namespace LoginModel
{
    /// <summary>
    /// 实体类，用于保存用户信息
    /// </summary>
    public class UserModel
    {
        public int Id { get; set; }
        public string UserName { get; set; }
        public string UserPwd { get; set; }
    }
}

private void btnLogin_Click(object sender, EventArgs e)
{
    try
    {
        ////取出用户界面的数据
        string userName = txtUserName.Text.Trim();
        string userPwd = txtUserPwd.Text;
        UserBLL userBll = new UserBLL();//实例化一个业务逻辑层BLL
        UserModel userModel = userBll.UserLogin(userName, userPwd);//使用用户界面数据，进行查找BLL数据
        MessageBox.Show("登陆用户：" + userModel.UserName);
    }
    catch (Exception ex) //如果登陆有异常 则登陆失败
    {
        MessageBox.Show(ex.Message);
    }
}

namespace LoginBLL
{
    /// <summary>
    /// //业务逻辑层
    /// </summary>
    public class UserBLL
    {
        public UserModel UserLogin(string userName, string userPwd)
        {
            UserDAL userDal = new UserDAL();//实例化一个数据访问层
            UserModel user = userDal.SelectUser(userName, userPwd);////通过ui中填写的内容 返回来相应的数据
            if (user != null)
            {
                return user;//如果数据库中有数据，则返回一个实体类
            }
            else
            {
                throw new Exception("登陆失败");
            }
        }
    }
}

namespace LoginDAL
{
    public class UserDAL
    {
        ////根据 ui 选择返回一个user
        public UserModel SelectUser(string userName, string userPwd)
        {
            using (SqlConnection conn = new SqlConnection(DbUtil.connString))
            {
                //创建一个命令对象，并添加命令
                SqlCommand cmd = conn.CreateCommand();
                cmd.CommandText = @"select Id,UserName,UserPwd from T_UserInfo where UserName=@UserName and UserPwd=@UserPwd";
                cmd.CommandType = CommandType.Text;
                cmd.Parameters.AddWithValue("@UserName",userName);
                cmd.Parameters.AddWithValue("@UserPwd", userPwd);
                conn.Open();//打开数据链接
                SqlDataReader reader = cmd.ExecuteReader();
                UserModel user = null; //用于保存读取的数据
                while (reader.Read()) //开始读取数据
                {
                    if (user == null) //如果没有，则重新生成一个
                    {
                        user = new UserModel();
                    }
                    user.Id = reader.GetInt32(0);
                    user.UserName = reader.GetString(1);
                    user.UserPwd = reader.GetString(2);
                }
                return user;
            }
        }
    }
}

namespace LoginDAL
{
    /// <summary>
    /// //数据访问层，用于保存 链接服务器的sql语句
    /// </summary>
    class DbUtil
    {
        public static string connString = @"Server=xxx;Database=xx;User ID=sa; Password=123";
    }
}