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OverviewEdit

Dependency Injection as a topic is covered in a separate article (see: Dependency Injection)

This is article demonstrates how Dependency Injection can be implemented without additional software libraries. Walking through the example covered here is a good way to understand what problem Dependency Injection is trying to solve.

Walk-throughEdit

Let's say you're writing the next blockbuster game, where noble warriors do battle for great glory. First, we'll need a weapon suitable for arming our warriors.

class Sword {
  public void Hit(string target) {
    Console.WriteLine("Chopped {0} clean in half", target);
  }
}

Then, let's create a class to represent our warriors themselves. In order to attack its foes, the warrior will need an Attack() method. When this method is called, it should use its Sword to strike its opponent.

class Samurai {
  private Sword _sword;
  public Samurai() {
    _sword = new Sword();
  }
  public void Attack(string target) {
    _sword.Hit(target);
  }
}

Now, we can create our Samurai and do battle!

class Program {
  public static void Main() {
    Samurai warrior = new Samurai();
    warrior.Attack("the evildoers");
  }
}

As you might imagine, this will print "Chopped the evildoers clean in half" to the console. This works just fine, but what if we wanted to arm our Samurai with another weapon? Since the Sword is created inside the Samurai class's constructor, we have to modify the implementation of the class in order to make this change.

When a class is dependent on a concrete dependency, it is said to be tightly coupled to that class. In this example, the Samurai class is tightly coupled to the Sword class. When classes are tightly coupled, they cannot be interchanged without altering their implementation. In order to avoid tightly coupling classes, we can use interfaces to provide a level of indirection. Let's create an interface to represent a weapon in our game.

interface IWeapon {
  void Hit(string target);
}

Then, our Sword class can implement this interface:

class Sword : IWeapon {
  public void Hit(string target) {
    Console.WriteLine("Chopped {0} clean in half", target);
  }
}

And we can alter our Samurai class:

class Samurai {
  private IWeapon _weapon;
  public Samurai() {
    _weapon = new Sword();
  }
  public void Attack(string target) {
    _weapon.Hit(target);
  }
}

Now our Samurai can be armed with different weapons. But wait! The Sword is still created inside the Samurai's constructor. Since we still need to alter the implementation of Samurai in order to give our warrior another weapon, Samurai is still tightly coupled to Sword.

Fortunately, there is an easy solution. Rather than creating the Sword from within the Samurai's constructor, we can expose it as a parameter of the constructor instead.

class Samurai {
  private IWeapon _weapon;
  public Samurai(IWeapon weapon) {
    _weapon = weapon;
  }
  public void Attack(string target) {
    _weapon.Hit(target);
  }
}

Then, to arm our warrior, we can inject the Sword via the Samurai's constructor. This is an example of Dependency Injection (specifically, constructor injection). Let's create another weapon that our Samurai could use:

class Shuriken : IWeapon {
  public void Hit(string target) {
    Console.WriteLine("Pierced {0}'s armor");
  }
}

Now, we can create an army of warriors:

class Program {
  public static void Main() {
    Samurai warrior1 = new Samurai(new Shuriken());
    Samurai warrior2 = new Samurai(new Sword());
    warrior1.Attack("the evildoers");
    warrior2.Attack("the evildoers");
  }
}

This results in the following output to be printed to the console:

Pierced the evildoers armor.
Chopped the evildoers clean in half.

SummaryEdit

This is called "Dependency Injection by hand", because each time you want to create a Samurai, you must first create some implementation of IWeapon and then pass it to the Samurai's constructor. Now that we can change the weapon the Samurai uses without having to modify its implementation, the Samurai class could be in a separate assembly from Sword -- in fact, we can create new weapons without needing the source code of the Samurai class!

Dependency Injection by hand is an effective strategy for small projects, but as your application grows in size and complexity, it becomes more and more cumbersome to wire all of your objects up. What happens when the dependencies have dependencies of their own? You can easily end up spending most of your time creating and wiring together objects, when you could be writing code that adds real value to your software. This is where Dependency Injection containers can help.

ReferencesEdit

See Dependency Injection.

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