Are you trying to place this hook in a ModNPC class, a ModProjectile class, or a ModPlayer class? That hook you have there is only for a ModPlayer class.Do you know any hook that triggers only if you hit npc with projectile ?
public override void DealtNPC(NPC npc, Projectile projectile, int hitDir, int dmgDealt, float knockback, bool crit)
This thing doesn't work at all.
If it's for a ModNPC class, then the correct hook is:
public override void DealtNPC(Projectile projectile, int hitDir, int dmgDealt, float knocback, bool crit)
If it's for a ModProjectile class, then the correct hook is:
public override void DealtNPC(NPC npc, int hitDir, int dmgDealt, float knockback, bool crit)