创建级联下拉列表(WinForm)

我正在使用以下代码来填充我的组合框，现在using代码可以正常工作，并且我正在获得带有国家/地区项目的国家/地区组合框，但是如果我用comboBox1_SelectedIndexChanged编写using代码，那么using代码不起作用，为什么会这样？ 正因为如此，我没有根据所选国家/地区获得状态下拉列表，应该怎么做？

public partial class RegPatient : Form
    {
        DBHandling db = new DBHandling();
        string cmbvalue="";
        public RegPatient()
        {
            InitializeComponent();
             using (DataTable dt = DBHandling.GetCountryDataTable())
            {
                comboBox1.DataSource = new BindingSource(dt, null);
                comboBox1.DisplayMember = "CountryName"; //column to show in comboBox
                comboBox1.ValueMember = "Code"; 
            }//here the table is disposed 
        }
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {            
            //using code not working here          
        } 
    private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
    {
        using (DataTable dt = DBHandling.GetStateDataTable(comboBox1.Text) )
        {
            // contine using dt
            comboBox2.DataSource = new BindingSource(dt, null);
            comboBox2.DisplayMember = "ProvinceName"; 
           }//here the table is disposed 
    }   
    }

用于获取数据表国家/地区和州/地区的代码

public static DataTable GetCountryDataTable()
        {
            DataTable countryTable = new DataTable();
            using (OleDbConnection con = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=sonorepo.mdb")) //use your conn. string here
            {
                using (OleDbDataAdapter da = new OleDbDataAdapter(@"SELECT CountryName, Code FROM Country", con))
                    da.Fill(countryTable);
            }
            return countryTable;
        }
        public static DataTable GetStateDataTable(string countryCode)
        {
            DataTable stateTable = new DataTable();
            using (OleDbConnection con = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=sonorepo.mdb"))
            {
                using (OleDbDataAdapter da = new OleDbDataAdapter(@"SELECT ProvinceName FROM Province where Country='" + countryCode + "'", con))
                    da.Fill(stateTable);
            }
            return stateTable;
        }

提前致谢