Unity Container Resolve

Question

I just starting with Unity Container and my registration looks like this:

static void UnityRegister()
{
      _container = new UnityContainer();
      _container.RegisterType<IBook, Book>();
      _container.RegisterType<IBookRepository, BookRepository>("Book");
      _container.RegisterType<IBookService, BookService>();
      _container.RegisterType<IBookRepository, DatabaseRepository>("Database");
}

Now when I try to resolve doing this:

var service = _container.Resolve<IBookService>("Database");

I get error below:

Resolution of the dependency failed, type = "UnityConsoleEx.IBookService", name = "Database". Exception occurred while: while resolving. Exception is: InvalidOperationException - The current type, UnityConsoleEx.IBookService, is an interface and cannot be constructed. Are you missing a type mapping?

At the time of the exception, the container was:

Resolving UnityConsoleEx.IBookService,Database

Can anyone point what I am doing wrong?

Answer 1

I think, you try to resolve BookService with should contains DatabaseRepository as a parameter. You can't do it in your way.

You can do it like this:

var service = _container.Resolve<IBookService>(new ParameterOverride("repository", _container.Resolve<IBookRepository>("Database")));

Maybe, better way is register repository once, conditionaly:

  _container = new UnityContainer();
  _container.RegisterType<IBook, Book>();
  if (useDatabase)
  {
      _container.RegisterType<IBookRepository, DatabaseRepository>();
  }
  else
  {
      _container.RegisterType<IBookRepository, BookRepository>();
  }
  _container.RegisterType<IBookService, BookService>();

And now, resolve service. Also, you can configure you container like this:

_container.RegisterType<IBookService, BookService>(
    new InjectionConstructor(                        // Explicitly specify a constructor
        new ResolvedParameter<IBookRepository>("Database") // Resolve parameter of type IBookRepository using name "Database"
    )
);

This will tell the container to resolve IBookService using constructor with single IBookRepository parameter, and resolve IBookRepository with name of Database.

Answer 2

The main issue is that you are not using a named instance for your BookService.

_container.RegisterType<IBookService, BookService>();

But you are trying to resolve with a named instance.

var service = _container.Resolve<IBookService>("Database");

You need to resolve without a name to get that instance.

var service = _container.Resolve<IBookService>();

But it is unclear from your example why you are using named instances in the first place. If you post the constructors of your services, it will be more clear how to make your configuration work.

Answer 3

I figured it out, I needed create named instance for the service and inject constructor, as such:

static void UnityRegister()
{
    _container = new UnityContainer();
    _container.RegisterType<IBook, Book>();
    _container.RegisterType<IBookRepository, BookRepository>();            
    _container.RegisterType<IBookRepository, DatabaseRepository>();

    _container.RegisterType<IBookService, BookService>("BookService", new InjectionConstructor(typeof(BookRepository)));
    _container.RegisterType<IBookService, BookService>("DatabaseService", new InjectionConstructor(typeof(DatabaseRepository)));
}

And resolve it as below:

var service = _container.Resolve<IBookService>("DatabaseService");