Fix the MOTHERLOAD of undetected issues that are now visible thanks to net6.0

osu.Game.Rulesets.Osu/Replays/OsuAutoGeneratorBase.cs

@@ -82,7 +82,7 @@ namespace osu.Game.Rulesets.Osu.Replays
 
         private class ReplayFrameComparer : IComparer<ReplayFrame>
         {
-            public int Compare(ReplayFrame f1, ReplayFrame f2)
+            public int Compare(ReplayFrame? f1, ReplayFrame? f2)
             {
                 if (f1 == null) throw new ArgumentNullException(nameof(f1));
                 if (f2 == null) throw new ArgumentNullException(nameof(f2));

osu.Game.Tournament/IPC/FileBasedIPC.cs

@@ -245,8 +245,10 @@ namespace osu.Game.Tournament.IPC
             {
                 string stableInstallPath;
 
+#pragma warning disable CA1416
                 using (RegistryKey key = Registry.ClassesRoot.OpenSubKey("osu"))
                     stableInstallPath = key?.OpenSubKey(@"shell\open\command")?.GetValue(string.Empty)?.ToString()?.Split('"')[1].Replace("osu!.exe", "");
+#pragma warning restore CA1416
 
                 if (ipcFileExistsInDirectory(stableInstallPath))
                     return stableInstallPath;

osu.Game.Tournament/SaveChangesOverlay.cs

@@ -70,7 +70,7 @@ namespace osu.Game.Tournament
 
         private async Task checkForChanges()
         {
-            string serialisedLadder = await Task.Run(() => tournamentGame.GetSerialisedLadder());
+            string serialisedLadder = await Task.Run(() => tournamentGame.GetSerialisedLadder()).ConfigureAwait(false);
 
             // If a save hasn't been triggered by the user yet, populate the initial value
             lastSerialisedLadder ??= serialisedLadder;

osu.Game/Audio/SampleInfo.cs

@@ -35,7 +35,7 @@ namespace osu.Game.Audio
         public bool Equals(SampleInfo? other)
             => other != null && sampleNames.SequenceEqual(other.sampleNames);
 
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
             => obj is SampleInfo other && Equals(other);
     }
 }

osu.Game/Beatmaps/BeatmapImporter.cs

@@ -44,7 +44,7 @@ namespace osu.Game.Beatmaps
 
         public override async Task<Live<BeatmapSetInfo>?> ImportAsUpdate(ProgressNotification notification, ImportTask importTask, BeatmapSetInfo original)
         {
-            var imported = await Import(notification, importTask);
+            var imported = await Import(notification, importTask).ConfigureAwait(false);
 
             if (!imported.Any())
                 return null;

osu.Game/Beatmaps/BeatmapUpdaterMetadataLookup.cs

@@ -178,7 +178,7 @@ namespace osu.Game.Beatmaps
             {
                 try
                 {
-                    await cacheDownloadRequest.PerformAsync();
+                    await cacheDownloadRequest.PerformAsync().ConfigureAwait(false);
                 }
                 catch
                 {

osu.Game/Beatmaps/ControlPoints/ControlPoint.cs

@@ -16,7 +16,7 @@ namespace osu.Game.Beatmaps.ControlPoints
 
         public void AttachGroup(ControlPointGroup pointGroup) => Time = pointGroup.Time;
 
-        public int CompareTo(ControlPoint other) => Time.CompareTo(other.Time);
+        public int CompareTo(ControlPoint? other) => Time.CompareTo(other?.Time);
 
         public virtual Color4 GetRepresentingColour(OsuColour colours) => colours.Yellow;
 
@@ -32,7 +32,7 @@ namespace osu.Game.Beatmaps.ControlPoints
         /// </summary>
         public ControlPoint DeepClone()
         {
-            var copy = (ControlPoint)Activator.CreateInstance(GetType());
+            var copy = (ControlPoint)Activator.CreateInstance(GetType())!;
 
             copy.CopyFrom(this);
 

osu.Game/Beatmaps/ControlPoints/ControlPointGroup.cs

@@ -26,7 +26,7 @@ namespace osu.Game.Beatmaps.ControlPoints
             Time = time;
         }
 
-        public int CompareTo(ControlPointGroup other) => Time.CompareTo(other.Time);
+        public int CompareTo(ControlPointGroup? other) => Time.CompareTo(other?.Time);
 
         public void Add(ControlPoint point)
         {

osu.Game/Beatmaps/DifficultyRecommender.cs

@@ -51,11 +51,11 @@ namespace osu.Game.Beatmaps
                 if (!recommendedDifficultyMapping.TryGetValue(r, out double recommendation))
                     continue;
 
-                BeatmapInfo beatmapInfo = beatmaps.Where(b => b.Ruleset.ShortName.Equals(r)).OrderBy(b =>
+                BeatmapInfo beatmapInfo = beatmaps.Where(b => b.Ruleset.ShortName.Equals(r, StringComparison.Ordinal)).MinBy(b =>
                 {
                     double difference = b.StarRating - recommendation;
                     return difference >= 0 ? difference * 2 : difference * -1; // prefer easier over harder
-                }).FirstOrDefault();
+                });
 
                 if (beatmapInfo != null)
                     return beatmapInfo;
@@ -90,7 +90,7 @@ namespace osu.Game.Beatmaps
                 return recommendedDifficultyMapping
                        .OrderByDescending(pair => pair.Value)
                        .Select(pair => pair.Key)
-                       .Where(r => !r.Equals(ruleset.Value.ShortName))
+                       .Where(r => !r.Equals(ruleset.Value.ShortName, StringComparison.Ordinal))
                        .Prepend(ruleset.Value.ShortName);
             }
         }

osu.Game/Configuration/SettingSourceAttribute.cs

@@ -91,15 +91,15 @@ namespace osu.Game.Configuration
             OrderPosition = orderPosition;
         }
 
-        public int CompareTo(SettingSourceAttribute other)
+        public int CompareTo(SettingSourceAttribute? other)
         {
-            if (OrderPosition == other.OrderPosition)
+            if (OrderPosition == other?.OrderPosition)
                 return 0;
 
             // unordered items come last (are greater than any ordered items).
             if (OrderPosition == null)
                 return 1;
-            if (other.OrderPosition == null)
+            if (other?.OrderPosition == null)
                 return -1;
 
             // ordered items are sorted by the order value.
@@ -113,7 +113,7 @@ namespace osu.Game.Configuration
         {
             foreach (var (attr, property) in obj.GetOrderedSettingsSourceProperties())
             {
-                object value = property.GetValue(obj);
+                object value = property.GetValue(obj)!;
 
                 if (attr.SettingControlType != null)
                 {
@@ -121,7 +121,7 @@ namespace osu.Game.Configuration
                     if (controlType.EnumerateBaseTypes().All(t => !t.IsGenericType || t.GetGenericTypeDefinition() != typeof(SettingsItem<>)))
                         throw new InvalidOperationException($"{nameof(SettingSourceAttribute)} had an unsupported custom control type ({controlType.ReadableName()})");
 
