Make RoomID non-bindable

Most important changes are to `RoomSubScreen` and `PlaylistsRoomSubScreen`, because those are the only two cases that now bind to the event instead.

osu.Game.Tests/NonVisual/Multiplayer/StatefulMultiplayerClientTest.cs

@@ -75,7 +75,7 @@ namespace osu.Game.Tests.NonVisual.Multiplayer
                 var newRoom = new Room();
                 newRoom.CopyFrom(SelectedRoom.Value);
 
-                newRoom.RoomID.Value = null;
+                newRoom.RoomID = null;
                 MultiplayerClient.RoomSetupAction = room =>
                 {
                     room.State = MultiplayerRoomState.Playing;

osu.Game.Tests/Visual/DailyChallenge/TestSceneDailyChallenge.cs

@@ -39,7 +39,7 @@ namespace osu.Game.Tests.Visual.DailyChallenge
         {
             var room = new Room
             {
-                RoomID = { Value = 1234 },
+                RoomID = 1234,
                 Name = { Value = "Daily Challenge: June 4, 2024" },
                 Playlist =
                 {
@@ -62,7 +62,7 @@ namespace osu.Game.Tests.Visual.DailyChallenge
         {
             var room = new Room
             {
-                RoomID = { Value = 1234 },
+                RoomID = 1234,
                 Name = { Value = "Daily Challenge: June 4, 2024" },
                 Playlist =
                 {
@@ -91,7 +91,7 @@ namespace osu.Game.Tests.Visual.DailyChallenge
         {
             var room = new Room
             {
-                RoomID = { Value = 1234 },
+                RoomID = 1234,
                 Name = { Value = "Daily Challenge: June 4, 2024" },
                 Playlist =
                 {

osu.Game.Tests/Visual/DailyChallenge/TestSceneDailyChallengeIntro.cs

@@ -68,7 +68,7 @@ namespace osu.Game.Tests.Visual.DailyChallenge
             {
                 API.Perform(new CreateRoomRequest(room = new Room
                 {
-                    RoomID = { Value = roomId },
+                    RoomID = roomId,
                     Name = { Value = "Daily Challenge: June 4, 2024" },
                     Playlist =
                     {

osu.Game.Tests/Visual/DailyChallenge/TestSceneDailyChallengeLeaderboard.cs

@@ -43,7 +43,7 @@ namespace osu.Game.Tests.Visual.DailyChallenge
                     return false;
                 };
             });
-            AddStep("create leaderboard", () => Child = leaderboard = new DailyChallengeLeaderboard(new Room { RoomID = { Value = 1 } }, new PlaylistItem(Beatmap.Value.BeatmapInfo))
+            AddStep("create leaderboard", () => Child = leaderboard = new DailyChallengeLeaderboard(new Room { RoomID = 1 }, new PlaylistItem(Beatmap.Value.BeatmapInfo))
             {
                 RelativeSizeAxes = Axes.Both,
                 Anchor = Anchor.Centre,
@@ -86,7 +86,7 @@ namespace osu.Game.Tests.Visual.DailyChallenge
                     return false;
                 };
             });
-            AddStep("create leaderboard", () => Child = leaderboard = new DailyChallengeLeaderboard(new Room { RoomID = { Value = 1 } }, new PlaylistItem(Beatmap.Value.BeatmapInfo))
+            AddStep("create leaderboard", () => Child = leaderboard = new DailyChallengeLeaderboard(new Room { RoomID = 1 }, new PlaylistItem(Beatmap.Value.BeatmapInfo))
             {
                 RelativeSizeAxes = Axes.Both,
                 Anchor = Anchor.Centre,

osu.Game.Tests/Visual/Menus/TestSceneMainMenu.cs

@@ -42,7 +42,7 @@ namespace osu.Game.Tests.Visual.Menus
                         beatmap.OnlineID = 1001;
                         getRoomRequest.TriggerSuccess(new Room
                         {
-                            RoomID = { Value = 1234 },
+                            RoomID = 1234,
                             Name = { Value = "Aug 8, 2024" },
                             Playlist =
                             {

osu.Game.Tests/Visual/Multiplayer/TestSceneMatchLeaderboard.cs

@@ -61,7 +61,7 @@ namespace osu.Game.Tests.Visual.Multiplayer
 
             AddStep("create leaderboard", () =>
             {
-                SelectedRoom.Value = new Room { RoomID = { Value = 3 } };
+                SelectedRoom.Value = new Room { RoomID = 3 };
 
                 Child = new MatchLeaderboard
                 {

osu.Game.Tests/Visual/Playlists/TestScenePlaylistsParticipantsList.cs

@@ -19,7 +19,7 @@ namespace osu.Game.Tests.Visual.Playlists
 
             AddStep("create list", () =>
             {
-                SelectedRoom.Value = new Room { RoomID = { Value = 7 } };
+                SelectedRoom.Value = new Room { RoomID = 7 };
 
                 for (int i = 0; i < 50; i++)
                 {

osu.Game.Tests/Visual/UserInterface/TestSceneMainMenuButton.cs

@@ -53,7 +53,7 @@ namespace osu.Game.Tests.Visual.UserInterface
                         beatmap.OnlineID = 1001;
                         getRoomRequest.TriggerSuccess(new Room
                         {
-                            RoomID = { Value = 1234 },
+                            RoomID = 1234,
                             Playlist =
                             {
                                 new PlaylistItem(beatmap)
@@ -131,7 +131,7 @@ namespace osu.Game.Tests.Visual.UserInterface
                         beatmap.OnlineID = 1001;
                         getRoomRequest.TriggerSuccess(new Room
                         {
-                            RoomID = { Value = 1234 },
+                            RoomID = 1234,
                             Playlist =
                             {
                                 new PlaylistItem(beatmap)

osu.Game/Online/Multiplayer/MultiplayerClient.cs

@@ -181,10 +181,10 @@ namespace osu.Game.Online.Multiplayer
 
             await joinOrLeaveTaskChain.Add(async () =>
             {
-                Debug.Assert(room.RoomID.Value != null);
+                Debug.Assert(room.RoomID != null);
 
