How to drag an object does not pass the camera's range in Unity C#. Pls Help Me.

public GameObject Dragon; public Transform PotisionSpawmDragon; private Vector3 screenPoint; private Vector3 offset; public Transform PotisionCamera; // Use this for initialization void Start () { print(GetInstanceID()); } // Update is called once per frame void Update () {

 }
 void OnMouseDown()
 {
     // Spawm Dragon
     Invoke ("SpawmDragon",0);
     // Turn off Camera and Check Potision Mouse
     ManagerCamera.checkDragCamera = false;
     screenPoint = Camera.main.WorldToScreenPoint(gameObject.transform.position);
     offset = gameObject.transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
 }
 // Drag Home to potision new
 void OnMouseDrag()
 {
     ManagerCamera.checkDragCamera = false;
     Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
     Vector3 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
     transform.position = curPosition;
 }
 void SpawmDragon()
 {
     Instantiate (Dragon,PotisionSpawmDragon.position,PotisionSpawmDragon.rotation);
 }
 void OnMouseExit()
 {
     ManagerCamera.checkDragCamera = true;
 }