Dependency Injection Unity - Conditional Resolving

Question

Conditional resolving is the last thing I don't understand at the moment.

Lets say we have an interface IAuthenticate:

public interface IAuthenticate{
    bool Login(string user, string pass);
}

Now I have two types of authentication.

Twitter auth

public class TwitterAuth : IAuthenticate
{
  bool Login(string user, string pass)
{
   //connect to twitter api
}

}

Facebook Auth

public class FacebookAuth: IAuthenticate
{
  bool Login(string user, string pass)
{
   //connect to fb api
}

}

Registering types in unity config:

unityContainer.RegisterType<IAuthenticate, TwitterAuth>();
unityContainer.RegisterType<IAuthenticate, FacebookAuth>();

inject objects via DI in our controller:

private readonly IAuthenticate _authenticate;

public AuthenticateController(IAuthenticate authenticate)
{
    _authenticate = authenticate;
}



// login with twitter
public virtual ActionResult Twitter(string user, string pass)
{
    bool success =
            _authenticate.Login(user, pass);
}



// login with fb
public virtual ActionResult Facebook(string user, string pass)
{
    bool success =
            _authenticate.Login(user, pass);
}



// login with google
public virtual ActionResult Google(string user, string pass)
{
    bool success =
            _authenticate.Login(user, pass);
}

How exactly will unity know which object does it have to resolve for different types of authentication? How do I do conditional resolving in this case?

I spoke with friend of mine, and he explained if this situation appears it is wrong design, but this is just factory pattern used.

Answer 1

A simple way to solve this is with the strategy pattern. Note that you can add or remove login providers without changing the design - you simply need to change the DI configuration.

Interfaces

public interface IAuthenticate{
    bool Login(string user, string pass);
    bool AppliesTo(string providerName);
}

public interface IAuthenticateStrategy
{
    bool Login(string providerName, string user, string pass);
}

Authenticate Providers

public class TwitterAuth : IAuthenticate
{
    bool Login(string user, string pass)
    {
        //connect to twitter api
    }
    
    bool AppliesTo(string providerName)
    {
        // I used the type name for this example, but
        // note that you could use any string or other
        // datatype to select the correct provider.
        return this.GetType().Name.Equals(providerName);
    }
}

public class FacebookAuth: IAuthenticate
{
    bool Login(string user, string pass)
    {
        //connect to fb api
    }

    bool AppliesTo(string providerName)
    {
        return this.GetType().Name.Equals(providerName);
    }
}

Strategy

public class AuthenticateStrategy: IAuthenticateStrategy
{
    private readonly IAuthenticate[] authenticateProviders;
    
    public AuthenticateStrategy(IAuthenticate[] authenticateProviders)
    {
        if (authenticateProviders == null)
            throw new ArgumentNullException("authenticateProviders");
            
        this.authenticateProviders = authenticateProviders;
    }

    public bool Login(string providerName, string user, string pass)
    {
        var provider = this.authenticateProviders
            .FirstOrDefault(x => x.AppliesTo(providerName));

        if (provider == null)
        {
            throw new Exception("Login provider not registered");
        }

        return provider.Login(user, pass);
    }
}

Unity Registration

// Note that the strings used here for instance names have nothing 
// to do with the strings used to select the instance in the strategy pattern
unityContainer.RegisterType<IAuthenticate, TwitterAuth>("twitterAuth");
unityContainer.RegisterType<IAuthenticate, FacebookAuth>("facebookAuth");
unityContainer.RegisterType<IAuthenticateStrategy, AuthenticateStrategy>(
    new InjectionConstructor(
        new ResolvedArrayParameter<IAuthenticate>(
            new ResolvedParameter<IAuthenticate>("twitterAuth"),
            new ResolvedParameter<IAuthenticate>("facebookAuth")
        )
    ));

Usage

private readonly IAuthenticateStrategy _authenticateStrategy;

public AuthenticateController(IAuthenticateStrategy authenticateStrategy)
{
    if (authenticateStrategy == null)
        throw new ArgumentNullException("authenticateStrategy");
        
    _authenticateStrategy = authenticateStrategy;
}



// login with twitter
public virtual ActionResult Twitter(string user, string pass)
{
    bool success =
            _authenticateStrategy.Login("TwitterAuth", user, pass);
}



// login with fb
public virtual ActionResult Facebook(string user, string pass)
{
    bool success =
            _authenticateStrategy.Login("FacebookAuth", user, pass);
}

unity.config

Instead of "Unity Registration" you could do this on your unity.config

<register type="IAuthenticate" mapTo="TwitterAuth" name="twitterAuth" />
<register type="IAuthenticate" mapTo="FacebookAuth" name="facebookAuth" />
<register type="IAuthenticateStrategy" mapTo="AuthenticateStrategy" />

Answer 2

Unity won't without your help. You could provide a name when you register your IAuthenticate types:

unityContainer.RegisterType<IAuthenticate, TwitterAuth>("Twitter");
unityContainer.RegisterType<IAuthenticate, FacebookAuth>("Facebook");

You'll no longer want to directly inject an IAuthenticate instance into your AuthenticateController. You'll either get the instance you want based on a condition right out of unity (service locator style):

myContainer.Resolve<IAuthenticate>("Twitter");

or you'll inject a Factory that does this for you (if you like a strict DI style).