https://github.com/discord-net/Discord.Net/blob/8b929690a9a89a9aa8a8b8e43ec0dd2e1ab685db/samples/TextCommandFramework/Services/CommandHandlingService.cs#L27 there are events within the DiscordSocketClient you can hook into for when a message is received/updated/deleted
GitHub
An unofficial .Net wrapper for the Discord API (https://discord.com/) - discord-net/Discord.Net