Add OnSerialize callbacks to POCOs

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

@@ -500,7 +500,13 @@ private string GenerateForObject(TypeGenerationSpec typeMetadata)
 
                 if (typeMetadata.GenerateSerializationLogic)
                 {
-                    serializeFuncSource = GenerateFastPathFuncForObject(typeCompilableName, serializeMethodName, typeMetadata.CanBeNull, properties);
+                    serializeFuncSource = GenerateFastPathFuncForObject(
+                        typeCompilableName,
+                        serializeMethodName,
+                        typeMetadata.CanBeNull,
+                        typeMetadata.ImplementsIJsonOnSerialized,
+                        typeMetadata.ImplementsIJsonOnSerializing,
+                        properties);
                     serializeFuncNamedArg = $@"serializeFunc: {serializeMethodName}";
                 }
                 else
@@ -635,6 +641,8 @@ private string GenerateFastPathFuncForObject(
                 string typeInfoTypeRef,
                 string serializeMethodName,
                 bool canBeNull,
+                bool implementsIJsonOnSerialized,
+                bool implementsIJsonOnSerializing,
                 List<PropertyGenerationSpec>? properties)
             {
                 JsonSourceGenerationOptionsAttribute options = _currentContext.GenerationOptions;
@@ -646,6 +654,12 @@ private string GenerateFastPathFuncForObject(
                 StringBuilder sb = new();
 
                 // Begin method definition
+                if (implementsIJsonOnSerializing)
+                {
+                    sb.Append($@"(({IJsonOnSerializingFullName}){ValueVarName}).OnSerializing();");
+                    sb.Append($@"{Environment.NewLine}    ");
+                }
+
                 sb.Append($@"{WriterVarName}.WriteStartObject();");
 
                 if (properties != null)
@@ -733,6 +747,12 @@ private string GenerateFastPathFuncForObject(
 
         {WriterVarName}.WriteEndObject();");
 
+                if (implementsIJsonOnSerialized)
+                {
+                    sb.Append($@"{Environment.NewLine}    ");
+                    sb.Append($@"(({IJsonOnSerializedFullName}){ValueVarName}).OnSerialized();");
+                };
+
                 return GenerateFastPathFuncForType(serializeMethodName, typeInfoTypeRef, sb.ToString(), canBeNull);
             }
 

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

@@ -494,6 +494,9 @@ private TypeGenerationSpec GetOrAddTypeGenerationSpec(Type type, JsonSourceGener
                 bool foundDesignTimeCustomConverter = false;
                 string? converterInstatiationLogic = null;
 
+                bool implementsIJsonOnSerialized = false;
+                bool implementsIJsonOnSerializing = false;
+
                 IList<CustomAttributeData> attributeDataList = CustomAttributeData.GetCustomAttributes(type);
                 foreach (CustomAttributeData attributeData in attributeDataList)
                 {
@@ -577,6 +580,11 @@ private TypeGenerationSpec GetOrAddTypeGenerationSpec(Type type, JsonSourceGener
                         constructionStrategy = ObjectConstructionStrategy.ParameterlessConstructor;
                     }
 
+                    // GetInterface() is currently not implemented, so we use GetInterfaces().
+                    IEnumerable<string> interfaces = type.GetInterfaces().Select(interfaceType => interfaceType.FullName);
+                    implementsIJsonOnSerialized = interfaces.FirstOrDefault(interfaceName => interfaceName == IJsonOnSerializedFullName) != null;
+                    implementsIJsonOnSerializing = interfaces.FirstOrDefault(interfaceName => interfaceName == IJsonOnSerializingFullName) != null;
+
                     for (Type? currentType = type; currentType != null; currentType = currentType.BaseType)
                     {
                         const BindingFlags bindingFlags =
@@ -627,7 +635,9 @@ private TypeGenerationSpec GetOrAddTypeGenerationSpec(Type type, JsonSourceGener
                     collectionValueTypeMetadata: collectionValueType != null ? GetOrAddTypeGenerationSpec(collectionValueType, generationMode) : null,
                     constructionStrategy,
                     nullableUnderlyingTypeMetadata: nullableUnderlyingType != null ? GetOrAddTypeGenerationSpec(nullableUnderlyingType, generationMode) : null,
-                    converterInstatiationLogic);
+                    converterInstatiationLogic,
+                    implementsIJsonOnSerialized,
+                    implementsIJsonOnSerializing);
 
