title: How to dependency inject a class / type?
body: <p>I'm struggling with a design problem and I don't want my code to become a mess because of a bad solution. Rather than give a poor analogy I'll just explain my exact case.</p>
  
  <p>I'm trying to write a clone of Wii Play Tanks, and I'm having trouble designing the Tank classes. <code>Tank</code> itself is the only such class, it uses dependency injection for its parts. The two parts right now are <code>TankAI</code> and <code>TankWeapon</code>. The AI handles decisions about movement and firing, the weapon describes how the weapon behaves - what projectiles it fires, and how often, etc. I have a factory class that builds tanks in different combinations.</p>
  
  <p>My projectile classes are set up under an abstract <code>Projectile</code> class. Each subclass describes the projectile's model, number of bounces, speed, etc.</p>
  
  <p>The problem I'm having is that each <code>TankWeapon</code> subclass is duplicating a lot of code around the area where they construct a new projectile, because they each construct a different class. I want to move this code into the base class, but I would have to somehow inject the projectile class itself that the weapon needs to construct. I know I could literally pass a Class to the base upon construction, but that feels like the wrong way to go.</p>
  
  <p>And while we're at it I have another design problem: How can I make my AI classes aware of the projectile class as well? Their decisions will depend on properties of the projectile being fired, such as how many times they can bounce off walls. Both the AI and Weapon classes are being given a reference to the parent <code>Tank</code> upon injection.</p>
  
  <h2>Edit:</h2>
  
  <p>It seems like my original question was a bit confusing, so I'll post code. I already have the DI set up for my tank.</p>
  
  <pre><code>public class Tank : ISolidObject
  {
      public TankAI AISystem { get; private set; }
      public TankWeapon Weapon { get; private set; }
  
      public Tank(TankAI aiSystem, TankWeapon weapon)
      {
          this.AISystem = aiSystem;
          this.AISystem.Tank = this;
  
          this.Weapon = weapon;
          this.Weapon.Tank = this;
      }
  }
  
  public abstract class TankAI
  {
      public Tank Tank { get; set; }
  
      public abstract void Think();
  }
  
  // TankAI implementations aren't important here
  
  public abstract class TankWeapon
  {
      protected int maxShotsOnScreen, shotsOnScreen;
  
      public Tank Tank { get; set; }
  
      public virtual void Shoot()
      {
          shotsOnScreen++;
  
          // I really want to put the projectile construction code in here
      }
  }
  
  public class BulletWeapon : TankWeapon
  {
      public BulletWeapon()
      {
          this.maxShotsOnScreen = 5;
          this.turnSpeed = 1;
      }
  
      public override void Shoot()
      {
          // here's my problem. Every weapon class duplicates this, because I can't put the projectile construction in the base weapon class.
          if (shotsOnScreen &gt;= maxShotsOnScreen) return;
  
          base.Shoot();
  
          // just create it, it will take care of the rest
          double bx = Tank.X - Math.Sin(Tank.AngleTurret * Math.PI / 180.0);
          double by = Tank.Y + Math.Cos(Tank.AngleTurret * Math.PI / 180.0);
          // note that projectiles subscribe themselves to the game entity handler, so  don't have to store it myself.
  
          // this weapon creates bullets. A different weapon might create rockets. How would the base class know which? Is there any way I can prevent this code from being duplicated?
          new Bullet(bx, by, Tank.AngleTurret).Death += ShotDeath;
      }
  
      private void ShotDeath(Projectile p)
      {
          p.Death -= ShotDeath;
          shotsOnScreen--;
      }
  }
  </code></pre>