Hello Mustafa,
So as per my implementation, when cascading filters are applied (button click) and if the filters combination are invalid , the text box will be blank. For a valid combination proper value will be returned.
So to address the usability concern, if there needs to be a warming message for the user when selecting members in the list boxes, we can address it by making a check during list box selection event whether the getdataasstring of Second Datasource(DS2) for the current filter combination returns a blank value and show a message to the user appropriately and reset the filters back.
Since am using 2 Datasources , I dont have the difficulty in handling this till now. Correct me if still the purpose is not solved.
Regards
Dinesh