So what's Ninject all about? First, let's examine the idea of dependency injection by walking through a simple example. 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.
C#:
class Sword {
public void Hit(string target) {
Console.WriteLine("Chopped {0} clean in half", target);
}
}
Public Class Sword
Public Sub Hit(ByVal target As String)
Console.WriteLine("Chopped {0} clean in half", target)
End Sub
End Class
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.
C#:
class Samurai {
private Sword _sword;
public Samurai() {
_sword = new Sword();
}
public void Attack(string target) {
_sword.Hit(target);
}
}
Public Class Samurai
Private _sword As Sword
Public Sub New()
_sword = New Sword()
End Sub
Public Sub Attack(ByVal target As String)
_sword.Hit(target)
End Sub
End Class
Now, we can create our Samurai and do battle!
C#:
class Program {
public static void Main() {
Samurai warrior = new Samurai();
warrior.Attack("the evildoers");
}
}
Module Module1
Sub Main()
Dim warrior As New Samurai()
warrior.Attack("the evildoers")
End Sub
End Module
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.
C#:
interface IWeapon {
void Hit(string target);
}
Public Interface IWeapon
Sub Hit(ByVal target As String)
End Interface
Then, our
Sword class can implement this interface:
C#:
class Sword : IWeapon {
public void Hit(string target) {
Console.WriteLine("Chopped {0} clean in half", target);
}
}
Public Class Sword
Implements IWeapon
Public Sub Hit(ByVal target As String) Implements IWeapon.Hit
Console.WriteLine("Chopped {0} clean in half", target)
End Sub
End Class
And we can alter our
Samurai class:
C#:
class Samurai {
private IWeapon _weapon;
public Samurai() {
_weapon = new Sword();
}
public void Attack(string target) {
_weapon.Hit(target);
}
}
Public Class Samurai
Private ReadOnly _sword As IWeapon
Public Sub New()
_sword = New Sword()
End Sub
Public Sub Attack(ByVal target As String)
_sword.Hit(target)
End Sub
End Class
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.
C#:
class Samurai {
private IWeapon _weapon;
public Samurai(IWeapon weapon) {
_weapon = weapon;
}
public void Attack(string target) {
_weapon.Hit(target);
}
}
Public Class Samurai
Private ReadOnly _sword As IWeapon
Public Sub New(ByVal weapon As IWeapon)
_sword = weapon
End Sub
Public Sub Attack(ByVal target As String)
_sword.Hit(target)
End Sub
End Class
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:
C#:
class Shuriken : IWeapon {
public void Hit(string target) {
Console.WriteLine("Pierced {0}'s armor", target);
}
}
Public Class Shuriken
Implements IWeapon
Public Sub Hit(ByVal target As String) Implements IWeapon.Hit
Console.WriteLine("Pierced {0}'s armor", target)
End Sub
End Class
Now, we can create an army of warriors:
C#:
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");
}
}
Module Module1
Sub Main()
Dim warrior1 As New Samurai(New Shuriken())
Dim warrior2 As New Samurai(New Sword())
warrior1.Attack("the evildoers")
warrior2.Attack("the evildoers")
End Sub
End Module
This results in the following output to be printed to the console:
Pierced the evildoers armor.
Chopped the evildoers clean in half.
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 injectors like Ninject can help.
Continue reading:
Dependency Injection With Ninject