public class UsersData    {        SqlConnection conn = null;        SqlCommand cmd = null;        public UsersData()        {            conn = new SqlConnection("server=.;database=Data0720;user=sa;pwd=123");            cmd = conn.CreateCommand();        }        /// <summary>        /// Users表添加方法        /// </summary>        /// <param name="u">要添加到数据库的Users对象</param>        public void Insert(Users u)        {            cmd.CommandText = "insert into Users values(@a,@b,@c,@d,@e,@f)";            cmd.Parameters.Clear();            cmd.Parameters.Add("@a", u.UserName);            cmd.Parameters.Add("@b", u.PassWord);            cmd.Parameters.Add("@c", u.NickName);            cmd.Parameters.Add("@d", u.Sex);            cmd.Parameters.Add("@e", u.Birthday);            cmd.Parameters.Add("@f", u.Nation);            conn.Open();            cmd.ExecuteNonQuery();            conn.Close();        }        public Users Select(string Uname)        {            Users u = null;            cmd.CommandText = "select *from Users where UserName=@a";            cmd.Parameters.Clear();            cmd.Parameters.Add("@a", Uname);            conn.Open();            SqlDataReader dr = cmd.ExecuteReader();            if (dr.HasRows)            {                dr.Read();                u = new Users();                u.UserName = dr["UserName"].ToString();                u.PassWord = dr["PassWord"].ToString();                u.NickName = dr["NickName"].ToString();                u.Sex = Convert.ToBoolean(dr["Sex"]);                u.Birthday = Convert.ToDateTime(dr["Birthday"]);                u.Nation = dr["Nation"].ToString();            }            conn.Close();            return u;        }        public bool Delete(string Uname)        {            bool delOK = false;            cmd.CommandText = "delete from Users where UserName =@a";            cmd.Parameters.Clear();            cmd.Parameters.Add("@a", Uname);            conn.Open();            try            {                cmd.ExecuteNonQuery();                delOK = true;            }            catch { }            conn.Close();            return delOK;        }        public List<Users> Select()        {            //泛型集合，放置所有Users数据对象            List<Users> list = new List<Users>();            cmd.CommandText = "select *from Users";            conn.Open();            SqlDataReader dr = cmd.ExecuteReader();            if (dr.HasRows)//如果数据表中有数据            {                while (dr.Read())//循环读取全部数据                {                    //没读取一行数据，就制作一个Users对象                    Users u = new Users();                    u.UserName = dr["UserName"].ToString();                    u.PassWord = dr["PassWord"].ToString();                    u.NickName = dr["NickName"].ToString();                    u.Sex = Convert.ToBoolean(dr["Sex"]);                    u.Birthday = Convert.ToDateTime(dr["Birthday"]);                    u.Nation = dr["Nation"].ToString();                    //注意，在循环中，将制作好的每一个对象，都放到集合中去                    list.Add(u);                }            }            conn.Close();            return list;        }        public void Select1()        {            cmd.CommandText = "select *from Users";            conn.Open();            SqlDataReader dr = cmd.ExecuteReader();            if (dr.HasRows)//如果数据表中有数据            {                while (dr.Read())//循环读取全部数据                {                    Console.WriteLine(dr["UserName"] + "  " + dr["PassWord"]);                }            }            conn.Close();        }    }}