[DataContract] 
    public abstract class Person
    {
        [DataMember]
        public int Code { get; set; }

        [DataMember]
        public string Name { get; set; }     
}

 [DataContract]  
    public class Student : Person
    {
        [DataMember]
        public int StudentId { get; set; }
    }

[DataContract]
    public class Teacher : Person
    {
        public int TeacherId { get; set; }
    }

[ServiceContract]
    public interface IStudentService
    {
        [OperationContract]
        IEnumerable<Person> GetAll();
    }

 public class StudentService : IStudentService
    {
        public IEnumerable<Person> GetAll()
        {
            List<Person> listOfPerson = new List<Person>();

            listOfPerson.Add( new Student() { Code = 1, StudentId = 123, Name = "Masoud Pakdel" } );
            listOfPerson.Add( new Student() { Code = 1, StudentId = 123, Name = "Mostafa Asgari"} );
            listOfPerson.Add( new Student() { Code = 1, StudentId = 123, Name = "Saeed Alizadeh"} );          

 
            listOfPerson.Add( new Teacher() { Code = 1, TeacherId = 321, Name = "Mahdi Rad"} );
            listOfPerson.Add( new Teacher() { Code = 1, TeacherId = 321, Name = "Mohammad Heydari" } );
            listOfPerson.Add( new Teacher() { Code = 1, TeacherId = 321, Name = "Saeed Khatami"} );

            return listOfPerson;
        }

 class Program
    {
        static void Main( string[] args )
        {
            StudentService.StudentServiceClient client = new StudentService.StudentServiceClient();

            client.GetAll().ToList().ForEach( _record => 
            {
                Console.Write( "Name : {0}", _record.Name );
                Console.WriteLine( "Code : {0}", _record.Code );
            } );

            Console.ReadLine();
        }
    }

   [DataContract]
    [KnownType( typeof( Student ) )]
    [KnownType( typeof(Teacher) )]
    public abstract class Person
    {
        [DataMember]
        public int Code { get; set; }

        [DataMember]
        public string Name { get; set; }
    }