[claude] Defer change-type freeze until JsonSerializerOptions is first used

src/SIL.Harmony.Tests/ConfigTests.cs

@@ -1,3 +1,5 @@
+using System.Text.Json;
+using SIL.Harmony.Changes;
 using SIL.Harmony.Sample.Changes;
 using SIL.Harmony.Sample.Models;
 using SIL.Harmony.Tests.Adapter;
@@ -29,4 +31,25 @@ public void CanGetChangeTypes()
         var types = config.ChangeTypes.ToArray();
         types.Should().BeEquivalentTo([typeof(NewDefinitionChange), typeof(SetWordTextChange)]);
     }
+
+    [Fact]
+    public void CanAddChangeTypesAfterReadingJsonSerializerOptions()
+    {
+        // Mirrors a consumer that reads JsonSerializerOptions mid-configuration (to layer its own
+        // TypeInfoResolver modifier onto ours) and then keeps registering change types.
+        var config = new CrdtConfig();
+        config.ChangeTypeListBuilder.Add<NewDefinitionChange>();
+
+        _ = config.JsonSerializerOptions;
+
+        config.ChangeTypeListBuilder.Add<SetWordTextChange>();
+
+        var options = config.JsonSerializerOptions;
+        var entityId = Guid.Parse("aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee");
+        var json = JsonSerializer.Serialize<IChange>(new SetWordTextChange(entityId, "hello"), options);
+
+        var roundTripped = JsonSerializer.Deserialize<IChange>(json, options);
+        roundTripped.Should().BeOfType<SetWordTextChange>()
+            .Which.Text.Should().Be("hello");
+    }
 }

src/SIL.Harmony/Changes/PeekThenConcreteChangeConverter.cs

@@ -13,13 +13,14 @@ namespace SIL.Harmony.Changes;
 /// </summary>
 internal sealed class PeekThenConcreteChangeConverter : JsonConverter<IChange>
 {
-    private readonly KnownType[] _known;
+    private readonly Lazy<KnownType[]> _known;
     private readonly byte[] _discriminatorPropertyUtf8;
 
-    public PeekThenConcreteChangeConverter(IReadOnlyDictionary<string, Type> known)
+    public PeekThenConcreteChangeConverter(Func<IReadOnlyDictionary<string, Type>> knownFactory)
     {
         _discriminatorPropertyUtf8 = Encoding.UTF8.GetBytes(CrdtConstants.ChangeDiscriminatorProperty);
-        _known = known.Select(kv => new KnownType(Encoding.UTF8.GetBytes(kv.Key), kv.Value)).ToArray();
+        _known = new Lazy<KnownType[]>(() =>
+            knownFactory().Select(kv => new KnownType(Encoding.UTF8.GetBytes(kv.Key), kv.Value)).ToArray());
     }
 
     public override IChange Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
@@ -46,7 +47,7 @@ public override IChange Read(ref Utf8JsonReader reader, Type typeToConvert, Json
             return ReadOpaque(ref reader, unknownTypeName!);
         }
 
-        ref var known = ref _known[knownIndex];
+        ref var known = ref _known.Value[knownIndex];
         var typeInfo = known.EnsureTypeInfo(options);
 
         // Real change types use parameterized constructors / get-only props — let STJ materialize.
@@ -83,9 +84,10 @@ private static OpaqueChange ReadOpaque(ref Utf8JsonReader reader, string typeNam
 
     private bool TryFindKnown(ref Utf8JsonReader reader, out int index, out string? unknownTypeName)
     {
-        for (var i = 0; i < _known.Length; i++)
+        var known = _known.Value;
+        for (var i = 0; i < known.Length; i++)
         {
-            if (reader.ValueTextEquals(_known[i].Utf8Discriminator))
+            if (reader.ValueTextEquals(known[i].Utf8Discriminator))
             {
                 index = i;
                 unknownTypeName = null;

src/SIL.Harmony/CrdtConfig.cs

@@ -39,13 +39,14 @@ public CrdtConfig()
 
     private JsonSerializerOptions CreateJsonSerializerOptions()
     {
-        var changeDiscriminators = _lazyChangeDiscriminatorMaps.Value;
-
         var options = new JsonSerializerOptions(JsonSerializerDefaults.General)
         {
             TypeInfoResolver = MakeJsonTypeResolver()
         };
-        options.Converters.Add(new PeekThenConcreteChangeConverter(changeDiscriminators.ByDiscriminator));
+        // A factory, not .Value: building the map freezes ChangeTypeListBuilder, but consumers read these
+        // options mid-configuration (to add their own TypeInfoResolver modifier) and keep registering
+        // change types. The converter pulls the map on first use, once registration is done.
+        options.Converters.Add(new PeekThenConcreteChangeConverter(() => _lazyChangeDiscriminatorMaps.Value.ByDiscriminator));
         return options;
     }