Populating JSON properties support

src/libraries/System.Text.Json/Common/JsonNumberHandling.cs

@@ -15,7 +15,7 @@ namespace System.Text.Json.Serialization
 #else
     public
 #endif
-     enum JsonNumberHandling
+    enum JsonNumberHandling
     {
         /// <summary>
         /// Numbers will only be read from <see cref="JsonTokenType.Number"/> tokens and will only be written as JSON numbers (without quotes).

src/libraries/System.Text.Json/Common/JsonObjectCreationHandling.cs

@@ -0,0 +1,27 @@
+// 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>
+    /// Determines how deserialization will handle object creation for fields or properties.
+    /// </summary>
+#if BUILDING_SOURCE_GENERATOR
+    internal
+#else
+    public
+#endif
+    enum JsonObjectCreationHandling
+    {
+        /// <summary>
+        /// A new instance will always be created when deserializing a field or property.
+        /// </summary>
+        Replace = 0,
+
+        /// <summary>
+        /// Attempt to populate any instances already found on a deserialized field or property.
+        /// </summary>
+        Populate = 1,
+    }
+}

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

@@ -32,6 +32,7 @@ private sealed partial class Emitter
             internal const string JsonContextVarName = "jsonContext";
             private const string NumberHandlingPropName = "NumberHandling";
             private const string UnmappedMemberHandlingPropName = "UnmappedMemberHandling";
+            private const string PreferredPropertyObjectCreationHandlingPropName = "PreferredPropertyObjectCreationHandling";
             private const string ObjectCreatorPropName = "ObjectCreator";
             private const string OptionsInstanceVariableName = "Options";
             private const string JsonTypeInfoReturnValueLocalVariableName = "jsonTypeInfo";
@@ -65,6 +66,7 @@ private sealed partial class Emitter
             private const string JsonCollectionInfoValuesTypeRef = "global::System.Text.Json.Serialization.Metadata.JsonCollectionInfoValues";
             private const string JsonIgnoreConditionTypeRef = "global::System.Text.Json.Serialization.JsonIgnoreCondition";
             private const string JsonNumberHandlingTypeRef = "global::System.Text.Json.Serialization.JsonNumberHandling";
+            private const string JsonObjectCreationHandlingTypeRef = "global::System.Text.Json.Serialization.JsonObjectCreationHandling";
             private const string JsonUnmappedMemberHandlingTypeRef = "global::System.Text.Json.Serialization.JsonUnmappedMemberHandling";
             private const string JsonMetadataServicesTypeRef = "global::System.Text.Json.Serialization.Metadata.JsonMetadataServices";
             private const string JsonObjectInfoValuesTypeRef = "global::System.Text.Json.Serialization.Metadata.JsonObjectInfoValues";
@@ -657,6 +659,14 @@ private string GenerateForObject(TypeGenerationSpec typeMetadata)
 """;
                 }
 
+                if (typeMetadata.PreferredPropertyObjectCreationHandling != null)
+                {
+                    objectInfoInitSource += $"""
+
+        {JsonTypeInfoReturnValueLocalVariableName}.{PreferredPropertyObjectCreationHandlingPropName} = {GetObjectCreationHandlingAsStr(typeMetadata.PreferredPropertyObjectCreationHandling.Value)};
+""";
+                }
+
                 string additionalSource = @$"{propMetadataInitFuncSource}{serializeFuncSource}{ctorParamMetadataInitFuncSource}";
 
                 return GenerateForType(typeMetadata, objectInfoInitSource, additionalSource);
@@ -762,6 +772,12 @@ private static string GeneratePropMetadataInitFunc(TypeGenerationSpec typeGenera
     {propertyInfoVarName}.IsRequired = true;");
                     }
 
