DamageVolume.cs

using UnityEngine;
using UniRx.Triggers;
using UniRx;

public class DamageVolume : MonoBehaviour {

    public bool IsNegateVolume = false;
	
    void Start () {
        Collider c = GetComponent<Collider>();

        // Add this volume to the player
        c.OnTriggerEnterAsObservable()
            .Select(other => other.GetComponent<DamageVolumePlayer>())
            .Where(damageVolumePlayer => damageVolumePlayer != null && damageVolumePlayer.CurrentDamageVolumes.Contains(this) == false )
            .Subscribe(damageVolumePlayer =>
                damageVolumePlayer.CurrentDamageVolumes.Add(this)
            ).AddTo(this);

        // Remove this volume from the player
        c.OnTriggerExitAsObservable()
            .Select(other => other.GetComponent<DamageVolumePlayer>())
            .Where(damageVolumePlayer => damageVolumePlayer != null && damageVolumePlayer.CurrentDamageVolumes.Contains(this) == true)
            .Subscribe(damageVolumePlayer =>
                damageVolumePlayer.CurrentDamageVolumes.Remove(this)
            ).AddTo(this);
    }
}