public void ReadXml(XmlReader reader)
        {
            reader.MoveToContent();

            Source = reader["Source"];

            if (reader.IsEmptyElement) {
                reader.ReadStartElement();
                return;
            }

            reader.ReadStartElement();
            while (!reader.EOF) {
                if (reader.IsStartElement()) {
                    switch (reader.Name) {
                        case "Sprite":
                            XmlSpriteElement sprite = new XmlSpriteElement();
                            sprite.ReadXml(reader);
                            Sprites.Add(sprite);
                            break;

                        default:
                            reader.Skip();
                            break;
                    }
                }
                else if (reader.NodeType == XmlNodeType.EndElement) {
                    reader.ReadEndElement();
                    break;
                }
                else
                    reader.Read();
            }
        }
        public void WriteXml(XmlWriter writer)
        {
            writer.WriteAttributeString("Source", Source);

            foreach (ISpriteElement sprite in Sprites) {
                XmlSpriteElement elem = sprite as XmlSpriteElement;
                if (elem == null)
                    elem = new XmlSpriteElement(sprite);

                writer.WriteStartElement("Sprite");
                elem.WriteXml(writer);
                writer.WriteEndElement();
            }
        }