Serialization, why does this work?

[EEKstream]

I have an object, we can call it ClassA, that are to be serialized. It's containing an arraylist. It looks something like this

[Serializable()]
public class ClassA {
      private ArrayList list;
      public ArrayList List {
                // get/set
      }

      public override void GetObjectData(SerializationInfo info, StreamingContext ctxt)
        {
              info.AddValue("list", list);
        }
}

and also a constructor for the deserialization. The thing is that I store objects of type ClassB in my ArrayList. I've marked this class (ClassB) as Serializable but it does not implement ISerializable or has a GetObjectData method. Now the question is: Why is this working? How can the ArrayList populated with ClassB-objects be serialized when there is no serialize-method for ClassB? Because everything works fine and when I deserialize my object, the arraylist contains the same stuff as it did before the serialization.