Discord.Net/docs/faq/1-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

My client keeps returning 401 upon logging in!

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 tokens start with Mj*. If it starts with any other characters, chances are, you are using the client secret, which has nothing to do with the login token.

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 in order 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's 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.