How to set value in a field according to another field in a grid of a form

if you want automatically to assign value to field_I  in a grid according to another field_II you can do it in the

modified method of field_II in your dataSource as follows:

public void modified()
{
    ;

    super();

    CategoryTable.CategoryName = Thy_DimensionsNivels::find(
                             CategoryTable.CategoryId).Description;
}