DragAndDrop.cs

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(MeshCollider))]

public class DragAndDrop : MonoBehaviour
{
	
	private Vector3 screenPoint;
	private Vector3 offset;
	
	void OnMouseDown()
	{
		//if (Turret.moveable_) {
			screenPoint = Camera.main.WorldToScreenPoint (gameObject.transform.position);
		
			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);
		transform.position = curPosition;
		
	}
	void OnMouseRelease()

	{

	}
}