                 // Join the server-side room.
-                var joinedRoom = await JoinRoom(room.RoomID.Value.Value, password ?? room.Password.Value).ConfigureAwait(false);
+                var joinedRoom = await JoinRoom(room.RoomID.Value, password ?? room.Password.Value).ConfigureAwait(false);
                 Debug.Assert(joinedRoom != null);
 
                 // Populate users.

osu.Game/Online/Rooms/JoinRoomRequest.cs

@@ -27,6 +27,6 @@ namespace osu.Game.Online.Rooms
             return req;
         }
 
-        protected override string Target => $@"rooms/{Room.RoomID.Value}/users/{User!.Id}";
+        protected override string Target => $@"rooms/{Room.RoomID}/users/{User!.Id}";
     }
 }

osu.Game/Online/Rooms/PartRoomRequest.cs

@@ -23,6 +23,6 @@ namespace osu.Game.Online.Rooms
             return req;
         }
 
-        protected override string Target => $"rooms/{room.RoomID.Value}/users/{User!.Id}";
+        protected override string Target => $"rooms/{room.RoomID}/users/{User!.Id}";
     }
 }

osu.Game/Online/Rooms/Room.cs

@@ -21,6 +21,15 @@ namespace osu.Game.Online.Rooms
     {
         public event PropertyChangedEventHandler? PropertyChanged;
 
+        /// <summary>
+        /// The online room ID. Will be <c>null</c> while the room has not yet been created.
+        /// </summary>
+        public long? RoomID
+        {
+            get => roomId;
+            set => SetField(ref roomId, value);
+        }
+
         /// <summary>
         /// Represents the current item selected within the room.
         /// </summary>
@@ -33,13 +42,12 @@ namespace osu.Game.Online.Rooms
             set => SetField(ref currentPlaylistItem, value);
         }
 
+        [JsonProperty("id")]
+        private long? roomId;
+
         [JsonProperty("current_playlist_item")]
         private PlaylistItem? currentPlaylistItem;
 
-        [Cached]
-        [JsonProperty("id")]
-        public readonly Bindable<long?> RoomID = new Bindable<long?>();
-
         [Cached]
         [JsonProperty("name")]
         public readonly Bindable<string> Name = new Bindable<string>();
@@ -196,7 +204,7 @@ namespace osu.Game.Online.Rooms
         /// <param name="other">The <see cref="Room"/> to copy values from.</param>
         public void CopyFrom(Room other)
         {
-            RoomID.Value = other.RoomID.Value;
+            RoomID = other.RoomID;
             Name.Value = other.Name.Value;
 
             Category.Value = other.Category.Value;

osu.Game/Screens/Menu/DailyChallengeButton.cs

@@ -155,9 +155,9 @@ namespace osu.Game.Screens.Menu
                     Room = room;
                     cover.OnlineInfo = TooltipContent = room.Playlist.FirstOrDefault()?.Beatmap.BeatmapSet as APIBeatmapSet;
 
-                    if (room.StartDate.Value != null && room.RoomID.Value != lastDailyChallengeRoomID)
+                    if (room.StartDate.Value != null && room.RoomID != lastDailyChallengeRoomID)
                     {
-                        lastDailyChallengeRoomID = room.RoomID.Value;
+                        lastDailyChallengeRoomID = room.RoomID;
 
                         // new challenge is live, reset intro played static.
                         statics.SetValue(Static.DailyChallengeIntroPlayed, false);

osu.Game/Screens/OnlinePlay/Components/RoomManager.cs

@@ -1,13 +1,10 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
-using JetBrains.Annotations;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Development;
@@ -20,18 +17,17 @@ namespace osu.Game.Screens.OnlinePlay.Components
 {
     public partial class RoomManager : Component, IRoomManager
     {
-        [CanBeNull]
+        public event Action? RoomsUpdated;
-        public event Action RoomsUpdated;
 
         private readonly BindableList<Room> rooms = new BindableList<Room>();
 
         public IBindableList<Room> Rooms => rooms;
 
-        protected IBindable<Room> JoinedRoom => joinedRoom;
+        protected IBindable<Room?> JoinedRoom => joinedRoom;
-        private readonly Bindable<Room> joinedRoom = new Bindable<Room>();
+        private readonly Bindable<Room?> joinedRoom = new Bindable<Room?>();
 
         [Resolved]
-        private IAPIProvider api { get; set; }
+        private IAPIProvider api { get; set; } = null!;
 
         public RoomManager()
         {
@@ -44,7 +40,7 @@ namespace osu.Game.Screens.OnlinePlay.Components
             PartRoom();
         }
 
-        public virtual void CreateRoom(Room room, Action<Room> onSuccess = null, Action<string> onError = null)
+        public virtual void CreateRoom(Room room, Action<Room>? onSuccess = null, Action<string>? onError = null)
         {
             room.Host.Value = api.LocalUser.Value;
 
@@ -69,9 +65,9 @@ namespace osu.Game.Screens.OnlinePlay.Components
             api.Queue(req);
         }
 
-        private JoinRoomRequest currentJoinRoomRequest;
+        private JoinRoomRequest? currentJoinRoomRequest;
 
-        public virtual void JoinRoom(Room room, string password = null, Action<Room> onSuccess = null, Action<string> onError = null)
+        public virtual void JoinRoom(Room room, string? password = null, Action<Room>? onSuccess = null, Action<string>? onError = null)
         {
             currentJoinRoomRequest?.Cancel();
             currentJoinRoomRequest = new JoinRoomRequest(room, password);
@@ -97,7 +93,7 @@ namespace osu.Game.Screens.OnlinePlay.Components
         {
             currentJoinRoomRequest?.Cancel();
 