+                    if (memberMetadata.ObjectCreationHandling != null)
+                    {
+                        sb.Append($@"
+    {propertyInfoVarName}.ObjectCreationHandling = {GetObjectCreationHandlingAsStr(memberMetadata.ObjectCreationHandling.Value)};");
+                    }
+
                     sb.Append($@"
     {PropVarName}[{i}] = {propertyInfoVarName};
 ");
@@ -1386,12 +1402,22 @@ private static string IndentSource(string source, int numIndentations)
             }
 
             private static string GetNumberHandlingAsStr(JsonNumberHandling? numberHandling) =>
-                 numberHandling.HasValue
-                    ? $"({JsonNumberHandlingTypeRef}){(int)numberHandling.Value}"
-                    : "default";
+                numberHandling switch
+                {
+                    null => "default",
+                    >= 0 => $"({JsonNumberHandlingTypeRef}){(int)numberHandling.Value}",
+                    < 0 => $"({JsonNumberHandlingTypeRef})({(int)numberHandling.Value})"
+                };
+
+            private static string GetObjectCreationHandlingAsStr(JsonObjectCreationHandling creationHandling) =>
+                creationHandling >= 0
+                ? $"({JsonObjectCreationHandlingTypeRef}){(int)creationHandling}"
+                : $"({JsonObjectCreationHandlingTypeRef})({(int)creationHandling})";
 
             private static string GetUnmappedMemberHandlingAsStr(JsonUnmappedMemberHandling unmappedMemberHandling) =>
-                $"({JsonUnmappedMemberHandlingTypeRef}){(int)unmappedMemberHandling}";
+                unmappedMemberHandling >= 0
+                ? $"({JsonUnmappedMemberHandlingTypeRef}){(int)unmappedMemberHandling}"
+                : $"({JsonUnmappedMemberHandlingTypeRef})({(int)unmappedMemberHandling})";
 
             private static string GetCreateValueInfoMethodRef(string typeCompilableName) => $"{CreateValueInfoMethodName}<{typeCompilableName}>";
 

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

@@ -38,6 +38,7 @@ private sealed class Parser
             private const string JsonIgnoreConditionFullName = "System.Text.Json.Serialization.JsonIgnoreCondition";
             private const string JsonIncludeAttributeFullName = "System.Text.Json.Serialization.JsonIncludeAttribute";
             private const string JsonNumberHandlingAttributeFullName = "System.Text.Json.Serialization.JsonNumberHandlingAttribute";
+            private const string JsonObjectCreationHandlingAttributeFullName = "System.Text.Json.Serialization.JsonObjectCreationHandlingAttribute";
             private const string JsonUnmappedMemberHandlingAttributeFullName = "System.Text.Json.Serialization.JsonUnmappedMemberHandlingAttribute";
             private const string JsonPropertyNameAttributeFullName = "System.Text.Json.Serialization.JsonPropertyNameAttribute";
             private const string JsonPropertyOrderAttributeFullName = "System.Text.Json.Serialization.JsonPropertyOrderAttribute";
@@ -708,6 +709,7 @@ private TypeGenerationSpec GetOrAddTypeGenerationSpec(Type type, JsonSourceGener
                 CollectionType collectionType = CollectionType.NotApplicable;
                 JsonNumberHandling? numberHandling = null;
                 JsonUnmappedMemberHandling? unmappedMemberHandling = null;
+                JsonObjectCreationHandling? preferredPropertyObjectCreationHandling = null;
                 bool foundDesignTimeCustomConverter = false;
                 string? converterInstatiationLogic = null;
                 bool implementsIJsonOnSerialized = false;
@@ -735,6 +737,12 @@ private TypeGenerationSpec GetOrAddTypeGenerationSpec(Type type, JsonSourceGener
                         unmappedMemberHandling = (JsonUnmappedMemberHandling)ctorArgs[0].Value!;
                         continue;
                     }
+                    else if (attributeTypeFullName == JsonObjectCreationHandlingAttributeFullName)
+                    {
+                        IList<CustomAttributeTypedArgument> ctorArgs = attributeData.ConstructorArguments;
+                        preferredPropertyObjectCreationHandling = (JsonObjectCreationHandling)ctorArgs[0].Value!;
+                        continue;
+                    }
                     else if (!foundDesignTimeCustomConverter && attributeType.GetCompatibleBaseClass(JsonConverterAttributeFullName) != null)
                     {
                         foundDesignTimeCustomConverter = true;
@@ -1139,6 +1147,7 @@ void CacheMemberHelper(Location memberLocation)
                     classType,
                     numberHandling,
                     unmappedMemberHandling,
+                    preferredPropertyObjectCreationHandling,
                     propGenSpecList,
                     paramGenSpecArray,
                     propertyInitializerSpecList,
@@ -1238,6 +1247,7 @@ private PropertyGenerationSpec GetPropertyGenerationSpec(
                     out string? jsonPropertyName,
                     out JsonIgnoreCondition? ignoreCondition,
                     out JsonNumberHandling? numberHandling,
+                    out JsonObjectCreationHandling? objectCreationHandling,
                     out string? converterInstantiationLogic,
                     out int order,
                     out bool hasFactoryConverter,
@@ -1287,6 +1297,7 @@ private PropertyGenerationSpec GetPropertyGenerationSpec(
                     SetterIsVirtual = setterIsVirtual,
                     DefaultIgnoreCondition = ignoreCondition,
                     NumberHandling = numberHandling,
+                    ObjectCreationHandling = objectCreationHandling,
                     Order = order,
                     HasJsonInclude = hasJsonInclude,
                     IsExtensionData = isExtensionData,
@@ -1320,6 +1331,7 @@ private void ProcessMemberCustomAttributes(
                 out string? jsonPropertyName,
                 out JsonIgnoreCondition? ignoreCondition,
                 out JsonNumberHandling? numberHandling,
+                out JsonObjectCreationHandling? objectCreationHandling,
                 out string? converterInstantiationLogic,
                 out int order,
                 out bool hasFactoryConverter,
@@ -1330,6 +1342,7 @@ private void ProcessMemberCustomAttributes(
                 jsonPropertyName = null;
                 ignoreCondition = default;
                 numberHandling = default;
+                objectCreationHandling = default;
                 converterInstantiationLogic = null;
                 order = 0;
                 isExtensionData = false;
@@ -1382,6 +1395,12 @@ private void ProcessMemberCustomAttributes(
                                     numberHandling = (JsonNumberHandling)ctorArgs[0].Value!;
                                 }
                                 break;
+                            case JsonObjectCreationHandlingAttributeFullName:
+                                {
+                                    IList<CustomAttributeTypedArgument> ctorArgs = attributeData.ConstructorArguments;
+                                    objectCreationHandling = (JsonObjectCreationHandling)ctorArgs[0].Value!;
+                                }
+                                break;
                             case JsonPropertyNameAttributeFullName:
                                 {
                                     IList<CustomAttributeTypedArgument> ctorArgs = attributeData.ConstructorArguments;

