Is it appropriate to use Property Injection in a base class when a dependency is only required in the base class?

Question

Example:

public abstract class BaseControler : Controller
{
    public IUnitOfWork UnitOfWork { get; set; }
}

public class HomeController : BaseControler
{
    readonly IUserRepository _userRepository;

    // :-)
    public HomeController(IUserRepository userRepository)
    {
        _userRepository = userRepository;
    }
}

We all know that we have to use Constructor Injection when the dependency is required. If it is an optional dependency we can use Property Injection instead.

But what should you do when only your base class requires the dependency?

When you would use Constructor Injection you would in my opinion pollute all derived classes.

public abstract class BaseControler : Controller
{
    readonly IUnitOfWork _unitOfWork;

    public BaseControler(IUnitOfWork unitOfWork)
    {
        _unitOfWork = unitOfWork;
    }
}

public class HomeController : BaseControler
{
    readonly IUserRepository _userRepository;

    // :-(
    public HomeController(IUserRepository userRepository, 
       IUnitOfWork unitOfWork) : base(unitOfWork)
    {
        _userRepository = userRepository;
    }
}

Is it approperiate to use Property Injection in a base class when a dependency is only required in the base class?

Answer 1

You are not polluting the derived class. You are explicitly stating to the consumer that this class cannot function without this dependency.

If the base class requires this dependency in order to function properly, since the derived class derives from this base class, it implicitly requires this dependency as well. So the second example is the correct way. You shouldn't be using property injection for something that is a required dependency.

Answer 2

Actually your derived class IS a base class. There is no some other object somewhere. When you pass something to base constructor, you actually initialize same object. And it shows to clients what your instance depends on:

public HomeController(IUserRepository userRepository, IUnitOfWork unitOfWork)

There is ONE instance, which depends on two things: user repository, and unit of work. Call to base class constructor just removes initialization duplication from your derived classes.