-            if (JoinedRoom.Value == null)
+            if (joinedRoom.Value == null)
                 return;
 
             if (api.State.Value == APIState.Online)
@@ -111,14 +107,14 @@ namespace osu.Game.Screens.OnlinePlay.Components
         public void AddOrUpdateRoom(Room room)
         {
             Debug.Assert(ThreadSafety.IsUpdateThread);
-            Debug.Assert(room.RoomID.Value != null);
+            Debug.Assert(room.RoomID != null);
 
-            if (ignoredRooms.Contains(room.RoomID.Value.Value))
+            if (ignoredRooms.Contains(room.RoomID.Value))
                 return;
 
             try
             {
-                var existing = rooms.FirstOrDefault(e => e.RoomID.Value == room.RoomID.Value);
+                var existing = rooms.FirstOrDefault(e => e.RoomID == room.RoomID);
                 if (existing == null)
                     rooms.Add(room);
                 else
@@ -128,7 +124,7 @@ namespace osu.Game.Screens.OnlinePlay.Components
             {
                 Logger.Error(ex, $"Failed to update room: {room.Name.Value}.");
 
-                ignoredRooms.Add(room.RoomID.Value.Value);
+                ignoredRooms.Add(room.RoomID.Value);
                 rooms.Remove(room);
             }
 

osu.Game/Screens/OnlinePlay/Components/SelectionPollingComponent.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System.Threading.Tasks;
 using osu.Game.Online.Rooms;
 
@@ -20,21 +18,21 @@ namespace osu.Game.Screens.OnlinePlay.Components
             this.room = room;
         }
 
-        private GetRoomRequest lastPollRequest;
+        private GetRoomRequest? lastPollRequest;
 
         protected override Task Poll()
         {
             if (!API.IsLoggedIn)
                 return base.Poll();
 
-            if (room.RoomID.Value == null)
+            if (room.RoomID == null)
                 return base.Poll();
 
             var tcs = new TaskCompletionSource<bool>();
 
             lastPollRequest?.Cancel();
 
-            var req = new GetRoomRequest(room.RoomID.Value.Value);
+            var req = new GetRoomRequest(room.RoomID.Value);
 
             req.Success += result =>
             {

osu.Game/Screens/OnlinePlay/DailyChallenge/DailyChallenge.cs

@@ -353,12 +353,12 @@ namespace osu.Game.Screens.OnlinePlay.DailyChallenge
         private void presentScore(long id)
         {
             if (this.IsCurrentScreen())
-                this.Push(new PlaylistItemScoreResultsScreen(room.RoomID.Value!.Value, playlistItem, id));
+                this.Push(new PlaylistItemScoreResultsScreen(room.RoomID!.Value, playlistItem, id));
         }
 
         private void onRoomScoreSet(MultiplayerRoomScoreSetEvent e)
         {
-            if (e.RoomID != room.RoomID.Value || e.PlaylistItemID != playlistItem.ID)
+            if (e.RoomID != room.RoomID || e.PlaylistItemID != playlistItem.ID)
                 return;
 
             userLookupCache.GetUserAsync(e.UserID).ContinueWith(t =>
@@ -410,7 +410,7 @@ namespace osu.Game.Screens.OnlinePlay.DailyChallenge
 
         private void dailyChallengeChanged(ValueChangedEvent<DailyChallengeInfo?> change)
         {
-            if (change.OldValue?.RoomID == room.RoomID.Value && change.NewValue == null && metadataClient.IsConnected.Value)
+            if (change.OldValue?.RoomID == room.RoomID && change.NewValue == null && metadataClient.IsConnected.Value)
             {
                 notificationOverlay?.Post(new SimpleNotification { Text = DailyChallengeStrings.ChallengeEndedNotification });
             }
@@ -437,7 +437,7 @@ namespace osu.Game.Screens.OnlinePlay.DailyChallenge
             roomManager.JoinRoom(room);
             startLoopingTrack(this, musicController);
 
-            metadataClient.BeginWatchingMultiplayerRoom(room.RoomID.Value!.Value).ContinueWith(t =>
+            metadataClient.BeginWatchingMultiplayerRoom(room.RoomID!.Value).ContinueWith(t =>
             {
                 if (t.Exception != null)
                 {
@@ -489,7 +489,7 @@ namespace osu.Game.Screens.OnlinePlay.DailyChallenge
             this.Delay(WaveContainer.DISAPPEAR_DURATION).FadeOut();
 
             roomManager.PartRoom();
-            metadataClient.EndWatchingMultiplayerRoom(room.RoomID.Value!.Value).FireAndForget();
+            metadataClient.EndWatchingMultiplayerRoom(room.RoomID!.Value).FireAndForget();
 
             return base.OnExiting(e);
         }

osu.Game/Screens/OnlinePlay/DailyChallenge/DailyChallengeLeaderboard.cs

@@ -138,7 +138,7 @@ namespace osu.Game.Screens.OnlinePlay.DailyChallenge
             if (request?.CompletionState == APIRequestCompletionState.Waiting)
                 return;
 
