public interface IAuthProvider
{
    AuthResult Authenticate(string username, string password);
}

public class DbAuthProvider : IAuthProvider
{
    public AuthResult Authenticate(string username, string password)
    {
        // Do authentication against database
    }
}

public class LdapAuthProvider : IAuthProvider
{
    public AuthResult Authenticate(string username, string password)
    {
        // Do authentication against LDAP
    }
}

/** in our controller **/

public class LoginController
{
    private IAuthProvider authProvider;

    public LoginController()
    {
        // We can instantiate the implementation of auth provider here
        // in the constructor
        authProvider = new DbAuthProvider();

        // Or better yet, use an IoC so the implementation can be
        // configured elsewhere.
        authProvider = IoC.GetInstance<IAuthProvider>();
    }

    public object Login(string username, string password)
    {
        // use authProvider
        authProvider.Authenticate(username, password);
        // bla.. bla.. bla..
    }
}