-                    var control = (Drawable)Activator.CreateInstance(controlType);
+                    var control = (Drawable)Activator.CreateInstance(controlType)!;
                     controlType.GetProperty(nameof(SettingsItem<object>.SettingSourceObject))?.SetValue(control, obj);
                     controlType.GetProperty(nameof(SettingsItem<object>.LabelText))?.SetValue(control, attr.Label);
                     controlType.GetProperty(nameof(SettingsItem<object>.TooltipText))?.SetValue(control, attr.Description);
@@ -188,7 +188,7 @@ namespace osu.Game.Configuration
 
                     case IBindable bindable:
                         var dropdownType = typeof(ModSettingsEnumDropdown<>).MakeGenericType(bindable.GetType().GetGenericArguments()[0]);
-                        var dropdown = (Drawable)Activator.CreateInstance(dropdownType);
+                        var dropdown = (Drawable)Activator.CreateInstance(dropdownType)!;
 
                         dropdownType.GetProperty(nameof(SettingsDropdown<object>.LabelText))?.SetValue(dropdown, attr.Label);
                         dropdownType.GetProperty(nameof(SettingsDropdown<object>.TooltipText))?.SetValue(dropdown, attr.Description);
@@ -231,7 +231,7 @@ namespace osu.Game.Configuration
                     // An unknown (e.g. enum) generic type.
                     var valueMethod = u.GetType().GetProperty(nameof(IBindable<int>.Value));
                     Debug.Assert(valueMethod != null);
-                    return valueMethod.GetValue(u);
+                    return valueMethod.GetValue(u)!;
 
                 default:
                     // fall back for non-bindable cases.

osu.Game/Database/LegacyImportManager.cs

@@ -66,16 +66,16 @@ namespace osu.Game.Database
             switch (content)
             {
                 case StableContent.Beatmaps:
-                    return await new LegacyBeatmapImporter(beatmaps).GetAvailableCount(stableStorage);
+                    return await new LegacyBeatmapImporter(beatmaps).GetAvailableCount(stableStorage).ConfigureAwait(false);
 
                 case StableContent.Skins:
-                    return await new LegacySkinImporter(skins).GetAvailableCount(stableStorage);
+                    return await new LegacySkinImporter(skins).GetAvailableCount(stableStorage).ConfigureAwait(false);
 
                 case StableContent.Collections:
-                    return await new LegacyCollectionImporter(realmAccess).GetAvailableCount(stableStorage);
+                    return await new LegacyCollectionImporter(realmAccess).GetAvailableCount(stableStorage).ConfigureAwait(false);
 
                 case StableContent.Scores:
-                    return await new LegacyScoreImporter(scores).GetAvailableCount(stableStorage);
+                    return await new LegacyScoreImporter(scores).GetAvailableCount(stableStorage).ConfigureAwait(false);
 
                 default:
                     throw new ArgumentException($"Only one {nameof(StableContent)} flag should be specified.");

osu.Game/Database/Live.cs

@@ -61,6 +61,6 @@ namespace osu.Game.Database
 
         public override int GetHashCode() => HashCode.Combine(ID);
 
-        public override string ToString() => PerformRead(i => i.ToString());
+        public override string? ToString() => PerformRead(i => i.ToString());
     }
 }

osu.Game/Database/ModelDownloader.cs

@@ -71,9 +71,9 @@ namespace osu.Game.Database
                     bool importSuccessful;
 
                     if (originalModel != null)
-                        importSuccessful = (await importer.ImportAsUpdate(notification, new ImportTask(filename), originalModel)) != null;
+                        importSuccessful = (await importer.ImportAsUpdate(notification, new ImportTask(filename), originalModel).ConfigureAwait(false)) != null;
                     else
-                        importSuccessful = (await importer.Import(notification, new ImportTask(filename))).Any();
+                        importSuccessful = (await importer.Import(notification, new ImportTask(filename)).ConfigureAwait(false)).Any();
 
                     // for now a failed import will be marked as a failed download for simplicity.
                     if (!importSuccessful)

osu.Game/Database/RealmAccess.cs

@@ -481,7 +481,7 @@ namespace osu.Game.Database
                 // server, which we don't use. May want to report upstream or revisit in the future.
                 using (var realm = getRealmInstance())
                     // ReSharper disable once AccessToDisposedClosure (WriteAsync should be marked as [InstantHandle]).
-                    await realm.WriteAsync(() => action(realm));
+                    await realm.WriteAsync(() => action(realm)).ConfigureAwait(false);
 
                 pendingAsyncWrites.Signal();
             });
@@ -558,7 +558,7 @@ namespace osu.Game.Database
 
                 return new InvokeOnDisposal(() => model.PropertyChanged -= onPropertyChanged);
 
-                void onPropertyChanged(object sender, PropertyChangedEventArgs args)
+                void onPropertyChanged(object? sender, PropertyChangedEventArgs args)
                 {
                     if (args.PropertyName == propertyName)
                         onChanged(propLookupCompiled(model));

osu.Game/Extensions/DrawableExtensions.cs

@@ -66,10 +66,10 @@ namespace osu.Game.Extensions
 
                 foreach (var (_, property) in component.GetSettingsSourceProperties())
                 {
-                    if (!info.Settings.TryGetValue(property.Name.ToSnakeCase(), out object settingValue))
+                    if (!info.Settings.TryGetValue(property.Name.ToSnakeCase(), out object? settingValue))
                         continue;
 
-                    skinnable.CopyAdjustedSetting((IBindable)property.GetValue(component), settingValue);
+                    skinnable.CopyAdjustedSetting(((IBindable)property.GetValue(component)!), settingValue);
                 }
             }
 

osu.Game/Extensions/TaskExtensions.cs

@@ -37,7 +37,7 @@ namespace osu.Game.Extensions
 
                 if (cancellationToken.IsCancellationRequested)
                 {
-                    tcs.SetCanceled();
+                    tcs.SetCanceled(cancellationToken);
                 }
                 else
                 {

osu.Game/Graphics/UserInterface/OsuTextBox.cs

@@ -277,7 +277,7 @@ namespace osu.Game.Graphics.UserInterface
         {
             var samples = sampleMap[feedbackSampleType];
 
-            if (samples == null || samples.Length == 0)
+            if (samples.Length == 0)
                 return null;
 
             return samples[RNG.Next(0, samples.Length)]?.GetChannel();

osu.Game/Models/RealmUser.cs

@@ -27,7 +27,7 @@ namespace osu.Game.Models
 
         public bool IsBot => false;
 
