Fix MRW JSON write core override for custom bases

Detect overridable JsonModelWriteCore methods on custom and framework base types so derived model serialization emits override instead of hiding the base member.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: live1206

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.Xml.cs

@@ -67,7 +67,7 @@ private MethodProvider BuildXmlModelWriteCoreMethod()
             MethodSignatureModifiers modifiers = _isStruct
                 ? MethodSignatureModifiers.Private
                 : MethodSignatureModifiers.Internal | MethodSignatureModifiers.Virtual;
-            if (_shouldOverrideMethods)
+            if (ShouldOverrideMethods)
             {
                 modifiers = MethodSignatureModifiers.Internal | MethodSignatureModifiers.Override;
             }
@@ -81,7 +81,7 @@ private MethodProvider BuildXmlModelWriteCoreMethod()
 
         private MethodBodyStatement[] BuildXmlModelWriteCoreMethodBody()
         {
-            var categorizedProperties = _shouldOverrideMethods
+            var categorizedProperties = ShouldOverrideMethods
                 ? CategorizedXmlProperties
                 : AllCategorizedXmlProperties;
             var statements = new List<MethodBodyStatement>
@@ -90,7 +90,7 @@ private MethodBodyStatement[] BuildXmlModelWriteCoreMethodBody()
                 MethodBodyStatement.EmptyLine
             };
 
-            if (_shouldOverrideMethods)
+            if (ShouldOverrideMethods)
             {
                 statements.Add(Base.Invoke(XmlModelWriteCoreMethodName, _xmlWriterParameter, _serializationOptionsParameter).Terminate());
             }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs

@@ -67,8 +67,8 @@ public partial class MrwSerializationTypeDefinition : TypeProvider
         private readonly bool _supportsXml;
         private ConstructorProvider? _serializationConstructor;
         // Flag to determine if the model should override the serialization methods
-        private readonly bool _shouldOverrideMethods;
-        private readonly bool _shouldSkipDerivedSerializationMethodOverrides;
+        private bool ShouldOverrideMethods => _model.BaseModelProvider != null && !_isStruct;
+        private bool ShouldSkipSerializationMethodOverrides => ShouldSkipDerivedSerializationMethodOverrides(_model.BaseModelProvider);
         private readonly Lazy<PropertyProvider[]> _additionalProperties;
 
         private CSharpType RootType => _rootType ??= GetRootModelType();
@@ -93,8 +93,6 @@ public MrwSerializationTypeDefinition(InputModelType inputModel, ModelProvider m
             _rawDataField = _model.Fields.FirstOrDefault(f => f.Name == AdditionalPropertiesHelper.AdditionalBinaryDataPropsFieldName);
             _additionalBinaryDataProperty = new(GetAdditionalBinaryDataPropertiesProp);
             _additionalProperties = new(() => [.. _model.Properties.Where(p => p.IsAdditionalProperties)]);
-            _shouldOverrideMethods = _model.BaseModelProvider != null && !_isStruct;
-            _shouldSkipDerivedSerializationMethodOverrides = ShouldSkipDerivedSerializationMethodOverrides(_model.BaseModelProvider);
             _utf8JsonWriterSnippet = _utf8JsonWriterParameter.As<Utf8JsonWriter>();
             _mrwOptionsParameterSnippet = _serializationOptionsParameter.As<ModelReaderWriterOptions>();
             _jsonElementParameterSnippet = _jsonElementDeserializationParam.As<JsonElement>();
@@ -530,7 +528,7 @@ internal MethodProvider BuildJsonModelWriteCoreMethod()
             MethodSignatureModifiers modifiers = _isStruct
                 ? MethodSignatureModifiers.Private
                 : MethodSignatureModifiers.Protected | MethodSignatureModifiers.Virtual;
-            if (_shouldOverrideMethods)
+            if (ShouldOverrideMethods)
             {
                 modifiers = MethodSignatureModifiers.Protected | MethodSignatureModifiers.Override;
             }
@@ -552,7 +550,7 @@ internal MethodProvider BuildPersistableModelWriteCoreMethod()
                 ? MethodSignatureModifiers.Private
                 : MethodSignatureModifiers.Protected | MethodSignatureModifiers.Virtual;
 
