Dependency Injection: Keeping It Simple

When I gave my talk at TriNug’s SIG about the SOLID principles, I whipped up an example of Dependency Injection (DI) that some people really liked.  I decided to blog about it.  Consider a basic console application that has a single Interface included – Animal.  The Animal interface looks like this:

public interface IAnimal
{
    void Talk();
    void Eat();
}

The Console application includes a couple of classes that implement the interface:

public class Elephant: IAnimal
{
    public void Talk()
    {
        Console.WriteLine("Trumpet");
    }

    public void Eat()
    {
        Console.WriteLine("Yummy Peanuts");
    }
}

public class Seal(): IAnimal 
{
    public void Talk()
    {
        Console.WriteLine("Ark Ark");
    }

    public void Eat()
    {
        Console.WriteLine("Yummy Fish");
    }
}

Then in the Main method, I want to inject in an animal at run time.  I could write something like this:

static void Main(string[] args)
{
    IAnimal animal = ????;
    animal.Talk();
    animal.Eat();
}

What I don’t want to do is to have a switch/if..then statement in the Main function because as new animals are added to my project, I would have to update existing code and violate the Open/Closed principle.  I know if a couple of ways around this problem:

  • Using a Dependency Injection Framework like Unity
  • Using Microsoft Extension Framework
  • Use a Factory Pattern
  • Injecting the name of the class and using Activator to resolve. Note that this name can come from a database call or a .config file.

A DI framework seems a bit too heavy for this kind of solution.  I tried MEF and quickly gave up, Using a factory pattern delays but does not alter the problem resolution.  I think the Activator keyword is the best solution to my problem.

To that end, I added a section in my .config file like this:

<configuration>
  <appSettings>
    <add key="animalTypeName" value="Tff.PoorMansDependencyInjection.Seal"/>
  </appSettings>
</configuration>

And then in my Main method:

String animalTypeName = ConfigurationManager.AppSettings["animalTypeName"].ToString();
Type animalType = Type.GetType(animalTypeName);
IAnimal animal = (IAnimal)Activator.CreateInstance(animalType);
animal.Talk();
animal.Eat();
Console.ReadKey();

And I get this:

image

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