Constructor Injection in C#/Unity?

Question

I'm using C# with Microsoft's Unity framework. I'm not quite sure how to solve this problem. It probably has something to do with my lack of understanding DI with Unity.

My problem can be summed up using the following example code:

class Train(Person p) { ... }

class Bus(Person p) { ... }

class Person(string name) { ... }

Person dad = new Person("joe");
Person son = new Person("timmy");

When I call the resolve method on Bus how can I be sure that the Person 'son' with the name 'timmy' is injected and when resolving Train how can I be sure that Person 'dad' with then name 'joe' is resolved?

I'm thinking maybe use named instances? But I'm at a loss. Any help would be appreciated.

As an aside, I would rather not create an IPerson interface.

Answer 1

Unless you register respectively "joe" and "timmy" as named dependencies, you can't be sure that "timmy" is injected into Schoolbus. In fact, if you attempt to register two instances of the same class as unnamed dependencies, you will have an ambiguous setup, and you will not be able to resolve Person at all.

In general, if you have to register a lot of named instances you are probably going about DI in the wrong way. The main idea of DI is to resolve Domain Services more than Domain Objects.

The primary idea of DI is to provide a mechanism that allows you to resolve abstract types (interfaces or abstract classes) into concrete types. Your example has no abstract types, so it doesn't really make a lot of sense.