Discord.Net/docs/guides/commands/samples/command_handler.cs

public class CommandHandler
{
    private readonly DiscordSocketClient _client;
    private readonly CommandService _commands;

    public CommandHandler(DiscordSocketClient client, CommandService commands)
    {
        _commands = commands;
        _client = client;
    }
    
    public async Task InstallCommandsAsync()
    {
        // Hook the MessageReceived event into our command handler
        _client.MessageReceived += HandleCommandAsync;

        // Here we discover all of the command modules in the entry 
        // assembly and load them. Starting from Discord.NET 2.0, a
        // service provider is required to be passed into the
        // module registration method to inject the 
        // required dependencies.
        //
        // If you do not use Dependency Injection, pass null.
        // See Dependency Injection guide for more information.
        await _commands.AddModulesAsync(assembly: Assembly.GetEntryAssembly(), 
                                        services: null);
    }

    private async Task HandleCommandAsync(SocketMessage messageParam)
    {
        // Don't process the command if it was a system message
        var message = messageParam as SocketUserMessage;
        if (message == null) return;

        // Create a number to track where the prefix ends and the command begins
        int argPos = 0;

        // Determine if the message is a command based on the prefix
        if (!(message.HasCharPrefix('!', ref argPos) || 
            message.HasMentionPrefix(_client.CurrentUser, ref argPos)))
            return;

        // Create a WebSocket-based command context based on the message
        var context = new SocketCommandContext(_client, message);

        // Execute the command with the command context we just
        // created, along with the service provider for precondition checks.
        
        // Keep in mind that result does not indicate a return value
        // rather an object stating if the command executed successfully.
        var result = await _command.ExecuteAsync(
            context: context, 
            argPos: argPos,
            services: null);

        // Optionally, we may inform the user if the command fails
        // to be executed; however, this may not always be desired,
        // as it may clog up the request queue should a user spam a
        // command.
        // if (!result.IsSuccess)
        // await context.Channel.SendMessageAsync(result.ErrorReason);
    }
}