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

using System;
using System.Threading.Tasks;
using System.Reflection;
using Discord;
using Discord.WebSocket;
using Discord.Commands;
using Microsoft.Extensions.DependencyInjection;

public class Program
{
    private CommandService _commands;
    private DiscordSocketClient _client;
    private IServiceProvider _services;

    private static void Main(string[] args) => new Program().StartAsync().GetAwaiter().GetResult();

    public async Task StartAsync()
    {
        _client = new DiscordSocketClient();
        _commands = new CommandService();

        // Avoid hard coding your token. Use an external source instead in your code.
        string token = "bot token here";

        _services = new ServiceCollection()
            .AddSingleton(_client)
            .AddSingleton(_commands)
            .BuildServiceProvider();

        await InstallCommandsAsync();

        await _client.LoginAsync(TokenType.Bot, token);
        await _client.StartAsync();

        await Task.Delay(-1);
    }

    public async Task InstallCommandsAsync()
    {
        // Hook the MessageReceived Event into our Command Handler
        _client.MessageReceived += HandleCommandAsync;
        // Discover all of the commands in this assembly and load them.
        await _commands.AddModulesAsync(Assembly.GetEntryAssembly(), _services);
    }

    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 if it starts with '!' or a mention prefix
        if (!(message.HasCharPrefix('!', ref argPos) || message.HasMentionPrefix(_client.CurrentUser, ref argPos))) return;
        // Create a Command Context
        var context = new SocketCommandContext(_client, message);
        // Execute the command. (result does not indicate a return value, 
        // rather an object stating if the command executed successfully)
        var result = await _commands.ExecuteAsync(context, argPos, _services);
        if (!result.IsSuccess)
            await context.Channel.SendMessageAsync(result.ErrorReason);
    }
}