DragObject.cs

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

public class DragObject : MonoBehaviour {

    private Vector3 mOffset;

    private float mZCoord;

    //private Vector3 currentPosition;

    void OnMouseDown()
    {
        mZCoord = Camera.main.WorldToScreenPoint(gameObject.transform.position).z;
        // Store offset = gameObject world pos - mouse world pos
        mOffset = gameObject.transform.position - GetMouseWorldPos();
       // currentPosition = gameObject.transform.position;
    }

    void OnMouseDrag()
    {
        transform.position = GetMouseWorldPos() + mOffset;
    }

    /*private void OnMouseUp()
    {
        transform.position = currentPosition;
    }*/

    private Vector3 GetMouseWorldPos()
    {
        //pixel coordinates (x,y)
        Vector3 mousePoint = Input.mousePosition;

        //z coordinate of game object on screen
        mousePoint.z = mZCoord;

        return Camera.main.ScreenToWorldPoint(mousePoint);

    }


}