protected override void OnFieldValueChanged(object originalValue, IEntityField2 field)

{

     base.OnFieldValueChanged(originalValue, field);

     switch ((SalesOrderFieldIndex)field.FieldIndex)

     {               

         case SalesOrderFieldIndex.CustomerNo:

              OnChangeCustomer((string)originalValue);

              break;

     }

}

public override void ValidateEntityBeforeSave(IEntityCore involvedEntity)

{

    base.ValidateEntityBeforeSave(involvedEntity);

    SalesOrderEntity salesOrder = (SalesOrderEntity)involvedEntity;

    if (string.IsNullOrEmpty(salesOrder.CustomerNo))

       throw new FieldValidationException("Customer No. is required");

}