                 return typeMetadata;
             }

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

@@ -18,6 +18,10 @@ namespace System.Text.Json.SourceGeneration
     [Generator]
     public sealed partial class JsonSourceGenerator : ISourceGenerator
     {
+        private const string SystemTextJsonSourceGenerationName = "System.Text.Json.SourceGeneration";
+        private const string IJsonOnSerializedFullName = "System.Text.Json.Serialization.IJsonOnSerialized";
+        private const string IJsonOnSerializingFullName = "System.Text.Json.Serialization.IJsonOnSerializing";
+
         /// <summary>
         /// Registers a syntax resolver to receive compilation units.
         /// </summary>
@@ -51,8 +55,6 @@ public void Execute(GeneratorExecutionContext executionContext)
             }
         }
 
-        private const string SystemTextJsonSourceGenerationName = "System.Text.Json.SourceGeneration";
-
         /// <summary>
         /// Helper for unit tests.
         /// </summary>

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

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <CLSCompliant>false</CLSCompliant>

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

@@ -33,6 +33,9 @@ internal class TypeGenerationSpec
 
         public ClassType ClassType { get; private set; }
 
+        public bool ImplementsIJsonOnSerialized { get; private set; }
+        public bool ImplementsIJsonOnSerializing { get; private set; }
+
         public bool IsValueType { get; private set; }
 
         public bool CanBeNull { get; private set; }
@@ -67,7 +70,9 @@ public void Initialize(
             TypeGenerationSpec? collectionValueTypeMetadata,
             ObjectConstructionStrategy constructionStrategy,
             TypeGenerationSpec? nullableUnderlyingTypeMetadata,
-            string? converterInstantiationLogic)
+            string? converterInstantiationLogic,
+            bool implementsIJsonOnSerialized,
+            bool implementsIJsonOnSerializing)
         {
             GenerationMode = generationMode;
             TypeRef = $"global::{typeRef}";
@@ -84,6 +89,8 @@ public void Initialize(
             ConstructionStrategy = constructionStrategy;
             NullableUnderlyingTypeMetadata = nullableUnderlyingTypeMetadata;
             ConverterInstantiationLogic = converterInstantiationLogic;
+            ImplementsIJsonOnSerialized = implementsIJsonOnSerialized;
+            ImplementsIJsonOnSerializing = implementsIJsonOnSerializing;
         }
 
         private bool FastPathIsSupported()

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

@@ -733,6 +733,22 @@ public abstract partial class JsonValue : System.Text.Json.Nodes.JsonNode
 }
 namespace System.Text.Json.Serialization
 {
+    public partial interface IJsonOnDeserialized
+    {
+        void OnDeserialized();
+    }
+    public partial interface IJsonOnDeserializing
+    {
+        void OnDeserializing();
+    }
+    public partial interface IJsonOnSerialized
+    {
+        void OnSerialized();
+    }
+    public partial interface IJsonOnSerializing
+    {
+        void OnSerializing();
+    }
     public abstract partial class JsonAttribute : System.Attribute
     {
         protected JsonAttribute() { }

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

@@ -92,6 +92,10 @@
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonPropertyNameAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\JsonMetadataServicesConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\IgnoreReferenceResolver.cs" />
+    <Compile Include="System\Text\Json\Serialization\IJsonOnDeserialized.cs" />
+    <Compile Include="System\Text\Json\Serialization\IJsonOnDeserializing.cs" />
+    <Compile Include="System\Text\Json\Serialization\IJsonOnSerialized.cs" />
+    <Compile Include="System\Text\Json\Serialization\IJsonOnSerializing.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializerContext.cs" />
     <Compile Include="System\Text\Json\Serialization\Metadata\JsonMetadataServices.Collections.cs" />
     <Compile Include="System\Text\Json\Serialization\Metadata\JsonMetadataServices.Converters.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Object/ObjectDefaultConverter.cs

@@ -36,6 +36,11 @@ internal override bool OnTryRead(ref Utf8JsonReader reader, Type typeToConvert,
 
                 obj = jsonTypeInfo.CreateObject!()!;
 
