Enforce readonly private members where possible.

osu.Game/Screens/Backgrounds/BackgroundScreenBeatmap.cs

@@ -72,7 +72,7 @@ namespace osu.Game.Screens.Backgrounds
 
         private class BeatmapBackground : Background
         {
-            private WorkingBeatmap beatmap;
+            private readonly WorkingBeatmap beatmap;
 
             public BeatmapBackground(WorkingBeatmap beatmap)
             {

osu.Game/Screens/GameScreenWhiteBox.cs

@@ -19,14 +19,14 @@ namespace osu.Game.Screens
 {
     public class ScreenWhiteBox : OsuScreen
     {
-        private BackButton popButton;
+        private readonly BackButton popButton;
 
         private const double transition_time = 1000;
 
         protected virtual IEnumerable<Type> PossibleChildren => null;
 
-        private Container textContainer;
-        private Box box;
+        private readonly Container textContainer;
+        private readonly Box box;
 
         protected override BackgroundScreen CreateBackground() => new BackgroundScreenCustom(@"Backgrounds/bg2");
 

osu.Game/Screens/Menu/Button.cs

@@ -26,13 +26,13 @@ namespace osu.Game.Screens.Menu
     /// </summary>
     public class Button : Container, IStateful<ButtonState>
     {
-        private Container iconText;
-        private Container box;
-        private Box boxHoverLayer;
-        private TextAwesome icon;
-        private string internalName;
-        private Action clickAction;
-        private Key triggerKey;
+        private readonly Container iconText;
+        private readonly Container box;
+        private readonly Box boxHoverLayer;
+        private readonly TextAwesome icon;
+        private readonly string internalName;
+        private readonly Action clickAction;
+        private readonly Key triggerKey;
         private SampleChannel sampleClick;
 
         protected override bool InternalContains(Vector2 screenSpacePos) => box.Contains(screenSpacePos);

osu.Game/Screens/Menu/ButtonSystem.cs

@@ -32,7 +32,7 @@ namespace osu.Game.Screens.Menu
 
         private Toolbar toolbar;
 
-        private FlowContainerWithOrigin buttonFlow;
+        private readonly FlowContainerWithOrigin buttonFlow;
 
         //todo: make these non-internal somehow.
         internal const float BUTTON_AREA_HEIGHT = 100;
@@ -41,16 +41,16 @@ namespace osu.Game.Screens.Menu
 
         public const int EXIT_DELAY = 3000;
 
-        private OsuLogo osuLogo;
-        private Drawable iconFacade;
-        private Container buttonArea;
-        private Box buttonAreaBackground;
+        private readonly OsuLogo osuLogo;
+        private readonly Drawable iconFacade;
+        private readonly Container buttonArea;
+        private readonly Box buttonAreaBackground;
 
-        private Button backButton;
-        private Button settingsButton;
+        private readonly Button backButton;
+        private readonly Button settingsButton;
 
-        private List<Button> buttonsTopLevel = new List<Button>();
-        private List<Button> buttonsPlay = new List<Button>();
+        private readonly List<Button> buttonsTopLevel = new List<Button>();
+        private readonly List<Button> buttonsPlay = new List<Button>();
 
         public ButtonSystem()
         {

osu.Game/Screens/Menu/Disclaimer.cs

@@ -16,7 +16,7 @@ namespace osu.Game.Screens.Menu
     internal class Disclaimer : OsuScreen
     {
         private Intro intro;
-        private TextAwesome icon;
+        private readonly TextAwesome icon;
         private Color4 iconColour;
 
         internal override bool ShowOverlays => false;

osu.Game/Screens/Menu/Intro.cs

@@ -16,7 +16,7 @@ namespace osu.Game.Screens.Menu
 {
     public class Intro : OsuScreen
     {
-        private OsuLogo logo;
+        private readonly OsuLogo logo;
 
         /// <summary>
         /// Whether we have loaded the menu previously.

osu.Game/Screens/Menu/MainMenu.cs

@@ -22,11 +22,11 @@ namespace osu.Game.Screens.Menu
 {
     public class MainMenu : OsuScreen
     {
-        private ButtonSystem buttons;
+        private readonly ButtonSystem buttons;
 
         internal override bool ShowOverlays => buttons.State != MenuState.Initial;
 
-        private BackgroundScreen background;
+        private readonly BackgroundScreen background;
         private Screen songSelect;
 
         protected override BackgroundScreen CreateBackground() => background;

osu.Game/Screens/Menu/OsuLogo.cs

@@ -25,22 +25,22 @@ namespace osu.Game.Screens.Menu
     {
         public Color4 OsuPink = OsuColour.FromHex(@"e967a1");
 
