Implementing a basic IoC container using C#

Implementing a basic IoC container using C#, step by step.

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    Improving LINQ code reusability: Select method

    Select method is used to project each element of a sequence into a new form, i.e. it can be used to map a collection of one type to a collection of another type. In this article I’ll show you a simple approach that will allow you to reuse the code used in the Select method.

    Table of contents

    The Problem

    Consider the following model:

    Let’s suppose that you have a services layer, so you don’t want to expose your domain objects directly to the client applications. Instead you create a set of data contracts (or DTOs, if you prefer):

    At some stage you’ll have to convert those Domain objects to data contracts. This is a common way of doing it:

    var details = repository.All<Album>().Select(album => new AlbumDetail {
        AlbumId = album.AlbumId,
        Price = album.Price,
        Title = album.Title,
    
        ArtistId = album.ArtistId,
        GenreId = album.GenreId,
        ArtistName = (album.Artist == null) ? null : album.Artist.Name,
        GenreName = (album.Genre == null) ? null : album.Genre.Name
    });
    

    There is a problem with this approach – if you need to query the same collection but using different criteria you have to duplicate the code inside the Select method.

    Solution 1 – Creating a method for the mapping

    In order to reuse the code, we can create a method that converts Album objects (Domain) to data contract objects:

    private static AlbumSummary CreateAlbumSummary(Album album)
    {
        return new AlbumSummary {
            AlbumId = album.AlbumId,
            Title = album.Title,
    
            ArtistName = (album.Artist == null) ? null : album.Artist.Name
        };
    }
    
    private static AlbumDetail CreateAlbumDetail(Album album)
    {
        return new AlbumDetail {
            AlbumId = album.AlbumId,
            Price = album.Price,
            Title = album.Title,
    
            ArtistId = album.ArtistId,
            GenreId = album.GenreId,
            ArtistName = (album.Artist == null) ? null : album.Artist.Name,
            GenreName = (album.Genre == null) ? null : album.Genre.Name
        };
    }
    

    Using the code:

    var albums = Albums.Select(CreateAlbumDetail);
    var albumsByGenre = Albums.Where(x => x.GenreId == genreId).Select(CreateAlbumDetail);
    
    // alternative way
    var albums2 = Albums.Select(x => CreateAlbumDetail(x));
    var albumsByGenre2 = Albums.Where(x => x.GenreId == genreId).Select(x => CreateAlbumDetail(x));
    

    Solution 2 – Creating a generic ObjectMapper object

    The previous solution solves the code reusability problem, but there’s still a tight coupling between components. Abstractions should be used to implement loose coupling between components – in this case, to abstract the mapping code.

    Step 1: define a contract (interface) with a method that converts one object of type TSource to an object of type TDestination:

    public interface IObjectMapper
    {
        TDestination Map<TSource, TDestination>(TSource source);
    }
    

    Step 2: create a class that implements IObjectMapper (click to expand):

    public class ObjectMapper : IObjectMapper
    {
        private Dictionary<Type, Func<object, object>> Mappers = new Dictionary<Type, Func<object, object>>
        {
            { typeof(Tuple<Album, AlbumDetail>), CreateAlbumDetail },
            { typeof(Tuple<Album, AlbumSummary>), CreateAlbumSummary }
    
            // more mappings here
            // ....
        };
    
    
        public TDestination Map<TSource, TDestination>(TSource source)
        {
            if(source == null)
                return default(TDestination);
    
            Func<object, object> mapper = null;
            Type key = typeof(Tuple<TSource, TDestination>);
    
            if(Mappers.TryGetValue(key, out mapper))
            {
                var newObject = mapper(source);
                return (TDestination) newObject;
            }
    
            string errorMessage = string.Format("Invalid mapping (Source: {0}, Destination: {1})";,
                                                typeof(TSource).FullName, 
                                                typeof(TDestination).FullName);
            
            throw new InvalidOperationException(errorMessage);
        }
    
    
        private static object CreateAlbumDetail(object source)
        {
            var album = source as Album;
    
            return new AlbumDetail {
                AlbumId = album.AlbumId,
                Price = album.Price,
                Title = album.Title,
    
                ArtistId = album.ArtistId,
                GenreId = album.GenreId,
                ArtistName = (album.Artist == null) ? null : album.Artist.Name,
                GenreName = (album.Genre == null) ? null : album.Genre.Name
            };
        }
    
        private static object CreateAlbumSummary(object source)
        {
            var album = source as Album;
    
            return new AlbumSummary {
                AlbumId = album.AlbumId,
                Title = album.Title,
                
                ArtistName = (album.Artist == null) ? null : album.Artist.Name
            };
        }
    }
    

    Example 1: Using LINQ

    Using the mapper in a LINQ expression – convert an Album collection to an AlbumSummary collection:

    IObjectMapper mapper = new ObjectMapper();
    
    IEnumerable<AlbumSummary> summaries = repository.All<Album>()
                                            .Select(mapper.Map<Album, AlbumSummary>);
    

    Example 1: Mapping a single object

    Using the mapper for a single object:

    var album = new Album {
        AlbumId = 1,
        Price = 10.0m,
        Title = "The Dreamer",
        Artist = new Artist { ArtistId = 1, Name = "José James" },
        Genre = new Genre { GenreId = 1, Name = "Jazz" }
    };
    
