Called by the framework when Excel opens the XLL. Programmer can return FALSE to halt further loading of the XLL.
[C++]
virtual BOOL OnXllOpenEx();
TRUE if the function completes successfully; FALSE otherwise.
This function is called whenever a user loads the add-in, 
            whether using the Excel Tools - Add-ins dialog or directly, 
            using the File - Open menu option. This is a good time to do any initialization that needs to precede 
            any use of your add-in functions, particularly password dialogs or data loading. 
            Return FALSE to indicate that initialization failed, TRUE otherwise. If you return FALSE, the add-in will not be loaded and will not be 
            used by Excel thereafter. The default implementation of this function does nothing.
Header: xllplus.h
CXllApp Class 
					  | CXllApp Methods			 
					  | CXllApp::OnXllOpen()			 
					  | CXllApp::OnXllClose()