private void btnRemove_Click(object sender, EventArgs e)
        {
            if (lstCriteria.SelectedItem != null)
            {
                CriterionItem selCriterionItem = (CriterionItem)lstCriteria.SelectedValue;
                criteriaBuilder.Remove(selCriterionItem);

                //refresh
                ListBoxRefresh();
            }
        }
        private void btnAdd_Click(object sender, EventArgs e)
        {
            if (!string.IsNullOrEmpty(txtValue.Text))
            {
                CriterionItem cr = GetValidCriterionItem();
                if (cr != null)
                {
                    criteriaBuilder.AddCriterionItem(cr);
                    index++;
                }

                //refresh
                ListBoxRefresh();
            }
        }