    IObjectMapper mapper = new ObjectMapper();
    
    AlbumDetail albumDetail = mapper.Map<Album, AlbumDetail>(album);
    

    Unit Testing

    Some NUnit tests:

    [Test]
    public void Given_a_non_existing_mapping_when_mapping_object_then_should_throw_InvalidOperationException()
    {
        // arrange
        IObjectMapper mapper = new ObjectMapper();
        var albumDetail = new AlbumDetail();
    
        // act/assert
        Assert.Throws<InvalidOperationException>(() => 
            // non-existing mapping
            mapper.Map<AlbumDetail, AlbumSummary>(albumDetail)
        );
    }
    
    [Test]
    public void Given_an_album_when_mapping_to_album_summary_should_equals_expected_album_summary()
    {
        // arrange
        IObjectMapper mapper = new ObjectMapper();
        
        var album = new Album {
            AlbumId = 4,
            Price = 10.0m,
            Title = "Heritage",
            Artist = new Artist { ArtistId = 4, Name = "Opeth" },
            Genre = new Genre { GenreId = 4, Name = "Metal" }
        };
    
        var expectedAlbumSummary = new AlbumSummary {
            AlbumId = 4,
            ArtistName = "Opeth",
            Title = "Heritage"
        };
        
        // act
        AlbumSummary albumSummary = mapper.Map<Album, AlbumSummary>(album);
        
        // assert
        Assert.AreEqual(albumSummary, expectedAlbumSummary);
    }
    

    Final thoughts

    In this article you learned how to reuse the code used in the Select method, and how you can use that code to map single objects. But writing mapping code is tedious and time consuming. There are mapping tools out there that can make your life easier – AutoMapper is one of them. I’ve used it in the past and I definitely recommend it. So, why use Automapper? Quoting their website:

    “What makes AutoMapper interesting is that it provides some interesting conventions to take the dirty work out of figuring out how to map type A to type B. As long as type B follows AutoMapper’s established convention, almost zero configuration is needed to map two types”

    “Mapping code is boring. Testing mapping code is even more boring. AutoMapper provides simple configuration of types, as well as simple testing of mappings”

    References

    Downloads

    Download the demo project (VS2010): LINQ-Select.zip

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    ASP.NET Web Services Dependency Injection using Unity

    Recently, I had to setup Unity IoC container in an existing ASP.NET 3.5 Web Forms application. This application had not only web pages (.aspx files), but also some Web Services (.asmx files). After some research, I found out David Hayden’s screencast: Unity IoC and ASP.NET screencast – Dependency Injection into Web Pages.

    You can use a similar technique for your Web Services – that’s what I’ll show you in this article.

    Table of contents

    Example – Adding logging to your application

    You have the following interface and its implementation:

    public interface ILogger
    {
        void Write(string message);
    }
    
    public class DebugLogger : ILogger
    {
        public void Write(string message)
        {
            Debug.WriteLine(message);
        }
    }
    

     

    Step 1: Setting up the container in Global.asax

    The first step is to setup Unity Container in Global.asax file. This is a good place to do it because it can be accessed either by web pages or by web services.
    The CreateContainer() method is the place where the dependencies are specified.

    public class Global : HttpApplication, IContainerAccessor
    {
        private static IUnityContainer _container;
    
        public static IUnityContainer Container
        {
            get
            {
                return _container;
            }
            private set
            {
                _container = value;
            }
        }
    
        IUnityContainer IContainerAccessor.Container
        {
            get
            {
                return Container;
            }
        }
    
        protected void Application_Start(object sender, EventArgs e)
        {
            CreateContainer();
        }
    
        protected virtual void CreateContainer()
        {
            IUnityContainer container = new UnityContainer();
            container.RegisterType<ILogger, DebugLogger>();
            
            Container = container;
        }
    }
    

     

    Step 2: Creating a base class for the services

    Create a generic BaseService that all your services will inherit from. The dependencies will be injected when you create an instance of the service (default constructor).

    public abstract class BaseService<T> : System.Web.Services.WebService where T : class
    {
        public BaseService()
        {
            InjectDependencies();
        }
    
        protected virtual void InjectDependencies()
        {
            HttpContext context = HttpContext.Current;
    
            if (context == null)
                return;
    
            IContainerAccessor accessor = context.ApplicationInstance as IContainerAccessor;
    
            if (accessor == null)
                return;
    
            IUnityContainer container = accessor.Container;
    
            if (container == null)
                throw new InvalidOperationException("Container on Global Application Class is Null. Cannot perform BuildUp.");
    
            container.BuildUp(this as T);
        }
    }
    

     

    Step 3: Setting up the services

    Now all you need to do is to inherit from the BaseService and invoke its base constructor.
    Don’t forget to add the [Dependency] attribute to your dependency, and it has to be public.

    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    public class DummyService : BaseService<DummyService>
    {
        [Dependency]
        public ILogger Logger
        {
            get;
            set;
        }
    
        public DummyService() : base()
        {
        }
    
        [WebMethod]
        public string HelloWorld(string name)
        {
            string message = string.Format("Hello World, {0}!", name);
    
            this.Logger.Write(message);
    
            return message;
        }
    }
    

    That’s it! Now you just need to compile and run the application and see it in action :)

    Feel free to download the demo application

    References

    Downloads

    Download the demo project (VS2010): UnityAsmxWebServices.zip