Frequently Asked Questions

I recommend you read all of these as you will learn many interesting things. Or you can use Ctrl-F to search for a specific topic.

If you're C# beginner, you should learn about async programming.

2. My update handler fails or stops executing at some point

You likely have an exception somewhere. You should place a try..catch around your whole update handler.
Also, you should learn to use a debugger and go step-by-step through your code to understand where and why an exception is raised. See next question.

3. Apparently my update handler gets a NullReferenceException

Not all updates are about an incoming Message, so update.Message could be null. (see also update.Type)
Not all messages are text messages, message.Text could be null (see also message.Type). etc...
So please use a debugger to check the content of your variables or structure fields and make sure your code can handle all cases.

4. How to add buttons under a message?

Pass an InlineKeyboardMarkup into the replyMarkup parameter when sending the message. You will likely need to create a List<List<InlineKeyboardButton>> for rows&columns
See also next question.

5. How to handle a click on such inline buttons?

For buttons with callback data, your update handler should handle update.CallbackQuery. (Remember that not all updates are about update.Message. See question #3)

Your code should answer to the query within 10 seconds, using AnswerCallbackQueryAsync (or else the button gets momentarily disabled)

6. How to show a popup text to the user?

It is only possible with inline callback button (see above questions).
Use AnswerCallbackQueryAsync with some text, and pass parameter showAlert: true to display the text as an alert box instead of a short popup.

7. How to fill the input textbox of the user with some text?

You can't. The closest you can do is setup a ReplyKeyboardMarkup for buttons with pre-made texts under the textbox

8. How to fetch previous messages?

You can't with Bot API but it's possible with WTelegramBot.
Normally, bots only get messages at the moment they are posted. You could archive them all in a database for later retrieval.

9. How to fetch a list of all users in chat?

You can't with Bot API but it's possible with WTelegramBot.
Normally, bots can only get the list of administrators (GetChatAdministratorsAsync) or detail about one specific member (GetChatMemberAsync)
Alternatively, you can keep track of users by observing new messages in a chat and saving user info into a database.

10. How to send a private message to some random user?

You can't. Bots can only send private messages to users that have already initiated a private chat with your bot.

11. How to detect if a user blocked my bot?

You would have received an update.MyChatMember with NewChatMember.Status == ChatMemberStatus.Kicked
If you didn't record that info, you can try to SendChatActionAsync and see if it raises an exception.

12. How to set a caption to a media group (album)?

Set the media.Caption (and media.ParseMode) on the first media

13. How to write a bot that make questions/answers with users?

Either you can code a complex state machine workflow, saving where each user is currently in the discussion.
Or you can just use YourEasyBot which makes sequential bots very simple to write... (or one of the other frameworks available for Telegram.Bot)

14. How to make font effects in message?

Pass a ParseMode.Html (or ParseMode.MarkDownV2) to argument parseMode. See formatting options.
⚠️ I highly recommend you choose HTML formatting because MarkDownV2 has A LOT of annoyingly reserved characters and you will regret it later.

15. Where can I host my bot online for cheap/free?

I would recommend you make an ASP.NET webhook bot and host it on some WebApp hosting service.
For example, Azure WebApp Service has a F1 Free plan including 1 GB disk, 1 GB ram, 60 minutes of daily cumulated active CPU usage (more than enough for most bots without heavy use). And publishing to Azure is very easy from VS.
A credit-card is necessary but you shouldn't get charged if you stay within quotas.
Other cloud providers might also offer similar services.

16. Is there some limitation/maximum about feature X?

See https://limits.tginfo.me for a list of limitations.

17. How to populate the bot Menu button / commands list?

You can either do this via @BotFather (static entries), or you can use SetMyCommandsAsync for more advanced settings
⚠️ This can only be filled with bot commands, starting with a / and containing only latin characters a-z_0-9

18. How to receive ChatMember updates?

You should specify all update types including ChatMember in AllowedUpdates array on StartReceiving:ReceiverOptions or SetWebhookAsync

19. How to get rid of past updates when I restart my bot?

Pass true into StartReceiving:ReceiverOptions:DropPendingUpdates or SetWebhookAsync:dropPendingUpdates

20. Difficulties to upload & send a file/media?

  • Make sure you await until the end of the send method before closing the file (a "using" clause would close the file on leaving the current { scope }
  • If you just filled a MemoryStream, make sure to rewind it to the beginning with ms.Position = 0; before sending
  • If you send a media group, make sure you specify different filenames on InputFile.FromStream

21. How to fetch all medias from an album/media group ?

Medias in a media group are received as separate consecutive messages having the same MediaGroupId property. You should collect them progressively as you receive those messages.
There is no way to know how many medias are in the album, so:

  • look for consecutive messages in that chat with same MediaGroupId and stop when it's not the same
  • stop after 10 media in the group (maximum)
  • use a timeout of a few seconds not receiving new messages in that chat to determine the end

22. How to send a custom emoji❓

⚠️ It costs about ~$5,000 !! 😱

  • First you need to buy a reserved username on Fragment.
  • Then you need to pay an additional upgrade fee of 1K TON to apply that username to your bot.
  • Finally, your bot can now post custom emojis using specific HTML or Markdown syntax (or entity).

To post to a specific group, there is an alternative solution:

  • Have premium members boost your group to Level 4.
  • Then you can assign a custom emoji pack to your group that your members AND bots can use freely in group messages.

23. How to upgrade my existing code? You keep breaking compatibility!

A new lead developer (Wizou) is now in charge of the library and commits to reduce code-breaking changes in the future.
Version 21.x of the library have been much improved to facilitate migration from previous versions of the library, and include a lot of helpers/implicit operators to simplify your code.

24. Can I use several apps/instance to manage my bot?

You can call API methods (like sending messages) from several instances in parallel
BUT only one instance can call method GetUpdates (or else you will receive Telegram API Error 409: Conflict: terminated by other getUpdates request)

25. How do I get the user id from a username?

You can't with Bot API but it's possible with WTelegramBot.
Alternatively, you could store in database the mapping of UserId<->Username.
Remember that not every user has a username.

26. How to receive messages from channels?

Your bot has to be added as administrator of the channel. You will then receive the messages as update.ChannelPost or update.EditedChannelPost.

27. How to sent the same media multiple times

The first time, you will send the media with a stream (upload). Next times, you will use its FileId:

var sent = await bot.SendVideoAsync(chatId, stream, ....);
var fileId = sent.Video.FileId

// next times:
await bot.SendVideoAsync(chatId2, fileId, ...);

For photos, use sent.Photo[^1].FileId

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