How can I restrict/constrain my drag and drop of sprites to only be on the screen?

I am trying to keep the sprites I drag and drop to be only in the screen. I don´t want it to be possible to drag and drop outside of the screen view. Is it a good way to solve this so I can change the screen size without any problems later?

This is my code so far :

         void OnMouseDown ()
         {
 
                 screenPoint = Camera.main.WorldToScreenPoint (gameObject.transform.position);
 
         }
 
         void OnMouseDrag ()
         {
                         Vector3 currentScreenPoint = new Vector3 (Input.mousePosition.x, 60, screenPoint.z);
                         Vector3 currentPos = Camera.main.ScreenToWorldPoint (currentScreenPoint);
                         transform.position = currentPos;
 
 
         }

This is to use a drag and drop constraint to the X axis to make my own slider. Now, I need it to stop where the screen ends on both sides of the X axis.