Правильная запись в XMLfile

abstract class Employee
    {
        public string Firstname;
        public string SecondName;
        public int salary;
        public Employee(string Firstname, string SecondName, int salary)
        {
            this.Firstname = Firstname;
            this.SecondName = SecondName;
            this.salary = salary;
        }      
    }

class Seller : Employee
    {
        public Seller(string Firstname, string SecondName, int salary) : base(Firstname, SecondName, salary)
        {
            this.Firstname = Firstname;
            this.SecondName = SecondName;
            this.salary = salary;
        }
    }

[XmlInclude(typeof(Seller))]
public abstract class Employee
{
    public Employee()
    {
 
    }
    public string Firstname { get; set; }
    public string SecondName { get; set; }
    public int Salary { get; set; }
    public Employee(string firstname, string secondName, int salary)
    {
        Firstname = firstname;
        SecondName = secondName;
        Salary = salary;
    }
}

public class Seller : Employee
{
    public Seller()
    {
 
    }
    public Seller(string firstName, string secondName, int salary)
        : base(firstName, secondName, salary)
    {
        Firstname = firstName;
        SecondName = secondName;
        Salary = salary;
    }
}

var staff = new List<Employee>(new Employee[]{
    new Seller("John", "Smith",1000),
    new Seller("James", "Jameson",1000),
    new Seller("Robert", "Paddington",1000),
    });
using (var writer = new StreamWriter("staff.xml"))
{
    var xmlser = new XmlSerializer(staff.GetType());
    xmlser.Serialize(writer, staff);
}

[XmlInclude(typeof(Seller))]

[KnownType(typeof(Seller))]