+                if (obj is IJsonOnDeserializing onDeserializing)
+                {
+                    onDeserializing.OnDeserializing();
+                }
+
                 // Process all properties.
                 while (true)
                 {
@@ -108,6 +113,11 @@ internal override bool OnTryRead(ref Utf8JsonReader reader, Type typeToConvert,
 
                     obj = jsonTypeInfo.CreateObject!()!;
 
+                    if (obj is IJsonOnDeserializing onDeserializing)
+                    {
+                        onDeserializing.OnDeserializing();
+                    }
+
                     state.Current.ReturnValue = obj;
                     state.Current.ObjectState = StackFrameObjectState.CreatedObject;
                 }
@@ -216,14 +226,21 @@ internal override bool OnTryRead(ref Utf8JsonReader reader, Type typeToConvert,
                 }
             }
 
+            if (obj is IJsonOnDeserialized onDeserialized)
+            {
+                onDeserialized.OnDeserialized();
+            }
+
+            // Unbox
+            Debug.Assert(obj != null);
+            value = (T)obj;
+
             // Check if we are trying to build the sorted cache.
             if (state.Current.PropertyRefCache != null)
             {
                 jsonTypeInfo.UpdateSortedPropertyCache(ref state.Current);
             }
 
-            value = (T)obj;
-
             return true;
         }
 
@@ -235,20 +252,24 @@ internal sealed override bool OnTryWrite(
         {
             JsonTypeInfo jsonTypeInfo = state.Current.JsonTypeInfo;
 
-            // Minimize boxing for structs by only boxing once here
-            object objectValue = value!;
+            object obj = value; // box once
 
             if (!state.SupportContinuation)
             {
                 writer.WriteStartObject();
                 if (options.ReferenceHandlingStrategy == ReferenceHandlingStrategy.Preserve)
                 {
-                    if (JsonSerializer.WriteReferenceForObject(this, objectValue, ref state, writer) == MetadataPropertyName.Ref)
+                    if (JsonSerializer.WriteReferenceForObject(this, obj, ref state, writer) == MetadataPropertyName.Ref)
                     {
                         return true;
                     }
                 }
 
+                if (obj is IJsonOnSerializing onSerializing)
+                {
+                    onSerializing.OnSerializing();
+                }
+
                 List<KeyValuePair<string, JsonPropertyInfo?>> properties = state.Current.JsonTypeInfo.PropertyCache!.List;
                 for (int i = 0; i < properties.Count; i++)
                 {
@@ -259,7 +280,7 @@ internal sealed override bool OnTryWrite(
                         state.Current.DeclaredJsonPropertyInfo = jsonPropertyInfo;
                         state.Current.NumberHandling = jsonPropertyInfo.NumberHandling;
 
-                        bool success = jsonPropertyInfo.GetMemberAndWriteJson(objectValue, ref state, writer);
+                        bool success = jsonPropertyInfo.GetMemberAndWriteJson(obj, ref state, writer);
                         // Converters only return 'false' when out of data which is not possible in fast path.
                         Debug.Assert(success);
 
@@ -275,14 +296,13 @@ internal sealed override bool OnTryWrite(
                     state.Current.DeclaredJsonPropertyInfo = dataExtensionProperty;
                     state.Current.NumberHandling = dataExtensionProperty.NumberHandling;
 
-                    bool success = dataExtensionProperty.GetMemberAndWriteJsonExtensionData(objectValue, ref state, writer);
+                    bool success = dataExtensionProperty.GetMemberAndWriteJsonExtensionData(obj, ref state, writer);
                     Debug.Assert(success);
 
                     state.Current.EndProperty();
                 }
 
                 writer.WriteEndObject();
-                return true;
             }
             else
             {
@@ -291,12 +311,17 @@ internal sealed override bool OnTryWrite(
                     writer.WriteStartObject();
                     if (options.ReferenceHandlingStrategy == ReferenceHandlingStrategy.Preserve)
                     {
-                        if (JsonSerializer.WriteReferenceForObject(this, objectValue, ref state, writer) == MetadataPropertyName.Ref)
+                        if (JsonSerializer.WriteReferenceForObject(this, obj, ref state, writer) == MetadataPropertyName.Ref)
                         {
                             return true;
                         }
                     }
 