src/libraries/System.Text.Json/gen/PropertyGenerationSpec.cs

@@ -90,6 +90,11 @@ internal sealed class PropertyGenerationSpec
         /// </summary>
         public JsonNumberHandling? NumberHandling { get; init; }
 
+        /// <summary>
+        /// The <see cref="JsonObjectCreationHandling"/> for the property.
+        /// </summary>
+        public JsonObjectCreationHandling? ObjectCreationHandling { get; init; }
+
         /// <summary>
         /// The serialization order of the property.
         /// </summary>

src/libraries/System.Text.Json/gen/System.Text.Json.SourceGeneration.targets

@@ -35,6 +35,7 @@
     <Compile Include="..\Common\JsonKebabCaseUpperNamingPolicy.cs" Link="Common\System\Text\Json\Serialization\JsonKebabCaseUpperNamingPolicy.cs" />
     <Compile Include="..\Common\JsonKnownNamingPolicy.cs" Link="Common\System\Text\Json\Serialization\JsonKnownNamingPolicy.cs" />
     <Compile Include="..\Common\JsonNumberHandling.cs" Link="Common\System\Text\Json\Serialization\JsonNumberHandling.cs" />
+    <Compile Include="..\Common\JsonObjectCreationHandling.cs" Link="Common\System\Text\Json\Serialization\JsonObjectCreationHandling.cs" />
     <Compile Include="..\Common\JsonSeparatorNamingPolicy.cs" Link="Common\System\Text\Json\Serialization\JsonSeparatorNamingPolicy.cs" />
     <Compile Include="..\Common\JsonSerializableAttribute.cs" Link="Common\System\Text\Json\Serialization\JsonSerializableAttribute.cs" />
     <Compile Include="..\Common\JsonSnakeCaseLowerNamingPolicy.cs" Link="Common\System\Text\Json\Serialization\JsonSnakeCaseLowerNamingPolicy.cs" />

src/libraries/System.Text.Json/gen/TypeGenerationSpec.cs

@@ -63,6 +63,7 @@ public TypeGenerationSpec(Type type)
 
         public JsonNumberHandling? NumberHandling { get; private set; }
         public JsonUnmappedMemberHandling? UnmappedMemberHandling { get; private set; }
+        public JsonObjectCreationHandling? PreferredPropertyObjectCreationHandling { get; private set; }
 
