System.Text.Json: Fix polymorphic state bug when using ReferenceHandler.IgnoreCycles

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverter.MetadataHandling.cs

@@ -148,6 +148,13 @@ internal bool TryHandleSerializedObjectReference(Utf8JsonWriter writer, object v
                     if (resolver.ContainsReferenceForCycleDetection(value))
                     {
                         writer.WriteNullValue();
+
+                        if (polymorphicConverter is not null)
+                        {
+                            // Clear out any polymorphic state.
+                            state.PolymorphicTypeDiscriminator = null;
+                            state.PolymorphicTypeResolver = null;
+                        }
                         return true;
                     }
 

src/libraries/System.Text.Json/tests/Common/ReferenceHandlerTests/ReferenceHandlerTests.IgnoreCycles.cs

@@ -429,6 +429,48 @@ public async Task IgnoreCycles_BoxedValueShouldNotBeIgnored()
             await Test_Serialize_And_SerializeAsync_Contains(root, expectedSubstring: @"""DayOfBirth"":15", expectedTimes: 2, s_optionsIgnoreCycles);
         }
 
+        [Fact]
+        public async Task IgnoreCycles_DerivedType_InArray()
+        {
+            var worker = new OfficeWorker
+            {
+                Office = new Office
+                {
+                    Dummy = new()
+                }
+            };
+
+            worker.Office.Staff = [worker, new RemoteWorker()];
+
+            await Test_Serialize_And_SerializeAsync(worker, """{"Office":{"Staff":[null,{"$type":"remote"}],"Dummy":{}}}""", s_optionsIgnoreCycles);
+
+            worker.Office.Staff = [worker];
+
+            await Test_Serialize_And_SerializeAsync(worker, """{"Office":{"Staff":[null],"Dummy":{}}}""", s_optionsIgnoreCycles);
+        }
+
+        [JsonDerivedType(typeof(OfficeWorker), "office")]
+        [JsonDerivedType(typeof(RemoteWorker), "remote")]
+        public abstract class EmployeeLocation
+        {
+        }
+
+        public class OfficeWorker : EmployeeLocation
+        {
+            public Office Office { get; set; }
+        }
+
+        public class RemoteWorker : EmployeeLocation
+        {
+        }
+
+        public class Office
+        {
+            public EmployeeLocation[] Staff { get; set; }
+
+            public EmptyClass Dummy { get; set; }
+        }
+
         [Fact]
         public async Task CycleDetectionStatePersistsAcrossContinuations()
         {

src/libraries/System.Text.Json/tests/Common/ReferenceHandlerTests/ReferenceHandlerTests.Serialize.cs

@@ -224,5 +224,49 @@ public async Task CustomHashCode()
             // otherwise objects would not be correctly identified when searching for them in the dictionary.
             Assert.Same(listCopy[0], listCopy[1]);
         }
+
+        [Fact]
+        public async Task Preserve_DerivedType_InArray()
+        {
+            var worker = new OfficeWorker
+            {
+                Office = new Office
+                {
+                    Dummy = new()
+                }
+            };
+
+            worker.Office.Staff = [worker, new RemoteWorker()];
+
+            string json = await Serializer.SerializeWrapper(worker, s_serializerOptionsPreserve);
+            Assert.Equal("""{"$id":"1","Office":{"$id":"2","Staff":[{"$ref":"1"},{"$id":"3","$type":"remote"}],"Dummy":{"$id":"4"}}}""", json);
+
+            worker.Office.Staff = [worker];
+
+            json = await Serializer.SerializeWrapper(worker, s_serializerOptionsPreserve);
+            Assert.Equal("""{"$id":"1","Office":{"$id":"2","Staff":[{"$ref":"1"}],"Dummy":{"$id":"3"}}}""", json);
+        }
+
+        [JsonDerivedType(typeof(OfficeWorker), "office")]
+        [JsonDerivedType(typeof(RemoteWorker), "remote")]
+        public abstract class EmployeeLocation
+        {
+        }
+
+        public class OfficeWorker : EmployeeLocation
+        {
+            public Office Office { get; set; }
+        }
+
+        public class RemoteWorker : EmployeeLocation
+        {
+        }
+
+        public class Office
+        {
+            public EmployeeLocation[] Staff { get; set; }
+
+            public EmptyClass Dummy { get; set; }
+        }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/ReferenceHandlerTests.IgnoreCycles.cs

@@ -89,6 +89,11 @@ public ReferenceHandlerTests_IgnoreCycles_Metadata(JsonSerializerWrapper seriali
         [JsonSerializable(typeof(TreeNode<List<string>>))]
         [JsonSerializable(typeof(TreeNode<object>))]
         [JsonSerializable(typeof(int))]
+        [JsonSerializable(typeof(EmployeeLocation))]
+        [JsonSerializable(typeof(EmployeeLocation[]))]
+        [JsonSerializable(typeof(OfficeWorker))]
+        [JsonSerializable(typeof(Office))]
+        [JsonSerializable(typeof(RemoteWorker))]
         internal sealed partial class ReferenceHandlerTests_IgnoreCyclesContext_Metadata : JsonSerializerContext
         {
         }
@@ -172,6 +177,11 @@ public ReferenceHandlerTests_IgnoreCycles_Default(JsonSerializerWrapper serializ
         [JsonSerializable(typeof(TreeNode<List<string>>))]
         [JsonSerializable(typeof(TreeNode<object>))]
         [JsonSerializable(typeof(int))]
+        [JsonSerializable(typeof(EmployeeLocation))]
+        [JsonSerializable(typeof(EmployeeLocation[]))]
+        [JsonSerializable(typeof(OfficeWorker))]
+        [JsonSerializable(typeof(Office))]
+        [JsonSerializable(typeof(RemoteWorker))]
         internal sealed partial class ReferenceHandlerTests_IgnoreCyclesContext_Default : JsonSerializerContext
         {
         }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/ReferenceHandlerTests.cs

@@ -139,6 +139,11 @@ public ReferenceHandlerTests_Metadata(JsonSerializerWrapper serializer)
         [JsonSerializable(typeof(ClassWithConflictingIdProperty))]
         [JsonSerializable(typeof(ClassWithIgnoredConflictingProperty))]
         [JsonSerializable(typeof(ClassWithExtensionDataConflictingProperty))]
+        [JsonSerializable(typeof(EmployeeLocation))]
+        [JsonSerializable(typeof(EmployeeLocation[]))]
+        [JsonSerializable(typeof(OfficeWorker))]
+        [JsonSerializable(typeof(Office))]
+        [JsonSerializable(typeof(RemoteWorker))]
         internal sealed partial class ReferenceHandlerTestsContext_Metadata : JsonSerializerContext
         {
         }
@@ -281,6 +286,11 @@ public ReferenceHandlerTests_Default(JsonSerializerWrapper serializer)
         [JsonSerializable(typeof(ClassWithConflictingIdProperty))]
         [JsonSerializable(typeof(ClassWithIgnoredConflictingProperty))]
         [JsonSerializable(typeof(ClassWithExtensionDataConflictingProperty))]
+        [JsonSerializable(typeof(EmployeeLocation))]
+        [JsonSerializable(typeof(EmployeeLocation[]))]
+        [JsonSerializable(typeof(OfficeWorker))]
+        [JsonSerializable(typeof(Office))]
+        [JsonSerializable(typeof(RemoteWorker))]
         internal sealed partial class ReferenceHandlerTestsContext_Default : JsonSerializerContext
         {
         }