using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace 省市选择
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void cmd省_SelectedIndexChanged(object sender, EventArgs e)
        {
            using (SqlConnection conn = new SqlConnection(@"data source=.\sqlexpress;Attachdbfilename=|datadirectory|ss.mdf;Integrated security=true;User Instance=true;"))
            {
                conn.Open();
                using (SqlCommand cmd = conn.CreateCommand())
                {
                     = "select * from city where proid=@proid;";
                    cmd.Parameters.Add(new SqlParameter("proid", index));
                    using (SqlDataReader reader = cmd.ExecuteReader())
                    {
                        while (reader.Read())
                        {
                            cmd市.Items.Add(reader.GetString(reader.GetOrdinal("cityname")));
                        }                        
                    }
                }
            }
           
        }
        public int index = 0;
        private void Form1_Load(object sender, EventArgs e)
        {
            cmd市.Items.Clear();
            using (SqlConnection conn = new SqlConnection(@"data source=.\sqlexpress;Attachdbfilename=|datadirectory|ss.mdf;Integrated security=true;User Instance=true;"))
            {
                conn.Open();
                using (SqlCommand cmd = conn.CreateCommand())
                {
                     = "select * from promary";
                    using (SqlDataReader reader = cmd.ExecuteReader())
                    {
                        while (reader.Read())
                        {
                            cmd省.Items.Add(reader.GetString(reader.GetOrdinal("ProName")));
                            index=reader.GetInt32(reader.GetOrdinal("proid"));
                        }
                    }
                }
            }
        }
    }
}

也是一样没有变化的，但是，现在自己弄懂了，谢谢您！