How to make the Dialog field mandatory in Axapta reports

Dialog field Mandatory

In the below , i have marked in yellow shows you , how to make the dialog field mandatory.

public Object dialog(Object _dialog)
{
    int                                  i;
    DialogRunbase              dialog = _dialog;
    DialogField                   checkField;
    FormDateControl          control;
    ;
    dialog.addGroup("@SYS119346");
    fieldConversionDate = dialog.addFieldValue(typeid(InventStdCostConvEndDate),  conversionDate);
    control = fieldConversionDate.fieldControl();
    control.mandatory(true);
    control.allowEdit(false);
      return dialog;
}