-        private Sprite logo;
-        private CircularContainer logoContainer;
-        private Container logoBounceContainer;
-        private Container logoHoverContainer;
+        private readonly Sprite logo;
+        private readonly CircularContainer logoContainer;
+        private readonly Container logoBounceContainer;
+        private readonly Container logoHoverContainer;
 
         private SampleChannel sampleClick;
 
-        private Container colourAndTriangles;
+        private readonly Container colourAndTriangles;
 
         public Action Action;
 
         public float SizeForFlow => logo == null ? 0 : logo.DrawSize.X * logo.Scale.X * logoBounceContainer.Scale.X * logoHoverContainer.Scale.X * 0.78f;
 
-        private Sprite ripple;
+        private readonly Sprite ripple;
 
-        private Container rippleContainer;
+        private readonly Container rippleContainer;
 
         public bool Triangles
         {
@@ -62,7 +62,7 @@ namespace osu.Game.Screens.Menu
         }
 
         public bool Interactive = true;
-        private Box flashLayer;
+        private readonly Box flashLayer;
 
         public OsuLogo()
         {

osu.Game/Screens/Play/KeyCounterCollection.cs

@@ -106,7 +106,7 @@ namespace osu.Game.Screens.Play
 
         public class Receptor : Drawable
         {
-            private KeyCounterCollection target;
+            private readonly KeyCounterCollection target;
 
             public Receptor(KeyCounterCollection target)
             {

osu.Game/Screens/Play/Pause/PauseProgressBar.cs

@@ -13,10 +13,10 @@ namespace osu.Game.Screens.Play.Pause
 {
     public class PauseProgressBar : Container
     {
-        private Color4 fillColour = new Color4(221, 255, 255, 255);
-        private Color4 glowColour = new Color4(221, 255, 255, 150);
+        private readonly Color4 fillColour = new Color4(221, 255, 255, 255);
+        private readonly Color4 glowColour = new Color4(221, 255, 255, 150);
 
-        private Container fill;
+        private readonly Container fill;
         private WorkingBeatmap current;
 
         [BackgroundDependencyLoader]

osu.Game/Screens/Play/Player.cs

@@ -42,7 +42,7 @@ namespace osu.Game.Screens.Play
 
         public int RestartCount;
 
-        private double pauseCooldown = 1000;
+        private readonly double pauseCooldown = 1000;
         private double lastPauseActionTime;
 
         private bool canPause => Time.Current >= lastPauseActionTime + pauseCooldown;

osu.Game/Screens/Play/PlayerInputManager.cs

@@ -9,7 +9,7 @@ namespace osu.Game.Screens.Play
 {
     public class PlayerInputManager : PassThroughInputManager
     {
-        private ManualClock clock = new ManualClock();
+        private readonly ManualClock clock = new ManualClock();
         private IFrameBasedClock parentClock;
 
         private ReplayInputHandler replayInputHandler;

osu.Game/Screens/Play/PlayerLoader.cs

@@ -21,7 +21,7 @@ namespace osu.Game.Screens.Play
     public class PlayerLoader : OsuScreen
     {
         private readonly Player player;
-        private OsuLogo logo;
+        private readonly OsuLogo logo;
         private BeatmapMetadataDisplay info;
 
         protected override BackgroundScreen CreateBackground() => new BackgroundScreenBeatmap(Beatmap);

osu.Game/Screens/Select/BeatmapCarousel.cs

@@ -60,18 +60,18 @@ namespace osu.Game.Screens.Select
             }
         }
 
-        private List<float> yPositions = new List<float>();
+        private readonly List<float> yPositions = new List<float>();
 
         /// <summary>
         /// Required for now unfortunately.
         /// </summary>
         private BeatmapDatabase database;
 
-        private Container<Panel> scrollableContent;
+        private readonly Container<Panel> scrollableContent;
 
-        private List<BeatmapGroup> groups = new List<BeatmapGroup>();
+        private readonly List<BeatmapGroup> groups = new List<BeatmapGroup>();
 
-        private List<Panel> panels = new List<Panel>();
+        private readonly List<Panel> panels = new List<Panel>();
 
         private BeatmapGroup selectedGroup;
 

osu.Game/Screens/Select/FilterControl.cs

@@ -24,9 +24,9 @@ namespace osu.Game.Screens.Select
     {
         public Action<FilterCriteria> FilterChanged;
 
-        private OsuTabControl<SortMode> sortTabs;
+        private readonly OsuTabControl<SortMode> sortTabs;
 
