example.cs

public abstract class A 
{
  public int Foo = 1;
}

public class B : A 
{
  public int Bar = 2;
}

public class C : A
{
  public int Baz = 3;
}

public class ToSerialize {
  public List<A> MyList;
  
  public ToSerialize() {
    MyList = new List<A>();
    MyList.Add(new B());
    MyList.Add(new C());
  }
}