Dependency Injection WIth Parameterless Constructor

Question

Let's assume that I have the following interfaces and classes:

public interface ITest { }

public class Test : ITest { }

In NinjectWebCommon.cs:

kernel.Bind<ITest>().To<Test>();

And after this I want to use my ITest interface:

public class Vehicle
{
    private ITest _test = null;
    public Vehicle(ITest test)
    {
        this._test = test;
    }
}

public class TestVehicle
{
    public Vehicle vehicle = new Vehicle();//this will throw an error, because I haven't parameterless cnstructor in Vehicle class;
}

I know that in Vehicle class I can to do the parameterless constructor with the following way:

public Vehicle()
{
    this._test = DependencyResolver.Current.GetService<ITest>();
}

But it seems to be a bad practice and doesn't follow to dependency injection pattern.

So, how can I to solve this?

Answer 1

"It's turtles all the way down".

Just as you're injecting ITest into Vehicle, you should inject Vehicle into TestVehicle - it doesn't matter that it's a concrete class rather than an interface, just let your DI framework do the work for you.

In fact, as you're using NInject, a self-binding will already have been created for Vehicle for you, so there's very little you need to do at all:

public class TestVehicle
{
  private readonly Vehicle _vehicle;

  public TestVehicle(Vehicle vehicle)
  {
    this._vehicle = vehicle;
  }
}