public int CompareTo(object obj)
        {
            var category = (Category)obj;
            if (productName == category.productName)
            {
                if (messageType == category.messageType)
                    return messageTopic.CompareTo(category.messageTopic);
                return messageType.CompareTo(category.messageType);
            }
            return productName.CompareTo(category.productName);
        }