How does Unity.Resolve know which constructor to use?

Question

Given a class with several constructors - how can I tell Resolve which constructor to use?

Consider the following example class:

public class Foo
{
    public Foo() { }
    public Foo(IBar bar)
    {
        Bar = bar;
    }
    public Foo(string name, IBar bar)
    {
        Bar = bar;
        Name = name;
    }
    public IBar Bar { get; set; }        
    public string Name { get; set; }
}

If I want to create an object of type Foo using Resolve how will Resolve know which constructor to use? And how can I tell it to use the right one? Let's say I have a container with an IBar registered - will it understand that it should favor the constructor taking IBar? And if I specify a string too - will it use the (string, IBar) constructor?

Foo foo = unityContainer.Resolve<Foo>(); 

And please ignore the fact that it would probably be easier if the class just had a single constructor...

Answer 1

When a target class contains more than one constructor, Unity will use the one that has the InjectionConstructor attribute applied. If there is more than one constructor, and none carries the InjectionConstructor attribute, Unity will use the constructor with the most parameters. If there is more than one such constructor (more than one of the “longest” with the same number of parameters), Unity will raise an exception.

Taken from link text

Answer 2

When you register the type, you can specify which constructor to use like this:

container.RegisterType<Foo>(
    new InjectionConstructor(
        new ResolvedParameter<IBar>()));

The above code is from memory, but that's the general principle. In this case I've picked the constructor that takes a single parameter of type IBar.

And please ignore the fact that it would probably be easier if the class just had a single constructor...

I can't ignore this. When it comes to Constructor Injection, ambiguity is a design smell. You are basically saying: I don't really know if I care about this dependency or not.

Sure, Unity is likely to figure it out for you, but then you would be relying on a specific container behavior instead of properly designing your API. Other containers may have a different behavior, so if you ever choose to migrate from Unity to a better container, subtle bugs may occur.

It's much safer to write your code in a DI-friendly, but container-agnostic manner.