自动填充文本框的结果.文本从数据库，基于2个不同的组合框

namespace Training
{
public partial class AddingNewData : Form
{
    public AddingNewData()
    {
        InitializeComponent();
        fillcombo1();
        fillcombo2();
        autopopulatedays();
}
string original_city, destination_city;
void fillcombo1()
{
    string constring = "datasource=localhost;port=3306;username=root;password=root";
    string Query = "SELECT * FROM itemdelivery.fee GROUP BY orig_city;";
    MySqlConnection conDataBase = new MySqlConnection(constring);
    MySqlCommand cmdDataBase = new MySqlCommand(Query, conDataBase);
    MySqlDataReader myReader;
    try
    {
        conDataBase.Open();
        myReader = cmdDataBase.ExecuteReader();
        while (myReader.Read())
        {
            string storig = myReader.GetString("orig_city");
            comboBox1.Items.Add(storig);
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}
void fillcombo2()
{
    string constring = "datasource=localhost;port=3306;username=root;password=root";
    string Query = "SELECT * FROM itemdelivery.fee GROUP BY dest_city;";
    MySqlConnection conDataBase = new MySqlConnection(constring);
    MySqlCommand cmdDataBase = new MySqlCommand(Query, conDataBase);
    MySqlDataReader myReader;
    try
    {
        conDataBase.Open();
        myReader = cmdDataBase.ExecuteReader();
        while (myReader.Read())
        {
            string stdest = myReader.GetString("dest_city");
            comboBox2.Items.Add(stdest);
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    original_city = comboBox1.Text;
}
private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
    destination_city = comboBox2.Text;
}
private void txt_deliverytime_TextChanged(object sender, EventArgs e)
{
}
void autopopulatedays()
{
    string constring = "datasource=localhost;port=3306;username=root;password=root";
    string Query = "SELECT `del_time` FROM `itemdelivery.fee` WHERE `orig_city` = @oc AND `dest_city`= @dc";
    MySqlConnection conDataBase = new MySqlConnection(constring);
    MySqlCommand cmdDataBase = new MySqlCommand(Query, conDataBase);
    try
    {
        conDataBase.Open();
        cmdDataBase.Parameters.AddWithValue("@oc", original_city);
        cmdDataBase.Parameters.AddWithValue("@dc", destination_city);
        object result = cmdDataBase.ExecuteScalar();
        if (result != null)
            txt_deliverytime.Text = result.ToString();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
  }
}
}

所以，我有两个组合框，选择其中的两个后，我把original_city和destination_city的值作为string，使用comboBox1_SelectedIndexChanged和comboBox2_SelectedIndexChanged。

然后在方法autopopulatedays()上，我试图通过使用数据库上的original_city和destination_city的值来匹配单个整数值来自动填充txt_deliverytime.Text查询。

但是，为什么我失败了呢?我得到的唯一错误是"No database selected"，这对我来说很奇怪，当我选择2个组合框选项时txt_deliverytime.Text不会自动填充。

========================================================================

// UPDATE VERSION
// Connected to database, and ignoring the code before(above),
// This code only meant to auto-populate txt_deliverytime.Text when
// combobox1.selectitem and combobox2.selectitem
// Why it's still wrong?.

    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        original_city = comboBox1.SelectedItem.ToString();
    }
    private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
    {
        destination_city = comboBox2.SelectedItem.ToString();
    // if the combobox1 is not empty and combobox2 is also not empty, then run this code
        if (comboBox1.SelectedItem.ToString() != null)
        {
            string constring = "datasource=localhost;port=3306;username=root;password=root";
            string Query = "SELECT del_time FROM itemdelivery.fee WHERE orig_city='" + original_city + "' AND dest_city='" + destination_city + "';";
             // if I run this query on MySQL, it will show only a column name del_time with only a single row, 
             // thus only show a value, I want to get that value to txt_deliverytime.Text
             MySqlConnection conDataBase = new MySqlConnection(constring);
             MySqlCommand cmdDataBase = new MySqlCommand(Query, conDataBase);
            conDataBase.Open();
            string getValue = cmdDataBase.ExecuteScalar().ToString();
            if (getValue != null)
            {
                txt_deliverytime.Text = getValue.ToString();
        // meant to change it here, but seems not successful
            }
            conDataBase.Close();
        }
    }