根据第一个组合框中的选择填充第二个组合框

Question

我有2张桌子。 1个表-带[ID_house]，[House_names]和外键[num_Area]的“房子”。 2个表-带[ID_area]，[Area_names]的“ Area”。

我尝试从第一个组合框中使用SelectedValue，但从中获得空值。 可能是我在输入正确的ValueMember时犯了一个错误。

以下是两个带有ValueMembers的组合框的代码：

            //1st combobox
            string query1 = "SELECT House.num_Area, House_names FROM House";
            OleDbCommand command1 = new OleDbCommand(query1, connection);
            OleDbDataReader reader1 = command1.ExecuteReader();
            while (reader1.Read())
            {  
                cmbhouse.Items.Add(reader1["House_names"].ToString());
            }
            cmbhouse.ValueMember = "House.num_Area";
            reader1.Close();    

            //2nd combobox
            string query2 = "SELECT * FROM Area";
            OleDbCommand command2 = new OleDbCommand(query2, connection);
            OleDbDataReader reader2 = command2.ExecuteReader();
            while (reader2.Read())
            {
                cmbarea.Items.Add(reader2["Area_names"].ToString());
            }
            cmbregion.ValueMember = "ID_area";
            reader2.Close(); 

在这里，我尝试使用此代码填充第二个组合框：

            if (cmbhouse.SelectedValue.ToString()!="")
            {
              int house_numArea = Convert.ToInt32(cmbhouse.SelectedValue.ToString());
              string query2 = "SELECT * FROM Area, House WHERE Area.ID_area='" + house_numArea + "'";
              OleDbCommand command2 = new OleDbCommand(query2, connection);
              OleDbDataReader reader2 = command2.ExecuteReader();

              while (reader2.Read())
              {
                cmbarea.Items.Add(reader2["Area_names"].ToString());
              }
              reader2.Close();
            }

如果我在第一个组合框中选择任何房子，例如，我想在第二个组合框中仅显示该房子所在的区域。