public class TestShape : ShapeNode
{
...

public long Id {get; set;}

...

protected override void SaveTo(System.IO.BinaryWriter writer, PersistContext ctx)
{
base.SaveTo(writer, ctx);
writer.Write(Id.ToString());
}
protected override void LoadFrom(System.IO.BinaryReader reader, PersistContext ctx)
{
base.LoadFrom(reader, ctx);
Id = Convert.ToInt64(reader.ReadString());
}
}