-            request = new IndexPlaylistScoresRequest(room.RoomID.Value!.Value, playlistItem.ID);
+            request = new IndexPlaylistScoresRequest(room.RoomID!.Value, playlistItem.ID);
 
             request.Success += req => Schedule(() =>
             {

osu.Game/Screens/OnlinePlay/Lounge/Components/RoomsContainer.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
@@ -11,6 +9,7 @@ using System.Linq;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Extensions.IEnumerableExtensions;
+using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Input.Bindings;
@@ -24,8 +23,8 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
 {
     public partial class RoomsContainer : CompositeDrawable, IKeyBindingHandler<GlobalAction>
     {
-        public readonly Bindable<Room> SelectedRoom = new Bindable<Room>();
+        public readonly Bindable<Room?> SelectedRoom = new Bindable<Room?>();
-        public readonly Bindable<FilterCriteria> Filter = new Bindable<FilterCriteria>();
+        public readonly Bindable<FilterCriteria?> Filter = new Bindable<FilterCriteria?>();
 
         public IReadOnlyList<DrawableRoom> Rooms => roomFlow.FlowingChildren.Cast<DrawableRoom>().ToArray();
 
@@ -33,7 +32,7 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
         private readonly FillFlowContainer<DrawableLoungeRoom> roomFlow;
 
         [Resolved]
-        private IRoomManager roomManager { get; set; }
+        private IRoomManager roomManager { get; set; } = null!;
 
         // handle deselection
         public override bool ReceivePositionalInputAt(Vector2 screenSpacePos) => true;
@@ -67,10 +66,10 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
 
             rooms.BindTo(roomManager.Rooms);
 
-            Filter?.BindValueChanged(criteria => applyFilterCriteria(criteria.NewValue), true);
+            Filter.BindValueChanged(criteria => applyFilterCriteria(criteria.NewValue), true);
         }
 
-        private void applyFilterCriteria(FilterCriteria criteria)
+        private void applyFilterCriteria(FilterCriteria? criteria)
         {
             roomFlow.Children.ForEach(r =>
             {
@@ -113,7 +112,7 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
             }
         }
 
-        private void roomsChanged(object sender, NotifyCollectionChangedEventArgs args)
+        private void roomsChanged(object? sender, NotifyCollectionChangedEventArgs args)
         {
             switch (args.Action)
             {
@@ -142,7 +141,7 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
             foreach (var room in rooms)
                 roomFlow.Add(new DrawableLoungeRoom(room) { SelectedRoom = SelectedRoom });
 
-            applyFilterCriteria(Filter?.Value);
+            applyFilterCriteria(Filter.Value);
         }
 
         private void removeRooms(IEnumerable<Room> rooms)
@@ -173,7 +172,7 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
                 roomFlow.SetLayoutPosition(room, room.Room.Category.Value > RoomCategory.Normal
                     // Always show spotlight playlists at the top of the listing.
                     ? float.MinValue
-                    : -(room.Room.RoomID.Value ?? 0));
+                    : -(room.Room.RoomID ?? 0));
             }
         }
 
@@ -213,7 +212,7 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
 
             var visibleRooms = Rooms.AsEnumerable().Where(r => r.IsPresent);
 
-            Room room;
+            Room? room;
 
             if (SelectedRoom.Value == null)
                 room = visibleRooms.FirstOrDefault()?.Room;
@@ -236,7 +235,7 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
         {
             base.Dispose(isDisposing);
 
-            if (roomManager != null)
+            if (roomManager.IsNotNull())
                 roomManager.RoomsUpdated -= updateSorting;
         }
     }

osu.Game/Screens/OnlinePlay/Lounge/LoungeSubScreen.cs

@@ -259,7 +259,7 @@ namespace osu.Game.Screens.OnlinePlay.Lounge
             selectionLease.Return();
             selectionLease = null;
 
-            if (SelectedRoom.Value?.RoomID.Value == null)
+            if (SelectedRoom.Value?.RoomID == null)
                 SelectedRoom.Value = new Room();
 
             music?.EnsurePlayingSomething();
@@ -326,19 +326,19 @@ namespace osu.Game.Screens.OnlinePlay.Lounge
         /// <param name="room">The room to copy.</param>
         public void OpenCopy(Room room)
         {
-            Debug.Assert(room.RoomID.Value != null);
+            Debug.Assert(room.RoomID != null);
 
             if (joiningRoomOperation != null)
                 return;
 
             joiningRoomOperation = ongoingOperationTracker?.BeginOperation();
 
-            var req = new GetRoomRequest(room.RoomID.Value.Value);
+            var req = new GetRoomRequest(room.RoomID.Value);
 
             req.Success += r =>
             {
                 // ID must be unset as we use this as a marker for whether this is a client-side (not-yet-created) room or not.
-                r.RoomID.Value = null;
+                r.RoomID = null;
 
                 // Null out dates because end date is not supported client-side and the settings overlay will populate a duration.
                 r.EndDate.Value = null;

osu.Game/Screens/OnlinePlay/Match/Components/MatchChatDisplay.cs

@@ -36,7 +36,7 @@ namespace osu.Game.Screens.OnlinePlay.Match.Components
 
         private void updateChannel()
         {
-            if (room.RoomID.Value == null || channelId.Value == 0)
+            if (room.RoomID == null || channelId.Value == 0)
                 return;
 
             Channel.Value = channelManager?.JoinChannel(new Channel { Id = channelId.Value, Type = ChannelType.Multiplayer, Name = $"#lazermp_{room.RoomID.Value}" });

osu.Game/Screens/OnlinePlay/Match/Components/RoomSettingsOverlay.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
@@ -23,7 +21,7 @@ namespace osu.Game.Screens.OnlinePlay.Match.Components
         protected const float TRANSITION_DURATION = 350;
         protected const float FIELD_PADDING = 25;
 
