C#

DataContractSerializer xs = new DataContractSerializer(typeof(List<Customer>));
        DataContractSerializer xs2 = new DataContractSerializer(typeof(List<Book>));

Book-class:

    [KnownType(typeof(FaktaBok))]
    [KnownType(typeof(BarnBok))]
    [KnownType(typeof(SportBok))]
    [DataContractAttribute()]
    public class Book
    {
        [DataMember]
        protected int isbn = 0;

        [DataMember]
        protected string titel = null;

        [DataMember]
        protected string author;

        [DataMember]
        protected int price;

        [DataMember]
        protected String isType = null;

        [DataMember]
        protected Kund biblioteksKund = null;

One derived class:

[DataContractAttribute()]
     public class SportBok : Bok
     {
         [DataMember]
         private string sport;


Serialize: 

 using (Stream s = File.Create(path))
                {
                    xs.WriteObject(s, CustomerList);
                }

                using (Stream s2 = File.Create(path2))
                {
                    xs2.WriteObject(s2, BookList);
                }


DeSerialize:

 if (File.Exists(path))
 {
      using (Stream s = File.OpenRead(path))
       {
             CustomerList = (List<Kund>)xs.ReadObject(s);
       }
 }

if (File.Exists(path2))
{
      using (Stream s2 = File.OpenRead(path2))
      {
          BookList = (List<Bok>)xs2.ReadObject(s2);
      }
}