Question

Lets say I have the following code in C#:

public class AppleTree
{
   public AppleTree()
   {
   }

   public string GetApple
   {
      return new Fruit("Apple").ToString();
   }
}

where Fruit is a third party class that does not have an interface.

I want to create a unit test for the AppleTree class but I do not want to run the Fruit class. Instead I would like to inject the Fruit class so that I can mock it in the tests.

How should I go about doing this? I can create a factory that creates apples and then add an interface to this factory like:

public class FruitFactory : IFruitFactory
{
   Fruit CreateApple()
   {
      return new Fruit("Apple");
   }
}

Now I can inject IFruitFactory into the AppleTree and use the CreateApple instead of new Fruit as:

public class AppleTree
{
   private readonly IFruitFactory _fruitFactory;

   public AppleTree(IFruitFactory fruitFactory)
   {
      _fruitFactory = fruitFactory
   }

   public string GetApple
   {
      return  _fruitFactory.CreateApple().ToString();
   }
}

Now to my question: Is there a good way of doing this without having to create the factory? Can I for example use a dependency injector famework like Ninject in some way?

Answer 1

See Ninject.Extensions.Factory – with it, you have Func<T> ctor arguments and the Abstract Factory is generated behind the scenes (a la @Nenad’s +1’d answer)

(You can also define an Abstract Factory interface and have Ninject.Extensions.Factory implement it.