Register null as instance in Unity container

Question

I have a repository class with optional dependency:

class MyRepository : BaseRepository, IMyRepository
{
    public MyRepository(IDataContext dataContext, ICacheProvider cacheProvider = null)
        : base(dataContext, cacheProvider)
    {}

    // …
}

The existence of cacheProvider parameter acts as strategy for the repository. I want setup Unity container like this:

Container.RegisterType<IDataContext, MyDataContext>(new PerResolveLifetimeManager(), new InjectionConstructor())
         .RegisterInstance<ICacheProvider>(null) // ???
         .RegisterType<IMyRepository, MyRepository>();

I.e. not pointing out particular InjectionConstructor with one parameter for MyRepository, but use default constructor with null as cacheProvider parameter.

Is there any way to do this?

Answer 1

I found that RegisterType, instead of Register instance, supports returning null.

container.RegisterType<IInterface>(new InjectionFactory((c) => null));

This was the most straightforward way of getting an actual null to be returned.

Answer 2

In the .RegisterType<IMyRepository, MyRepository>() call, specify the InjectionConstructor with an OptionalParameter, as in

.RegisterType<IMyRepository, MyRepository>(new InjectionConstructor(
new ResolvedParameter<IDataContext>(), 
new OptionalParameter<ICacheProvider>()));