-            if (_shouldOverrideMethods && !_shouldSkipDerivedSerializationMethodOverrides)
+            if (ShouldOverrideMethods && !ShouldSkipSerializationMethodOverrides)
             {
                 modifiers = MethodSignatureModifiers.Protected | MethodSignatureModifiers.Override;
             }
@@ -576,7 +574,7 @@ internal MethodProvider BuildPersistableModelCreateCoreMethod()
                 ? MethodSignatureModifiers.Private
                 : MethodSignatureModifiers.Protected | MethodSignatureModifiers.Virtual;
 
-            if (_shouldOverrideMethods && !_shouldSkipDerivedSerializationMethodOverrides)
+            if (ShouldOverrideMethods && !ShouldSkipSerializationMethodOverrides)
             {
                 modifiers = MethodSignatureModifiers.Protected | MethodSignatureModifiers.Override;
             }
@@ -624,7 +622,7 @@ internal MethodProvider BuildJsonModelCreateCoreMethod()
                 ? MethodSignatureModifiers.Private
                 : MethodSignatureModifiers.Protected | MethodSignatureModifiers.Virtual;
 
-            if (_shouldOverrideMethods && !_shouldSkipDerivedSerializationMethodOverrides)
+            if (ShouldOverrideMethods && !ShouldSkipSerializationMethodOverrides)
             {
                 modifiers = MethodSignatureModifiers.Protected | MethodSignatureModifiers.Override;
             }
@@ -1055,7 +1053,7 @@ private MethodBodyStatement[] BuildPersistableModelCreateCoreMethodBody()
         private MethodBodyStatement CallBaseJsonModelWriteCore(bool isDynamicModelWithNonDynamicBase)
         {
             // base.<JsonModelWriteCore>()
-            bool callBaseWriteMethod = _shouldOverrideMethods
+            bool callBaseWriteMethod = ShouldOverrideMethods
                 && (_jsonPatchProperty is null || !isDynamicModelWithNonDynamicBase);
             return callBaseWriteMethod ?
                 Base.Invoke(JsonModelWriteCoreMethodName, [_utf8JsonWriterParameter, _serializationOptionsParameter]).Terminate()

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/SystemObjectModelSerializationTests.cs

@@ -121,6 +121,27 @@ public void JsonModelWriteCore_IsOverride_WhenBaseIsRegularModel()
                 "JsonModelWriteCore should be 'override' with regular base too");
         }
 
+        [Test]
+        public void JsonModelWriteCore_IsOverride_WhenBaseProviderIsResolvedAfterSerialization()
+        {
+            var baseInputModel = InputFactory.Model("Resource");
+            var derivedInputModel = InputFactory.Model("TrackedResource", properties: [InputFactory.Property("Location", InputPrimitiveType.String)]);
+            MockHelpers.LoadMockGenerator(inputModels: () => [baseInputModel, derivedInputModel]);
+
+            var derived = new DelayedBaseModelProvider(derivedInputModel);
+            var serialization = new MrwSerializationTypeDefinition(derivedInputModel, derived);
+
+            // The serialization provider can be constructed before later visitors/customization
+            // resolution make the base model provider available.
+            derived.BaseModel = new SystemObjectModelProvider(new CSharpType(typeof(object)), baseInputModel);
+
+            var method = serialization.BuildJsonModelWriteCoreMethod();
+
+            Assert.IsTrue(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Override),
+                "JsonModelWriteCore should evaluate BaseModelProvider when the method is built, not when serialization is constructed");
+            Assert.IsFalse(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Virtual));
+        }
+
         // -------------------------------------------------------------------
         // PersistableModelWriteCore: 'virtual' with system base, 'override' with regular
         // (the framework base already implements this; derived model re-introduces it)
@@ -328,5 +349,14 @@ FakeMrwBase IPersistableModel<FakeMrwBase>.Create(BinaryData data, ModelReaderWr
             string IPersistableModel<FakeMrwBase>.GetFormatFromOptions(ModelReaderWriterOptions options)
                 => "J";
         }
+
+        private class DelayedBaseModelProvider(InputModelType inputModel) : ModelProvider(inputModel)
+        {
+            public ModelProvider? BaseModel { get; set; }
+
+            protected override ModelProvider? BuildBaseModelProvider() => BaseModel;
+
+            protected override CSharpType? BuildBaseType() => BaseModel?.Type;
+        }
     }
 }