How do I use an HTTP/Socks proxy?

Look at the wiki page: Working Behind a Proxy.

I got a '409' error. What do I do?

You are trying to receive updates multiple times at the same time. Either you are calling GetUpdates from two instances of the bot, or you are calling GetUpdates while a web hook is already set. That is not supported by the API, only receive on one instance.

How do I get the user id from a username?

There is no way to do that with the API directly. You could store a list of known usernames, mapped to ids. This is not recommended, because usernames can be changed.

How do I get updates in channels?

If you are using polling, you will have to subscribe to the OnUpdate event. Check the UpdateType of the Update. If it is UpdateType.ChannelPost then the Update.ChannelPost property will be set.

I have serialization issues or null values in Update object in my webhook. What do I do?

If you're using ASP.NET Core 3.0+ you need to install additional Nuget package: Microsoft.AspNetCore.Mvc.NewtonsoftJson. For more information read this page about migrating from previous versions of ASP.NET Core.

Is there a way to get a list of users in a group or a channel?

There's no API to get all users in a chat, there's only getChatMember request to obtain a ChatMember object knowing it's user_id.

You can keep track of users observing new messages in a chat and saving user info into a database.

This FAQ doesn't have my question on it. Where can I get my torch and pitchfork?

Check the Bots FAQ by Telegram and if that doesn't pan out, feel free to let us know in the public group chat.