Primeros pasos DAO

 class Alumno
    {
        public int Id { get; set; }
        public string Nombre { get; set; }
        public string DNI { get; set; }
        public string Email { get; set; }

        public Alumno(int id, string nombre, string dni, string email)
        {
            Id = id;
            Nombre = nombre;
            DNI = dni;
            Email = email;
        }
        public override string ToString()
        {
            return Id+" - "+Nombre+" - "+DNI+" - "+Email;
        }
    }

 internal class DAOAlumno
    {
        public DAOAlumno() { }
        public List<Alumno> getAll()
        {
            var connection = new SqlConnection();
            connection.ConnectionString = "Data Source=.\\SQLEXPRESS;" +
                "Initial Catalog=pasiona;Integrated Security=True;" +
                "TrustServerCertificate=True";
            connection.Open();
            String sql = "select * from alumno";
            SqlCommand lectura = new SqlCommand(sql, connection);
            SqlDataReader sqlDataReader = lectura.ExecuteReader();
            List<Alumno> alumnos=new List<Alumno>();
            while (sqlDataReader.Read())
            {
                Alumno temp = new Alumno(sqlDataReader.GetInt32(0),
                    sqlDataReader.GetString(1),
                    sqlDataReader.GetString(2),
                    sqlDataReader.GetString(3));
                alumnos.Add(temp); 
            }

            sqlDataReader.Close();
            connection.Close();
            return alumnos;
        }
    }

  DAOAlumno dao=new DAOAlumno();
            List<Alumno> clase = dao.getAll();
            Console.WriteLine(String.Join(" | ",clase));