Serializing probs with abstr. node class

I am not able to copy objects derived from GraphNode if the GraphNode class is abstract, why is this?
I would like to keep this abstract behaviour, to represent a correct model.
What could I do to make this work? Just set the constructors of the abstract classes to private or protected and removing the abstract key word would not do it.

Could you describe exactly how you are trying to copy the objects and what the problem is?
GoObject and GoShape are both abstract classes, and clearly GoDocument.CopyFromCollection or AddCopy work. (They call GoObject.CopyObject.) So just declaring a base class “abstract” isn’t by itself going to cause any problems.
By the way, if you declare all constructors private, you won’t be able to define a class inheriting from that class.