Better default values + don't set Mode from outside.

osu.Desktop.VisualTests/Tests/TestCasePlaySongSelect.cs

@@ -8,6 +8,7 @@
 using osu.Game.Database;
 using osu.Game.Screens.Select;
 using osu.Game.Screens.Select.Filter;
+using osu.Game.Modes;
 
 namespace osu.Desktop.VisualTests.Tests
 {
@@ -71,7 +72,7 @@ private BeatmapSetInfo createTestBeatmapSet(int i)
                     new BeatmapInfo
                     {
                         OnlineBeatmapID = 1234 + i,
-                        Mode = 0,
+                        Ruleset = RulesetCollection.GetRuleset(0),
                         Path = "normal.osu",
                         Version = "Normal",
                         Difficulty = new BeatmapDifficulty
@@ -82,7 +83,7 @@ private BeatmapSetInfo createTestBeatmapSet(int i)
                     new BeatmapInfo
                     {
                         OnlineBeatmapID = 1235 + i,
-                        Mode = 0,
+                        Ruleset = RulesetCollection.GetRuleset(0),
                         Path = "hard.osu",
                         Version = "Hard",
                         Difficulty = new BeatmapDifficulty
@@ -93,7 +94,7 @@ private BeatmapSetInfo createTestBeatmapSet(int i)
                     new BeatmapInfo
                     {
                         OnlineBeatmapID = 1236 + i,
-                        Mode = 0,
+                        Ruleset = RulesetCollection.GetRuleset(0),
                         Path = "insane.osu",
                         Version = "Insane",
                         Difficulty = new BeatmapDifficulty

osu.Game/Database/BeatmapInfo.cs

@@ -56,7 +56,13 @@ public class BeatmapInfo : IEquatable<BeatmapInfo>, IJsonSerializable
         public bool SpecialStyle { get; set; }
 
         public int Mode { get; set; }
-        public Ruleset Ruleset => RulesetCollection.GetRuleset(Mode);
+
+        [Ignore]
+        public Ruleset Ruleset
+        {
+            get { return RulesetCollection.GetRuleset(Mode); }
+            set { Mode = RulesetCollection.GetId(value); }
+        }
         
         public bool LetterboxInBreaks { get; set; }
         public bool WidescreenStoryboard { get; set; }

osu.Game/Modes/RulesetCollection.cs

@@ -8,13 +8,16 @@
 
 namespace osu.Game.Modes
 {
+    /// <summary>
+    /// Todo: All of this needs to be moved to a RulesetDatabase.
+    /// </summary>
     public static class RulesetCollection
     {
         private static readonly ConcurrentDictionary<int, Ruleset> available_rulesets = new ConcurrentDictionary<int, Ruleset>();
 
         public static void Register(Type type)
         {
-            Ruleset ruleset = Activator.CreateInstance(type) as Ruleset;
+            var ruleset = Activator.CreateInstance(type) as Ruleset;
 
             if (ruleset == null)
                 return;
@@ -25,9 +28,9 @@ public static void Register(Type type)
         public static Ruleset GetRuleset(int rulesetId)
         {
             Ruleset ruleset;
-            
+
             if (!available_rulesets.TryGetValue(rulesetId, out ruleset))
-                throw new InvalidOperationException($"Ruleset id {rulesetId} doesn't exist. How did you trigger this?");
+                return null;
 
             return ruleset;
         }

osu.Game/Overlays/Mods/ModSelectOverlay.cs

@@ -37,7 +37,7 @@ public class ModSelectOverlay : WaveOverlayContainer
 
         public readonly Bindable<IEnumerable<Mod>> SelectedMods = new Bindable<IEnumerable<Mod>>();
 
-        public readonly Bindable<Ruleset> Ruleset = new Bindable<Ruleset>();
+        public readonly Bindable<Ruleset> Ruleset = new Bindable<Ruleset>(RulesetCollection.GetRuleset(0));
 
         private void rulesetChanged(Ruleset newRuleset)
         {

osu.Game/Screens/Select/FilterControl.cs

@@ -163,7 +163,7 @@ public void Activate()
             searchTextBox.HoldFocus = true;
         }
 
-        private readonly Bindable<Ruleset> ruleset = new Bindable<Ruleset>();
+        private readonly Bindable<Ruleset> ruleset = new Bindable<Ruleset>(RulesetCollection.GetRuleset(0));
 
         [BackgroundDependencyLoader(permitNulls:true)]
         private void load(OsuColour colours, OsuGame osu)

osu.Game/Screens/Select/SongSelect.cs

@@ -29,7 +29,7 @@ namespace osu.Game.Screens.Select
 {
     public abstract class SongSelect : OsuScreen
     {
-        private readonly Bindable<Ruleset> ruleset = new Bindable<Ruleset>();
+        private readonly Bindable<Ruleset> ruleset = new Bindable<Ruleset>(RulesetCollection.GetRuleset(0));
         private BeatmapDatabase database;
         protected override BackgroundScreen CreateBackground() => new BackgroundScreenBeatmap(Beatmap);