types such as controllers or API controllers (unless you want to

///change the defaults), as Unity allows resolving a concrete type even if it was not previously registered.</remarks>

public static void RegisterTypes(IUnityContainer container)

{

//NOTE: To load from web.config uncomment the line below. Make sure to add a Microsoft.Practices.Unity.Configuration to the using statements.

//container.LoadConfiguration();

//TODO: Register your types here

//container.RegisterType<IProductRepository, ProductRepository>();

container.RegisterType<ISanta, SantaAndHisReindeer>();

}

}

}

UnityMvcActivator - > also with nice comments which say that this class integrates Unity with ASP.NET MVC

using System.Linq; using System.Web.Mvc;

using Microsoft.Practices.Unity.Mvc;

[assembly: WebActivatorEx.PreApplicationStartMethod(typeof(Vegan.WebUi.App_Start.UnityWebActivator), "Start")] [assembly: WebActivatorEx.ApplicationShutdownMethod(typeof(Vegan.WebUi.App_Start.UnityWebActivator), "Shutdown")]

namespace Vegan.WebUi.App_Start

{

/// <summary>Provides the bootstrapping for integrating Unity with ASP.NET MVC.</summary> public static class UnityWebActivator

{

/// <summary>Integrates Unity when the application starts.</summary> public static void Start()

{

var container = UnityConfig.GetConfiguredContainer();

FilterProviders.Providers.Remove(FilterProviders.Providers.OfType<FilterAttributeFilterProvider>().

First());

FilterProviders.Providers.Add(new UnityFilterAttributeFilterProvider(container));

DependencyResolver.SetResolver(new UnityDependencyResolver(container));

// TODO: Uncomment if you want to use PerRequestLifetimeManager

//

Microsoft.Web.Infrastructure.DynamicModuleHelper.DynamicModuleUtility.RegisterModule(typeof(UnityPerR

equestHttpModule));

}

/// <summary>Disposes the Unity container when the application is shut down.</summary> public static void Shutdown()

{

var container = UnityConfig.GetConfiguredContainer(); container.Dispose();

}

}

}

Now we can decouple our Controller from class SantAndHisReindeer :)

public class AnimalController()

{

private readonly SantaAndHisReindeer _SantaAndHisReindeer;

public AnimalController(SantaAndHisReindeer SantaAndHisReindeer){

_SantAndHisReindeer = SantaAndHisReindeer;

}

}

There is one final thing we must do before running our application.

In Global.asax.cs we must add new line: UnityWebActivator.Start() which will start, configure Unity and register our types.

using System;

using System.Collections.Generic; using System.Linq;

using System.Web; using System.Web.Mvc;

using System.Web.Optimization; using System.Web.Routing; using Vegan.WebUi.App_Start;

namespace Vegan.WebUi

{

public class MvcApplication : System.Web.HttpApplication

{

protected void Application_Start()

{

AreaRegistration.RegisterAllAreas(); FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters); RouteConfig.RegisterRoutes(RouteTable.Routes); BundleConfig.RegisterBundles(BundleTable.Bundles); UnityWebActivator.Start();

}

}

}

Section 44.2: Dependency injection using MEF

public interface ILogger

{

void Log(string message);

}

[Export(typeof(ILogger))]

[ExportMetadata("Name", "Console")] public class ConsoleLogger:ILogger

{

public void Log(string message)

{

Console.WriteLine(message);

}

}

[Export(typeof(ILogger))] [ExportMetadata("Name", "File")] public class FileLogger:ILogger

{

public void Log(string message)

{

//Write the message to file

}

}

public class User

{

private readonly ILogger logger; public User(ILogger logger)

{

this.logger = logger;

}

public void LogUser(string message)

{

logger.Log(message) ;

}

}

public interface ILoggerMetaData

{

string Name { get; }

}

internal class Program

{

private CompositionContainer _container;

[ImportMany]

private IEnumerable<Lazy<ILogger, ILoggerMetaData>> _loggers;

private static void Main()

{

ComposeLoggers();

Lazy<ILogger, ILoggerMetaData> loggerNameAndLoggerMapping = _ loggers.First((n) => ((n.Metadata.Name.ToUpper() =="Console"));

ILogger logger= loggerNameAndLoggerMapping.Value var user = new User(logger);

user.LogUser("user name");

}

private void ComposeLoggers()

{

//An aggregate catalog that combines multiple catalogs var catalog = new AggregateCatalog();

string loggersDllDirectory =Path.Combine(Utilities.GetApplicationDirectory(), "Loggers"); if (!Directory.Exists(loggersDllDirectory ))

{

Directory.CreateDirectory(loggersDllDirectory );

}

//Adds all the parts found in the same assembly as the PluginManager class catalog.Catalogs.Add(new AssemblyCatalog(typeof(Program).Assembly)); catalog.Catalogs.Add(new DirectoryCatalog(loggersDllDirectory ));

//Create the CompositionContainer with the parts in the catalog

_container = new CompositionContainer(catalog);

//Fill the imports of this object try

{

this._container.ComposeParts(this);

}

catch (CompositionException compositionException)

{

throw new CompositionException(compositionException.Message);

}

}

}