         public List<PropertyGenerationSpec>? PropertyGenSpecList { get; private set; }
 
@@ -131,6 +132,7 @@ public void Initialize(
             ClassType classType,
             JsonNumberHandling? numberHandling,
             JsonUnmappedMemberHandling? unmappedMemberHandling,
+            JsonObjectCreationHandling? preferredPropertyObjectCreationHandling,
             List<PropertyGenerationSpec>? propertyGenSpecList,
             ParameterGenerationSpec[]? ctorParamGenSpecArray,
             List<PropertyInitializerGenerationSpec>? propertyInitializerSpecList,
@@ -156,6 +158,7 @@ public void Initialize(
             IsPolymorphic = isPolymorphic;
             NumberHandling = numberHandling;
             UnmappedMemberHandling = unmappedMemberHandling;
+            PreferredPropertyObjectCreationHandling = preferredPropertyObjectCreationHandling;
             PropertyGenSpecList = propertyGenSpecList;
             PropertyInitializerSpecList = propertyInitializerSpecList;
             CtorParamGenSpecArray = ctorParamGenSpecArray;

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

@@ -384,6 +384,7 @@ public JsonSerializerOptions(System.Text.Json.JsonSerializerOptions options) { }
         public bool IsReadOnly { get { throw null; } }
         public int MaxDepth { get { throw null; } set { } }
         public System.Text.Json.Serialization.JsonNumberHandling NumberHandling { get { throw null; } set { } }
+        public System.Text.Json.Serialization.JsonObjectCreationHandling PreferredObjectCreationHandling { get { throw null; } set { } }
         public bool PropertyNameCaseInsensitive { get { throw null; } set { } }
         public System.Text.Json.JsonNamingPolicy? PropertyNamingPolicy { get { throw null; } set { } }
         public System.Text.Json.JsonCommentHandling ReadCommentHandling { get { throw null; } set { } }
@@ -969,6 +970,17 @@ public sealed partial class JsonNumberHandlingAttribute : System.Text.Json.Seria
         public JsonNumberHandlingAttribute(System.Text.Json.Serialization.JsonNumberHandling handling) { }
         public System.Text.Json.Serialization.JsonNumberHandling Handling { get { throw null; } }
     }
+    public enum JsonObjectCreationHandling
+    {
+        Replace = 0,
+        Populate = 1,
+    }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Field | System.AttributeTargets.Property | AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface, AllowMultiple = false)]
+    public sealed partial class JsonObjectCreationHandlingAttribute : System.Text.Json.Serialization.JsonAttribute
+    {
+        public JsonObjectCreationHandlingAttribute(System.Text.Json.Serialization.JsonObjectCreationHandling handling) { }
+        public System.Text.Json.Serialization.JsonObjectCreationHandling Handling { get { throw null; } }
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Class | System.AttributeTargets.Interface, AllowMultiple=false, Inherited=false)]
     public sealed partial class JsonPolymorphicAttribute : System.Text.Json.Serialization.JsonAttribute
     {
@@ -1210,6 +1222,7 @@ public abstract partial class JsonPropertyInfo
     {
         internal JsonPropertyInfo() { }
         public System.Reflection.ICustomAttributeProvider? AttributeProvider { get { throw null; } set { } }
+        public System.Text.Json.Serialization.JsonObjectCreationHandling? ObjectCreationHandling { get { throw null; } set { } }
         public System.Text.Json.Serialization.JsonConverter? CustomConverter { get { throw null; } set { } }
         public System.Func<object, object?>? Get { get { throw null; } set { } }
         public bool IsExtensionData { get { throw null; } set { } }
@@ -1249,6 +1262,7 @@ internal JsonTypeInfo() { }
         public bool IsReadOnly { get { throw null; } }
         public System.Text.Json.Serialization.Metadata.JsonTypeInfoKind Kind { get { throw null; } }
         public System.Text.Json.Serialization.JsonNumberHandling? NumberHandling { get { throw null; } set { } }
+        public System.Text.Json.Serialization.JsonObjectCreationHandling? PreferredPropertyObjectCreationHandling { get { throw null; } set { } }
         public System.Action<object>? OnDeserialized { get { throw null; } set { } }
         public System.Action<object>? OnDeserializing { get { throw null; } set { } }
         public System.Action<object>? OnSerialized { get { throw null; } set { } }