Add JsonNamingPolicyAttribute for member-level naming policies in System.Text.Json

src/libraries/System.Text.Json/gen/Helpers/KnownTypeSymbols.cs

@@ -231,6 +231,9 @@ public KnownTypeSymbols(Compilation compilation)
         public INamedTypeSymbol? JsonNumberHandlingAttributeType => GetOrResolveType("System.Text.Json.Serialization.JsonNumberHandlingAttribute", ref _JsonNumberHandlingAttributeType);
         private Option<INamedTypeSymbol?> _JsonNumberHandlingAttributeType;
 
+        public INamedTypeSymbol? JsonNamingPolicyAttributeType => GetOrResolveType("System.Text.Json.Serialization.JsonNamingPolicyAttribute", ref _JsonNamingPolicyAttributeType);
+        private Option<INamedTypeSymbol?> _JsonNamingPolicyAttributeType;
+
         public INamedTypeSymbol? JsonObjectCreationHandlingAttributeType => GetOrResolveType("System.Text.Json.Serialization.JsonObjectCreationHandlingAttribute", ref _JsonObjectCreationHandlingAttributeType);
         private Option<INamedTypeSymbol?> _JsonObjectCreationHandlingAttributeType;
 

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs

@@ -587,6 +587,7 @@ private TypeGenerationSpec ParseTypeGenerationSpec(in TypeToGenerate typeToGener
                     out JsonNumberHandling? numberHandling,
                     out JsonUnmappedMemberHandling? unmappedMemberHandling,
                     out JsonObjectCreationHandling? preferredPropertyObjectCreationHandling,
+                    out JsonKnownNamingPolicy? typeNamingPolicy,
                     out JsonIgnoreCondition? typeIgnoreCondition,
                     out bool foundJsonConverterAttribute,
                     out TypeRef? customConverterType,
@@ -692,7 +693,7 @@ private TypeGenerationSpec ParseTypeGenerationSpec(in TypeToGenerate typeToGener
                     implementsIJsonOnSerialized = _knownSymbols.IJsonOnSerializedType.IsAssignableFrom(type);
 
                     ctorParamSpecs = ParseConstructorParameters(typeToGenerate, constructor, out constructionStrategy, out constructorSetsRequiredMembers);
-                    propertySpecs = ParsePropertyGenerationSpecs(contextType, typeToGenerate, typeIgnoreCondition, options, out hasExtensionDataProperty, out fastPathPropertyIndices);
+                    propertySpecs = ParsePropertyGenerationSpecs(contextType, typeToGenerate, typeIgnoreCondition, options, typeNamingPolicy, out hasExtensionDataProperty, out fastPathPropertyIndices);
                     propertyInitializerSpecs = ParsePropertyInitializers(ctorParamSpecs, propertySpecs, constructorSetsRequiredMembers, ref constructionStrategy);
                 }
 
