.NET - Pure DI examples

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);
    }
}

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

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

public class CompositionRoot : IDisposable, IControllerFactory
{
    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 IController CreateController(RequestContext requestContext, string controllerName)
    {
        if (controllerName.ToLower() == "home")
        {
            return new HomeController(_singleton, new TransientDependency());
        }

        var argumentException = new ArgumentException(@"Unexpected controller! " + controllerName);
        Log.Error("don't know how to instantiate MVC controller: {controllerType}. redirecting to help", controllerName);
        throw argumentException; // Alternatively would return some default Page Not Found placeholder controller;
    }

    public SessionStateBehavior GetControllerSessionBehavior(RequestContext requestContext, string controllerName)
    {
        return SessionStateBehavior.Default; 
    }

    public void ReleaseController(IController controller)
    {
        // anything to clean up?
    }
}

public static class DependencyInjection
{
    public static void WireUp()
    {
        var compositionRoot = new CompositionRoot();
        System.Web.Mvc.ControllerBuilder.Current.SetControllerFactory(compositionRoot);
    }
}

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    // Add framework services.
    services.AddMvc();
    
    var controllerActivator = new CompositionRoot();
    services.AddSingleton<IControllerActivator>(controllerActivator);
}

public class CompositionRoot : IControllerActivator, IDisposable
    {
        // Singletons
        private readonly ISingleton _singleton;

        public CompositionRoot()
        {
            // Create singletons
            _singleton = new Singleton();
        }

        public object Create(ControllerContext c) => this.Create(c.ActionDescriptor.ControllerTypeInfo.AsType());
        public void Release(ControllerContext c, object controller) => (controller as IDisposable)?.Dispose();

        public Controller Create(Type type)
        {
            // scoped 
            var scoped = new Scoped();
            // transient get new()-ed up below
            if (type == typeof(HomeController))
                return new HomeController(_singleton, scoped, new Transient());
            
            throw new InvalidOperationException($"Unknown controller {type}.");
        }

        public void Dispose()
        {
            // dispose stuff
        }
    }