[http-client-csharp] Add CustomCodeAttributeDefinition base for custom-code attribute providers (#10997)

Custom-code attribute providers contributed via
`CodeModelGenerator.AddCustomCodeAttributeProvider` are written into the
custom-code workspace before `SourceInputModel` is initialized. For a
normal `TypeProvider` outside the generator assembly, writing the
provider touches the lazy `CustomCodeView`/`LastContractView`, which
evaluate `SourceInputModel` and crash with *"SourceInputModel has not
been initialized yet"*. Built-in providers avoid this by overriding
`BuildCustomCodeView`/`BuildLastContractView` to return `null`, but
those are `private protected` and unreachable from derived generators in
other assemblies (e.g. Azure management), forcing reflection
workarounds.

### Changes
- **New base type `CustomCodeAttributeDefinition`**
(`Microsoft.TypeSpec.Generator.Providers`): public abstract
`TypeProvider` that `sealed override`s
`BuildCustomCodeView`/`BuildLastContractView` to `null`. Living in the
generator assembly lets it override the `private protected` members;
derived generators inherit the opt-out with no reflection.
- **Built-in definitions**
(`CodeGenType`/`CodeGenMember`/`CodeGenSuppress`/`CodeGenSerialization`AttributeDefinition)
now inherit from the base, dropping their duplicated per-class opt-outs.
Generated output is unchanged.
- **`CodeModelGenerator` APIs**: `CustomCodeAttributeProviders` is now
`List<CustomCodeAttributeDefinition>` and
`AddCustomCodeAttributeProvider` takes a
`CustomCodeAttributeDefinition`, so contributed providers are required
to inherit the base and the source-input opt-out is enforced at the type
level rather than relying on the caller.
- **Tests**: `TestCustomCodeAttributeDefinition` inherits the new base;
added a test that sets `SourceInputModel` to throw and asserts a
`CustomCodeAttributeDefinition`'s views/`Name`/`Type` resolve without
throwing, while a plain `TypeProvider` still throws.

### Usage
```csharp
public class CodeGenResourceDataAttributeDefinition : CustomCodeAttributeDefinition
{
    protected override string BuildName() => "CodeGenResourceDataAttribute";
    // BuildCustomCodeView / BuildLastContractView are sealed to null by the base
}
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jorgerangel-msft <102122018+jorgerangel-msft@users.noreply.github.com>

Author: Copilot

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

@@ -100,12 +100,12 @@ internal set
         public IReadOnlyList<string> SharedSourceDirectories => _sharedSourceDirectories;
 
         /// <summary>
-        /// The list of <see cref="TypeProvider"/> instances that define custom-code attributes. These attribute
+        /// The list of <see cref="CustomCodeAttributeDefinition"/> instances that define custom-code attributes. These attribute
         /// definitions are generated into the SDK project and are made available to the compiler while it compiles
         /// custom code. Derived generators can contribute additional providers via
-        /// <see cref="AddCustomCodeAttributeProvider(TypeProvider)"/>.
+        /// <see cref="AddCustomCodeAttributeProvider(CustomCodeAttributeDefinition)"/>.
         /// </summary>
-        internal List<TypeProvider> CustomCodeAttributeProviders { get; } =
+        internal List<CustomCodeAttributeDefinition> CustomCodeAttributeProviders { get; } =
         [
             new CodeGenTypeAttributeDefinition(),
             new CodeGenMemberAttributeDefinition(),
@@ -171,8 +171,8 @@ public virtual void AddSharedSourceDirectory(string sharedSourceDirectory)
         /// generator-specific attribute definitions. The provider's attribute definition is generated into
         /// the SDK project and made available to the compiler while it compiles custom code.
