How to detect if an object is under a shadow casted by a directional light

I'm assuming we can't treat a directional light as any other form of light, that is as a single point, but as a plane with the same orientation as the directional light. so instead of casting a ray from my object towards the center of the DirectionalLight, I'm casting it from myobject.transform.position to myObject.transform.position - directection of DirectionalLight so that I'm casting lines parallel to the light as in the picture

and if the ray hits a wall on it's way then set a bool value underSun= true else underSun=false and to increase the accuracy of the detection, instead of casting a single ray from the center of the object ,I'm casting four each from the it's top corners and on everyUpdate if a rayCast from any of these corners doesn't hit a wall then break and set underSun= true

this does make sense to me, but I'm getting some weird unwanted results with my code and I would appreciate if anyone can point me to the right direction. see in gif below

wanted behaviour

Unwanted Behaviour

the code is here

 using UnityEngine;
 using System.Collections;
 
 public class NewPlayer : MonoBehaviour {
 
    
     public NewSun sun;
 
     private MeshRenderer mesh;
     private RaycastHit hit;
     private bool underSun = false;
     
     // Use this for initialization
     void Start () {
         mesh = GetComponent<MeshRenderer>();
     }
     
     // Update is called once per frame
     void Update () {
         underSun = false;
         Vector3 sunDir = sun.transform.forward;
         sunDir.Normalize();
         sunDir *= 100;
    
         foreach (Transform child in transform) {
 
             if (!Physics.Raycast(child.position, child.position - sunDir, 30, LayerMask.GetMask("Wall")))
             {
 
                 Debug.DrawLine(child.position, child.position - sunDir, Color.red);
                 underSun = true;
 
             }
             else {
                 Debug.DrawLine(child.position, child.position - sunDir, Color.green);
             }
            
         }
 
        
 
         if (underSun)
         {
             mesh.material.color = Color.red;
         }
         else {
             mesh.material.color = Color.green;
         }
        
 
     }
 }