Notification function called by the ODA Drawings API for DGN immediately before an object is opened. 

This function is notified just before an object is to be opened; giving this function the ability to cancel the object being opened. 

Returns eOk if and only if open() is to continue. 

Overriding this function in a child class allows a child instance to be notified ach time an object is opened. 

When overriding this function: 

1) If the OdDgElement's state is incorrect, return something other than eOk. 

2) If the parent class's subOpen() returns anything other than eOk, immediately return it. 

3) If other actions are required before open, do them. 

4) Return eOk. 

The default implementation of this function does nothing but return eOk. This function can be overridden in custom classes.