         /// </summary>
-        /// <param name="provider">The <see cref="TypeProvider"/> that defines the custom-code attribute.</param>
-        protected void AddCustomCodeAttributeProvider(TypeProvider provider)
+        /// <param name="provider">The <see cref="CustomCodeAttributeDefinition"/> that defines the custom-code attribute.</param>
+        protected void AddCustomCodeAttributeProvider(CustomCodeAttributeDefinition provider)
         {
             CustomCodeAttributeProviders.Add(provider);
         }

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

@@ -11,17 +11,14 @@
 
 namespace Microsoft.TypeSpec.Generator.Providers
 {
-    internal class CodeGenMemberAttributeDefinition : TypeProvider
+    internal class CodeGenMemberAttributeDefinition : CustomCodeAttributeDefinition
     {
         protected override string BuildRelativeFilePath() => Path.Combine("src", "Generated", "Internal", $"{Name}.cs");
 
         protected override string BuildName() => "CodeGenMemberAttribute";
 
         protected override string BuildNamespace() => CodeModelGenerator.CustomizationAttributeNamespace;
 
-        private protected sealed override NamedTypeSymbolProvider? BuildCustomCodeView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
-        private protected sealed override NamedTypeSymbolProvider? BuildLastContractView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
-
         protected override TypeSignatureModifiers BuildDeclarationModifiers() =>
             TypeSignatureModifiers.Internal | TypeSignatureModifiers.Class;
 

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

@@ -11,17 +11,14 @@
 
 namespace Microsoft.TypeSpec.Generator.Providers
 {
-    internal class CodeGenSerializationAttributeDefinition : TypeProvider
+    internal class CodeGenSerializationAttributeDefinition : CustomCodeAttributeDefinition
     {
         protected override string BuildRelativeFilePath() => Path.Combine("src", "Generated", "Internal", $"{Name}.cs");
 
         protected override string BuildName() => "CodeGenSerializationAttribute";
 
         protected override string BuildNamespace() => CodeModelGenerator.CustomizationAttributeNamespace;
 
-        private protected sealed override NamedTypeSymbolProvider? BuildCustomCodeView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
-        private protected sealed override NamedTypeSymbolProvider? BuildLastContractView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
-
         protected override TypeSignatureModifiers BuildDeclarationModifiers() =>
             TypeSignatureModifiers.Internal | TypeSignatureModifiers.Class;
 

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

@@ -11,17 +11,14 @@
 
 namespace Microsoft.TypeSpec.Generator.Providers
 {
-    internal class CodeGenSuppressAttributeDefinition : TypeProvider
+    internal class CodeGenSuppressAttributeDefinition : CustomCodeAttributeDefinition
     {
         protected override string BuildRelativeFilePath() => Path.Combine("src", "Generated", "Internal", $"{Name}.cs");
 
         protected override string BuildName() => "CodeGenSuppressAttribute";
 
         protected override string BuildNamespace() => CodeModelGenerator.CustomizationAttributeNamespace;
 
-        private protected sealed override NamedTypeSymbolProvider? BuildCustomCodeView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
-        private protected sealed override NamedTypeSymbolProvider? BuildLastContractView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
-
         protected override TypeSignatureModifiers BuildDeclarationModifiers() =>
             TypeSignatureModifiers.Internal | TypeSignatureModifiers.Class;
 

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

@@ -11,17 +11,14 @@
 
 namespace Microsoft.TypeSpec.Generator.Providers
 {
-    internal class CodeGenTypeAttributeDefinition : TypeProvider
+    internal class CodeGenTypeAttributeDefinition : CustomCodeAttributeDefinition
     {
         protected override string BuildRelativeFilePath() => Path.Combine("src", "Generated", "Internal", $"{Name}.cs");
 
         protected override string BuildName() => "CodeGenTypeAttribute";
 
         protected override string BuildNamespace() => CodeModelGenerator.CustomizationAttributeNamespace;
 
-        private protected sealed override NamedTypeSymbolProvider? BuildCustomCodeView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
-        private protected sealed override NamedTypeSymbolProvider? BuildLastContractView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
-
         protected override TypeSignatureModifiers BuildDeclarationModifiers() =>
             TypeSignatureModifiers.Internal | TypeSignatureModifiers.Class;
 

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

@@ -0,0 +1,15 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+namespace Microsoft.TypeSpec.Generator.Providers
+{
+    /// <summary>
+    /// A base type for custom-code attribute definitions that are generated into the SDK project and made available
+    /// to the compiler while it compiles custom code.
