Coordinates in-game is different from the unity editor

I've running into a problem where the coordinates inside the canvas while playing the game is different from the outside
(from inside, (0,0) is in the left-bottom corner, from outside, (0,0) is on the center)

I can't just put the left-bottom corner of the canvas on the center of the editor because I don't know exactly how big will be the screen that will play the game, and when I stretch the canvas by the sides, both extremes get stretched ( I think it would work if could stretch only the right side)

So, when I instantiate an object, I get the mouse coordinates in the canvas, which is different from the outside.

My Canvas Render Mode is set to "Screen Space - Camera"

here it is the code that instantiate the object

Code (CSharp):

1.  
2. using System.Collections;
3. using System.Collections.Generic;
4. using UnityEngine;
5.  
6. public class ClickStart : MonoBehaviour
7. {
8.     public GameObject prefab;
9.     int count = 0;
10.     // Update is called once per frame
11.     void Update()
12.     {
13.      
14.         // if mouse button down
15.         if ((Input.GetMouseButtonDown(0)) && (count == 0))
16.         {
17.             // get mouse position
18.             Vector3 mousePos = Input.mousePosition;
19.          
20.             //instantiates a new object at the mouse position
21.             GameObject p = Instantiate(prefab, mousePos, Quaternion.identity);
22.             //set parent to canvas          
23.             count = 1;
24.          
25.         }
26.  
27.     }
28. }
29.  

 

Realize that there are many different coordinate systems in play:

- screen
- canvas (and this has many possibilities based on Canvas and Canvas Scaler settings)
- world

Identify clearly what you are doing. It looks like your code above presumes screen space equals canvas space and that is most certainly not the case.

Also checkout the RectTransformUtility class for some extra helpers.