to display error message abap webdynpro

Hi,

I need to display an error message when the field is not selected from the drop down list..when the default value - please select -- is there.. n not other value is selected...

I did some coding where in I am getting the message for the first time... when I press a button but when I press the button for the second time after selecting some value from the drop down box.. but when I am changing it again to -

please select----, its not giving me the error message....

any demo code will be very useful.

Thanks and Regards

Tenzin