-        public bool Equals(RealmUser other)
+        public bool Equals(RealmUser? other)
         {
             if (ReferenceEquals(null, other)) return false;
             if (ReferenceEquals(this, other)) return true;

osu.Game/Online/API/APIMod.cs

@@ -39,7 +39,7 @@ namespace osu.Game.Online.API
 
             foreach (var (_, property) in mod.GetSettingsSourceProperties())
             {
-                var bindable = (IBindable)property.GetValue(mod);
+                var bindable = (IBindable)property.GetValue(mod)!;
 
                 if (!bindable.IsDefault)
                     Settings.Add(property.Name.ToSnakeCase(), bindable.GetUnderlyingSettingValue());
@@ -60,16 +60,16 @@ namespace osu.Game.Online.API
             {
                 foreach (var (_, property) in resultMod.GetSettingsSourceProperties())
                 {
-                    if (!Settings.TryGetValue(property.Name.ToSnakeCase(), out object settingValue))
+                    if (!Settings.TryGetValue(property.Name.ToSnakeCase(), out object? settingValue))
                         continue;
 
                     try
                     {
-                        resultMod.CopyAdjustedSetting((IBindable)property.GetValue(resultMod), settingValue);
+                        resultMod.CopyAdjustedSetting((IBindable)property.GetValue(resultMod)!, settingValue);
                     }
                     catch (Exception ex)
                     {
-                        Logger.Log($"Failed to copy mod setting value '{settingValue ?? "null"}' to \"{property.Name}\": {ex.Message}");
+                        Logger.Log($"Failed to copy mod setting value '{settingValue}' to \"{property.Name}\": {ex.Message}");
                     }
                 }
             }
@@ -79,7 +79,7 @@ namespace osu.Game.Online.API
 
         public bool ShouldSerializeSettings() => Settings.Count > 0;
 
-        public bool Equals(APIMod other)
+        public bool Equals(APIMod? other)
         {
             if (ReferenceEquals(null, other)) return false;
             if (ReferenceEquals(this, other)) return true;

osu.Game/Online/API/Requests/Responses/APIBeatmap.cs

@@ -143,7 +143,7 @@ namespace osu.Game.Online.API.Requests.Responses
 
             public bool Equals(IRulesetInfo? other) => other is APIRuleset r && this.MatchesOnlineID(r);
 
-            public int CompareTo(IRulesetInfo other)
+            public int CompareTo(IRulesetInfo? other)
             {
                 if (!(other is APIRuleset ruleset))
                     throw new ArgumentException($@"Object is not of type {nameof(APIRuleset)}.", nameof(other));

osu.Game/Online/Chat/MessageFormatter.cs

@@ -71,7 +71,7 @@ namespace osu.Game.Online.Chat
             {
                 int index = m.Index - captureOffset;
 
-                string? displayText = string.Format(display,
+                string displayText = string.Format(display,
                     m.Groups[0],
                     m.Groups["text"].Value,
                     m.Groups["url"].Value).Trim();
@@ -109,7 +109,7 @@ namespace osu.Game.Online.Chat
             foreach (Match m in regex.Matches(result.Text, startIndex))
             {
                 int index = m.Index;
-                string? linkText = m.Groups["link"].Value;
+                string linkText = m.Groups["link"].Value;
                 int indexLength = linkText.Length;
 
                 var details = GetLinkDetails(linkText);
@@ -125,7 +125,7 @@ namespace osu.Game.Online.Chat
 
         public static LinkDetails GetLinkDetails(string url)
         {
-            string[]? args = url.Split('/', StringSplitOptions.RemoveEmptyEntries);
+            string[] args = url.Split('/', StringSplitOptions.RemoveEmptyEntries);
             args[0] = args[0].TrimEnd(':');
 
             switch (args[0])
@@ -362,6 +362,6 @@ namespace osu.Game.Online.Chat
 
         public bool Overlaps(Link otherLink) => Index < otherLink.Index + otherLink.Length && otherLink.Index < Index + Length;
 
-        public int CompareTo(Link otherLink) => Index > otherLink.Index ? 1 : -1;
+        public int CompareTo(Link? otherLink) => Index > otherLink?.Index ? 1 : -1;
     }
 }

osu.Game/Online/Chat/MessageNotifier.cs

@@ -5,6 +5,7 @@
 
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Diagnostics;
 using System.Linq;
 using System.Text.RegularExpressions;
 using osu.Framework.Allocation;
@@ -61,12 +62,16 @@ namespace osu.Game.Online.Chat
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
+                    Debug.Assert(e.NewItems != null);
+
                     foreach (var channel in e.NewItems.Cast<Channel>())
                         channel.NewMessagesArrived += checkNewMessages;
 
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
+                    Debug.Assert(e.OldItems != null);
+
                     foreach (var channel in e.OldItems.Cast<Channel>())
                         channel.NewMessagesArrived -= checkNewMessages;
 

osu.Game/Online/Metadata/OnlineMetadataClient.cs

@@ -68,7 +68,7 @@ namespace osu.Game.Online.Metadata
                         while (true)
                         {
                             Logger.Log($"Requesting catch-up from {lastQueueId.Value}");
-                            var catchUpChanges = await GetChangesSince(lastQueueId.Value);
+                            var catchUpChanges = await GetChangesSince(lastQueueId.Value).ConfigureAwait(true);
 
                             lastQueueId.Value = catchUpChanges.LastProcessedQueueID;
 
@@ -78,7 +78,7 @@ namespace osu.Game.Online.Metadata
                                 break;
                             }
 
-                            await ProcessChanges(catchUpChanges.BeatmapSetIDs);
+                            await ProcessChanges(catchUpChanges.BeatmapSetIDs).ConfigureAwait(true);
                         }
                     }
                     catch (Exception e)
@@ -101,7 +101,7 @@ namespace osu.Game.Online.Metadata
             if (!catchingUp)
                 lastQueueId.Value = updates.LastProcessedQueueID;
 
-            await ProcessChanges(updates.BeatmapSetIDs);
+            await ProcessChanges(updates.BeatmapSetIDs).ConfigureAwait(false);
         }
 
         public override Task<BeatmapUpdates> GetChangesSince(int queueId)

osu.Game/Online/Multiplayer/MultiplayerClient.cs

@@ -822,7 +822,7 @@ namespace osu.Game.Online.Multiplayer
             {
                 if (cancellationToken.IsCancellationRequested)
                 {
-                    tcs.SetCanceled();
+                    tcs.SetCanceled(cancellationToken);
                     return;
                 }
 

osu.Game/Online/Multiplayer/MultiplayerRoomUser.cs

@@ -54,10 +54,10 @@ namespace osu.Game.Online.Multiplayer
             return UserID == other.UserID;
         }
 
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             if (ReferenceEquals(this, obj)) return true;
-            if (obj.GetType() != GetType()) return false;
+            if (obj?.GetType() != GetType()) return false;
 
             return Equals((MultiplayerRoomUser)obj);
         }

osu.Game/Online/Multiplayer/OnlineMultiplayerClient.cs

@@ -80,7 +80,7 @@ namespace osu.Game.Online.Multiplayer
 
             try
             {
-                return await connection.InvokeAsync<MultiplayerRoom>(nameof(IMultiplayerServer.JoinRoomWithPassword), roomId, password ?? string.Empty);
+                return await connection.InvokeAsync<MultiplayerRoom>(nameof(IMultiplayerServer.JoinRoomWithPassword), roomId, password ?? string.Empty).ConfigureAwait(false);
             }
             catch (HubException exception)
             {
@@ -88,8 +88,8 @@ namespace osu.Game.Online.Multiplayer
                 {
                     Debug.Assert(connector != null);
 
