Custom Command Handler

Important

Writing your own handler logic should only be done if you know what you're doing.
You will be responsible for command execution and preventing deadlocks.

Disable Default Handler

To begin, we'll need to disable the default command handler provided by CommandsNext.
This is done by setting the UseDefaultCommandHandler configuration property to false.

var discord = new DiscordClient();
var commands = discord.UseCommandsNext(new CommandsNextConfiguration()
{
    UseDefaultCommandHandler = false
});

Create Event Handler

We'll then write a new handler for the MessageCreated event fired from DiscordClient.

discord.MessageCreated += CommandHandler;

// ...

private Task CommandHandler(DiscordClient client, MessageCreateEventArgs e)
{
    // See below ...
}

This event handler will be our command handler, and you'll need to write the logic for it.

Handle Commands

Start by parsing the message content for a prefix and command string

var cnext = client.GetCommandsNext();
var msg = e.Message;

// Check if message has valid prefix.
var cmdStart = msg.GetStringPrefixLength("!");
if (cmdStart == -1) return;

// Retrieve prefix.
var prefix = msg.Content.Substring(0, cmdStart);

// Retrieve full command string.
var cmdString = msg.Content.Substring(cmdStart);

Then provide the command string to CommandsNextExtension#FindCommand

var command = cnext.FindCommand(cmdString, out var args);

Create a command context using our message and prefix, along with the command and its arguments

var ctx = cnext.CreateContext(msg, prefix, command, args);

And pass the context to CommandsNextExtension#ExecuteCommandAsync to execute the command.

_ = Task.Run(async () => await cnext.ExecuteCommandAsync(ctx));
// Wrapped in Task.Run() to prevent deadlocks.

Finished Product

Altogether, your implementation should function similarly to the following:

private Task CommandHandler(DiscordClient client, MessageCreateEventArgs e)
{
    var cnext = client.GetCommandsNext();
    var msg = e.Message;

    var cmdStart = msg.GetStringPrefixLength("!");
    if (cmdStart == -1) return Task.CompletedTask;

    var prefix = msg.Content.Substring(0, cmdStart);
    var cmdString = msg.Content.Substring(cmdStart);

    var command = cnext.FindCommand(cmdString, out var args);
    if (command == null) return Task.CompletedTask;

    var ctx = cnext.CreateContext(msg, prefix, command, args);
    Task.Run(async () => await cnext.ExecuteCommandAsync(ctx));

    return Task.CompletedTask;
}