Add polymorphic serialization to System.Text.Json

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

@@ -267,6 +267,7 @@ public JsonSerializerOptions(System.Text.Json.JsonSerializerOptions options) { }
         public bool IncludeFields { get { throw null; } set { } }
         public int MaxDepth { get { throw null; } set { } }
         public System.Text.Json.Serialization.JsonNumberHandling NumberHandling { get { throw null; } set { } }
+        public System.Func<Type, bool> SupportedPolymorphicTypes { 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 { } }
@@ -796,6 +797,11 @@ 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; } }
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Class | System.AttributeTargets.Interface, AllowMultiple = false, Inherited = false)]
+    public sealed partial class JsonPolymorphicTypeAttribute : System.Text.Json.Serialization.JsonAttribute
+    {
+        public JsonPolymorphicTypeAttribute() { }
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Field | System.AttributeTargets.Property, AllowMultiple=false)]
     public sealed partial class JsonPropertyNameAttribute : System.Text.Json.Serialization.JsonAttribute
     {

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

@@ -88,6 +88,7 @@
     <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\JsonPolymorphicTypeAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonPropertyNameAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Attributes\JsonSerializableAttribute.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\JsonMetadataServicesConverter.cs" />

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

@@ -0,0 +1,14 @@
+// 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 declaration, indicates that instances of that type
+    /// should be serialized using the schema based on their derived runtime types.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface, AllowMultiple = false, Inherited = false)]
+    public sealed class JsonPolymorphicTypeAttribute : JsonAttribute
+    {
+    }
+}

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/ArrayConverter.cs

@@ -38,7 +38,7 @@ protected override bool OnWriteResume(Utf8JsonWriter writer, TCollection value,
             int index = state.Current.EnumeratorIndex;
 
             JsonConverter<TElement> elementConverter = GetElementConverter(ref state);
-            if (elementConverter.CanUseDirectReadOrWrite && state.Current.NumberHandling == null)
+            if (GetElementTypeInfo(ref state).CanUseDirectWrite && state.Current.NumberHandling == null)
             {
                 // Fast path that avoids validation and extra indirection.
                 for (; index < array.Length; index++)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/DictionaryDefaultConverter.cs

@@ -67,7 +67,7 @@ internal sealed override bool OnTryRead(
                 CreateCollection(ref reader, ref state);
 
                 _valueConverter ??= GetConverter<TValue>(elementTypeInfo);
-                if (_valueConverter.CanUseDirectReadOrWrite && state.Current.NumberHandling == null)
+                if (elementTypeInfo.CanUseDirectRead && state.Current.NumberHandling == null)
                 {
                     // Process all elements.
                     while (true)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/DictionaryOfTKeyTValueConverter.cs

@@ -55,7 +55,7 @@ protected internal override bool OnWriteResume(
             _keyConverter ??= GetConverter<TKey>(typeInfo.KeyTypeInfo!);
             _valueConverter ??= GetConverter<TValue>(typeInfo.ElementTypeInfo!);
 
-            if (!state.SupportContinuation && _valueConverter.CanUseDirectReadOrWrite && state.Current.NumberHandling == null)
+            if (!state.SupportContinuation && typeInfo.ElementTypeInfo!.CanUseDirectWrite && state.Current.NumberHandling == null)
             {
                 // Fast path that avoids validation and extra indirection.
                 do

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/IEnumerableDefaultConverter.cs

@@ -33,6 +33,14 @@ protected static JsonConverter<TElement> GetElementConverter(ref WriteStack stat
             return converter;
         }
 
+        protected static JsonTypeInfo GetElementTypeInfo(ref WriteStack state)
+        {
+            JsonTypeInfo typeInfo = state.Current.DeclaredJsonPropertyInfo!.RuntimeTypeInfo;
+            Debug.Assert(typeInfo != null); // It should not be possible to have a null JsonTypeInfo
+
+            return typeInfo;
+        }
+
         internal override bool OnTryRead(
             ref Utf8JsonReader reader,
             Type typeToConvert,
@@ -54,7 +62,7 @@ internal override bool OnTryRead(
                 CreateCollection(ref reader, ref state, options);
 
                 JsonConverter<TElement> elementConverter = GetElementConverter(elementTypeInfo);
-                if (elementConverter.CanUseDirectReadOrWrite && state.Current.NumberHandling == null)
+                if (elementTypeInfo.CanUseDirectRead && state.Current.NumberHandling == null)
                 {
                     // Fast path that avoids validation and extra indirection.
                     while (true)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/IListConverter.cs

@@ -61,7 +61,7 @@ protected override bool OnWriteResume(Utf8JsonWriter writer, TCollection value,
             int index = state.Current.EnumeratorIndex;
             JsonConverter<object?> elementConverter = GetElementConverter(ref state);
 
-            if (elementConverter.CanUseDirectReadOrWrite && state.Current.NumberHandling == null)
+            if (GetElementTypeInfo(ref state).CanUseDirectWrite && state.Current.NumberHandling == null)
             {
                 // Fast path that avoids validation and extra indirection.
                 for (; index < list.Count; index++)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/ListOfTConverter.cs

@@ -33,7 +33,7 @@ protected override bool OnWriteResume(Utf8JsonWriter writer, TCollection value,
             int index = state.Current.EnumeratorIndex;
             JsonConverter<TElement> elementConverter = GetElementConverter(ref state);
 
-            if (elementConverter.CanUseDirectReadOrWrite && state.Current.NumberHandling == null)
+            if (GetElementTypeInfo(ref state).CanUseDirectWrite && state.Current.NumberHandling == null)
             {
                 // Fast path that avoids validation and extra indirection.
                 for (; index < list.Count; index++)

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

@@ -312,7 +312,9 @@ internal sealed override bool OnTryWrite(
 
                         if (!jsonPropertyInfo.GetMemberAndWriteJson(objectValue!, ref state, writer))
                         {
-                            Debug.Assert(jsonPropertyInfo.ConverterBase.ConverterStrategy != ConverterStrategy.Value);
+                            Debug.Assert(jsonPropertyInfo.ConverterBase.ConverterStrategy != ConverterStrategy.Value ||
+                                         jsonPropertyInfo.ConverterBase.TypeToConvert == JsonTypeInfo.ObjectType);
+
                             return false;
                         }
 

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/ObjectConverter.cs

@@ -24,7 +24,10 @@ public ObjectConverter()
 
         public override void Write(Utf8JsonWriter writer, object? value, JsonSerializerOptions options)
         {
-            throw new InvalidOperationException();
+            Debug.Assert(value?.GetType() == typeof(object));
+
+            writer.WriteStartObject();
+            writer.WriteEndObject();
         }
 
         internal override object ReadWithQuotes(ref Utf8JsonReader reader)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverter.cs

@@ -22,18 +22,11 @@ internal JsonConverter() { }
 
         internal abstract ConverterStrategy ConverterStrategy { get; }
 
-        /// <summary>
-        /// Can direct Read or Write methods be called (for performance).
-        /// </summary>
-        internal bool CanUseDirectReadOrWrite { get; set; }
-
         /// <summary>
         /// Can the converter have $id metadata.
         /// </summary>
         internal virtual bool CanHaveIdMetadata => true;
 
-        internal bool CanBePolymorphic { get; set; }
-
         /// <summary>
         /// Used to support JsonObject as an extension property in a loosely-typed, trimmable manner.
         /// </summary>