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

using System.Threading.Tasks;
using System.Reflection;
using Discord;
using Discord.WebSocket;
using Discord.Commands;

public class Program
{
    private CommandService commands;
    private DiscordSocketClient client;
    private DependencyMap map;

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

    public async Task Start()
    {
        client = new DiscordSocketClient();
        commands = new CommandService();

        string token = "bot token here";

        map = new DependencyMap();

        await InstallCommands();

        await client.LoginAsync(TokenType.Bot, token);
        await client.ConnectAsync();

        await Task.Delay(-1);
    }
    public async Task InstallCommands()
    {
        // Hook the MessageReceived Event into our Command Handler
        client.MessageReceived += HandleCommand;
		// Discover all of the commands in this assembly and load them.
        await commands.AddModulesAsync(Assembly.GetEntryAssembly());
    }
    public async Task HandleCommand(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 CommandContext(client, message);
        // Execute the command. (result does not indicate a return value, 
        // rather an object stating if the command executed succesfully)
        var result = await commands.ExecuteAsync(context, argPos, map);
        if (!result.IsSuccess)
            await context.Channel.SendMessageAsync(result.ErrorReason);
	}

}