using System;

public class Program

{

      public Form1()

        {

            InitializeComponent();

            comboBox1.DataSource =new string[] { "Ha Noi", "TP HCM" };

            string[] tpHCMList = { "Quan 1", "Quan 2", "Quan 3" };

​

        }

​

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)

        {

            string[] haNoiList = { "Hoan Kiem", "Long Bien", "Cau Giay" };

            string[] tpHCMList = { "Quan 1", "Quan 2", "Quan 3" };

            if (comboBox1.SelectedItem == "Ha Noi")

            {

                comboBox2.DataSource = haNoiList;

            }

            if (comboBox1.SelectedItem == "TP HCM")

            {

                comboBox2.DataSource = tpHCMList;

            }

        }

}