-                    await connector.Reconnect();
+                    await connector.Reconnect().ConfigureAwait(false);
-                    return await JoinRoom(roomId, password);
+                    return await JoinRoom(roomId, password).ConfigureAwait(false);
                 }
 
                 throw;

osu.Game/Online/Notifications/NotificationsClientConnector.cs

@@ -27,7 +27,7 @@ namespace osu.Game.Online.Notifications
 
         protected sealed override async Task<PersistentEndpointClient> BuildConnectionAsync(CancellationToken cancellationToken)
         {
-            var client = await BuildNotificationClientAsync(cancellationToken);
+            var client = await BuildNotificationClientAsync(cancellationToken).ConfigureAwait(false);
 
             client.ChannelJoined = c => ChannelJoined?.Invoke(c);
             client.ChannelParted = c => ChannelParted?.Invoke(c);

osu.Game/Online/Notifications/WebSocket/WebSocketNotificationsClient.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Diagnostics;
+using System.Net;
 using System.Net.WebSockets;
 using System.Text;
 using System.Threading;
@@ -36,11 +37,11 @@ namespace osu.Game.Online.Notifications.WebSocket
         public override async Task ConnectAsync(CancellationToken cancellationToken)
         {
             await socket.ConnectAsync(new Uri(endpoint), cancellationToken).ConfigureAwait(false);
-            await sendMessage(new StartChatRequest(), CancellationToken.None);
+            await sendMessage(new StartChatRequest(), CancellationToken.None).ConfigureAwait(false);
 
             runReadLoop(cancellationToken);
 
-            await base.ConnectAsync(cancellationToken);
+            await base.ConnectAsync(cancellationToken).ConfigureAwait(false);
         }
 
         private void runReadLoop(CancellationToken cancellationToken) => Task.Run(async () =>
@@ -52,7 +53,7 @@ namespace osu.Game.Online.Notifications.WebSocket
             {
                 try
                 {
-                    WebSocketReceiveResult result = await socket.ReceiveAsync(buffer, cancellationToken);
+                    WebSocketReceiveResult result = await socket.ReceiveAsync(buffer, cancellationToken).ConfigureAwait(false);
 
                     switch (result.MessageType)
                     {
@@ -72,7 +73,7 @@ namespace osu.Game.Online.Notifications.WebSocket
                                     break;
                                 }
 
-                                await onMessageReceivedAsync(message);
+                                await onMessageReceivedAsync(message).ConfigureAwait(false);
                             }
 
                             break;
@@ -81,12 +82,12 @@ namespace osu.Game.Online.Notifications.WebSocket
                             throw new NotImplementedException("Binary message type not supported.");
 
                         case WebSocketMessageType.Close:
-                            throw new Exception("Connection closed by remote host.");
+                            throw new WebException("Connection closed by remote host.");
                     }
                 }
                 catch (Exception ex)
                 {
-                    await InvokeClosed(ex);
+                    await InvokeClosed(ex).ConfigureAwait(false);
                     return;
                 }
             }
@@ -109,7 +110,7 @@ namespace osu.Game.Online.Notifications.WebSocket
             if (socket.State != WebSocketState.Open)
                 return;
 
-            await socket.SendAsync(Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(message)), WebSocketMessageType.Text, true, cancellationToken);
+            await socket.SendAsync(Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(message)), WebSocketMessageType.Text, true, cancellationToken).ConfigureAwait(false);
         }
 
         private async Task onMessageReceivedAsync(SocketMessage message)
@@ -141,7 +142,7 @@ namespace osu.Game.Online.Notifications.WebSocket
                     Debug.Assert(messageData != null);
 
                     foreach (var msg in messageData.Messages)
-                        HandleChannelJoined(await getChannel(msg.ChannelId));
+                        HandleChannelJoined(await getChannel(msg.ChannelId).ConfigureAwait(false));
 
                     HandleMessages(messageData.Messages);
                     break;
@@ -150,7 +151,7 @@ namespace osu.Game.Online.Notifications.WebSocket
 
         private async Task<Channel> getChannel(long channelId)
         {
-            if (channelsMap.TryGetValue(channelId, out Channel channel))
+            if (channelsMap.TryGetValue(channelId, out Channel? channel))
                 return channel;
 
             var tsc = new TaskCompletionSource<Channel>();
@@ -166,13 +167,13 @@ namespace osu.Game.Online.Notifications.WebSocket
 
             API.Queue(req);
 
-            return await tsc.Task;
+            return await tsc.Task.ConfigureAwait(false);
         }
 
         public override async ValueTask DisposeAsync()
         {
-            await base.DisposeAsync();
+            await base.DisposeAsync().ConfigureAwait(false);
-            await closeAsync();
+            await closeAsync().ConfigureAwait(false);
             socket.Dispose();
         }
     }

osu.Game/Online/Notifications/WebSocket/WebSocketNotificationsClientConnector.cs

@@ -32,7 +32,7 @@ namespace osu.Game.Online.Notifications.WebSocket
             req.Failure += ex => tcs.SetException(ex);
             api.Queue(req);
 
-            string endpoint = await tcs.Task;
+            string endpoint = await tcs.Task.ConfigureAwait(false);
 
             ClientWebSocket socket = new ClientWebSocket();
             socket.Options.SetRequestHeader(@"Authorization", @$"Bearer {api.AccessToken}");

osu.Game/Online/PersistentEndpointClientConnector.cs

@@ -65,11 +65,11 @@ namespace osu.Game.Online
             {
                 case APIState.Failing:
                 case APIState.Offline:
-                    await disconnect(true);
+                    await disconnect(true).ConfigureAwait(true);
                     break;
 
                 case APIState.Online:
-                    await connect();
+                    await connect().ConfigureAwait(true);
                     break;
             }
         }
@@ -147,7 +147,7 @@ namespace osu.Game.Online
         {
             bool hasBeenCancelled = cancellationToken.IsCancellationRequested;
 
-            await disconnect(true);
+            await disconnect(true).ConfigureAwait(false);
 
             if (ex != null)
                 await handleErrorAndDelay(ex, CancellationToken.None).ConfigureAwait(false);

osu.Game/Online/SignalRDerivedTypeWorkaroundJsonConverter.cs

@@ -33,7 +33,8 @@ namespace osu.Game.Online
 
             object? instance = Activator.CreateInstance(resolvedType);
 
