Make IBeatmapConverter an abstract class instead of an interface.

osu.Game.Modes.Catch/Beatmaps/CatchBeatmapConverter.cs

@@ -10,11 +10,11 @@ using osu.Game.Modes.Beatmaps;
 
 namespace osu.Game.Modes.Catch.Beatmaps
 {
-    internal class CatchBeatmapConverter : IBeatmapConverter<CatchBaseHit>
+    internal class CatchBeatmapConverter : BeatmapConverter<CatchBaseHit>
     {
-        public IEnumerable<Type> ValidConversionTypes { get; } = new[] { typeof(IHasXPosition) };
+        public override IEnumerable<Type> ValidConversionTypes { get; } = new[] { typeof(IHasXPosition) };
 
-        public Beatmap<CatchBaseHit> Convert(Beatmap original)
+        public override Beatmap<CatchBaseHit> Convert(Beatmap original)
         {
             return new Beatmap<CatchBaseHit>(original)
             {

osu.Game.Modes.Catch/CatchDifficultyCalculator.cs

@@ -20,6 +20,6 @@ namespace osu.Game.Modes.Catch
             return 0;
         }
 
-        protected override IBeatmapConverter<CatchBaseHit> CreateBeatmapConverter() => new CatchBeatmapConverter();
+        protected override BeatmapConverter<CatchBaseHit> CreateBeatmapConverter() => new CatchBeatmapConverter();
     }
 }

osu.Game.Modes.Catch/UI/CatchHitRenderer.cs

@@ -22,7 +22,7 @@ namespace osu.Game.Modes.Catch.UI
 
         public override ScoreProcessor CreateScoreProcessor() => new CatchScoreProcessor(this);
 
-        protected override IBeatmapConverter<CatchBaseHit> CreateBeatmapConverter() => new CatchBeatmapConverter();
+        protected override BeatmapConverter<CatchBaseHit> CreateBeatmapConverter() => new CatchBeatmapConverter();
 
         protected override IBeatmapProcessor<CatchBaseHit> CreateBeatmapProcessor() => new CatchBeatmapProcessor();
 

osu.Game.Modes.Mania/Beatmaps/ManiaBeatmapConverter.cs

@@ -10,11 +10,11 @@ using osu.Game.Modes.Beatmaps;
 
 namespace osu.Game.Modes.Mania.Beatmaps
 {
-    internal class ManiaBeatmapConverter : IBeatmapConverter<ManiaBaseHit>
+    internal class ManiaBeatmapConverter : BeatmapConverter<ManiaBaseHit>
     {
-        public IEnumerable<Type> ValidConversionTypes { get; } = new[] { typeof(IHasColumn) };
+        public override IEnumerable<Type> ValidConversionTypes { get; } = new[] { typeof(IHasColumn) };
 
-        public Beatmap<ManiaBaseHit> Convert(Beatmap original)
+        public override Beatmap<ManiaBaseHit> Convert(Beatmap original)
         {
             return new Beatmap<ManiaBaseHit>(original)
             {

osu.Game.Modes.Mania/ManiaDifficultyCalculator.cs

@@ -21,6 +21,6 @@ namespace osu.Game.Modes.Mania
             return 0;
         }
 
-        protected override IBeatmapConverter<ManiaBaseHit> CreateBeatmapConverter() => new ManiaBeatmapConverter();
+        protected override BeatmapConverter<ManiaBaseHit> CreateBeatmapConverter() => new ManiaBeatmapConverter();
     }
 }

osu.Game.Modes.Mania/UI/ManiaHitRenderer.cs

@@ -25,7 +25,7 @@ namespace osu.Game.Modes.Mania.UI
 
         public override ScoreProcessor CreateScoreProcessor() => new ManiaScoreProcessor(this);
 
