abstract class CombatBehavior
{
}

class SwordfightBehavior : CombatBehavior
{
}

class BowBehavior : CombatBehavior
{
}

class NPC
{
    private CombatBehavior _combatBehavior;

    public void EquipWeapon(Weapon weapon)
    {
        _combatBehavior = the appropriate behavior for this weapon;
    }
}