[C# Generator] Populate attributes from custom code properties and fields in NamedTypeSymbolProvider (#10322)

`NamedTypeSymbolProvider.BuildProperties()` and `BuildFields()` never
passed symbol attributes to the `PropertyProvider`/`FieldProvider`
constructors, so `CustomCodeView.Properties[*].Attributes` and
`CustomCodeView.Fields[*].Attributes` were always empty. This blocked
downstream consumers (e.g., mgmt generator) from detecting attributes
like `[Obsolete]` on custom code properties and fields.

### Changes

- **`NamedTypeSymbolProvider.cs`**: Extract `AttributeData` from each
`IPropertySymbol` and `IFieldSymbol` and pass as `attributes:` parameter
when constructing `PropertyProvider` and `FieldProvider`. All attributes
(including internal CodeGen attributes) are passed through without
filtering. Uses `.ToArray()` because the constructor's `(attributes as
IReadOnlyList<AttributeStatement>) ?? []` silently drops unmaterialized
`IEnumerable`.

```csharp
new PropertyProvider(
    ...,
    this,
    attributes: propertySymbol.GetAttributes().Select(a => new AttributeStatement(a)).ToArray())

new FieldProvider(
    ...,
    attributes: fieldSymbol.GetAttributes().Select(a => new AttributeStatement(a)).ToArray())
```

- **`FieldProvider.cs`**: Updated `InitializeParameter()` to filter out
codegen-related attributes (from the
`Microsoft.TypeSpec.Generator.Customizations` namespace) when
propagating field attributes to constructor parameters via
`AsParameter`. This ensures user-facing attributes like `[Obsolete]`
propagate to generated constructor parameters, while internal codegen
attributes like `[CodeGenMember]` do not appear in generated code. The
field itself retains all attributes for downstream consumers.

```csharp
var paramAttributes = Attributes.Where(a => a.Type.Namespace != CodeModelGenerator.CustomizationAttributeNamespace).ToArray();
_parameter = new(() => new ParameterProvider(
    ..., attributes: paramAttributes));
```

- **Test**: Added `CanReadPropertyAttributes` with a custom code file
declaring `[Obsolete]` (with a named argument `DiagnosticId`),
`[EditorBrowsable]`, and a custom non-system attribute on a property,
plus `[Obsolete]` on a field. The test validates attribute types using
`CSharpType.Equals`, checks argument counts, validates
`PositionalArguments`, and ensures the generator does not throw for
custom non-system attributes.

---------

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/Providers/FieldProvider.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
+using System.Linq;
 using Microsoft.TypeSpec.Generator.Expressions;
 using Microsoft.TypeSpec.Generator.Input.Extensions;
 using Microsoft.TypeSpec.Generator.Primitives;
@@ -147,8 +148,9 @@ public void Update(
         [MemberNotNull(nameof(_parameter))]
         private void InitializeParameter()
         {
+            var paramAttributes = Attributes.Where(a => a.Type.Namespace != CodeModelGenerator.CustomizationAttributeNamespace).ToArray();
             _parameter = new(() => new ParameterProvider(
-                Name.ToVariableName(), Description ?? FormattableStringHelpers.Empty, Type, field: this, wireInfo: WireInfo, attributes: Attributes));
+                Name.ToVariableName(), Description ?? FormattableStringHelpers.Empty, Type, field: this, wireInfo: WireInfo, attributes: paramAttributes));
         }
 
         private MemberExpression? _asMember;

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

@@ -122,7 +122,8 @@ protected internal override FieldProvider[] BuildFields()
                         fieldSymbol.Name,
                         this,
                         GetSymbolXmlDoc(fieldSymbol, "summary"),
-                        initializationValue: GetFieldInitializer(fieldSymbol))
+                        initializationValue: GetFieldInitializer(fieldSymbol),
+                        attributes: fieldSymbol.GetAttributes().Select(a => new AttributeStatement(a)).ToArray())
                     {
                         OriginalName = GetOriginalName(fieldSymbol)
                     };
@@ -146,7 +147,8 @@ protected internal override PropertyProvider[] BuildProperties()
                     new AutoPropertyBody(
                         propertySymbol.SetMethod is not null,
                         InitializationExpression: GetPropertyInitializer(propertySymbol)),
-                    this)
+                    this,
+                    attributes: propertySymbol.GetAttributes().Select(a => new AttributeStatement(a)).ToArray())
                 {
                     OriginalName = GetOriginalName(propertySymbol),
                     CustomProvider = new(() => propertySymbol.Type is INamedTypeSymbol propertyNamedTypeSymbol