-            jsonSerializer.Populate(obj["$value"]!.CreateReader(), instance);
+            if (instance != null)
+                jsonSerializer.Populate(obj["$value"]!.CreateReader(), instance);
 
             return instance;
         }

osu.Game/Online/Spectator/OnlineSpectatorClient.cs

@@ -56,7 +56,7 @@ namespace osu.Game.Online.Spectator
 
             try
             {
-                await connection.InvokeAsync(nameof(ISpectatorServer.BeginPlaySession), scoreToken, state);
+                await connection.InvokeAsync(nameof(ISpectatorServer.BeginPlaySession), scoreToken, state).ConfigureAwait(false);
             }
             catch (Exception exception)
             {
@@ -64,8 +64,8 @@ namespace osu.Game.Online.Spectator
                 {
                     Debug.Assert(connector != null);
 
-                    await connector.Reconnect();
+                    await connector.Reconnect().ConfigureAwait(false);
-                    await BeginPlayingInternal(scoreToken, state);
+                    await BeginPlayingInternal(scoreToken, state).ConfigureAwait(false);
                 }
 
                 // Exceptions can occur if, for instance, the locally played beatmap doesn't have a server-side counterpart.

osu.Game/Online/Spectator/SpectatorScoreProcessor.cs

@@ -184,7 +184,7 @@ namespace osu.Game.Online.Spectator
                 Header = header;
             }
 
-            public int CompareTo(TimedFrame other) => Time.CompareTo(other.Time);
+            public int CompareTo(TimedFrame? other) => Time.CompareTo(other?.Time);
         }
     }
 }

osu.Game/Overlays/ChatOverlay.cs

@@ -352,11 +352,13 @@ namespace osu.Game.Overlays
 
         protected virtual DrawableChannel CreateDrawableChannel(Channel newChannel) => new DrawableChannel(newChannel);
 
-        private void joinedChannelsChanged(object sender, NotifyCollectionChangedEventArgs args)
+        private void joinedChannelsChanged(object? sender, NotifyCollectionChangedEventArgs args)
         {
             switch (args.Action)
             {
                 case NotifyCollectionChangedAction.Add:
+                    Debug.Assert(args.NewItems != null);
+
                     IEnumerable<Channel> newChannels = args.NewItems.OfType<Channel>().Where(isChatChannel);
 
                     foreach (var channel in newChannels)
@@ -365,6 +367,8 @@ namespace osu.Game.Overlays
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
+                    Debug.Assert(args.OldItems != null);
+
                     IEnumerable<Channel> leftChannels = args.OldItems.OfType<Channel>().Where(isChatChannel);
 
                     foreach (var channel in leftChannels)
@@ -384,7 +388,7 @@ namespace osu.Game.Overlays
             }
         }
 
-        private void availableChannelsChanged(object sender, NotifyCollectionChangedEventArgs args)
+        private void availableChannelsChanged(object? sender, NotifyCollectionChangedEventArgs args)
             => channelListing.UpdateAvailableChannels(channelManager.AvailableChannels);
 
         private void handleChatMessage(string message)

osu.Game/Overlays/Comments/DrawableComment.cs

@@ -19,6 +19,7 @@ using System;
 using osu.Framework.Graphics.Shapes;
 using osu.Framework.Extensions.IEnumerableExtensions;
 using System.Collections.Specialized;
+using System.Diagnostics;
 using osu.Framework.Localisation;
 using osu.Framework.Platform;
 using osu.Game.Graphics.UserInterface;
@@ -359,6 +360,8 @@ namespace osu.Game.Overlays.Comments
                 switch (args.Action)
                 {
                     case NotifyCollectionChangedAction.Add:
+                        Debug.Assert(args.NewItems != null);
+
                         onRepliesAdded(args.NewItems.Cast<DrawableComment>());
                         break;
 

osu.Game/Overlays/FirstRunSetupOverlay.cs

@@ -301,7 +301,7 @@ namespace osu.Game.Overlays
 
             if (currentStepIndex < steps.Count)
             {
-                var nextScreen = (Screen)Activator.CreateInstance(steps[currentStepIndex.Value]);
+                var nextScreen = (Screen)Activator.CreateInstance(steps[currentStepIndex.Value])!;
 
                 loadingShowDelegate = Scheduler.AddDelayed(() => loading.Show(), 200);
                 nextScreen.OnLoadComplete += _ =>

osu.Game/Overlays/Profile/Sections/Historical/DrawableMostPlayedBeatmap.cs

@@ -3,7 +3,6 @@
 
 #nullable disable
 
-using System.Diagnostics;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
@@ -131,8 +130,6 @@ namespace osu.Game.Overlays.Profile.Sections.Historical
             {
                 var metadata = beatmapInfo.Metadata;
 
-                Debug.Assert(metadata != null);
-
                 return new Drawable[]
                 {
                     new OsuSpriteText

osu.Game/Overlays/Profile/Sections/Ranks/DrawableProfileScore.cs

@@ -4,7 +4,6 @@
 #nullable disable
 
 using System;
-using System.Diagnostics;
 using System.Linq;
 using JetBrains.Annotations;
 using osu.Framework.Allocation;
@@ -269,8 +268,6 @@ namespace osu.Game.Overlays.Profile.Sections.Ranks
             {
                 var metadata = beatmapInfo.Metadata;
 
-                Debug.Assert(metadata != null);
-
                 return new Drawable[]
                 {
                     new OsuSpriteText

osu.Game/Overlays/Settings/Sections/Input/RulesetBindingsSection.cs

@@ -3,7 +3,6 @@
 
 #nullable disable
 
-using System.Diagnostics;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Sprites;
 using osu.Framework.Localisation;
@@ -29,8 +28,6 @@ namespace osu.Game.Overlays.Settings.Sections.Input
 
             var r = ruleset.CreateInstance();
 
-            Debug.Assert(r != null);
-
             foreach (int variant in r.AvailableVariants)
                 Add(new VariantBindingsSubsection(ruleset, variant));
         }

osu.Game/Rulesets/Mods/IMod.cs

@@ -55,6 +55,6 @@ namespace osu.Game.Rulesets.Mods
         /// <summary>
         /// Create a fresh <see cref="Mod"/> instance based on this mod.
         /// </summary>
-        Mod CreateInstance() => (Mod)Activator.CreateInstance(GetType());
+        Mod CreateInstance() => (Mod)Activator.CreateInstance(GetType())!;
     }
 }

osu.Game/Rulesets/Mods/Mod.cs

@@ -70,7 +70,7 @@ namespace osu.Game.Rulesets.Mods
 
                 foreach ((SettingSourceAttribute attr, PropertyInfo property) in this.GetOrderedSettingsSourceProperties())
                 {
-                    var bindable = (IBindable)property.GetValue(this);
+                    var bindable = (IBindable)property.GetValue(this)!;
 
                     if (!bindable.IsDefault)
                         tooltipTexts.Add($"{attr.Label} {bindable}");
@@ -134,7 +134,7 @@ namespace osu.Game.Rulesets.Mods
         /// </summary>
         public virtual Mod DeepClone()
         {
-            var result = (Mod)Activator.CreateInstance(GetType());
+            var result = (Mod)Activator.CreateInstance(GetType())!;
             result.CopyFrom(this);
             return result;
         }
@@ -150,8 +150,8 @@ namespace osu.Game.Rulesets.Mods
 
             foreach (var (_, prop) in this.GetSettingsSourceProperties())
             {
-                var targetBindable = (IBindable)prop.GetValue(this);
+                var targetBindable = (IBindable)prop.GetValue(this)!;
-                var sourceBindable = (IBindable)prop.GetValue(source);
+                var sourceBindable = (IBindable)prop.GetValue(source)!;
 
