How to implement classes which all use the same variables

class Form : Document { ... }

// Later on
Controls.Add(new DocumentDisplay(form));

class Form
{
    private Document Document { get; set; }
    // Other stuff
}

// Later on
Controls.Add (new DocumentDisplay(form.Document));

class Document
{
   List<Field> m_Fields;
   public void AddField(Field f) {... }
   public void AddFields(Field[] f) {... }
}

class DocumentFactory()
{
  public static Document GetDocument()
  {
     Document d = new Document();
     d.AddFields(GetDocumentFields());  	// private helper .. returns fields 1-8
     return d;
  }
  public static Document GetForm()
  {
     Document d = new Document();
     AddDocumentFields(d);
     d.AddField(FIELD_9);
     d.AddField(FIELD_10);
  }
  // and so on..
}