I have been able to define an External List through a WCF Service. This works well, but I am now moving onto more advanced scenarios - I would like my Create, Update and Delete methods to implement validations and business logic.
For example, when a list item is created, I would like to perform validations to ensure certain fields are unique across all rows in the table and enforce business rules such as "If cost field of list item > 1 million dollars and the current status of parent record in the DB is 'In Progress', prevent creation of current list item".
Can someone please provide some insight into how I can return error / business rule failure information to SharePoint? I tried throwing an exception, but the exception message is not displayed to the user.