                 CopyAdjustedSetting(targetBindable, sourceBindable);
             }
@@ -183,9 +183,9 @@ namespace osu.Game.Rulesets.Mods
             }
         }
 
-        public bool Equals(IMod other) => other is Mod them && Equals(them);
+        public bool Equals(IMod? other) => other is Mod them && Equals(them);
 
-        public bool Equals(Mod other)
+        public bool Equals(Mod? other)
         {
             if (ReferenceEquals(null, other)) return false;
             if (ReferenceEquals(this, other)) return true;
@@ -209,16 +209,16 @@ namespace osu.Game.Rulesets.Mods
         /// <summary>
         /// Reset all custom settings for this mod back to their defaults.
         /// </summary>
-        public virtual void ResetSettingsToDefaults() => CopyFrom((Mod)Activator.CreateInstance(GetType()));
+        public virtual void ResetSettingsToDefaults() => CopyFrom((Mod)Activator.CreateInstance(GetType())!);
 
         private class ModSettingsEqualityComparer : IEqualityComparer<IBindable>
         {
             public static ModSettingsEqualityComparer Default { get; } = new ModSettingsEqualityComparer();
 
-            public bool Equals(IBindable x, IBindable y)
+            public bool Equals(IBindable? x, IBindable? y)
             {
-                object xValue = x.GetUnderlyingSettingValue();
+                object? xValue = x?.GetUnderlyingSettingValue();
-                object yValue = y.GetUnderlyingSettingValue();
+                object? yValue = y?.GetUnderlyingSettingValue();
 
                 return EqualityComparer<object>.Default.Equals(xValue, yValue);
             }

osu.Game/Rulesets/Objects/SliderPath.cs

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Diagnostics;
 using System.Linq;
 using Newtonsoft.Json;
 using osu.Framework.Bindables;
@@ -57,12 +58,16 @@ namespace osu.Game.Rulesets.Objects
                 switch (args.Action)
                 {
                     case NotifyCollectionChangedAction.Add:
+                        Debug.Assert(args.NewItems != null);
+
                         foreach (var c in args.NewItems.Cast<PathControlPoint>())
                             c.Changed += invalidate;
                         break;
 
                     case NotifyCollectionChangedAction.Reset:
                     case NotifyCollectionChangedAction.Remove:
+                        Debug.Assert(args.OldItems != null);
+
                         foreach (var c in args.OldItems.Cast<PathControlPoint>())
                             c.Changed -= invalidate;
                         break;

osu.Game/Rulesets/Ruleset.cs

@@ -89,6 +89,8 @@ namespace osu.Game.Rulesets
                                                        // Confine all mods of each mod type into a single IEnumerable<Mod>
                                                        .SelectMany(GetModsFor)
                                                        // Filter out all null mods
+                                                       // This is to handle old rulesets which were doing mods bad. Can be removed at some point we are sure nulls will not appear here.
+                                                       // ReSharper disable once ConditionIsAlwaysTrueOrFalseAccordingToNullableAPIContract
                                                        .Where(mod => mod != null)
                                                        // Resolve MultiMods as their .Mods property
                                                        .SelectMany(mod => (mod as MultiMod)?.Mods ?? new[] { mod });

osu.Game/Rulesets/RulesetInfo.cs

@@ -53,21 +53,21 @@ namespace osu.Game.Rulesets
 
         public bool Equals(IRulesetInfo? other) => other is RulesetInfo r && Equals(r);
 
-        public int CompareTo(RulesetInfo other)
+        public int CompareTo(RulesetInfo? other)
         {
-            if (OnlineID >= 0 && other.OnlineID >= 0)
+            if (OnlineID >= 0 && other?.OnlineID >= 0)
                 return OnlineID.CompareTo(other.OnlineID);
 
             // Official rulesets are always given precedence for the time being.
             if (OnlineID >= 0)
                 return -1;
-            if (other.OnlineID >= 0)
+            if (other?.OnlineID >= 0)
                 return 1;
 
-            return string.Compare(ShortName, other.ShortName, StringComparison.Ordinal);
+            return string.Compare(ShortName, other?.ShortName, StringComparison.Ordinal);
         }
 
-        public int CompareTo(IRulesetInfo other)
+        public int CompareTo(IRulesetInfo? other)
         {
             if (!(other is RulesetInfo ruleset))
                 throw new ArgumentException($@"Object is not of type {nameof(RulesetInfo)}.", nameof(other));

osu.Game/Rulesets/RulesetStore.cs

@@ -127,7 +127,7 @@ namespace osu.Game.Rulesets
 
         private void loadRulesetFromFile(string file)
         {
-            string? filename = Path.GetFileNameWithoutExtension(file);
+            string filename = Path.GetFileNameWithoutExtension(file);
 
             if (LoadedAssemblies.Values.Any(t => Path.GetFileNameWithoutExtension(t.Assembly.Location) == filename))
                 return;
@@ -158,7 +158,7 @@ namespace osu.Game.Rulesets
             }
             catch (Exception e)
             {
-                LogFailedLoad(assembly.GetName().Name.Split('.').Last(), e);
+                LogFailedLoad(assembly.GetName().Name!.Split('.').Last(), e);
             }
         }
 

osu.Game/Rulesets/Scoring/HealthProcessor.cs

@@ -80,7 +80,7 @@ namespace osu.Game.Rulesets.Scoring
             {
                 foreach (var condition in FailConditions.GetInvocationList())
                 {
-                    bool conditionResult = (bool)condition.Method.Invoke(condition.Target, new object[] { this, result });
+                    bool conditionResult = (bool)condition.Method.Invoke(condition.Target, new object[] { this, result })!;
                     if (conditionResult)
                         return true;
                 }

osu.Game/Scoring/ScoreInfo.cs

@@ -317,7 +317,7 @@ namespace osu.Game.Scoring
 
         #endregion
 
-        public bool Equals(ScoreInfo other) => other.ID == ID;
+        public bool Equals(ScoreInfo? other) => other?.ID == ID;
 
         public override string ToString() => this.GetDisplayTitle();
     }

osu.Game/Scoring/ScoreModelDownloader.cs

@@ -17,7 +17,7 @@ namespace osu.Game.Scoring
 
         protected override ArchiveDownloadRequest<IScoreInfo> CreateDownloadRequest(IScoreInfo score, bool minimiseDownload) => new DownloadReplayRequest(score);
 
