Introduce private APIRuleset for online ID equality comparison

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

@@ -98,13 +98,37 @@ private double lengthInSeconds
 
         public string MD5Hash => Checksum;
 
-        public IRulesetInfo Ruleset => new RulesetInfo { OnlineID = RulesetID };
+        public IRulesetInfo Ruleset => new APIRuleset { OnlineID = RulesetID };
 
         [JsonIgnore]
         public string Hash => throw new NotImplementedException();
 
         #endregion
 
         public bool Equals(IBeatmapInfo? other) => other is APIBeatmap b && this.MatchesOnlineID(b);
+
+        private class APIRuleset : IRulesetInfo
+        {
+            public int OnlineID { get; set; } = -1;
+
+            public string Name => $@"{nameof(APIRuleset)} (ID: {OnlineID})";
+            public string ShortName => nameof(APIRuleset);
+            public string InstantiationInfo => string.Empty;
+
+            public Ruleset CreateInstance() => throw new NotImplementedException();
+
+            public bool Equals(IRulesetInfo? other) => other is APIRuleset r && this.MatchesOnlineID(r);
+
+            public int CompareTo(IRulesetInfo other)
+            {
+                if (!(other is APIRuleset ruleset))
+                    throw new ArgumentException($@"Object is not of type {nameof(APIRuleset)}.", nameof(other));
+
+                return OnlineID.CompareTo(ruleset.OnlineID);
+            }
+
+            // ReSharper disable once NonReadonlyMemberInGetHashCode
+            public override int GetHashCode() => OnlineID;
+        }
     }
 }

osu.Game/Rulesets/RulesetInfo.cs

@@ -44,9 +44,6 @@ public bool Equals(RulesetInfo? other)
             if (ReferenceEquals(this, other)) return true;
             if (other == null) return false;
 
-            if (OnlineID >= 0 && other.OnlineID >= 0)
-                return OnlineID == other.OnlineID;
-
             return ShortName == other.ShortName;
         }