Discord.Net/docs/faq/ClientBasics.md

# 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 might be 
	using the *client secret*, which has nothing to do with the login 
	token.

[TokenType]: xref:Discord.TokenType

## 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.

[Ready]: xref:Discord.WebSocket.DiscordSocketClient#Discord_WebSocket_DiscordSocketClient_Ready
[GuildAvailable]: xref:Discord.WebSocket.BaseSocketClient#Discord_WebSocket_BaseSocketClient_GuildAvailable

## 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](../guides/concepts/events.md#cacheable).
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.

[MessageCacheSize]: xref:Discord.WebSocket.DiscordSocketConfig#Discord_WebSocket_DiscordSocketConfig_MessageCacheSize
[DiscordSocketConfig]: xref:Discord.WebSocket.DiscordSocketConfig
[MessageUpdated]: xref:Discord.WebSocket.BaseSocketClient#Discord_WebSocket_BaseSocketClient_MessageUpdated