namespace WindowsFormsApplication1{    public partial class Login : Form    {        public string username;//定义一个变量给子窗体传值        public Login()        {            InitializeComponent();        }        private void button1_Click(object sender, EventArgs e)        {            //取出用户名和密码            string uid = txtuid.Text;            string pwd = txtpwd.Text;            if (uid != "" && pwd != "")            {                LoginDA da = new LoginDA(); //连接数据库                if (da.DengLu(uid, pwd)) //登录查询                {                    //将用户名赋值给成员变量username                    username = uid;                    //将该窗体的返回值变为OK                    this.DialogResult = DialogResult.OK;                    //将该窗体关闭                    this.Close();                }                else                {                    MessageBox.Show("用户名或密码错误！");                }            }            else            {                MessageBox.Show("用户名和密码不能为空！");            }        }        private void Login_Load(object sender, EventArgs e)        {        }    }}

namespace WindowsFormsApplication1{    static class Program    {        /// <summary>        /// 应用程序的主入口点。        /// </summary>        [STAThread]        static void Main()        {            Application.EnableVisualStyles();            Application.SetCompatibleTextRenderingDefault(false);            //造一个登录窗体            Login log = new Login();            if (log.ShowDialog() == DialogResult.OK)            {                Application.Run(new Main(log.username));//main窗体的构造方法            }        }    }}

namespace WindowsFormsApplication1{    public partial class Main : Form    {        private string uid;//造一个变量接收Login窗体的值        private ChinaDA cda;        public Main()        {            InitializeComponent();        }        public Main(string username)        {            InitializeComponent();            uid = username;            cda = new ChinaDA();//连接数据库        }        private void Main_Load(object sender, EventArgs e)        {            label1.Text = uid;//把接收到的值传给label1显示一下            //填充省的下拉列表            FillSheng();            //填充市的下拉列表            FillShi();            //填充区的下拉列表            FillQu();        }        private void cmbsheng_SelectedIndexChanged(object sender, EventArgs e)        {            //填充市的下拉列表            FillShi();            //填充区的下拉列表            FillQu();        }        private void cmbshi_SelectedIndexChanged(object sender, EventArgs e)        {            //填充区的下拉列表            FillQu();        }        //填充省的方法        public void FillSheng()        {            List<China> list = cda.Select("0001");            cmbsheng.DataSource = list;            cmbsheng.DisplayMember = "Name";            cmbsheng.ValueMember = "Code";        }        //填充市的方法        public void FillShi()        {            List<China> list = cda.Select(cmbsheng.SelectedValue.ToString());            cmbshi.DataSource = list;            cmbshi.DisplayMember = "Name";            cmbshi.ValueMember = "Code";        }        //填充区的方法        public void FillQu()        {            List<China> list = cda.Select(cmbshi.SelectedValue.ToString());            cmbqu.DataSource = list;            cmbqu.DisplayMember = "Name";            cmbqu.ValueMember = "Code";        }        private void button1_Click(object sender, EventArgs e)        {            MessageBox.Show(cmbsheng.SelectedValue.ToString());        }    }}