How to drag gameobject (only x y) snapped to a grid?

Hi, this is the code that i use for the drag, could you show me how to implement your code?

I need this working on run time when i move the object

 using UnityEngine;
 using System.Collections;
  
 [RequireComponent(typeof(BoxCollider))]
 [RequireComponent(typeof(MeshRenderer))]
  
  
 public class MouseDrag : MonoBehaviour {
      
     
     Vector3 screenPoint;
     Vector3 offset;
     float yPos;
 
     
     void OnMouseDown() {
         Vector3 scanPos = gameObject.transform.position;
         screenPoint = Camera.main.WorldToScreenPoint(scanPos);
         //Debug.Log (Input.mousePosition.x);
         
         
         offset = scanPos - Camera.main.ScreenToWorldPoint(
         new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
     }
  
  
     void OnMouseDrag() {
         Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
         Vector3 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
         
         //Debug.Log(curPosition[0]);
         
             transform.position = curPosition;
     }
     
  
 }

In Unity, the X axis is generally left-to-right, the Y axis is down-to-up, and the Z axis is back-to-front.

If you're placing units on a flat world (like a strategy game map), you want the units to move on the X-Z plane, with a constant Y (probably Y=0). To do this, I recommend using Camera.ScreenPointToRay to see what point on the map the ray intersects. Then just round to whatever your scale is -- for example, if each square is 10 world units:

     x = ((int)(x/10))*10
     z = ((int)(z/10))*10
     y = 0

If you really want to place units on the X-Y plane, like they're on a wall (really the inside of a sphere) where all the points are equally far (Z distance) from the camera, then use Camera.ScreenToWorldPoint:

     worldPos = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, zDistanceFromCamera));

Hi thanks for the answer, im close to reach what i need with the code below:

 using UnityEngine;
 using System.Collections;
  
 [RequireComponent(typeof(BoxCollider))]
 [RequireComponent(typeof(MeshRenderer))]
  
  
 public class MouseDrag : MonoBehaviour {
      
     
     Vector3
             screenPoint,
             offset,
             scanPos,
             curPosition,
             curScreenPoint;
 
     
     float
             gridSize = 0.20f;
 
 
     
     void OnMouseDown() {
         scanPos = gameObject.transform.position;
         screenPoint = Camera.main.WorldToScreenPoint(scanPos);
         offset = scanPos - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
         
     }
     
 
  
  
     void OnMouseDrag() {
         curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
         curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
     
         
         curPosition.x = (float)(Mathf.Round(curPosition.x) * gridSize);
         
            transform.position = curPosition;
     }
 
  
 }

Now the object follow a grid snap, but the mouse cursor is too far away from the object when i make large move on the screen, have any idea in order to resolve this issue?

If the Game view isn't fullscreen, you can use ViewportToWorldPoint and clip the mouse input values. Or just clamp screenPoint.z to a maximum value. Hope that helps.

Could you write a code example?