-        protected OnlinePlayComposite Settings { get; set; }
+        protected OnlinePlayComposite Settings { get; set; } = null!;
 
         protected override bool BlockScrollInput => false;
 

osu.Game/Screens/OnlinePlay/Match/RoomSubScreen.cs

@@ -5,6 +5,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Diagnostics;
 using System.Linq;
 using JetBrains.Annotations;
@@ -29,6 +30,7 @@ using osu.Game.Rulesets.Mods;
 using osu.Game.Screens.Menu;
 using osu.Game.Screens.OnlinePlay.Match.Components;
 using osu.Game.Screens.OnlinePlay.Multiplayer;
+using Container = osu.Framework.Graphics.Containers.Container;
 
 namespace osu.Game.Screens.OnlinePlay.Match
 {
@@ -59,8 +61,6 @@ namespace osu.Game.Screens.OnlinePlay.Match
         /// </summary>
         protected readonly Bindable<IReadOnlyList<Mod>> UserMods = new Bindable<IReadOnlyList<Mod>>(Array.Empty<Mod>());
 
-        protected readonly IBindable<long?> RoomId = new Bindable<long?>();
-
         [Resolved(CanBeNull = true)]
         private IOverlayManager overlayManager { get; set; }
 
@@ -82,6 +82,9 @@ namespace osu.Game.Screens.OnlinePlay.Match
         [Resolved]
         private PreviewTrackManager previewTrackManager { get; set; } = null!;
 
+        [Resolved(canBeNull: true)]
+        private IDialogOverlay dialogOverlay { get; set; }
+
         [Cached]
         private readonly OnlinePlayBeatmapAvailabilityTracker beatmapAvailabilityTracker = new OnlinePlayBeatmapAvailabilityTracker();
 
@@ -109,8 +112,6 @@ namespace osu.Game.Screens.OnlinePlay.Match
             this.allowEdit = allowEdit;
 
             Padding = new MarginPadding { Top = Header.HEIGHT };
-
-            RoomId.BindTo(room.RoomID);
         }
 
         [BackgroundDependencyLoader]
@@ -252,9 +253,27 @@ namespace osu.Game.Screens.OnlinePlay.Match
         {
             base.LoadComplete();
 
-            RoomId.BindValueChanged(id =>
+            SelectedItem.BindValueChanged(_ => Scheduler.AddOnce(selectedItemChanged));
+            UserMods.BindValueChanged(_ => Scheduler.AddOnce(UpdateMods));
+
+            beatmapAvailabilityTracker.SelectedItem.BindTo(SelectedItem);
+            beatmapAvailabilityTracker.Availability.BindValueChanged(_ => updateWorkingBeatmap());
+
+            userModsSelectOverlayRegistration = overlayManager?.RegisterBlockingOverlay(UserModsSelectOverlay);
+
+            Room.PropertyChanged += onRoomPropertyChanged;
+            updateSetupState();
+        }
+
+        private void onRoomPropertyChanged(object sender, PropertyChangedEventArgs e)
         {
-                if (id.NewValue == null)
+            if (e.PropertyName == nameof(Room.RoomID))
+                updateSetupState();
+        }
+
+        private void updateSetupState()
+        {
+            if (Room.RoomID == null)
             {
                 // A new room is being created.
                 // The main content should be hidden until the settings overlay is hidden, signaling the room is ready to be displayed.
@@ -266,15 +285,6 @@ namespace osu.Game.Screens.OnlinePlay.Match
                 mainContent.Show();
                 settingsOverlay.Hide();
             }
-            }, true);
-
-            SelectedItem.BindValueChanged(_ => Scheduler.AddOnce(selectedItemChanged));
-            UserMods.BindValueChanged(_ => Scheduler.AddOnce(UpdateMods));
-
-            beatmapAvailabilityTracker.SelectedItem.BindTo(SelectedItem);
-            beatmapAvailabilityTracker.Availability.BindValueChanged(_ => updateWorkingBeatmap());
-
-            userModsSelectOverlayRegistration = overlayManager?.RegisterBlockingOverlay(UserModsSelectOverlay);
         }
 
         protected override IReadOnlyDependencyContainer CreateChildDependencies(IReadOnlyDependencyContainer parent)
@@ -285,14 +295,11 @@ namespace osu.Game.Screens.OnlinePlay.Match
             };
         }
 
-        [Resolved(canBeNull: true)]
-        private IDialogOverlay dialogOverlay { get; set; }
-
         protected virtual bool IsConnected => api.State.Value == APIState.Online;
 
         public override bool OnBackButton()
         {
-            if (Room.RoomID.Value == null)
+            if (Room.RoomID == null)
             {
                 if (!ensureExitConfirmed())
                     return true;
@@ -365,7 +372,7 @@ namespace osu.Game.Screens.OnlinePlay.Match
             if (!IsConnected)
                 return true;
 
-            bool hasUnsavedChanges = Room.RoomID.Value == null && Room.Playlist.Count > 0;
+            bool hasUnsavedChanges = Room.RoomID == null && Room.Playlist.Count > 0;
 
             if (dialogOverlay == null || !hasUnsavedChanges)
                 return true;
@@ -538,6 +545,7 @@ namespace osu.Game.Screens.OnlinePlay.Match
             base.Dispose(isDisposing);
 
             userModsSelectOverlayRegistration?.Dispose();
+            Room.PropertyChanged -= onRoomPropertyChanged;
         }
     }
 }

osu.Game/Screens/OnlinePlay/Multiplayer/Match/MultiplayerMatchSettingsOverlay.cs

@@ -353,7 +353,6 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Match
                 RoomName.BindValueChanged(name => NameField.Text = name.NewValue, true);
                 Type.BindValueChanged(type => TypePicker.Current.Value = type.NewValue, true);
                 MaxParticipants.BindValueChanged(count => MaxParticipantsField.Text = count.NewValue?.ToString(), true);
-                RoomID.BindValueChanged(roomId => playlistContainer.Alpha = roomId.NewValue == null ? 1 : 0, true);
                 Password.BindValueChanged(password => PasswordTextBox.Text = password.NewValue ?? string.Empty, true);
                 QueueMode.BindValueChanged(mode => QueueModeDropdown.Current.Value = mode.NewValue, true);
                 AutoStartDuration.BindValueChanged(duration => startModeDropdown.Current.Value = (StartMode)(int)duration.NewValue.TotalSeconds, true);
@@ -382,6 +381,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Match
                 base.Update();
 
