handle standard dropdown by key field dynamically.

Hi Experts ,

I'm enhancing dropdown by key field in web-dynpro from editable to read only state.

The default value is already set in init method.

but i am not able to handle it dynamically, purpose is that user can not change the drop down in create/edit mode.

Already tried below code but getting errors:

data: lo_inp type ref to cl_wd_DROPDOWN_BY_KEY.

lo_inp ?= view->get_element( 'FOLLOW_ON_DOC_KEY' ).

call method lo_inp->set_read_only( exporting  value =  abap_true ).

please help.