Con't use capplication object

Hi everyone.
I have developed an application.This application has a excel file list
control.When i double click in the excel file list control,the excel
application will run and open the corresponding excel file, and another
window which has another list control is opened. When I selected a cell in
the excel window and then double click in the list control,the selected list
control item value will be put into the selected cell.I use the following
code :

CRange cr=m_ExcelApplication.get_ActiveCell();
cr.put_Value2(COleVariant(value));

m_ExcelApplication is the excel application instance.

The question is :After I double click in excel work sheet and cursor is
appear in a cell,the "m_ExcelApplication" is con't be used,that is I con't
use "m_ExcelApplication" to call any CApplication class function.Why?

thanks

yes