-        public override ArchiveDownloadRequest<IScoreInfo> GetExistingDownload(IScoreInfo model)
+        public override ArchiveDownloadRequest<IScoreInfo>? GetExistingDownload(IScoreInfo model)
             => CurrentDownloads.Find(r => r.Model.MatchesOnlineID(model));
     }
 }

osu.Game/Screens/Edit/Components/Timelines/Summary/Parts/ControlPointPart.cs

@@ -5,6 +5,7 @@
 
 using System;
 using System.Collections.Specialized;
+using System.Diagnostics;
 using System.Linq;
 using osu.Framework.Bindables;
 using osu.Game.Beatmaps.ControlPoints;
@@ -33,6 +34,8 @@ namespace osu.Game.Screens.Edit.Components.Timelines.Summary.Parts
                         break;
 
                     case NotifyCollectionChangedAction.Add:
+                        Debug.Assert(args.NewItems != null);
+
                         foreach (var group in args.NewItems.OfType<ControlPointGroup>())
                         {
                             // as an optimisation, don't add a visualisation if there are already groups with the same types in close proximity.
@@ -47,6 +50,8 @@ namespace osu.Game.Screens.Edit.Components.Timelines.Summary.Parts
                         break;
 
                     case NotifyCollectionChangedAction.Remove:
+                        Debug.Assert(args.OldItems != null);
+
                         foreach (var group in args.OldItems.OfType<ControlPointGroup>())
                         {
                             var matching = Children.SingleOrDefault(gv => ReferenceEquals(gv.Group, group));

osu.Game/Screens/Edit/Compose/Components/BlueprintContainer.cs

@@ -58,6 +58,8 @@ namespace osu.Game.Screens.Edit.Compose.Components
                 switch (args.Action)
                 {
                     case NotifyCollectionChangedAction.Add:
+                        Debug.Assert(args.NewItems != null);
+
                         foreach (object o in args.NewItems)
                         {
                             if (blueprintMap.TryGetValue((T)o, out var blueprint))
@@ -67,6 +69,8 @@ namespace osu.Game.Screens.Edit.Compose.Components
                         break;
 
                     case NotifyCollectionChangedAction.Remove:
+                        Debug.Assert(args.OldItems != null);
+
                         foreach (object o in args.OldItems)
                         {
                             if (blueprintMap.TryGetValue((T)o, out var blueprint))

osu.Game/Screens/Edit/Compose/Components/Timeline/TimelineControlPointDisplay.cs

@@ -4,6 +4,7 @@
 #nullable disable
 
 using System.Collections.Specialized;
+using System.Diagnostics;
 using System.Linq;
 using osu.Framework.Bindables;
 using osu.Game.Beatmaps.ControlPoints;
@@ -33,11 +34,15 @@ namespace osu.Game.Screens.Edit.Compose.Components.Timeline
                         break;
 
                     case NotifyCollectionChangedAction.Add:
+                        Debug.Assert(args.NewItems != null);
+
                         foreach (var group in args.NewItems.OfType<ControlPointGroup>())
                             Add(new TimelineControlPointGroup(group));
                         break;
 
                     case NotifyCollectionChangedAction.Remove:
+                        Debug.Assert(args.OldItems != null);
+
                         foreach (var group in args.OldItems.OfType<ControlPointGroup>())
                         {
                             var matching = Children.SingleOrDefault(gv => ReferenceEquals(gv.Group, group));

osu.Game/Screens/Edit/EditorBeatmap.cs

@@ -304,7 +304,7 @@ namespace osu.Game.Screens.Edit
         /// <param name="index">The index of the <see cref="HitObject"/> to remove.</param>
         public void RemoveAt(int index)
         {
-            var hitObject = (HitObject)mutableHitObjects[index];
+            HitObject hitObject = (HitObject)mutableHitObjects[index]!;
 
             mutableHitObjects.RemoveAt(index);
 

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

@@ -4,6 +4,7 @@
 #nullable disable
 
 using System.Collections.Specialized;
+using System.Diagnostics;
 using System.Linq;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
@@ -197,11 +198,15 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
+                    Debug.Assert(e.NewItems != null);
+
                     foreach (var added in e.NewItems.OfType<APIUser>())
                         addUser(added);
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
+                    Debug.Assert(e.OldItems != null);
+
                     foreach (var removed in e.OldItems.OfType<APIUser>())
                         removeUser(removed);
                     break;

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

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Diagnostics;
 using System.Linq;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
@@ -117,10 +118,14 @@ namespace osu.Game.Screens.OnlinePlay.Lounge.Components
             switch (args.Action)
             {
                 case NotifyCollectionChangedAction.Add:
+                    Debug.Assert(args.NewItems != null);
+
                     addRooms(args.NewItems.Cast<Room>());
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
+                    Debug.Assert(args.OldItems != null);
+
                     removeRooms(args.OldItems.Cast<Room>());
                     break;
             }

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/MultiSpectatorScreen.cs

@@ -34,7 +34,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
         /// <summary>
         /// Whether all spectating players have finished loading.
         /// </summary>
-        public bool AllPlayersLoaded => instances.All(p => p?.PlayerLoaded == true);
+        public bool AllPlayersLoaded => instances.All(p => p.PlayerLoaded);
 
         protected override UserActivity InitialActivity => new UserActivity.SpectatingMultiplayerGame(Beatmap.Value.BeatmapInfo, Ruleset.Value);
 

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

@@ -349,7 +349,7 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
 
             public void SelectBeatmap() => editPlaylistButton.TriggerClick();
 
-            private void onPlaylistChanged(object sender, NotifyCollectionChangedEventArgs e) =>
+            private void onPlaylistChanged(object? sender, NotifyCollectionChangedEventArgs e) =>
                 playlistLength.Text = $"Length: {Playlist.GetTotalDuration()}";
 
             private bool hasValidSettings => RoomID.Value == null && NameField.Text.Length > 0 && Playlist.Count > 0;

osu.Game/Screens/Play/HUD/MultiplayerGameplayLeaderboard.cs

@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Diagnostics;
 using System.Linq;
 using System.Threading;
 using osu.Framework.Allocation;
@@ -153,11 +154,13 @@ namespace osu.Game.Screens.Play.HUD
             }
         }
 
