Allow customization of base model when spec defines a base model

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

@@ -242,32 +242,32 @@ private static bool IsDiscriminator(InputProperty property)
 
         private ModelProvider? BuildBaseModelProvider()
         {
-            if (_inputModel.BaseModel == null)
+            // consider models that have been customized to inherit from a different model
+            if (CustomCodeView?.BaseType != null)
             {
-                // consider models that have been customized to inherit from a different model
-                if (CustomCodeView?.BaseType != null)
-                {
-                    var baseType = CustomCodeView.BaseType;
+                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))
-                    {
-                        if (CodeModelGenerator.Instance.TypeFactory.InputModelTypeNameMap.TryGetValue(baseType.Name, out var baseInputModel))
-                        {
-                            baseType = CodeModelGenerator.Instance.TypeFactory.CreateCSharpType(baseInputModel);
-                        }
-                    }
-                    if (baseType != null && CodeModelGenerator.Instance.TypeFactory.CSharpTypeMap.TryGetValue(
-                            baseType,
-                            out var customBaseType) &&
-                        customBaseType is ModelProvider customBaseModel)
+                // 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))
+                {
+                    if (CodeModelGenerator.Instance.TypeFactory.InputModelTypeNameMap.TryGetValue(baseType.Name, out var baseInputModel))
                     {
-                        return customBaseModel;
+                        baseType = CodeModelGenerator.Instance.TypeFactory.CreateCSharpType(baseInputModel);
                     }
                 }
+                if (baseType != null && CodeModelGenerator.Instance.TypeFactory.CSharpTypeMap.TryGetValue(
+                        baseType,
+                        out var customBaseType) &&
+                    customBaseType is ModelProvider customBaseModel)
+                {
+                    return customBaseModel;
+                }
+            }
 
+            if (_inputModel.BaseModel == null)
+            {
                 return null;
             }
 

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

@@ -1441,6 +1441,38 @@ public async Task DiscriminatorPropertyNotGeneratedIfOnCustomizedBase()
             Assert.AreEqual("Sample.Models", modelProvider.BaseType!.Namespace);
         }
 
+        [Test]
+        public async Task CanCustomizeBaseModelWithSpecBase()
+        {
+            var specBaseModel = InputFactory.Model(
+                "specBaseModel",
+                properties: [InputFactory.Property("specBaseProp", InputPrimitiveType.String)],
+                usage: InputModelTypeUsage.Json);
+            var customBaseModel = InputFactory.Model(
+                "customBaseModel",
+                properties: [InputFactory.Property("customBaseProp", InputPrimitiveType.String)],
+                usage: InputModelTypeUsage.Json);
+            var childModel = InputFactory.Model(
+                "mockInputModel",
+                properties: [InputFactory.Property("childProp", InputPrimitiveType.String)],
+                baseModel: specBaseModel,
+                usage: InputModelTypeUsage.Json);
+
+            var mockGenerator = await MockHelpers.LoadMockGeneratorAsync(
+                inputModelTypes: [childModel, specBaseModel, customBaseModel],
+                compilation: async () => await Helpers.GetCompilationFromDirectoryAsync());
+
+            var modelProvider = mockGenerator.Object.OutputLibrary.TypeProviders.Single(t => t.Name == "MockInputModel");
+
+            // should have customized base type, not the spec base type
+            Assert.IsNotNull(modelProvider.BaseType);
+            Assert.IsNotNull(modelProvider.BaseTypeProvider);
+            Assert.AreEqual("CustomBaseModel", modelProvider.BaseType!.Name);
+            Assert.AreEqual("Sample.Models", modelProvider.BaseType!.Namespace);
+            Assert.AreEqual(1, modelProvider.BaseTypeProvider!.Properties.Count);
+            Assert.AreEqual("CustomBaseProp", modelProvider.BaseTypeProvider.Properties[0].Name);
+        }
+
         private class NameSpaceVisitor : LibraryVisitor
         {
             protected override TypeProvider? VisitType(TypeProvider type)

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelCustomizationTests/CanCustomizeBaseModelWithSpecBase/MockInputModel.cs

@@ -0,0 +1,11 @@
+#nullable disable
+
+using Sample;
+using SampleTypeSpec;
+
+namespace Sample.Models
+{
+    public partial class MockInputModel : CustomBaseModel
+    {
+    }
+}