dependency-injection .NET - Pure DI examples Web Api

Example

public interface ISingleton : IDisposable { }
public class TransientDependency { }

public class Singleton : ISingleton
{
    public void Dispose() { } 
}

public class CompositionRoot : IDisposable, IHttpControllerActivator
{
    private readonly ISingleton _singleton;

    // pass in any true singletons i.e. cross application instance singletons
    public CompositionRoot()
    {
        // intitialise any application instance singletons
        _singleton = new Singleton();
    }

    public void Dispose()
    {
        _singleton.Dispose();
    }

    public IHttpController Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)
    {
        // Per-Request-scoped services are declared and initialized here
        if (controllerType == typeof(SomeApiController))
        {
            // Transient services are created and directly injected here
            return new SomeApiController(_singleton, new TransientDependency());
        }

        var argumentException = new ArgumentException(@"Unexpected controller type! " + controllerType.Name,
            nameof(controllerType));
        Log.Error(argumentException, "don't know how to instantiate API controller: {controllerType}", controllerType.Name);
        throw argumentException;
    }
}

public static class DependencyInjection
{
    public static void WireUp()
    {
        var compositionRoot = new CompositionRoot();
        System.Web.Http.GlobalConfiguration.Configuration.Services.Replace(typeof (IHttpControllerActivator), compositionRoot);
    }
}