+    /// </summary>
+    public abstract class CustomCodeAttributeDefinition : TypeProvider
+    {
+        private protected sealed override TypeProvider? BuildCustomCodeView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
+        private protected sealed override TypeProvider? BuildLastContractView(string? generatedTypeName = default, string? generatedTypeNamespace = default) => null;
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/GeneratorTests.cs

@@ -1,7 +1,11 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System;
 using System.Linq;
+using Microsoft.TypeSpec.Generator.Providers;
+using Microsoft.TypeSpec.Generator.Tests.Common;
+using Moq;
 using NUnit.Framework;
 
 namespace Microsoft.TypeSpec.Generator.Tests
@@ -113,13 +117,38 @@ public void CanAddCustomCodeAttributeProvider()
             var mockGenerator = new TestGenerator();
             var initialCount = mockGenerator.CustomCodeAttributeProviders.Count;
 
-            var provider = new TestTypeProvider();
+            var provider = new TestCustomCodeAttributeDefinition();
             mockGenerator.AddCustomCodeAttributeProviderForTest(provider);
 
             Assert.AreEqual(initialCount + 1, mockGenerator.CustomCodeAttributeProviders.Count);
             Assert.AreSame(provider, mockGenerator.CustomCodeAttributeProviders[^1]);
         }
 
+        // Reproduces the real CSharpGen.ExecuteAsync ordering where contributed custom-code attribute providers are
+        // written before SourceInputModel is initialized. A CustomCodeAttributeDefinition disables the source-input
+        // views, so accessing its Name/Type (and the views directly) must not throw.
+        [Test]
+        public void CustomCodeAttributeDefinitionDoesNotEvaluateSourceInputViews()
+        {
+            var mockGenerator = MockHelpers.LoadMockGenerator();
+            // Simulate SourceInputModel not being initialized yet, as is the case when the contributed attribute
+            // provider is written into the custom-code workspace.
+            mockGenerator
+                .Setup(p => p.SourceInputModel)
+                .Throws(new InvalidOperationException("SourceInputModel has not been initialized yet"));
+
+            var attributeDefinition = new TestCustomCodeAttributeDefinition();
+
+            Assert.IsNull(attributeDefinition.CustomCodeView);
+            Assert.IsNull(attributeDefinition.LastContractView);
+            Assert.DoesNotThrow(() => _ = attributeDefinition.Name);
+            Assert.DoesNotThrow(() => _ = attributeDefinition.Type);
+
+            // A regular TypeProvider would still attempt to resolve the source-input views and therefore throw.
+            var regularProvider = new TestTypeProvider();
+            Assert.Throws<InvalidOperationException>(() => _ = regularProvider.CustomCodeView);
+        }
+
         private class DerivedTestLibraryVisitor : TestLibraryVisitor { }
     }
 }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/TestHelpers/TestCustomCodeAttributeDefinition.cs

@@ -16,7 +16,7 @@ namespace Microsoft.TypeSpec.Generator.Tests
     /// via <c>AddCustomCodeAttributeProvider</c>. Used to validate that custom code referencing the attribute is
     /// parsed and the attribute is registered on the parsed type.
     /// </summary>
-    public class TestCustomCodeAttributeDefinition : TypeProvider
+    public class TestCustomCodeAttributeDefinition : CustomCodeAttributeDefinition
     {
         public const string AttributeNamespace = "Sample.Customizations";
 

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/TestHelpers/TestGenerator.cs

@@ -12,7 +12,7 @@ public TestGenerator()
         {
         }
 
-        public void AddCustomCodeAttributeProviderForTest(TypeProvider provider)
+        public void AddCustomCodeAttributeProviderForTest(CustomCodeAttributeDefinition provider)
             => AddCustomCodeAttributeProvider(provider);
     }
 }