how to use before-fill

There's a fairly substantial discussion of ProDataSets -- including this topic -- in its own book available through the documentation set that you can find online through PSDN, which you should certainly look at. You can associate a BEFORE-FILL event with an entire ProDataSet, so the event handler code could be used for prep work to do before you do the FILL -- attaching DATA-SOURCEs and the like. You could also associate this event handler with an individual table, similarly to do preparation work for filling the table. In an extreme case where there are no standard DATA-SOURCES at all, because the data is not coming out of a database, the BEFORE-FILL event handler can do all the work of bringing in the data from wherever it comes from.