public void linkAge(ComboBox source)
        {
            switch (source.AccessibleName)
            {
                case "type":
                    typeTableAdapter typeAda = new typeTableAdapter();
                    AssetDS.typeDataTable table = typeAda.GetData();());                    
                    source.DataSource = table;
                    source.DisplayMember = table.tnameColumn.ColumnName;
                    source.ValueMember = table.tidColumn.ColumnName;
                    break;
            }
        }
这个ComboBox里原来是有一个默认项的,但是绑定到DataTable后,原来的默认项就丢失了,请教如何解决?