                 ApplyButton.Enabled.Value = Playlist.Count > 0 && NameField.Text.Length > 0 && !operationInProgress.Value;
+                playlistContainer.Alpha = room.RoomID == null ? 1 : 0;
             }
 
             private void apply()

osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerMatchSubScreen.cs

@@ -54,7 +54,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
         public MultiplayerMatchSubScreen(Room room)
             : base(room)
         {
-            Title = room.RoomID.Value == null ? "New room" : room.Name.Value;
+            Title = room.RoomID == null ? "New room" : room.Name.Value;
             Activity.Value = new UserActivity.InLobby(room);
         }
 

osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerPlayer.cs

@@ -1,14 +1,13 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Diagnostics;
 using System.Linq;
 using System.Threading.Tasks;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
+using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Logging;
 using osu.Framework.Screens;
 using osu.Game.Graphics.UserInterface;
@@ -29,17 +28,15 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
         protected override UserActivity InitialActivity => new UserActivity.InMultiplayerGame(Beatmap.Value.BeatmapInfo, Ruleset.Value);
 
         [Resolved]
-        private MultiplayerClient client { get; set; }
+        private MultiplayerClient client { get; set; } = null!;
 
-        private IBindable<bool> isConnected;
+        private IBindable<bool> isConnected = null!;
 
         private readonly TaskCompletionSource<bool> resultsReady = new TaskCompletionSource<bool>();
-
         private readonly MultiplayerRoomUser[] users;
 
-        private LoadingLayer loadingDisplay;
+        private LoadingLayer loadingDisplay = null!;
-
+        private MultiplayerGameplayLeaderboard multiplayerLeaderboard = null!;
-        private MultiplayerGameplayLeaderboard multiplayerLeaderboard;
 
         /// <summary>
         /// Construct a multiplayer player.
@@ -153,7 +150,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
             GameplayClockContainer.Reset();
         }
 
-        private void failAndBail(string message = null)
+        private void failAndBail(string? message = null)
         {
             if (!string.IsNullOrEmpty(message))
                 Logger.Log(message, LoggingTarget.Runtime, LogLevel.Important);
@@ -196,14 +193,14 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
 
         protected override ResultsScreen CreateResults(ScoreInfo score)
         {
-            Debug.Assert(Room.RoomID.Value != null);
+            Debug.Assert(Room.RoomID != null);
 
             return multiplayerLeaderboard.TeamScores.Count == 2
-                ? new MultiplayerTeamResultsScreen(score, Room.RoomID.Value.Value, PlaylistItem, multiplayerLeaderboard.TeamScores)
+                ? new MultiplayerTeamResultsScreen(score, Room.RoomID.Value, PlaylistItem, multiplayerLeaderboard.TeamScores)
                 {
                     ShowUserStatistics = true,
                 }
-                : new MultiplayerResultsScreen(score, Room.RoomID.Value.Value, PlaylistItem)
+                : new MultiplayerResultsScreen(score, Room.RoomID.Value, PlaylistItem)
                 {
                     ShowUserStatistics = true
                 };
@@ -213,7 +210,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
         {
             base.Dispose(isDisposing);
 
-            if (client != null)
+            if (client.IsNotNull())
             {
                 client.GameplayStarted -= onGameplayStarted;
                 client.ResultsReady -= onResultsReady;

osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerRoomManager.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Diagnostics;
 using osu.Framework.Allocation;
@@ -18,12 +16,12 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
     public partial class MultiplayerRoomManager : RoomManager
     {
         [Resolved]
-        private MultiplayerClient multiplayerClient { get; set; }
+        private MultiplayerClient multiplayerClient { get; set; } = null!;
 
-        public override void CreateRoom(Room room, Action<Room> onSuccess = null, Action<string> onError = null)
+        public override void CreateRoom(Room room, Action<Room>? onSuccess = null, Action<string>? onError = null)
             => base.CreateRoom(room, r => joinMultiplayerRoom(r, r.Password.Value, onSuccess, onError), onError);
 
-        public override void JoinRoom(Room room, string password = null, Action<Room> onSuccess = null, Action<string> onError = null)
+        public override void JoinRoom(Room room, string? password = null, Action<Room>? onSuccess = null, Action<string>? onError = null)
         {
             if (!multiplayerClient.IsConnected.Value)
             {
@@ -51,9 +49,9 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
             multiplayerClient.LeaveRoom();
         }
 
-        private void joinMultiplayerRoom(Room room, string password, Action<Room> onSuccess = null, Action<string> onError = null)
+        private void joinMultiplayerRoom(Room room, string? password, Action<Room>? onSuccess = null, Action<string>? onError = null)
         {
-            Debug.Assert(room.RoomID.Value != null);
+            Debug.Assert(room.RoomID != null);
 
             multiplayerClient.JoinRoom(room, password).ContinueWith(t =>
             {

osu.Game/Screens/OnlinePlay/OnlinePlayComposite.cs

@@ -16,9 +16,6 @@ namespace osu.Game.Screens.OnlinePlay
     /// </summary>
     public partial class OnlinePlayComposite : CompositeDrawable
     {
-        [Resolved(typeof(Room))]
-        protected Bindable<long?> RoomID { get; private set; } = null!;
-
         [Resolved(typeof(Room), nameof(Room.Name))]
         protected Bindable<string> RoomName { get; private set; } = null!;
 

osu.Game/Screens/OnlinePlay/Playlists/PlaylistsPlayer.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Diagnostics;
 using System.Linq;
@@ -21,11 +19,11 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
 {
     public partial class PlaylistsPlayer : RoomSubmittingPlayer
     {
-        public Action Exited;
+        public Action? Exited;
 
         protected override UserActivity InitialActivity => new UserActivity.InPlaylistGame(Beatmap.Value.BeatmapInfo, Ruleset.Value);
 
