Which design pattern should I use?

I have 3 types of weapon(A,B,C) and 3 types of monsters (D,E,F). Each type of weapon can only interact with certain types of monsters. For example, bullet A can only interact with monsters D, E, bullet B can only interact with monsters E, F,... So can I ask how should I handle this case? Thank you very much

 

If under "interacting" you mean collision detection, unity have Layer system which is useful because the physics system won't even take into account the collision which aren't as your setup.

So at the inspector you can find layer field, and then you can create layer for each weapon type AND for each enemy type.
So for example you create layerA, layerB, and layerC for the weapons , and layer D, E ,F for each enemy.
And the simply go to Edit-> Project settings-> Physics and at very bottom you can find the "Layer Collision Matrix", and the rest I think is self explanatory.

The problem could only occur if you a large quantity of weapon types and enemy types (because limit of layers is 32, also it would look unorganized).
The solution would be just checking is the collision possible with simple IF function (in onCollisionEnter).
If your collision scripts is at the weapon then check (if enemy.type==D||enemy.type==E), regardless of the bullet type.

 

"interacting" means there is a collision and there is handling at the time of the collision. For example, when bullets "interacting" with monsters, the bullets will be destroyed.
So in the case that in addition to colliding with the monster, but also being destroyed when colliding with the platform, how should I handle it?

 

If your "bullets" are physics objects, you can use OnCollisionEnter or OnTriggerEnter, and check the collided object.

In the following pseudocode, I am assuming:
- an enum of bullet types
- a Bullet class which contains the basic properties of a bullet, most important in this case being its bullet type
- your bullets are physical projectiles with trigger collision.

Code (CSharp):

1. //on enemy
2. BulletType weakness = BulletType.ice
3.  
4. void OnTriggerEnter(Collider col){
5.     Bullet bullet = col.getComponent<Bullet>();
6.     if(bullet.bulletType == weakness){
7.         //Oh no, I take damage
8.     }
9. }

Code (CSharp):

1. //on bullet
2. void OnTriggerEnter(Collider col){
3.     //some visual effect for the bullet disintegrating
4.     Destroy(this.gameObject);
5. }

This has nothing to do with design patterns, this is just making use of the collision functions Unity has provided. In my snipplets, the bullet is responsible for destroying itself, while the enemy is responsible for determining how it reacts to bullets.