linq怎样实现用户登录？

linq怎样实现用户登录，三层架构方式？我以前不是没用过linq，下面是我实现登录方法：
封装的方法：
 public static OracleDataReader GetExecuteReader(string sqlstring, params OracleParameter[] parames)
        {
            OracleCommand command = new OracleCommand(sqlstring, GetConnection());
            OracleDataReader datareader = null;
            if (parames != null)
            {
                foreach (OracleParameter parms in parames)
                {
                    command.Parameters.Add(parms);
                }
            }
            datareader = command.ExecuteReader();
            return datareader;
        }
DAL层：
public Model.UserInfo UserInfoIsLogin(string username,string userpwd,string typeno)
        {
            Model.UserInfo userinfomodel = null;
            OracleDataReader datareader = null;
                string sqlstr = "select * from view_userinfo where username=:username and userpwd=:userpwd";
                OracleParameter[] parames ={
                                      new OracleParameter("username",OracleType.VarChar,20),
                                      new OracleParameter("userpwd",OracleType.VarChar,20)
                                      };
                parames[0].Value = username;
                parames[1].Value = userpwd;
                datareader = OrclHelper.GetExecuteReader(sqlstr, parames);
                userinfomodel = new Model.UserInfo();
                if (datareader.HasRows)
                {
                    while (datareader.Read())
                    {
                        userinfomodel.Comment = datareader["res"].ToString();
                        userinfomodel.Typeno = datareader["typeno"].ToString();
                        userinfomodel.Typename = datareader["typename"].ToString();
                        userinfomodel.Username = datareader["username"].ToString();
                        userinfomodel.Userpwd = datareader["userpwd"].ToString();
                    }
                }
          return userinfomodel;
            }
Model.UserInfo是实体层中用户数据；
其它层就简单了，如果用linq应该怎样写？