I am pretty new in .Net and has a question about the update the field of parent table like when an invoice is issued, the onhand quantity of the product is updated. In VFP, the trigger (create, update, delete) is used to handling this. Since the BO is used in SF, where should I put the update procedure like this ? The trigger or BO ? The BO is better than trigger in providing the more informatic error message.

Other than the base methods for querying the database, SF does not provide any methods specific to RI support.  If you want to add a "Trigger" to a business object, you will be able to do that through the "Changed" events on the business object.  To add changed events to a business object, go to the BOMapper and configure the properties on a business object.  Make sure you can see the "Advanced Options" and select either single event for all properties or unique event for each property within the combo box for Changed Events.  Then, rebuild the partial class and you will be able to add triggers to the changed events of fields.