@@ -749,6 +750,7 @@ private void ProcessTypeCustomAttributes(
                 out JsonNumberHandling? numberHandling,
                 out JsonUnmappedMemberHandling? unmappedMemberHandling,
                 out JsonObjectCreationHandling? objectCreationHandling,
+                out JsonKnownNamingPolicy? namingPolicy,
                 out JsonIgnoreCondition? typeIgnoreCondition,
                 out bool foundJsonConverterAttribute,
                 out TypeRef? customConverterType,
@@ -757,6 +759,7 @@ private void ProcessTypeCustomAttributes(
                 numberHandling = null;
                 unmappedMemberHandling = null;
                 objectCreationHandling = null;
+                namingPolicy = null;
                 typeIgnoreCondition = null;
                 customConverterType = null;
                 foundJsonConverterAttribute = false;
@@ -781,6 +784,22 @@ private void ProcessTypeCustomAttributes(
                         objectCreationHandling = (JsonObjectCreationHandling)attributeData.ConstructorArguments[0].Value!;
                         continue;
                     }
+                    else if (_knownSymbols.JsonNamingPolicyAttributeType?.IsAssignableFrom(attributeType) == true)
+                    {
+                        if (attributeData.ConstructorArguments.Length == 1 &&
+                            attributeData.ConstructorArguments[0].Value is int knownPolicyValue)
+                        {
+                            namingPolicy = (JsonKnownNamingPolicy)knownPolicyValue;
+                        }
+                        else
+                        {
+                            // The attribute uses a custom naming policy that can't be resolved at compile time.
+                            // Use Unspecified to prevent the global naming policy from incorrectly applying.
+                            namingPolicy = JsonKnownNamingPolicy.Unspecified;
+                        }
+
+                        continue;
+                    }
                     else if (!foundJsonConverterAttribute && _knownSymbols.JsonConverterAttributeType.IsAssignableFrom(attributeType))
                     {
                         customConverterType = GetConverterTypeFromJsonConverterAttribute(contextType, typeToGenerate.Type, attributeData);
@@ -1004,6 +1023,7 @@ private List<PropertyGenerationSpec> ParsePropertyGenerationSpecs(
                 in TypeToGenerate typeToGenerate,
                 JsonIgnoreCondition? typeIgnoreCondition,
                 SourceGenerationOptionsSpec? options,
+                JsonKnownNamingPolicy? typeNamingPolicy,
                 out bool hasExtensionDataProperty,
                 out List<int>? fastPathPropertyIndices)
             {
@@ -1088,7 +1108,8 @@ void AddMember(
                         typeIgnoreCondition,
                         ref hasExtensionDataProperty,
                         generationMode,
-                        options);
+                        options,
+                        typeNamingPolicy);
 
                     if (propertySpec is null)
                     {
@@ -1237,7 +1258,8 @@ private bool IsValidDataExtensionPropertyType(ITypeSymbol type)
                 JsonIgnoreCondition? typeIgnoreCondition,
                 ref bool typeHasExtensionDataProperty,
                 JsonSourceGenerationMode? generationMode,
-                SourceGenerationOptionsSpec? options)
+                SourceGenerationOptionsSpec? options,
+                JsonKnownNamingPolicy? typeNamingPolicy)
             {
                 Debug.Assert(memberInfo is IFieldSymbol or IPropertySymbol);
 
@@ -1250,6 +1272,7 @@ private bool IsValidDataExtensionPropertyType(ITypeSymbol type)
                     out JsonIgnoreCondition? ignoreCondition,
                     out JsonNumberHandling? numberHandling,
                     out JsonObjectCreationHandling? objectCreationHandling,
+                    out JsonKnownNamingPolicy? memberNamingPolicy,
                     out TypeRef? converterType,
                     out int order,
                     out bool isExtensionData,
@@ -1319,9 +1342,18 @@ private bool IsValidDataExtensionPropertyType(ITypeSymbol type)
                     return null;
                 }
 
-                string effectiveJsonPropertyName = DetermineEffectiveJsonPropertyName(memberInfo.Name, jsonPropertyName, options);
+                string effectiveJsonPropertyName = DetermineEffectiveJsonPropertyName(memberInfo.Name, jsonPropertyName, memberNamingPolicy, typeNamingPolicy, options);
                 string propertyNameFieldName = DeterminePropertyNameFieldName(effectiveJsonPropertyName);
 