-        public PlaylistsPlayer(Room room, PlaylistItem playlistItem, PlayerConfiguration configuration = null)
+        public PlaylistsPlayer(Room room, PlaylistItem playlistItem, PlayerConfiguration? configuration = null)
             : base(room, playlistItem, configuration)
         {
         }
@@ -57,8 +55,8 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
 
         protected override ResultsScreen CreateResults(ScoreInfo score)
         {
-            Debug.Assert(Room.RoomID.Value != null);
+            Debug.Assert(Room.RoomID != null);
-            return new PlaylistItemUserResultsScreen(score, Room.RoomID.Value.Value, PlaylistItem)
+            return new PlaylistItemUserResultsScreen(score, Room.RoomID.Value, PlaylistItem)
             {
                 AllowRetry = true,
                 ShowUserStatistics = true,

osu.Game/Screens/OnlinePlay/Playlists/PlaylistsRoomSettingsOverlay.cs

@@ -372,7 +372,7 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
             private void onPlaylistChanged(object? sender, NotifyCollectionChangedEventArgs e) =>
                 playlistLength.Text = $"Length: {Playlist.GetTotalDuration(rulesets)}";
 
-            private bool hasValidSettings => RoomID.Value == null && NameField.Text.Length > 0 && Playlist.Count > 0
+            private bool hasValidSettings => room.RoomID == null && NameField.Text.Length > 0 && Playlist.Count > 0
                                              && hasValidDuration;
 
             private bool hasValidDuration => DurationField.Current.Value <= TimeSpan.FromDays(14) || localUser.Value.IsSupporter;

osu.Game/Screens/OnlinePlay/Playlists/PlaylistsRoomSubScreen.cs

@@ -1,11 +1,9 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
+using System.ComponentModel;
-
 using System.Diagnostics;
 using System.Linq;
-using JetBrains.Annotations;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics;
@@ -22,6 +20,7 @@ using osu.Game.Screens.Play;
 using osu.Game.Screens.Play.HUD;
 using osu.Game.Users;
 using osuTK;
+using Container = osu.Framework.Graphics.Containers.Container;
 
 namespace osu.Game.Screens.OnlinePlay.Playlists
 {
@@ -33,20 +32,22 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
 
         private readonly IBindable<bool> isIdle = new BindableBool();
 
-        private MatchLeaderboard leaderboard;
+        [Resolved(CanBeNull = true)]
-        private SelectionPollingComponent selectionPollingComponent;
+        private IdleTracker? idleTracker { get; set; }
 
-        private FillFlowContainer progressSection;
+        private MatchLeaderboard leaderboard = null!;
+        private SelectionPollingComponent selectionPollingComponent = null!;
+        private FillFlowContainer progressSection = null!;
 
         public PlaylistsRoomSubScreen(Room room)
             : base(room, false) // Editing is temporarily not allowed.
         {
-            Title = room.RoomID.Value == null ? "New playlist" : room.Name.Value;
+            Title = room.RoomID == null ? "New playlist" : room.Name.Value;
             Activity.Value = new UserActivity.InLobby(room);
         }
 
-        [BackgroundDependencyLoader(true)]
+        [BackgroundDependencyLoader]
-        private void load([CanBeNull] IdleTracker idleTracker)
+        private void load()
         {
             if (idleTracker != null)
                 isIdle.BindTo(idleTracker.IsIdle);
@@ -59,17 +60,26 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
             base.LoadComplete();
 
             isIdle.BindValueChanged(_ => updatePollingRate(), true);
-            RoomId.BindValueChanged(id =>
+            Room.MaxAttempts.BindValueChanged(_ => progressSection.Alpha = Room.MaxAttempts.Value != null ? 1 : 0, true);
+
+            Room.PropertyChanged += onRoomPropertyChanged;
+            updateSetupState();
+        }
+
+        private void onRoomPropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
-                if (id.NewValue != null)
+            if (e.PropertyName == nameof(Room.RoomID))
+                updateSetupState();
+        }
+
+        private void updateSetupState()
+        {
+            if (Room.RoomID != null)
             {
                 // Set the first playlist item.
                 // This is scheduled since updating the room and playlist may happen in an arbitrary order (via Room.CopyFrom()).
                 Schedule(() => SelectedItem.Value = Room.Playlist.FirstOrDefault());
             }
-            }, true);
-
-            Room.MaxAttempts.BindValueChanged(_ => progressSection.Alpha = Room.MaxAttempts.Value != null ? 1 : 0, true);
         }
 
         protected override Drawable CreateMainContent() => new Container
@@ -92,7 +102,7 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
                     },
                     Content = new[]
                     {
-                        new Drawable[]
+                        new Drawable?[]
                         {
                             // Playlist items column
                             new GridContainer
@@ -113,8 +123,8 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
                                             AllowShowingResults = true,
                                             RequestResults = item =>
                                             {
-                                                Debug.Assert(RoomId.Value != null);
+                                                Debug.Assert(Room.RoomID != null);
-                                                ParentScreen?.Push(new PlaylistItemUserResultsScreen(null, RoomId.Value.Value, item));
+                                                ParentScreen?.Push(new PlaylistItemUserResultsScreen(null, Room.RoomID.Value, item));
                                             }
                                         }
                                     },
@@ -248,5 +258,11 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
         {
             Exited = () => leaderboard.RefetchScores()
         });
+
+        protected override void Dispose(bool isDisposing)
+        {
+            base.Dispose(isDisposing);
+            Room.PropertyChanged -= onRoomPropertyChanged;
+        }
     }
 }

osu.Game/Screens/Play/RoomSubmittingPlayer.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System.Diagnostics;
 using osu.Game.Extensions;
 using osu.Game.Online.API;
@@ -19,16 +17,16 @@ namespace osu.Game.Screens.Play
         protected readonly PlaylistItem PlaylistItem;
         protected readonly Room Room;
 
