Fix duplicate discriminator property in derived types with matching serialized names

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

@@ -423,12 +423,22 @@ protected internal override PropertyProvider[] BuildProperties()
             var propertiesCount = _inputModel.Properties.Count;
             var properties = new List<PropertyProvider>(propertiesCount + 1);
             Dictionary<string, InputModelProperty> baseProperties = EnumerateBaseModels().SelectMany(m => m.Properties).GroupBy(x => x.Name).Select(g => g.First()).ToDictionary(p => p.Name) ?? [];
+            // Build a set of serialized names for base discriminator properties to handle cases where
+            // the derived model has a discriminator with a different C# name but the same wire name
+            HashSet<string> baseDiscriminatorSerializedNames = EnumerateBaseModels()
+                .SelectMany(m => m.Properties)
+                .Where(p => p.IsDiscriminator && p.SerializedName is not null)
+                .Select(p => p.SerializedName)
+                .ToHashSet();
             for (int i = 0; i < propertiesCount; i++)
             {
                 var property = _inputModel.Properties[i];
                 var isDiscriminator = IsDiscriminator(property);
 
-                if (isDiscriminator && baseProperties.ContainsKey(property.Name))
+                // Skip discriminator properties that already exist in the base class
+                // Check both by C# property name and by serialized name to handle cases where
+                // the derived model has a discriminator with a different C# name but the same wire name
+                if (isDiscriminator && (baseProperties.ContainsKey(property.Name) || (property.SerializedName is not null && baseDiscriminatorSerializedNames.Contains(property.SerializedName))))
                 {
                     continue;
                 }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/DiscriminatorTests.cs

@@ -452,5 +452,90 @@ public async Task CanCustomizeDiscriminator()
             Assert.IsNotNull(discriminatorParam);
             Assert.IsTrue(discriminatorParam!.Property?.IsDiscriminator);
         }
+
+        // This test validates that a derived model does not re-declare the discriminator property
+        // when the base model has a discriminator property with a different C# name but the same serialized name.
+        // This scenario can occur when the base model has a discriminator property named "OdataType"
+        // and the derived model has a discriminator property also named "OdataType" with the same
+        // serialized name "@odata.type".
+        [Test]
+        public void DerivedDoesNotDuplicateDiscriminatorWithSameSerializedName()
+        {
+            // Base model with discriminator named "OdataType" with serialized name "@odata.type"
+            var baseSkillModel = InputFactory.Model(
+                "SearchIndexerSkill",
+                properties: [InputFactory.Property("OdataType", InputPrimitiveType.String, isRequired: true, isDiscriminator: true, serializedName: "@odata.type")],
+                discriminatedModels: new Dictionary<string, InputModelType>());
+
+            // Derived model with discriminator named "OdataType" with same serialized name "@odata.type"
+            var derivedSkillModel = InputFactory.Model(
+                "ContentUnderstandingSkill",
+                discriminatedKind: "#Microsoft.Skills.Util.ContentUnderstandingSkill",
+                baseModel: baseSkillModel,
+                properties:
+                [
+                    InputFactory.Property("OdataType", InputPrimitiveType.String, isRequired: true, isDiscriminator: true, serializedName: "@odata.type"),
+                    InputFactory.Property("Inputs", InputPrimitiveType.String, isRequired: true)
+                ]);
+
+            MockHelpers.LoadMockGenerator(inputModelTypes: [baseSkillModel, derivedSkillModel]);
+            var outputLibrary = CodeModelGenerator.Instance.OutputLibrary;
+
+            var derivedModel = outputLibrary.TypeProviders.OfType<ModelProvider>().FirstOrDefault(t => t.Name == "ContentUnderstandingSkill");
+            Assert.IsNotNull(derivedModel);
+
+            // The derived model should NOT have the OdataType property since it's already defined in the base class
+            var odataTypeProperty = derivedModel!.Properties.FirstOrDefault(p => p.Name == "OdataType");
+            Assert.IsNull(odataTypeProperty, "Derived model should not re-declare the discriminator property that exists in the base class");
+
+            // The public constructor should correctly pass the discriminator value to the base class
+            var publicCtor = derivedModel.Constructors.FirstOrDefault(c => c.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Public));
+            Assert.IsNotNull(publicCtor);
+            // The public ctor should not have an odataType parameter
+            Assert.IsFalse(publicCtor!.Signature.Parameters.Any(p => p.Name.Equals("odataType", System.StringComparison.OrdinalIgnoreCase)));
+        }
+
+        // This test validates that a derived model does not re-declare the discriminator property
+        // when the base model has a discriminator property with a different C# name but the same serialized name.
+        // This scenario can occur when the base model has a discriminator property named "Type"
+        // and the derived model has a discriminator property named "OdataType" with the same
+        // serialized name "@odata.type".
+        [Test]
+        public void DerivedDoesNotDuplicateDiscriminatorWithDifferentNameButSameSerializedName()
+        {
+            // Base model with discriminator named "Type" with serialized name "@odata.type"
+            var baseSkillModel = InputFactory.Model(
+                "SearchIndexerSkill",
+                properties: [InputFactory.Property("Type", InputPrimitiveType.String, isRequired: true, isDiscriminator: true, serializedName: "@odata.type")],
+                discriminatedModels: new Dictionary<string, InputModelType>());
+
+            // Derived model with discriminator named "OdataType" with same serialized name "@odata.type"
+            var derivedSkillModel = InputFactory.Model(
+                "ContentUnderstandingSkill",
+                discriminatedKind: "#Microsoft.Skills.Util.ContentUnderstandingSkill",
+                baseModel: baseSkillModel,
+                properties:
+                [
+                    InputFactory.Property("OdataType", InputPrimitiveType.String, isRequired: true, isDiscriminator: true, serializedName: "@odata.type"),
+                    InputFactory.Property("Inputs", InputPrimitiveType.String, isRequired: true)
+                ]);
+
+            MockHelpers.LoadMockGenerator(inputModelTypes: [baseSkillModel, derivedSkillModel]);
+            var outputLibrary = CodeModelGenerator.Instance.OutputLibrary;
+
+            var derivedModel = outputLibrary.TypeProviders.OfType<ModelProvider>().FirstOrDefault(t => t.Name == "ContentUnderstandingSkill");
+            Assert.IsNotNull(derivedModel);
+
+            // The derived model should NOT have the OdataType property since it's the same discriminator 
+            // (same serialized name) as the base class "Type" property
+            var odataTypeProperty = derivedModel!.Properties.FirstOrDefault(p => p.Name == "OdataType");
+            Assert.IsNull(odataTypeProperty, "Derived model should not re-declare the discriminator property when base has same serialized name");
+
+            // The public constructor should correctly pass the discriminator value to the base class
+            var publicCtor = derivedModel.Constructors.FirstOrDefault(c => c.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Public));
+            Assert.IsNotNull(publicCtor);
+            // The public ctor should not have an odataType parameter
+            Assert.IsFalse(publicCtor!.Signature.Parameters.Any(p => p.Name.Equals("odataType", System.StringComparison.OrdinalIgnoreCase)));
+        }
     }
 }