-        private TabControl<GroupMode> groupTabs;
+        private readonly TabControl<GroupMode> groupTabs;
 
         private SortMode sort = SortMode.Title;
         public SortMode Sort
@@ -66,7 +66,7 @@ namespace osu.Game.Screens.Select
 
         public Action Exit;
 
-        private SearchTextBox searchTextBox;
+        private readonly SearchTextBox searchTextBox;
 
         protected override bool InternalContains(Vector2 screenSpacePos) => base.InternalContains(screenSpacePos) || groupTabs.Contains(screenSpacePos) || sortTabs.Contains(screenSpacePos);
 

osu.Game/Screens/Select/Footer.cs

@@ -17,7 +17,7 @@ namespace osu.Game.Screens.Select
 {
     public class Footer : Container
     {
-        private Box modeLight;
+        private readonly Box modeLight;
 
         private const float play_song_select_button_width = 100;
         private const float play_song_select_button_height = 50;
@@ -29,7 +29,7 @@ namespace osu.Game.Screens.Select
         public Action OnBack;
         public Action OnStart;
 
-        private FillFlowContainer buttons;
+        private readonly FillFlowContainer buttons;
 
         public OsuLogo StartButton;
 

osu.Game/Screens/Select/FooterButton.cs

@@ -51,9 +51,9 @@ namespace osu.Game.Screens.Select
             }
         }
 
-        private SpriteText spriteText;
-        private Box box;
-        private Box light;
+        private readonly SpriteText spriteText;
+        private readonly Box box;
+        private readonly Box light;
 
         public FooterButton()
         {

osu.Game/Screens/Select/Leaderboards/DrawableRank.cs

@@ -13,7 +13,7 @@ namespace osu.Game.Screens.Select.Leaderboards
 {
     public class DrawableRank : Container
     {
-        private Sprite sprite;
+        private readonly Sprite sprite;
 
         public ScoreRank Rank { get; private set; }
 

osu.Game/Screens/Select/Leaderboards/Leaderboard.cs

@@ -16,8 +16,8 @@ namespace osu.Game.Screens.Select.Leaderboards
 {
     public class Leaderboard : Container
     {
-        private ScrollContainer scrollContainer;
-        private FillFlowContainer<LeaderboardScore> scrollFlow;
+        private readonly ScrollContainer scrollContainer;
+        private readonly FillFlowContainer<LeaderboardScore> scrollFlow;
 
         private IEnumerable<Score> scores;
         public IEnumerable<Score> Scores

osu.Game/Screens/Select/Leaderboards/LeaderboardScore.cs

@@ -30,14 +30,16 @@ namespace osu.Game.Screens.Select.Leaderboards
         private const float background_alpha = 0.25f;
         private const float rank_width = 30;
 
-        private Box background;
-        private Container content, avatar;
-        private DrawableRank scoreRank;
-        private OsuSpriteText nameLabel;
-        private GlowingSpriteText scoreLabel;
-        private ScoreComponentLabel maxCombo, accuracy;
-        private Container flagBadgeContainer;
-        private FillFlowContainer<ScoreModIcon> modsContainer;
+        private readonly Box background;
+        private readonly Container content;
+        private readonly Container avatar;
+        private readonly DrawableRank scoreRank;
+        private readonly OsuSpriteText nameLabel;
+        private readonly GlowingSpriteText scoreLabel;
+        private readonly ScoreComponentLabel maxCombo;
+        private readonly ScoreComponentLabel accuracy;
+        private readonly Container flagBadgeContainer;
+        private readonly FillFlowContainer<ScoreModIcon> modsContainer;
 
         private Visibility state;
         public Visibility State

osu.Game/Screens/Select/Options/BeatmapOptionsButton.cs

@@ -20,10 +20,12 @@ namespace osu.Game.Screens.Select.Options
     {
         private static readonly float width = 130;
 
-        private Box background, flash;
-        private TextAwesome iconText;
-        private OsuSpriteText firstLine, secondLine;
-        private Container box;
+        private readonly Box background;
+        private readonly Box flash;
+        private readonly TextAwesome iconText;
+        private readonly OsuSpriteText firstLine;
+        private readonly OsuSpriteText secondLine;
+        private readonly Container box;
 
         public Color4 ButtonColour
         {

osu.Game/Screens/Select/Options/BeatmapOptionsOverlay.cs

@@ -24,8 +24,8 @@ namespace osu.Game.Screens.Select.Options
 
         private const float height = 100;
 