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

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -1698,6 +1699,50 @@ public async Task CanCustomizeBaseModelToSystemType()
                 "System.Exception is from a referenced assembly and should use SystemObjectTypeProvider");
         }
 
+        [Test]
+        public async Task CanReadPropertyAttributes()
+        {
+            await MockHelpers.LoadMockGeneratorAsync(compilation: async () => await Helpers.GetCompilationFromDirectoryAsync());
+
+            var props = new[]
+            {
+                InputFactory.Property("Prop1", InputPrimitiveType.String)
+            };
+
+            var inputModel = InputFactory.Model("mockInputModel", properties: props);
+            var modelTypeProvider = new ModelProvider(inputModel);
+            var customCodeView = modelTypeProvider.CustomCodeView;
+
+            Assert.IsNotNull(customCodeView);
+            Assert.AreEqual(1, customCodeView!.Properties.Count);
+
+            var customProperty = customCodeView.Properties[0];
+            Assert.AreEqual("Prop1", customProperty.Name);
+
+            // Verify that attributes from custom code are populated, including a custom non-system attribute
+            Assert.AreEqual(3, customProperty.Attributes.Count);
+
+            // Validate [Obsolete("This property is now deprecated.", DiagnosticId = "OBS001")] - type, arguments, and positional arguments
+            var obsoleteAttr = customProperty.Attributes.Single(a => new CSharpType(typeof(ObsoleteAttribute)).Equals(a.Type));
+            Assert.AreEqual(1, obsoleteAttr.Arguments.Count);
+            Assert.AreEqual(1, obsoleteAttr.PositionalArguments.Count);
+            Assert.AreEqual("DiagnosticId", obsoleteAttr.PositionalArguments[0].Key);
+
+            // Validate [EditorBrowsable(EditorBrowsableState.Never)] - type and arguments
+            var editorBrowsableAttr = customProperty.Attributes.Single(a => new CSharpType(typeof(System.ComponentModel.EditorBrowsableAttribute)).Equals(a.Type));
+            Assert.AreEqual(1, editorBrowsableAttr.Arguments.Count);
+
+            // Validate [Custom("custom message")] - custom non-system attribute does not throw
+            var customAttr = customProperty.Attributes.Single(a => a.Type.Name == "CustomAttribute");
+            Assert.AreEqual(1, customAttr.Arguments.Count);
+
+            // Verify that field attributes from custom code are populated
+            var customField = customCodeView.Fields.Single(f => f.Name == "_customField");
+            Assert.AreEqual(1, customField.Attributes.Count);
+            var fieldObsoleteAttr = customField.Attributes.Single(a => new CSharpType(typeof(ObsoleteAttribute)).Equals(a.Type));
+            Assert.AreEqual(1, fieldObsoleteAttr.Arguments.Count);
+        }
+
         private class TestNameVisitor : NameVisitor
         {
             public TypeProvider? InvokeVisit(TypeProvider type)

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

@@ -0,0 +1,23 @@
+#nullable disable
+
+using System;
+using System.ComponentModel;
+
+namespace Sample.Models
+{
+    public class CustomAttribute : Attribute
+    {
+        public CustomAttribute(string message) { }
+    }
+
+    public partial class MockInputModel
+    {
+        [Obsolete("This property is now deprecated.", DiagnosticId = "OBS001")]
+        [EditorBrowsable(EditorBrowsableState.Never)]
+        [Custom("custom message")]
+        public string Prop1 { get; set; }
+
+        [Obsolete("This field is now deprecated.")]
+        private int _customField;
+    }
+}