Serializing to XML

Hey I trying to write a save game system, and my idea is to write a lot of data to an xml then loading from the xml for a load game. Anyway I'm having some problems writing an array of T2DAnimatedSprites to xml, I can get everything else to work except for the array. heres the code

public struct SaveStruct
        {
            public string playerFaction;
            public int playerCurrency;
            public ArrayList units;
        }

        public void Save(ArrayList saveUnits,string playerFaction,int playerCurrency,string saveName)
        {

            SaveStruct gameSave = new SaveStruct();
            gameSave.playerCurrency = playerCurrency;
            gameSave.playerFaction = playerFaction;
            gameSave.units = saveUnits;

            Type[] extraTypes = new Type[4];
            extraTypes[0] = typeof(String);
            extraTypes[1] = typeof(int);
            extraTypes[2] = typeof(ArrayList);
            extraTypes[3] = typeof(T2DAnimatedSprite);

            StorageDevice device = StorageDevice.ShowStorageDeviceGuide();

            //open storage container
            StorageContainer container = device.OpenContainer("RTS");

            //get the path of the save game
            string filename = Path.Combine(container.Path, saveName+".xml");


            //creates a new file
            FileStream stream = File.Create(filename);
            //convert to xml data and put into stream
            XmlSerializer serializer = new XmlSerializer(typeof(SaveStruct));
            serializer.Serialize(stream, gameSave);


            stream.Close();

            container.Dispose();
        }