Simple way to calculate closest distance to a cube surface

DistanceCalculator.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class DistanceCalculator : MonoBehaviour {

	public GameObject[] targets;

	void Update () {
		foreach(GameObject target in targets) {
			Collider collider = target.GetComponent<Collider>();
			if(collider is BoxCollider) {
				Vector3 vector = target.transform.position - transform.position;
				Ray ray = new Ray(transform.position, vector);
				Debug.DrawRay(transform.position, vector, Color.yellow);

				RaycastHit hit;
				collider.Raycast(ray, out hit, vector.magnitude);
				Vector3 projection = Vector3.Project(hit.point - transform.position, hit.normal);
				Debug.DrawRay(transform.position, projection, Color.white);
			}
		}
	}
}