[csharp] Make ModelProvider.BuildBaseModelProvider virtual to keep BaseType and BaseModelProvider consistent

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

@@ -291,7 +291,13 @@ private static bool IsDiscriminator(InputProperty property)
             return property is InputModelProperty modelProperty && modelProperty.IsDiscriminator;
         }
 
-        private ModelProvider? BuildBaseModelProvider()
+        /// <summary>
+        /// Builds the <see cref="ModelProvider"/> representing the base model of this model.
+        /// Emitters that override <see cref="BuildBaseType"/> to redirect the generated C# base
+        /// class should also override this method so that <see cref="BaseType"/> and
+        /// <see cref="BaseModelProvider"/> stay consistent.
+        /// </summary>
+        protected virtual ModelProvider? BuildBaseModelProvider()
         {
             // consider models that have been customized to inherit from a different generated model
             if (CustomCodeView?.BaseType != null)

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

@@ -422,6 +422,45 @@ public void BuildBaseType()
             Assert.AreEqual(baseModel!.Type, derivedModel!.Type.BaseType);
         }
 
+        // Verifies an emitter can override BuildBaseType and BuildBaseModelProvider together to
+        // redirect the C# base class to a custom ModelProvider, keeping BaseType and
+        // BaseModelProvider in agreement.
+        [Test]
+        public void OverridingBuildBaseTypeAndBuildBaseModelProvider_KeepsBaseTypeConsistent()
+        {
+            var inputBase = InputFactory.Model("baseModel", usage: InputModelTypeUsage.Input, properties: []);
+            var inputDerived = InputFactory.Model("derivedModel", usage: InputModelTypeUsage.Input, properties: [], baseModel: inputBase);
+            CodeModelGenerator.Instance.TypeFactory.CreateModel(inputBase);
+
+            var customBaseProvider = new CustomBaseModelProvider(inputBase);
+            var derivedProvider = new BuildBaseTypeOverridingModelProvider(inputDerived, customBaseProvider);
+
+            Assert.AreEqual(customBaseProvider.Type, derivedProvider.BaseType);
+            Assert.AreSame(customBaseProvider, derivedProvider.BaseModelProvider);
+        }
+
+        private class CustomBaseModelProvider : ModelProvider
+        {
+            public CustomBaseModelProvider(InputModelType inputModel) : base(inputModel) { }
+
+            protected override string BuildName() => "CustomBase";
+        }
+
+        private class BuildBaseTypeOverridingModelProvider : ModelProvider
+        {
+            private readonly ModelProvider _redirectedBaseProvider;
+
+            public BuildBaseTypeOverridingModelProvider(InputModelType inputModel, ModelProvider redirectedBaseProvider) : base(inputModel)
+            {
+                _redirectedBaseProvider = redirectedBaseProvider;
+            }
+
+            protected override CSharpType? BuildBaseType() => _redirectedBaseProvider?.Type;
+
+            // The emitter overrides BuildBaseModelProvider to keep it in sync with the redirected BaseType.
+            protected override ModelProvider? BuildBaseModelProvider() => _redirectedBaseProvider;
+        }
+
         [Test]
         public void BuildModelAsStruct()
         {