-        protected override IBeatmapConverter<ManiaBaseHit> CreateBeatmapConverter() => new ManiaBeatmapConverter();
+        protected override BeatmapConverter<ManiaBaseHit> CreateBeatmapConverter() => new ManiaBeatmapConverter();
 
         protected override IBeatmapProcessor<ManiaBaseHit> CreateBeatmapProcessor() => new ManiaBeatmapProcessor();
 

osu.Game.Modes.Osu/Beatmaps/OsuBeatmapConverter.cs

@@ -14,11 +14,11 @@ using osu.Game.Modes.Beatmaps;
 
 namespace osu.Game.Modes.Osu.Beatmaps
 {
-    internal class OsuBeatmapConverter : IBeatmapConverter<OsuHitObject>
+    internal class OsuBeatmapConverter : BeatmapConverter<OsuHitObject>
     {
-        public IEnumerable<Type> ValidConversionTypes { get; } = new[] { typeof(IHasPosition) };
+        public override IEnumerable<Type> ValidConversionTypes { get; } = new[] { typeof(IHasPosition) };
 
-        public Beatmap<OsuHitObject> Convert(Beatmap original)
+        public override Beatmap<OsuHitObject> Convert(Beatmap original)
         {
             return new Beatmap<OsuHitObject>(original)
             {

osu.Game.Modes.Osu/OsuDifficultyCalculator.cs

@@ -181,7 +181,7 @@ namespace osu.Game.Modes.Osu
             return difficulty;
         }
 
-        protected override IBeatmapConverter<OsuHitObject> CreateBeatmapConverter() => new OsuBeatmapConverter();
+        protected override BeatmapConverter<OsuHitObject> CreateBeatmapConverter() => new OsuBeatmapConverter();
 
         // Those values are used as array indices. Be careful when changing them!
         public enum DifficultyType

osu.Game.Modes.Osu/UI/OsuHitRenderer.cs

@@ -25,7 +25,7 @@ namespace osu.Game.Modes.Osu.UI
 
         public override ScoreProcessor CreateScoreProcessor() => new OsuScoreProcessor(this);
 
-        protected override IBeatmapConverter<OsuHitObject> CreateBeatmapConverter() => new OsuBeatmapConverter();
+        protected override BeatmapConverter<OsuHitObject> CreateBeatmapConverter() => new OsuBeatmapConverter();
 
         protected override IBeatmapProcessor<OsuHitObject> CreateBeatmapProcessor() => new OsuBeatmapProcessor();
 

osu.Game.Modes.Taiko/Beatmaps/TaikoBeatmapConverter.cs

@@ -15,7 +15,7 @@ using osu.Game.Modes.Beatmaps;
 
 namespace osu.Game.Modes.Taiko.Beatmaps
 {
-    internal class TaikoBeatmapConverter : IBeatmapConverter<TaikoHitObject>
+    internal class TaikoBeatmapConverter : BeatmapConverter<TaikoHitObject>
     {
         /// <summary>
         /// osu! is generally slower than taiko, so a factor is added to increase
@@ -39,9 +39,9 @@ namespace osu.Game.Modes.Taiko.Beatmaps
         /// </summary>
         private const float taiko_base_distance = 100;
 
-        public IEnumerable<Type> ValidConversionTypes { get; } = new[] { typeof(HitObject) };
+        public override IEnumerable<Type> ValidConversionTypes { get; } = new[] { typeof(HitObject) };
 
-        public Beatmap<TaikoHitObject> Convert(Beatmap original)
+        public override Beatmap<TaikoHitObject> Convert(Beatmap original)
         {
             BeatmapInfo info = original.BeatmapInfo.DeepClone<BeatmapInfo>();
             info.Difficulty.SliderMultiplier *= legacy_velocity_multiplier;

osu.Game.Modes.Taiko/TaikoDifficultyCalculator.cs

@@ -20,6 +20,6 @@ namespace osu.Game.Modes.Taiko
             return 0;
         }
 
-        protected override IBeatmapConverter<TaikoHitObject> CreateBeatmapConverter() => new TaikoBeatmapConverter();
+        protected override BeatmapConverter<TaikoHitObject> CreateBeatmapConverter() => new TaikoBeatmapConverter();
     }
 }

osu.Game.Modes.Taiko/UI/TaikoHitRenderer.cs

@@ -115,7 +115,7 @@ namespace osu.Game.Modes.Taiko.UI
 
         public override ScoreProcessor CreateScoreProcessor() => new TaikoScoreProcessor(this);
 
