pass datatable as ref

Question

I am facing a problem . I have a Parent Form . Where i am creating 20 Datatable that is using to bind the combobox for child form. So i want that i create a public function .I want to pass the datatable that is created on my Parent form i can use on child form as ref . So how can i do that . Present i am using this code but every time when child form every time its fire my parent page function. What i am doing wrong, What will be the best approach to do this.

I also take a reference form my previous Question and convert my dataset to LIST Type but the load time of form not decrease.

Parent Form

  public DataTable _tableZip (ref  DataTable zip)
    {
         if (_forderdac == null)
             _forderdac = new foOrderDAC();
             DataTable _tblZip = new DataTable();
            _tblZip = _forderdac.GetZipPostal(appDirectory, this.FindForm().Name, "frmCometFastOrderEntry").Tables[0];

        return _tblZip;
    }

Child form

            DataTable _tableZip = new DataTable();
            cmblookupPickZip.DataSource = _frmcometfast._tableZip(ref _tableZip);

            if (_tableZip.Rows.Count > 0)
            {
                cmblookupDropZip.DisplayMember = "ZIPPOSTAL";
                cmblookupPickZip.ValueMember = "AREAID";
                cmblookupPickZip.SelectedIndex = -1;
            }

thanks for your comments .