Interface inheritance and Unity resolve

Question

I am trying to resolve a object through it's base interface using Unity. If I have the following interfaces and classes:

interface IFoo { }
interface IBar : IFoo { }
class MyBar : IBar { }

and I want to inject MyBar into multiple classes like so:

class DoSomething
{
    public DoSomething(IFoo myBar) 
    { 
        // Execute a method from IFoo implementation 
    }
}

class DoSomethingElse
{
    public DoSomethingElse(IBar myBar) 
    { 
        // Execute a method from IBar **AND** IFoo implementation 
    }
}

If I register MyBar like this:

container.RegisterType<IBar, MyBar>();

Unity throws an error trying to resolve for IFoo (see the DoSomething constructor). But IBar inherits from IFoo?

I could register MyBar twice with the container like this:

container.RegisterType<IFoo, MyBar>();
container.RegisterType<IBar, MyBar>();

but it feels like I should not have to do this. I might be wrong here.

So my question is if Unity can resolve a class from its base interface?

If you want to use singleton registration for MyBar you should look for a way to tell the service container that in order to resolve IFoo, it can resolve IBar and cast down, otherwise each of the two interfaces will typically resolve to two distinct instances of the MyBar class. — Lasse V. Karlsen

HimBromBeere HimBromBeere · Accepted Answer · 2018-09-28T11:50:19

Basically a DI-container tries to create instances of interfaces by some sort of mapping. So when you write

container.RegisterType<IBar, MyBar>();

the container knows, whenever I am requested to create some instance for IBar I resolve that call by instantiating MyBar.

However the container doesn´t know what to do what do if he is requested for an instance of IFoo, because not every IFoo is an IBar. So what should be returned when writing this:

container.GetInstance<IFoo>();

In your case only a single type is possible, IBar for which in turn also only a single type is possible. This however contradicts the purpose of interfaces, as in fact it´s allways bounded to one specific type, while it should work with any type implementing that interface.

That´s why you have to determine a mapping for both interfaces.

Interface inheritance and Unity resolve

1 Answers