Discord.Net/docs/faq/basics/client-basics.md

• Client Basics Questions
• • My client keeps returning 401 upon logging in!
  • How do I do X, Y, Z when my bot connects/logs on? Why do I get a NullReferenceException upon calling any client methods after connect?
  • How do I get a message's previous content when that message is edited?

Client Basics Questions

In the following section, you will find commonly asked questions and
answers about common issues that you may face when utilizing the
various clients offered by the library.

My client keeps returning 401 upon logging in!

[!WARNING]
Userbot/selfbot (logging in with a user token) is no
longer supported with this library starting from 2.0, as
logging in under a user account may result in account termination.

For more information, see issue 827 & 958, as well as the official
Discord API Terms of Service.

There are few possible reasons why this may occur.

1. You are not using the appropriate TokenType. If you are using a
   bot account created from the Discord Developer portal, you should
   be using TokenType.Bot.
2. You are not using the correct login credentials. Please keep in
   mind that a token is different from a client secret.

How do I do X, Y, Z when my bot connects/logs on? Why do I get a NullReferenceException upon calling any client methods after connect?

Your bot should not attempt to interact in any way with
guilds/servers until the Ready event fires. When the bot
connects, it first has to download guild information from
Discord for you to get access to any server
information; the client is not ready at this point.

Technically, the GuildAvailable event fires once the data for a
particular guild has downloaded; however, it is best to wait for all
guilds to be downloaded. Once all downloads are complete, the Ready
event is triggered, then you can proceed to do whatever you like.

How do I get a message's previous content when that message is edited?

If you need to do anything with messages (e.g., checking Reactions,
checking the content of edited/deleted messages), you must set the
MessageCacheSize in your DiscordSocketConfig settings in order to
use the cached message entity. Read more about it here.

1. Message Cache must be enabled.
2. Hook the MessageUpdated event. This event provides a before and
   after object.
3. Only messages received after the bot comes online will be
   available in the cache.