-        private Box holder;
-        private FillFlowContainer<BeatmapOptionsButton> buttonsContainer;
+        private readonly Box holder;
+        private readonly FillFlowContainer<BeatmapOptionsButton> buttonsContainer;
 
         protected override void PopIn()
         {

osu.Game/Screens/Select/PlaySongSelect.cs

@@ -19,8 +19,8 @@ namespace osu.Game.Screens.Select
     public class PlaySongSelect : SongSelect
     {
         private OsuScreen player;
-        private ModSelectOverlay modSelect;
-        private Leaderboard leaderboard;
+        private readonly ModSelectOverlay modSelect;
+        private readonly Leaderboard leaderboard;
 
         public PlaySongSelect()
         {

osu.Game/Screens/Select/SongSelect.cs

@@ -31,20 +31,19 @@ namespace osu.Game.Screens.Select
 {
     public abstract class SongSelect : OsuScreen
     {
-        private Bindable<PlayMode> playMode = new Bindable<PlayMode>();
+        private readonly Bindable<PlayMode> playMode = new Bindable<PlayMode>();
         private BeatmapDatabase database;
         protected override BackgroundScreen CreateBackground() => new BackgroundScreenBeatmap(Beatmap);
 
-        private BeatmapCarousel carousel;
+        private readonly BeatmapCarousel carousel;
         private TrackManager trackManager;
         private DialogOverlay dialogOverlay;
 
-
         private static readonly Vector2 wedged_container_size = new Vector2(0.5f, 245);
 
         private const float left_area_padding = 20;
 
-        private BeatmapInfoWedge beatmapInfoWedge;
+        private readonly BeatmapInfoWedge beatmapInfoWedge;
 
         protected Container LeftContent;
 

osu.Game/Screens/Tournament/Components/VisualiserContainer.cs

@@ -30,7 +30,7 @@ namespace osu.Game.Screens.Tournament.Components
             }
         }
 
-        private List<VisualiserLine> allLines = new List<VisualiserLine>();
+        private readonly List<VisualiserLine> allLines = new List<VisualiserLine>();
 
         private float offset;
 

osu.Game/Screens/Tournament/Drawings.cs

@@ -37,7 +37,7 @@ namespace osu.Game.Screens.Tournament
         private GroupContainer groupsContainer;
         private OsuSpriteText fullTeamNameText;
 
-        private List<Country> allTeams = new List<Country>();
+        private readonly List<Country> allTeams = new List<Country>();
 
         private DrawingsConfigManager drawingsConfig;
 

osu.Game/Screens/Tournament/Group.cs

@@ -23,9 +23,9 @@ namespace osu.Game.Screens.Tournament
 
         public int TeamsCount { get; private set; }
 
-        private FlowContainer<GroupTeam> teams;
+        private readonly FlowContainer<GroupTeam> teams;
 
-        private List<GroupTeam> allTeams = new List<GroupTeam>();
+        private readonly List<GroupTeam> allTeams = new List<GroupTeam>();
 
         public Group(string name)
         {
@@ -122,10 +122,10 @@ namespace osu.Game.Screens.Tournament
 
         private class GroupTeam : Container
         {
-            public Country Team;
+            public readonly Country Team;
 
-            private FillFlowContainer innerContainer;
-            private Sprite flagSprite;
+            private readonly FillFlowContainer innerContainer;
+            private readonly Sprite flagSprite;
 
             public GroupTeam(Country team)
             {

osu.Game/Screens/Tournament/GroupContainer.cs

@@ -14,9 +14,9 @@ namespace osu.Game.Screens.Tournament
 {
     public class GroupContainer : Container
     {
-        private List<Group> groups = new List<Group>();
+        private readonly List<Group> groups = new List<Group>();
 
-        private int maxTeams;
+        private readonly int maxTeams;
         private int currentGroup;
 
         public GroupContainer(int numGroups, int teamsPerGroup)

osu.Game/Screens/Tournament/ScrollingTeamContainer.cs

@@ -26,7 +26,7 @@ namespace osu.Game.Screens.Tournament
 
         private readonly List<Country> availableTeams = new List<Country>();
 
-        private Container tracker;
+        private readonly Container tracker;
 
         private float speed;
         private int expiredCount;
@@ -328,8 +328,8 @@ namespace osu.Game.Screens.Tournament
 
             public Country Team;
 
-            private Sprite flagSprite;
-            private Box outline;
+            private readonly Sprite flagSprite;
+            private readonly Box outline;
 
             private bool selected;
             public bool Selected

osu.Game/Screens/Tournament/Teams/StorageBackedTeamList.cs

@@ -14,7 +14,7 @@ namespace osu.Game.Screens.Tournament.Teams
     {
         private const string teams_filename = "drawings.txt";
 
-        private Storage storage;
+        private readonly Storage storage;
 
         public StorageBackedTeamList(Storage storage)
         {