Ported https://gist.github.com/seishun/6706470 into C#

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using SteamKit2;
using SteamKit2.Internal; // brings in our protobuf client messages


namespace DAE_WANT_STEAM_MACHINES
{
    class Program
    {
        static SteamClient steamClient;

        static SteamUser steamUser;

        static bool isRunning;

        static string user, pass;
        
        static SteamFriends steamFriends;


        static void Main(string[] args)
        {
            if (args.Length < 2)
            {
                Console.WriteLine("Enter Username:");
                user = Console.ReadLine();
                Console.WriteLine("Enter Password:");
                pass = Console.ReadLine();
            }
            else
            {
                // save our logon details
                user = args[0];
                pass = args[1];
            }


            // create our steamclient instance
            steamClient = new SteamClient();

            // get the steamuser handler, which is used for logging on after successfully connecting
            steamUser = steamClient.GetHandler<SteamUser>();

            // get the steam friends handler, which is used for interacting with friends on the network after logging on
            steamFriends = steamClient.GetHandler<SteamFriends>();

            isRunning = true;

            Console.WriteLine("Connecting to Steam...");

            // initiate the connection
            steamClient.Connect();

            // create our callback handling loop
            while (isRunning)
            {
                // wait for a callback to be posted
                var callback = steamClient.WaitForCallback(true);

                // handle the callback
                // the Handle function will only call the passed in handler
                // if the callback type matches the generic type
                callback.Handle<SteamClient.ConnectedCallback>(c =>
                {
                    if (c.Result != EResult.OK)
                    {
                        Console.WriteLine("Unable to connect to Steam: {0}", c.Result);

                        isRunning = false;
                        return;
                    }

                    Console.WriteLine("Connected to Steam! Logging in '{0}'...", user);

                    steamUser.LogOn(new SteamUser.LogOnDetails
                    {
                        Username = user,
                        Password = pass,
                    });
                });

                callback.Handle<SteamClient.DisconnectedCallback>(c =>
                {
                    Console.WriteLine("Disconnected from Steam");

                    isRunning = false;
                });

                callback.Handle<SteamUser.LoggedOnCallback>(c =>
                {
                    if (c.Result != EResult.OK)
                    {
                        if (c.Result == EResult.AccountLogonDenied)
                        {
                            // if we recieve AccountLogonDenied or one of it's flavors (AccountLogonDeniedNoMailSent, etc)
                            // then the account we're logging into is SteamGuard protected
                            // see sample 6 for how SteamGuard can be handled

                            Console.WriteLine("Unable to logon to Steam: This account is SteamGuard protected.");

                            isRunning = false;
                            return;
                        }

                        Console.WriteLine("Unable to logon to Steam: {0} / {1}", c.Result, c.ExtendedResult);

                        isRunning = false;
                        return;
                    }

                    Console.WriteLine("Successfully logged on!");

                    // at this point, we'd be able to perform actions on Steam
                    steamFriends.SetPersonaState(EPersonaState.Online);

                    var ui = new ClientMsgProtobuf<CMsgClientUIMode>(EMsg.ClientCurrentUIMode);

                    ui.Body.uimode.Equals(1);

                    // steamkit doesn't expose the "play game" message through any handler, so we'll just send the message manually
                    var playGame = new ClientMsgProtobuf<CMsgClientGamesPlayed>(EMsg.ClientGamesPlayed);

                    playGame.Body.games_played.Add(new CMsgClientGamesPlayed.GamePlayed
                    {
                        game_id = new GameID(570),
                        game_flags = 5120,
                    });

                    Console.WriteLine("Sending UI state");
                    // send it off
                    // notice here we're sending this message directly using the SteamClient
                    steamClient.Send(ui);

                    Console.WriteLine("Playing dota 2");
                    // send it off
                    // notice here we're sending this message directly using the SteamClient
                    steamClient.Send(playGame);

                    //steamUser.LogOff();
                });

                callback.Handle<SteamUser.LoggedOffCallback>(c =>
                {
                    Console.WriteLine("Logged off of Steam: {0}", c.Result);
                });
            }
        }
    }
}