+                // For metadata-based serialization, embed the effective property name when a naming policy
+                // attribute is present so that the runtime DeterminePropertyName uses it instead of
+                // falling back to the global PropertyNamingPolicy. JsonPropertyNameAttribute values are
+                // already captured in jsonPropertyName and take highest precedence.
+                string? metadataJsonPropertyName = jsonPropertyName
+                    ?? (memberNamingPolicy is not null || typeNamingPolicy is not null
+                        ? effectiveJsonPropertyName
+                        : null);
+
                 // Enqueue the property type for generation, unless the member is ignored.
                 TypeRef propertyTypeRef = ignoreCondition != JsonIgnoreCondition.Always
                     ? EnqueueType(memberType, generationMode)
@@ -1334,7 +1366,7 @@ private bool IsValidDataExtensionPropertyType(ITypeSymbol type)
                     IsProperty = memberInfo is IPropertySymbol,
                     IsPublic = isAccessible,
                     IsVirtual = memberInfo.IsVirtual(),
-                    JsonPropertyName = jsonPropertyName,
+                    JsonPropertyName = metadataJsonPropertyName,
                     EffectiveJsonPropertyName = effectiveJsonPropertyName,
                     PropertyNameFieldName = propertyNameFieldName,
                     IsReadOnly = isReadOnly,
@@ -1366,6 +1398,7 @@ private void ProcessMemberCustomAttributes(
                 out JsonIgnoreCondition? ignoreCondition,
                 out JsonNumberHandling? numberHandling,
                 out JsonObjectCreationHandling? objectCreationHandling,
+                out JsonKnownNamingPolicy? memberNamingPolicy,
                 out TypeRef? converterType,
                 out int order,
                 out bool isExtensionData,
@@ -1378,6 +1411,7 @@ private void ProcessMemberCustomAttributes(
                 ignoreCondition = default;
                 numberHandling = default;
                 objectCreationHandling = default;
+                memberNamingPolicy = default;
                 converterType = null;
                 order = 0;
                 isExtensionData = false;
@@ -1396,6 +1430,20 @@ private void ProcessMemberCustomAttributes(
                     {
                         converterType = GetConverterTypeFromJsonConverterAttribute(contextType, memberInfo, attributeData, memberType);
                     }
+                    else if (memberNamingPolicy is null && _knownSymbols.JsonNamingPolicyAttributeType?.IsAssignableFrom(attributeType) == true)
+                    {
+                        if (attributeData.ConstructorArguments.Length == 1 &&
+                            attributeData.ConstructorArguments[0].Value is int knownPolicyValue)
+                        {
+                            memberNamingPolicy = (JsonKnownNamingPolicy)knownPolicyValue;
+                        }
+                        else
+                        {
+                            // The attribute uses a custom naming policy that can't be resolved at compile time.
+                            // Use Unspecified to prevent the global naming policy from incorrectly applying.
+                            memberNamingPolicy = JsonKnownNamingPolicy.Unspecified;
+                        }
+                    }
                     else if (attributeType.ContainingAssembly.Name == SystemTextJsonNamespace)
                     {
                         switch (attributeType.ToDisplayString())
@@ -1843,14 +1891,23 @@ private static int GetTotalTypeParameterCount(INamedTypeSymbol unboundType)
                 return constructedContainingType;
             }
 
-            private static string DetermineEffectiveJsonPropertyName(string propertyName, string? jsonPropertyName, SourceGenerationOptionsSpec? options)
+            private static string DetermineEffectiveJsonPropertyName(
+                string propertyName,
+                string? jsonPropertyName,
+                JsonKnownNamingPolicy? memberNamingPolicy,
+                JsonKnownNamingPolicy? typeNamingPolicy,
+                SourceGenerationOptionsSpec? options)
             {
                 if (jsonPropertyName != null)
                 {
                     return jsonPropertyName;
                 }
 
-                JsonNamingPolicy? instance = options?.GetEffectivePropertyNamingPolicy() switch
+                JsonKnownNamingPolicy? effectiveKnownPolicy = memberNamingPolicy
+                    ?? typeNamingPolicy
+                    ?? options?.GetEffectivePropertyNamingPolicy();
+
+                JsonNamingPolicy? instance = effectiveKnownPolicy switch
                 {
                     JsonKnownNamingPolicy.CamelCase => JsonNamingPolicy.CamelCase,
                     JsonKnownNamingPolicy.SnakeCaseLower => JsonNamingPolicy.SnakeCaseLower,

src/libraries/System.Text.Json/ref/System.Text.Json.cs

@@ -1079,6 +1079,13 @@ public enum JsonKnownNamingPolicy
         KebabCaseLower = 4,
         KebabCaseUpper = 5,
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Class | System.AttributeTargets.Field | System.AttributeTargets.Interface | System.AttributeTargets.Property | System.AttributeTargets.Struct, AllowMultiple=false)]
+    public partial class JsonNamingPolicyAttribute : System.Text.Json.Serialization.JsonAttribute
+    {
+        public JsonNamingPolicyAttribute(System.Text.Json.Serialization.JsonKnownNamingPolicy namingPolicy) { }
+        protected JsonNamingPolicyAttribute(System.Text.Json.JsonNamingPolicy namingPolicy) { }
+        public System.Text.Json.JsonNamingPolicy NamingPolicy { get { throw null; } }
+    }
     public enum JsonKnownReferenceHandler
     {
         Unspecified = 0,

