OnSceneGUI mouse Dragging

I have a plane. And I wanna to use the mouse Drag Event to select all the vertices in center area at the Editor time.

some thing like this

I try to use OnSceneGUI to do this. but OnSceneGUI cannot receive a mouse drag event. Do anyone can help?

 private void OnSceneGUI()
 {
             var eventType = Event.current.type;
             if (eventType == EventType.DragUpdated || eventType == EventType.DragPerform)
             {
                 Debug.Log("Drag");
             }      
    }