Dependency Injection Unity - Conditional Resolving
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.
Solution 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" />
Solution 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).