Extension on modified form field for D365FO

 class PriceDiscAdmTable_ExtHandler

{

      [FormDataFieldEventHandler(formDataFieldStr(PriceDiscAdm, PriceDiscAdmTrans, relation), FormDataFieldEventType::Modified)]

    public static void relation_OnModified(FormDataObject sender, FormDataFieldEventArgs e)

    {

        FormDataSource priceDiscAdmTrans_ds = sender.datasource();

        PriceDiscAdmTrans _priceDiscAdmTrans = priceDiscAdmTrans_ds.cursor();

        if( _priceDiscAdmTrans.relation != PriceType::LineDiscPurch && 

            _priceDiscAdmTrans.relation != PriceType::PricePurch &&

            _priceDiscAdmTrans.relation != PriceType::LineDiscSales &&

            _priceDiscAdmTrans.relation != PriceType::PriceSales

          )

        {

            error('Relation: Only price or line discount is valid selection!');

            throw exception::Error;

        }

    }

}