-        protected override IBeatmapConverter<TaikoHitObject> CreateBeatmapConverter() => new TaikoBeatmapConverter();
+        protected override BeatmapConverter<TaikoHitObject> CreateBeatmapConverter() => new TaikoBeatmapConverter();
 
         protected override IBeatmapProcessor<TaikoHitObject> CreateBeatmapProcessor() => new TaikoBeatmapProcessor();
 

osu.Game/Beatmaps/DifficultyCalculator.cs

@@ -42,6 +42,6 @@ namespace osu.Game.Beatmaps
         {
         }
 
-        protected abstract IBeatmapConverter<T> CreateBeatmapConverter();
+        protected abstract BeatmapConverter<T> CreateBeatmapConverter();
     }
 }

osu.Game/Modes/Beatmaps/BeatmapConverter.cs

@@ -13,30 +13,25 @@ namespace osu.Game.Modes.Beatmaps
     /// Converts a Beatmap for another mode.
     /// </summary>
     /// <typeparam name="T">The type of HitObject stored in the Beatmap.</typeparam>
-    public interface IBeatmapConverter<T> where T : HitObject
+    public abstract class BeatmapConverter<T> where T : HitObject
     {
         /// <summary>
         /// The types of HitObjects that can be converted to be used for this Beatmap.
         /// </summary>
-        IEnumerable<Type> ValidConversionTypes { get; }
+        public abstract IEnumerable<Type> ValidConversionTypes { get; }
 
         /// <summary>
         /// Converts a Beatmap to another mode.
         /// </summary>
         /// <param name="original">The original Beatmap.</param>
         /// <returns>The converted Beatmap.</returns>
-        Beatmap<T> Convert(Beatmap original);
+        public abstract Beatmap<T> Convert(Beatmap original);
-    }
 
-    public static class BeatmapConverterExtensions
-    {
         /// <summary>
         /// Checks if a Beatmap can be converted using this Beatmap Converter.
         /// </summary>
-        /// <param name="converter">The Beatmap Converter.</param>
         /// <param name="beatmap">The Beatmap to check.</param>
-        /// <returns>Whether the Beatmap can be converted using <paramref name="converter"/>.</returns>
+        /// <returns>Whether the Beatmap can be converted using this Beatmap Converter.</returns>
-        public static bool CanConvert<TObject>(this IBeatmapConverter<TObject> converter, Beatmap beatmap) where TObject : HitObject
+        public bool CanConvert(Beatmap beatmap) => ValidConversionTypes.All(t => beatmap.HitObjects.Any(h => t.IsAssignableFrom(h.GetType())));
-            => converter.ValidConversionTypes.All(t => beatmap.HitObjects.Any(h => t.IsAssignableFrom(h.GetType())));
     }
 }

osu.Game/Modes/UI/HitRenderer.cs

@@ -120,7 +120,7 @@ namespace osu.Game.Modes.UI
 
             RelativeSizeAxes = Axes.Both;
 
-            IBeatmapConverter<TObject> converter = CreateBeatmapConverter();
+            BeatmapConverter<TObject> converter = CreateBeatmapConverter();
             IBeatmapProcessor<TObject> processor = CreateBeatmapProcessor();
 
             // Check if the beatmap can be converted
@@ -158,7 +158,7 @@ namespace osu.Game.Modes.UI
         /// Creates a converter to convert Beatmap to a specific mode.
         /// </summary>
         /// <returns>The Beatmap converter.</returns>
-        protected abstract IBeatmapConverter<TObject> CreateBeatmapConverter();
+        protected abstract BeatmapConverter<TObject> CreateBeatmapConverter();
 
         /// <summary>
         /// Creates a processor to perform post-processing operations

osu.Game/osu.Game.csproj

@@ -72,7 +72,7 @@
     <Compile Include="Audio\SampleInfo.cs" />
     <Compile Include="Beatmaps\Drawables\BeatmapBackgroundSprite.cs" />
     <Compile Include="Beatmaps\DifficultyCalculator.cs" />
-    <Compile Include="Modes\Beatmaps\IBeatmapConverter.cs" />
+    <Compile Include="Modes\Beatmaps\BeatmapConverter.cs" />
     <Compile Include="Modes\Beatmaps\IBeatmapProcessor.cs" />
     <Compile Include="Beatmaps\Legacy\LegacyBeatmap.cs" />
     <Compile Include="Beatmaps\Timing\TimeSignatures.cs" />