Merge pull request #6396 from bdach/allow-fallback-decoder-overwrite

Allow fallback decoder overwrite

osu.Game.Tests/Beatmaps/Formats/LegacyBeatmapDecoderTest.cs

@@ -591,5 +591,27 @@ namespace osu.Game.Tests.Beatmaps.Formats
                 Assert.Throws<IOException>(() => Decoder.GetDecoder<Beatmap>(stream));
             }
         }
+
+        [Test]
+        public void TestAllowFallbackDecoderOverwrite()
+        {
+            Decoder<Beatmap> decoder = null;
+
+            using (var resStream = TestResources.OpenResource("corrupted-header.osu"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                Assert.DoesNotThrow(() => decoder = Decoder.GetDecoder<Beatmap>(stream));
+                Assert.IsInstanceOf<LegacyBeatmapDecoder>(decoder);
+            }
+
+            Assert.DoesNotThrow(LegacyDifficultyCalculatorBeatmapDecoder.Register);
+
+            using (var resStream = TestResources.OpenResource("corrupted-header.osu"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                Assert.DoesNotThrow(() => decoder = Decoder.GetDecoder<Beatmap>(stream));
+                Assert.IsInstanceOf<LegacyDifficultyCalculatorBeatmapDecoder>(decoder);
+            }
+        }
     }
 }

osu.Game/Beatmaps/Formats/Decoder.cs

@@ -93,14 +93,12 @@ namespace osu.Game.Beatmaps.Formats
         /// <summary>
         /// Registers a fallback decoder instantiation function.
         /// The fallback will be returned if the first non-empty line of the decoded stream does not match any known magic.
+        /// Calling this method will overwrite any existing global fallback registration for type <see cref="T"/> - use with caution.
         /// </summary>
         /// <typeparam name="T">Type of object being decoded.</typeparam>
         /// <param name="constructor">A function that constructs the fallback<see cref="Decoder"/>.</param>
         protected static void SetFallbackDecoder<T>(Func<Decoder> constructor)
         {
-            if (fallback_decoders.ContainsKey(typeof(T)))
-                throw new InvalidOperationException($"A fallback decoder was already added for type {typeof(T)}.");
-
             fallback_decoders[typeof(T)] = constructor;
         }
     }