telegram-bot/modules/UrlLimitations/MessageExtension.cs

using System;
using System.Linq;
using System.Threading.Tasks;
using Telegram.Bot;
using Telegram.Bot.Types;
using Telegram.Bot.Types.Enums;

namespace Kruzya.TelegramBot.UrlLimitations
{
    public static class MessageExtension
    {
        public static async Task<bool> HasUnknownMention(this Message message, ITelegramBotClient bot)
        {
            foreach (var mention in message.Entities.Where(e =>
                e.Type == MessageEntityType.Mention || e.Type == MessageEntityType.TextMention))
            {
                long userId = 0;
                if (mention.Type == MessageEntityType.TextMention)
                {
                    userId = mention.User.Id;
                }
                else
                {
                    try
                    {
                        var chat = await bot.GetChatAsync(
                            new ChatId(message.Text.Substring(mention.Offset, mention.Length)));

                        if (chat != null)
                        {
                            if (chat.Type != ChatType.Private)
                            {
                                return true;
                            }

                            userId = Convert.ToInt64(chat.Id);
                        }
                    }
                    catch (Exception)
                    {
                        return true;
                    }
                }

                var status = (await bot.GetChatMemberAsync(message.Chat, userId)).Status;

                if ((new ChatMemberStatus[]
                {
                    ChatMemberStatus.Administrator, ChatMemberStatus.Creator, ChatMemberStatus.Member,
                    ChatMemberStatus.Restricted

                    // i'm not sure about "Restricted"
                    // maybe leaved members with some restrictions also can be "Restricted".
                }).Any(e => e == status) == false)
                {
                    return true;
                }
            }

            return false;
        }
    }
}