Question

How do you debug dependency injection (using Unity DI) when the dependancy does not instantiate?

eg Given a service class with dependencies:

public class FooService : IFooService
{
    [Dependency]
    public BarService BarService { get; set; }
    [Dependency]
    public AnotherService AnotherService { get; set; }

    // other code fails because BarService and AnotherService are null
}

And in Global.asax.cs

private void ConfigureIoC()
{
    container
        .ConfigureAutoRegistration()
        .LoadAssembliesFrom(assemblyPaths)
        .ExcludeSystemAssemblies()
        .Include(If.Any, Then.Register()) 
        .ApplyAutoRegistration();

    var serviceLocator = new UnityServiceLocator(container);
    ServiceLocator.SetLocatorProvider(() => serviceLocator);
}

The IFooService is also instantiated by Unity, but that uses constructor injection instead (and it works):

public class FooController : Controller
{
    private readonly IFooService _fooService;
    public FooController(IFooService fooService)
    {
        _fooService = fooService;
    }
}

How can I debug this to see why the dependencies are failing to instantiate. No exceptions are being thrown (or if they are then Elmah is not catching and logging them).

Answer 1

The dependency is not injected because the DependencyAttribute is on the concrete class instead of the interface.

As DI attributes can be harmful I would recommend you change the registration to

container.RegisterType<IFooService,FooService>(new InjectionProperty("BarService"), new InjectionProperty("AnotherService"));

Resolving IFooService will then return an instance of FooService with the injected dependencies.