how to make a dragable object in unity2D

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(BoxCollider))]

public class Drag : MonoBehaviour {
    private Vector3 screenPoint;
    private Vector3 offset;

	void OnMouseDown() {

    	offset = gameObject.transform.position - 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;
      transform.position = curPosition;
	}
}