-        protected RoomSubmittingPlayer(Room room, PlaylistItem playlistItem, PlayerConfiguration configuration = null)
+        protected RoomSubmittingPlayer(Room room, PlaylistItem playlistItem, PlayerConfiguration? configuration = null)
             : base(configuration)
         {
             Room = room;
             PlaylistItem = playlistItem;
         }
 
-        protected override APIRequest<APIScoreToken> CreateTokenRequest()
+        protected override APIRequest<APIScoreToken>? CreateTokenRequest()
         {
-            if (!(Room.RoomID.Value is long roomId))
+            if (Room.RoomID is not long roomId)
                 return null;
 
             int beatmapId = Beatmap.Value.BeatmapInfo.OnlineID;
@@ -45,8 +43,8 @@ namespace osu.Game.Screens.Play
 
         protected override APIRequest<MultiplayerScore> CreateSubmissionRequest(Score score, long token)
         {
-            Debug.Assert(Room.RoomID.Value != null);
+            Debug.Assert(Room.RoomID != null);
-            return new SubmitRoomScoreRequest(score.ScoreInfo, token, Room.RoomID.Value.Value, PlaylistItem.ID);
+            return new SubmitRoomScoreRequest(score.ScoreInfo, token, Room.RoomID.Value, PlaylistItem.ID);
         }
     }
 }

osu.Game/Tests/Visual/Multiplayer/TestMultiplayerClient.cs

@@ -214,7 +214,7 @@ namespace osu.Game.Tests.Visual.Multiplayer
             roomId = clone(roomId);
             password = clone(password);
 
-            ServerAPIRoom = roomManager.ServerSideRooms.Single(r => r.RoomID.Value == roomId);
+            ServerAPIRoom = roomManager.ServerSideRooms.Single(r => r.RoomID == roomId);
 
             if (password != ServerAPIRoom.Password.Value)
                 throw new InvalidOperationException("Invalid password.");

osu.Game/Tests/Visual/OnlinePlay/TestRoomManager.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using osu.Game.Beatmaps;
 using osu.Game.Online.API.Requests.Responses;
@@ -17,23 +15,23 @@ namespace osu.Game.Tests.Visual.OnlinePlay
     /// </summary>
     public partial class TestRoomManager : RoomManager
     {
-        public Action<Room, string> JoinRoomRequested;
+        public Action<Room, string?>? JoinRoomRequested;
 
         private int currentRoomId;
 
-        public override void JoinRoom(Room room, string password = null, Action<Room> onSuccess = null, Action<string> onError = null)
+        public override void JoinRoom(Room room, string? password = null, Action<Room>? onSuccess = null, Action<string>? onError = null)
         {
             JoinRoomRequested?.Invoke(room, password);
             base.JoinRoom(room, password, onSuccess, onError);
         }
 
-        public void AddRooms(int count, RulesetInfo ruleset = null, bool withPassword = false, bool withSpotlightRooms = false)
+        public void AddRooms(int count, RulesetInfo? ruleset = null, bool withPassword = false, bool withSpotlightRooms = false)
         {
             for (int i = 0; i < count; i++)
             {
                 var room = new Room
                 {
-                    RoomID = { Value = -currentRoomId },
+                    RoomID = -currentRoomId,
                     Name = { Value = $@"Room {currentRoomId}" },
                     Host = { Value = new APIUser { Username = @"Host" } },
                     EndDate = { Value = DateTimeOffset.Now + TimeSpan.FromSeconds(10) },

osu.Game/Tests/Visual/OnlinePlay/TestRoomRequestsHandler.cs

@@ -64,7 +64,7 @@ namespace osu.Game.Tests.Visual.OnlinePlay
 
                 case JoinRoomRequest joinRoomRequest:
                 {
-                    var room = ServerSideRooms.Single(r => r.RoomID.Value == joinRoomRequest.Room.RoomID.Value);
+                    var room = ServerSideRooms.Single(r => r.RoomID == joinRoomRequest.Room.RoomID);
 
                     if (joinRoomRequest.Password != room.Password.Value)
                     {
@@ -162,7 +162,7 @@ namespace osu.Game.Tests.Visual.OnlinePlay
                     return true;
 
                 case GetRoomRequest getRoomRequest:
-                    getRoomRequest.TriggerSuccess(createResponseRoom(ServerSideRooms.Single(r => r.RoomID.Value == getRoomRequest.RoomId), true));
+                    getRoomRequest.TriggerSuccess(createResponseRoom(ServerSideRooms.Single(r => r.RoomID == getRoomRequest.RoomId), true));
                     return true;
 
                 case CreateRoomScoreRequest createRoomScoreRequest:
@@ -261,7 +261,7 @@ namespace osu.Game.Tests.Visual.OnlinePlay
         /// <param name="host">The room host.</param>
         public void AddServerSideRoom(Room room, APIUser host)
         {
-            room.RoomID.Value ??= currentRoomId++;
+            room.RoomID ??= currentRoomId++;
             room.Host.Value = host;
 
             for (int i = 0; i < room.Playlist.Count; i++)

osu.Game/Users/UserActivity.cs

@@ -248,7 +248,7 @@ namespace osu.Game.Users
 
             public InLobby(Room room)
             {
-                RoomID = room.RoomID.Value ?? -1;
+                RoomID = room.RoomID ?? -1;
                 RoomName = room.Name.Value;
             }