-        private void playingUsersChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void playingUsersChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Remove:
+                    Debug.Assert(e.OldItems != null);
+
                     foreach (int userId in e.OldItems.OfType<int>())
                     {
                         spectatorClient.StopWatchingUser(userId);

osu.Game/Screens/Play/HUD/SkinnableInfo.cs

@@ -67,7 +67,7 @@ namespace osu.Game.Screens.Play.HUD
 
             foreach (var (_, property) in component.GetSettingsSourceProperties())
             {
-                var bindable = (IBindable)property.GetValue(component);
+                var bindable = (IBindable)property.GetValue(component)!;
 
                 if (!bindable.IsDefault)
                     Settings.Add(property.Name.ToSnakeCase(), bindable.GetUnderlyingSettingValue());
@@ -88,7 +88,7 @@ namespace osu.Game.Screens.Play.HUD
         {
             try
             {
-                Drawable d = (Drawable)Activator.CreateInstance(Type);
+                Drawable d = (Drawable)Activator.CreateInstance(Type)!;
                 d.ApplySkinnableInfo(this);
                 return d;
             }

osu.Game/Screens/Utility/LatencyCertifierScreen.cs

@@ -259,10 +259,7 @@ namespace osu.Game.Screens.Utility
 
             var displayMode = host.Window?.CurrentDisplayMode.Value;
 
-            string exclusive = "unknown";
+            string exclusive = (host.Renderer as IWindowsRenderer)?.FullscreenCapability.ToString() ?? "unknown";
-
-            if (host.Renderer is IWindowsRenderer windowsRenderer)
-                exclusive = windowsRenderer.FullscreenCapability.ToString();
 
             statusText.Clear();
 

osu.Game/Skinning/Editor/SkinComponentToolbox.cs

@@ -2,7 +2,6 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
-using System.Diagnostics;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
@@ -59,9 +58,7 @@ namespace osu.Game.Skinning.Editor
         {
             try
             {
-                var instance = (Drawable)Activator.CreateInstance(type);
+                Drawable instance = (Drawable)Activator.CreateInstance(type)!;
-
-                Debug.Assert(instance != null);
 
                 if (!((ISkinnableDrawable)instance).IsEditable) return;
 

osu.Game/Skinning/GameplaySkinComponentLookup.cs

@@ -16,7 +16,7 @@ namespace osu.Game.Skinning
         }
 
         protected virtual string RulesetPrefix => string.Empty;
-        protected virtual string ComponentName => Component.ToString();
+        protected virtual string ComponentName => Component.ToString() ?? string.Empty;
 
         public string LookupName =>
             string.Join('/', new[] { "Gameplay", RulesetPrefix, ComponentName }.Where(s => !string.IsNullOrEmpty(s)));

osu.Game/Skinning/LegacySkin.cs

@@ -105,7 +105,7 @@ namespace osu.Game.Skinning
                     return SkinUtils.As<TValue>(GetComboColour(Configuration, comboColour.ColourIndex, comboColour.Combo));
 
                 case SkinCustomColourLookup customColour:
-                    return SkinUtils.As<TValue>(getCustomColour(Configuration, customColour.Lookup.ToString()));
+                    return SkinUtils.As<TValue>(getCustomColour(Configuration, customColour.Lookup.ToString() ?? string.Empty));
 
                 case LegacyManiaSkinConfigurationLookup maniaLookup:
                     if (!AllowManiaSkin)
@@ -277,7 +277,7 @@ namespace osu.Game.Skinning
             => source.CustomColours.TryGetValue(lookup, out var col) ? new Bindable<Color4>(col) : null;
 
         private IBindable<string>? getManiaImage(LegacyManiaSkinConfiguration source, string lookup)
-            => source.ImageLookups.TryGetValue(lookup, out string image) ? new Bindable<string>(image) : null;
+            => source.ImageLookups.TryGetValue(lookup, out string? image) ? new Bindable<string>(image) : null;
 
         private IBindable<TValue>? legacySettingLookup<TValue>(SkinConfiguration.LegacySetting legacySetting)
             where TValue : notnull
@@ -298,7 +298,7 @@ namespace osu.Game.Skinning
         {
             try
             {
-                if (Configuration.ConfigDictionary.TryGetValue(lookup.ToString(), out string val))
+                if (Configuration.ConfigDictionary.TryGetValue(lookup.ToString() ?? string.Empty, out string? val))
                 {
                     // special case for handling skins which use 1 or 0 to signify a boolean state.
                     // ..or in some cases 2 (https://github.com/ppy/osu/issues/18579).

osu.Game/Skinning/RealmBackedResourceStore.cs

@@ -52,7 +52,7 @@ namespace osu.Game.Skinning
 
         private string? getPathForFile(string filename)
         {
-            if (fileToStoragePathMapping.Value.TryGetValue(filename.ToLowerInvariant(), out string path))
+            if (fileToStoragePathMapping.Value.TryGetValue(filename.ToLowerInvariant(), out string? path))
                 return path;
 
             return null;

osu.Game/Skinning/SkinInfo.cs

@@ -56,7 +56,7 @@ namespace osu.Game.Skinning
                 return new TrianglesSkin(this, resources);
             }
 
-            return (Skin)Activator.CreateInstance(type, this, resources);
+            return (Skin)Activator.CreateInstance(type, this, resources)!;
         }
 
         public IList<RealmNamedFileUsage> Files { get; } = null!;

osu.Game/Tests/Beatmaps/BeatmapConversionTest.cs

@@ -185,7 +185,7 @@ namespace osu.Game.Tests.Beatmaps
 
         private Stream openResource(string name)
         {
-            string localPath = Path.GetDirectoryName(Uri.UnescapeDataString(new UriBuilder(Assembly.GetExecutingAssembly().CodeBase).Path)).AsNonNull();
+            string localPath = Path.GetDirectoryName(Uri.UnescapeDataString(new UriBuilder(Assembly.GetExecutingAssembly().Location).Path)).AsNonNull();
             return Assembly.LoadFrom(Path.Combine(localPath, $"{ResourceAssembly}.dll")).GetManifestResourceStream($@"{ResourceAssembly}.Resources.{name}");
         }
 

osu.Game/Tests/Beatmaps/DifficultyCalculatorTest.cs

@@ -54,7 +54,7 @@ namespace osu.Game.Tests.Beatmaps
 
         private Stream openResource(string name)
         {
-            string localPath = Path.GetDirectoryName(Uri.UnescapeDataString(new UriBuilder(Assembly.GetExecutingAssembly().CodeBase).Path)).AsNonNull();
+            string localPath = Path.GetDirectoryName(Uri.UnescapeDataString(new UriBuilder(Assembly.GetExecutingAssembly().Location).Path)).AsNonNull();
             return Assembly.LoadFrom(Path.Combine(localPath, $"{ResourceAssembly}.dll")).GetManifestResourceStream($@"{ResourceAssembly}.Resources.{name}");
         }
 

osu.Game/Tests/PollingNotificationsClient.cs

@@ -24,8 +24,8 @@ namespace osu.Game.Tests
             {
                 while (!cancellationToken.IsCancellationRequested)
                 {
-                    await API.PerformAsync(CreateInitialFetchRequest());
+                    await API.PerformAsync(CreateInitialFetchRequest()).ConfigureAwait(true);
-                    await Task.Delay(1000, cancellationToken);
+                    await Task.Delay(1000, cancellationToken).ConfigureAwait(true);
                 }
             }, cancellationToken);