private readonly CommandService _service = new CommandService();

public void Setup()
    => _service.AddModule<CommandModule>();

// Imagine this being a message callback, whether it be from an IRC bot,
// a Discord bot, or any other chat based service.
private async Task MessageReceivedAsync(Message message)
{
    if (!CommandUtilities.HasPrefix(message.Content, '!', out string output))
        return;
        
    IResult result = await _service.ExecuteAsync(output, new CustomCommandContext(message));
    if (result is FailedResult failedResult)
        await message.Channel.SendMessageAsync(failedResult.Reason); 
}

public sealed class CustomCommandContext : CommandContext
{
    public Message Message { get; }
    
    public Channel Channel => Message.Channel;
  
    // Pass your service provider to the base command context.
    public CustomCommandContext(Message message, IServiceProvider provider = null) : base(provider)
    {
        Message = message;
    }
}

public sealed class CommandModule : ModuleBase<CustomCommandContext>
{
    // Dependency Injection via the constructor or public settable properties.
    // CommandService and IServiceProvider self-inject into modules,
    // properties and other types are requested from the provided IServiceProvider
    public CommandService Service { get; set; }

    // Invoked with:   !help
    // Responds with:  `help` - Lists available commands.
    //                 `sum` - Sums two given numbers.
    //                 `echo` - Echoes given text.
    [Command("help", "commands")]
    [Description("Lists available commands.")]
    public Task HelpAsync()
        => Context.Channel.SendMessageAsync(
            string.Join('\n', Service.GetAllCommands().Select(x => $"`{x.Name}` - {x.Description}")));

    // Invoked with:  !sum 3 5
    // Responds with: 3 + 5 = 8
    [Command("sum")]
    [Description("Sums two given numbers.")]
    public Task SumAsync(int firstNumber, int secondNumber)
      => Context.Channel.SendMessageAsync(
          $"{firstNumber} + {secondNumber} = {firstNumber + secondNumber}");

    // Invoked with:  !echo Hello, world.
    // Responds with: Hello, world.
    [Command("echo")]
    [Description("Echoes given text.")]
    public Task EchoAsync([Remainder] string text)
      => Context.Channel.SendMessageAsync(text);
}