AppInfo.cs

using SteamKit2;
using System;
using System.Diagnostics;
using System.Threading.Tasks;
using System.Linq;

namespace BeatSaberNoUpdate {
    class AppInfo {
        // Beat Saber's app id
        public const uint APPID = 620980;
        public const uint DEPOT_ID = 620981;

        private static SteamClient steamClient;
        private static SteamApps steamApps;
        private static SteamUser steamUser;
        private static CallbackManager manager;

        private static bool isRunning;

        private static JobID infoRequest = JobID.Invalid;
        private static string manifestId = null;

        public async Task<string> TryRetrieve() {
            manifestId = null;

            // initialize a client and a callback manager for responding to events
            steamClient = new SteamClient();
            manager = new CallbackManager(steamClient);

            // get a handler and register to callbacks we are interested in
            steamUser = steamClient.GetHandler<SteamUser>();
            steamApps = steamClient.GetHandler<SteamApps>();
            manager.Subscribe<SteamApps.PICSProductInfoCallback>(OnProductInfo);
            manager.Subscribe<SteamClient.ConnectedCallback>(OnConnected);
            manager.Subscribe<SteamClient.DisconnectedCallback>(OnDisconnected);
            manager.Subscribe<SteamUser.LoggedOnCallback>(OnLoggedOn);
            manager.Subscribe<SteamUser.LoggedOffCallback>(OnLoggedOff);

            isRunning = true;

            await Task.Run(() => {
                Debug.WriteLine("Connecting to Steam...");

                steamClient.Connect();

                while(isRunning)
                    manager.RunWaitCallbacks(TimeSpan.FromSeconds(1));
            });

            return manifestId;
        }


        static void OnConnected(SteamClient.ConnectedCallback callback) {
            Debug.WriteLine("Successfully connected to Steam");
            // log on anonymously
            steamUser.LogOnAnonymous();
        }

        static void OnDisconnected(SteamClient.DisconnectedCallback callback) {
            Debug.WriteLine("Disconnected from Steam");
            isRunning = false;
        }

        static void OnLoggedOn(SteamUser.LoggedOnCallback callback) {
            if(callback.Result != EResult.OK) {
                if(callback.Result == EResult.AccountLogonDenied) {
                    // this shouldn't happen when logging in anonymously
                    Debug.WriteLine("Unable to login to Steam");
                    isRunning = false;
                    return;
                }
                // if there is another error
                Debug.WriteLine("Unable to login to Steam: {0} / {1}", callback.Result, callback.ExtendedResult);
                isRunning = false;
                return;
            }

            Debug.WriteLine("Successfully logged in to Steam");

            // request product info
            infoRequest = steamApps.PICSGetProductInfo(APPID, null);
        }

        private void OnProductInfo(SteamApps.PICSProductInfoCallback callback) {
            Debug.WriteLine("In OnProductInfo callback");
            if(callback.JobID != infoRequest)
                return;

            // not sure if this is game specific or applies to other games
            var _manifest = callback?.Apps[APPID]?.KeyValues.Children
                .FirstOrDefault(x => x.Name == "depots")?.Children
                .FirstOrDefault(x => x.Name == DEPOT_ID.ToString())?.Children
                .FirstOrDefault(x => x.Name == "manifests")?.Children
                .FirstOrDefault(x => x.Name == "public");

            var manifest = 
                _manifest?.Value ?? 
                _manifest?.Children.FirstOrDefault(x => x.Name == "gid")?.Value;

            if(manifest?.Length >= 16)
                manifestId = manifest;

            Debug.WriteLine(manifestId);
            // log off after getting the manifest
            steamUser.LogOff();
        }

        static void OnLoggedOff(SteamUser.LoggedOffCallback callback) {
            Debug.WriteLine("Logged off Steam: {0}", callback.Result);
            isRunning = false;
        }
    }
}