How can I make also the Y (Height) to be random ?

Code (csharp):

1.  
2. using System;
3. using UnityEngine;
4. using Random = UnityEngine.Random;
5.  
6. [ExecuteInEditMode]
7. public class SpawnObjects : MonoBehaviour
8. {
9.     // for tracking properties change
10.     private Vector3 _extents;
11.     private int _objectCount;
12.     private float _objectSize;
13.  
14.     public GameObject objectToSpawn;
15.  
16.     /// <summary>
17.     ///     How far to place spheres randomly.
18.     /// </summary>
19.     public Vector3 Extents;
20.  
21.     /// <summary>
22.     ///     How many spheres wanted.
23.     /// </summary>
24.     public int ObjectCount;
25.  
26.     public float ObjectSize;
27.  
28.     private void OnValidate()
29.     {
30.         // prevent wrong values to be entered
31.         Extents = new Vector3(Mathf.Max(0.0f, Extents.x), Mathf.Max(0.0f, Extents.y), Mathf.Max(0.0f, Extents.z));
32.         ObjectCount = Mathf.Max(0, ObjectCount);
33.         ObjectSize = Mathf.Max(0.0f, ObjectSize);
34.     }
35.  
36.     private void Reset()
37.     {
38.         Extents = new Vector3(250.0f, 20.0f, 250.0f);
39.         ObjectCount = 100;
40.         ObjectSize = 20.0f;
41.     }
42.  
43.     private void Update()
44.     {
45.         UpdateSpheres();
46.     }
47.  
48.     private void UpdateSpheres()
49.     {
50.         if (Extents == _extents && ObjectCount == _objectCount && Mathf.Approximately(ObjectSize, _objectSize))
51.             return;
52.  
53.         // cleanup
54.         var spheres = GameObject.FindGameObjectsWithTag("SpawnObject");
55.         foreach (var t in spheres)
56.         {
57.             if (Application.isEditor)
58.             {
59.                 DestroyImmediate(t);
60.             }
61.             else
62.             {
63.                 Destroy(t);
64.             }
65.         }
66.  
67.         var withTag = GameObject.FindWithTag("Terrain");
68.         if (withTag == null)
69.             throw new InvalidOperationException("Terrain not found");
70.  
71.         for (var i = 0; i < ObjectCount; i++)
72.         {
73.             var o = Instantiate(objectToSpawn);
74.             o.tag = "SpawnObject";
75.             o.transform.localScale = new Vector3(ObjectSize, ObjectSize, ObjectSize);
76.  
77.             // get random position
78.             var x = Random.Range(-Extents.x, Extents.x);
79.             var y = Random.Range(-Extents.y, Extents.y); //Extents.y; // sphere altitude relative to terrain below
80.             var z = Random.Range(-Extents.z, Extents.z);
81.  
82.             // now send a ray down terrain to adjust Y according terrain below
83.             var height = 10000.0f; // should be higher than highest terrain altitude
84.             var origin = new Vector3(x, height, z);
85.             var ray = new Ray(origin, Vector3.down);
86.             RaycastHit hit;
87.             var maxDistance = 20000.0f;
88.             var nameToLayer = LayerMask.NameToLayer("Terrain");
89.             var layerMask = 1 << nameToLayer;
90.             if (Physics.Raycast(ray, out hit, maxDistance, layerMask))
91.             {
92.                 var distance = hit.distance;
93.                 y = height - distance + y; // adjust
94.             }
95.             else
96.             {
97.                 Debug.LogWarning("Terrain not hit, using default height !");
98.             }
99.  
100.             // place !
101.             o.transform.position = new Vector3(x, y, z);
102.         }
103.  
104.         _extents = Extents;
105.         _objectCount = ObjectCount;
106.         _objectSize = ObjectSize;
107.     }
108. }
109.  

I changed the line:

Code (csharp):

1.  
2. var y = Random.Range(-Extents.y, Extents.y); //Extents.y; // sphere altitude relative to terrain below
3.  

But what I'm getting is part of the objects above the terrain and some under the terrain:



And I want the random height to be only above the terrain.
For example if I change the Extents Y value to 0 all objects will be on terrain ground if I change it to 100 all objects will be random heights from 0(ground) to 100 range.

 

I'm not sure I understand your issue. If you want them all to be above, then you need to set your range to be only values available above. So if that is 0 to extents.y, then set your random to use 0 and extents.y.