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -116,6 +116,7 @@ The System.Text.Json library is built-in as part of the shared framework in .NET
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonIgnoreAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonIncludeAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonNumberHandlingAttribute.cs" />
+    <Compile Include="System\Text\Json\Serialization\Attributes\JsonNamingPolicyAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonPolymorphicAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonPropertyNameAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonRequiredAttribute.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Attributes/JsonNamingPolicyAttribute.cs

@@ -0,0 +1,72 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Text.Json.Serialization
+{
+    /// <summary>
+    /// When placed on a type, property, or field, indicates what <see cref="JsonNamingPolicy"/>
+    /// should be used to convert property names.
+    /// </summary>
+    /// <remarks>
+    /// When placed on a property or field, the naming policy specified by this attribute
+    /// takes precedence over the type-level attribute and the <see cref="JsonSerializerOptions.PropertyNamingPolicy"/>.
+    /// When placed on a type, the naming policy specified by this attribute takes precedence
+    /// over the <see cref="JsonSerializerOptions.PropertyNamingPolicy"/>.
+    /// The <see cref="JsonPropertyNameAttribute"/> takes precedence over this attribute.
+    /// </remarks>
+    [AttributeUsage(
+        AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface |
+        AttributeTargets.Property | AttributeTargets.Field,
+        AllowMultiple = false)]
+    public class JsonNamingPolicyAttribute : JsonAttribute
+    {
+        /// <summary>
+        /// Initializes a new instance of <see cref="JsonNamingPolicyAttribute"/>
+        /// with the specified known naming policy.
+        /// </summary>
+        /// <param name="namingPolicy">The known naming policy to use for name conversion.</param>
+        /// <exception cref="ArgumentOutOfRangeException">
+        /// The specified <paramref name="namingPolicy"/> is not a valid known naming policy value.
+        /// </exception>
+        public JsonNamingPolicyAttribute(JsonKnownNamingPolicy namingPolicy)
+        {
+            NamingPolicy = ResolveNamingPolicy(namingPolicy);
+        }
+
+        /// <summary>
+        /// Initializes a new instance of <see cref="JsonNamingPolicyAttribute"/>
+        /// with a custom naming policy.
+        /// </summary>
+        /// <param name="namingPolicy">The naming policy to use for name conversion.</param>
+        /// <exception cref="ArgumentNullException">
+        /// <paramref name="namingPolicy"/> is <see langword="null"/>.
+        /// </exception>
+        protected JsonNamingPolicyAttribute(JsonNamingPolicy namingPolicy)
+        {
+            if (namingPolicy is null)
+            {
+                ThrowHelper.ThrowArgumentNullException(nameof(namingPolicy));
+            }
+
+            NamingPolicy = namingPolicy;
+        }
+
+        /// <summary>
+        /// Gets the naming policy to use for name conversion.
+        /// </summary>
+        public JsonNamingPolicy NamingPolicy { get; }
+
+        internal static JsonNamingPolicy ResolveNamingPolicy(JsonKnownNamingPolicy namingPolicy)
+        {
+            return namingPolicy switch
+            {
+                JsonKnownNamingPolicy.CamelCase => JsonNamingPolicy.CamelCase,
+                JsonKnownNamingPolicy.SnakeCaseLower => JsonNamingPolicy.SnakeCaseLower,
+                JsonKnownNamingPolicy.SnakeCaseUpper => JsonNamingPolicy.SnakeCaseUpper,
+                JsonKnownNamingPolicy.KebabCaseLower => JsonNamingPolicy.KebabCaseLower,
+                JsonKnownNamingPolicy.KebabCaseUpper => JsonNamingPolicy.KebabCaseUpper,
+                _ => throw new ArgumentOutOfRangeException(nameof(namingPolicy)),
+            };
+        }
+    }
+}