+                    if (obj is IJsonOnSerializing onSerializing)
+                    {
+                        onSerializing.OnSerializing();
+                    }
+
                     state.Current.ProcessedStartToken = true;
                 }
 
@@ -310,7 +335,7 @@ internal sealed override bool OnTryWrite(
                         state.Current.DeclaredJsonPropertyInfo = jsonPropertyInfo;
                         state.Current.NumberHandling = jsonPropertyInfo.NumberHandling;
 
-                        if (!jsonPropertyInfo.GetMemberAndWriteJson(objectValue!, ref state, writer))
+                        if (!jsonPropertyInfo.GetMemberAndWriteJson(obj!, ref state, writer))
                         {
                             Debug.Assert(jsonPropertyInfo.ConverterBase.ConverterStrategy != ConverterStrategy.Value ||
                                          jsonPropertyInfo.ConverterBase.TypeToConvert == JsonTypeInfo.ObjectType);
@@ -342,7 +367,7 @@ internal sealed override bool OnTryWrite(
                         state.Current.DeclaredJsonPropertyInfo = dataExtensionProperty;
                         state.Current.NumberHandling = dataExtensionProperty.NumberHandling;
 
-                        if (!dataExtensionProperty.GetMemberAndWriteJsonExtensionData(objectValue, ref state, writer))
+                        if (!dataExtensionProperty.GetMemberAndWriteJsonExtensionData(obj, ref state, writer))
                         {
                             return false;
                         }
@@ -366,9 +391,16 @@ internal sealed override bool OnTryWrite(
                     state.Current.ProcessedEndToken = true;
                     writer.WriteEndObject();
                 }
+            }
 
-                return true;
+            if (obj is IJsonOnSerialized onSerialized)
+            {
+                onSerialized.OnSerialized();
             }
+
+            value = (T)obj; // unbox
+
+            return true;
         }
 
         // AggressiveInlining since this method is only called from two locations and is on a hot path.
@@ -437,6 +469,11 @@ internal sealed override void CreateInstanceForReferenceResolver(ref Utf8JsonRea
 
             object obj = state.Current.JsonTypeInfo.CreateObject!()!;
             state.Current.ReturnValue = obj;
+
+            if (obj is IJsonOnDeserializing onDeserializing)
+            {
+                onDeserializing.OnDeserializing();
+            }
         }
     }
 }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Object/ObjectWithParameterizedConstructorConverter.cs

@@ -32,7 +32,12 @@ internal sealed override bool OnTryRead(ref Utf8JsonReader reader, Type typeToCo
 
                 ReadConstructorArguments(ref state, ref reader, options);
 
-                obj = CreateObject(ref state.Current);
+                obj = (T)CreateObject(ref state.Current);
+
+                if (obj is IJsonOnDeserializing onDeserializing)
+                {
+                    onDeserializing.OnDeserializing();
+                }
 
                 if (argumentState.FoundPropertyCount > 0)
                 {
@@ -91,7 +96,12 @@ internal sealed override bool OnTryRead(ref Utf8JsonReader reader, Type typeToCo
                     return false;
                 }
 
-                obj = CreateObject(ref state.Current);
+                obj = (T)CreateObject(ref state.Current);
+
+                if (obj is IJsonOnDeserializing onDeserializing)
+                {
+                    onDeserializing.OnDeserializing();
+                }
 
                 if (argumentState.FoundPropertyCount > 0)
                 {
@@ -128,6 +138,17 @@ internal sealed override bool OnTryRead(ref Utf8JsonReader reader, Type typeToCo
                 }
             }
 
+            if (obj is IJsonOnDeserialized onDeserialized)
+            {
+                onDeserialized.OnDeserialized();
+            }
+
+            EndRead(ref state);
+
+            // Unbox
+            Debug.Assert(obj != null);
+            value = (T)obj;
+
             // Check if we are trying to build the sorted cache.
             if (state.Current.PropertyRefCache != null)
             {
@@ -140,10 +161,6 @@ internal sealed override bool OnTryRead(ref Utf8JsonReader reader, Type typeToCo
                 state.Current.JsonTypeInfo.UpdateSortedParameterCache(ref state.Current);
             }
 
-            EndRead(ref state);
-
-            value = (T)obj;
-
             return true;
         }