public string AddNewFriendSetting(FriendSetting newFriend)
        {
            if (newFriend != null)
            {
                if (GetFriendSettingFromAgreeById(newFriend.GetId()) != null)
                {
                    return("Friend: " + newFriend.GetNote() + "[" + newFriend.GetId() + "] is not a new friend.");
                }
                else
                {
                    FriendSettings.Add(newFriend);
                    return(null);
                }
            }

            return("Input null.");
        }
        public string UpdateFriendSetting(FriendSetting newFriendSetting)
        {
            foreach (var friendSetting in FriendSettings)
            {
                if (friendSetting.GetStatus() == FriendSettingStatus.AGREE)
                {
                    if (friendSetting.GetId().Equals(newFriendSetting.GetId()))
                    {
                        friendSetting.SetNote(newFriendSetting.GetNote());
                        friendSetting.SetDisablePost(newFriendSetting.IsDisablePost());
                        return(null);
                    }
                }
            }

            return("Can't find the friend: " + newFriendSetting.GetNote() + "[" + newFriendSetting.GetId() + "]");
        }