[csharp] Make ModelProvider.BuildBaseModelProvider virtual

.chronus/changes/fix-build-base-model-provider-virtual-2026-4-25.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-csharp"
+---
+
+Make `ModelProvider.BaseModelProvider` derive from `BaseType` so the two cannot return mismatched values when a derived `ModelProvider` overrides `BuildBaseType` or `BaseType`.

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

@@ -183,9 +183,69 @@ protected override string BuildNamespace() => string.IsNullOrEmpty(_inputModel.N
             CodeModelGenerator.Instance.TypeFactory.PrimaryNamespace :
             CodeModelGenerator.Instance.TypeFactory.GetCleanNameSpace(_inputModel.Namespace);
 
+        public override CSharpType? BaseType => _baseType ??= ResolveBaseType();
+
+        private CSharpType? ResolveBaseType()
+        {
+            // CustomCodeView wins over the spec-derived base, mirroring the precedence the
+            // pre-existing BaseModelProvider logic enforced.
+            var customBase = CustomCodeView?.BaseType;
+            if (customBase != null)
+            {
+                // If the custom base has no resolved namespace, Roslyn could not see the base
+                // symbol (typical when the base is a generated model that wasn't itself defined
+                // in custom code). Resolve it by name against the generated providers.
+                if (string.IsNullOrEmpty(customBase.Namespace))
+                {
+                    var resolved = ResolveModelProviderByName(customBase.Name);
+                    if (resolved != null)
+                    {
+                        return resolved.Type;
+                    }
+                }
+
+                return customBase;
+            }
+
+            return BuildBaseType();
+        }
+
+        private static ModelProvider? ResolveModelProviderByName(string name)
+        {
+            // Cheap check: the base model may already be created and registered under the right name.
+            if (CodeModelGenerator.Instance.TypeFactory.TypeProvidersByName.TryGetValue(
+                    name, out var resolvedProvider) &&
+                resolvedProvider is ModelProvider resolvedModel)
+            {
+                return resolvedModel;
+            }
+
+            // Force-create all input models so that visitors run (which may rename models
+            // via TypeProvider.Update) and TypeProvidersByName is fully populated.
+            // This is a no-op for models that have already been created.
+            foreach (var model in CodeModelGenerator.Instance.InputLibrary.InputNamespace.Models)
+            {
+                CodeModelGenerator.Instance.TypeFactory.CreateModel(model);
+            }
+
+            if (CodeModelGenerator.Instance.TypeFactory.TypeProvidersByName.TryGetValue(
+                    name, out resolvedProvider) &&
+                resolvedProvider is ModelProvider resolvedAfterCreate)
+            {
+                return resolvedAfterCreate;
+            }
+
+            return null;
+        }
+
         protected override CSharpType? BuildBaseType()
         {
-            return BaseModelProvider?.Type;
+            if (_inputModel.BaseModel == null)
+            {
+                return null;
+            }
+
+            return CodeModelGenerator.Instance.TypeFactory.CreateModel(_inputModel.BaseModel)?.Type;
         }
 
         protected override TypeProvider[] BuildSerializationProviders()
@@ -289,65 +349,25 @@ private static bool IsDiscriminator(InputProperty property)
             return property is InputModelProperty modelProperty && modelProperty.IsDiscriminator;
         }
 
-        private ModelProvider? BuildBaseModelProvider()
+        protected ModelProvider? BuildBaseModelProvider()
         {
-            // consider models that have been customized to inherit from a different generated model
-            if (CustomCodeView?.BaseType != null)
+            // BaseType is the source of truth: it already merges the spec-derived base with any
+            // CustomCodeView base (and resolves namespace-less custom bases). BaseModelProvider is
+            // simply the matching generated ModelProvider for that BaseType, or null if BaseType
+            // is external / hand-written.
+            var baseType = BaseType;
+            if (baseType == null)
             {
-                var baseType = CustomCodeView.BaseType;
-
-                // If the custom base type doesn't have a resolved namespace, then try to resolve it from the input model map.
-                // This will happen if a model is customized to inherit from another generated model, but that generated model
-                // was not also defined in custom code so Roslyn does not recognize it.
-                if (string.IsNullOrEmpty(baseType.Namespace))
-                {
-                    // Cheap check: the base model may already be created and registered under the right name.
-                    if (CodeModelGenerator.Instance.TypeFactory.TypeProvidersByName.TryGetValue(
-                            baseType.Name, out var resolvedProvider) &&
-                        resolvedProvider is ModelProvider resolvedModel)
-                    {
-                        return resolvedModel;
-                    }
-
-                    // Force-create all input models so that visitors run (which may rename models
-                    // via TypeProvider.Update) and TypeProvidersByName is fully populated.
-                    // This is a no-op for models that have already been created.
-                    foreach (var model in CodeModelGenerator.Instance.InputLibrary.InputNamespace.Models)
-                    {
-                        CodeModelGenerator.Instance.TypeFactory.CreateModel(model);
-                    }
-
-                    if (CodeModelGenerator.Instance.TypeFactory.TypeProvidersByName.TryGetValue(
-                            baseType.Name, out resolvedProvider) &&
-                        resolvedProvider is ModelProvider resolvedAfterCreate)
-                    {
-                        return resolvedAfterCreate;
-                    }
-                }
-
-                // Try to find the base type in the CSharpTypeMap
-                if (baseType != null && CodeModelGenerator.Instance.TypeFactory.CSharpTypeMap.TryGetValue(
-                        baseType,
-                        out var customBaseType) &&
-                    customBaseType is ModelProvider customBaseModel)
-                {
-                    return customBaseModel;
-                }
-
-                // If the custom base type has a namespace (external type), we don't return it here
-                // as it's handled by BuildBaseTypeProvider() which returns a TypeProvider
-                if (!string.IsNullOrEmpty(baseType?.Namespace))
-                {
-                    return null;
-                }
+                return null;
             }
 
-            if (_inputModel.BaseModel == null)
+            if (CodeModelGenerator.Instance.TypeFactory.CSharpTypeMap.TryGetValue(baseType, out var baseProvider) &&
+                baseProvider is ModelProvider baseModel)
             {
-                return null;
+                return baseModel;
             }
 
-            return CodeModelGenerator.Instance.TypeFactory.CreateModel(_inputModel.BaseModel);
+            return null;
         }
 
         private List<FieldProvider> BuildAdditionalPropertyFields()

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

@@ -210,8 +210,8 @@ private TypeSignatureModifiers BuildDeclarationModifiersInternal()
 
         private protected virtual bool FilterCustomizedMembers => true;
 
-        public CSharpType? BaseType => _baseType ??= BuildBaseType() ?? CustomCodeView?.BaseType;
-        private CSharpType? _baseType;
+        public virtual CSharpType? BaseType => _baseType ??= BuildBaseType() ?? CustomCodeView?.BaseType;
+        private protected CSharpType? _baseType;
 
         public WhereExpression? WhereClause => _whereClause ??= BuildWhereClause();
         private WhereExpression? _whereClause;