Implement a heuristic for using fast-path serialization in streaming JsonSerializer methods.

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

@@ -130,6 +130,8 @@ The System.Text.Json library is built-in as part of the shared framework in .NET
     <Compile Include="System\Text\Json\Serialization\Metadata\IJsonTypeInfoResolver.cs" />
     <Compile Include="System\Text\Json\Serialization\Metadata\JsonDerivedType.cs" />
     <Compile Include="System\Text\Json\Serialization\Metadata\JsonPolymorphismOptions.cs" />
+    <Compile Include="System\Text\Json\Serialization\Metadata\JsonTypeInfoOfT.ReadHelper.cs" />
+    <Compile Include="System\Text\Json\Serialization\Metadata\JsonTypeInfoOfT.WriteHelpers.cs" />
     <Compile Include="System\Text\Json\Serialization\Metadata\JsonTypeInfoResolver.cs" />
     <Compile Include="System\Text\Json\Serialization\Metadata\JsonTypeInfoKind.cs" />
     <Compile Include="System\Text\Json\Serialization\Metadata\CustomJsonTypeInfoOfT.cs" />
@@ -226,14 +228,12 @@ The System.Text.Json library is built-in as part of the shared framework in .NET
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Helpers.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.HandleMetadata.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.HandlePropertyName.cs" />
-    <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.Helpers.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.Span.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.Stream.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.String.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Read.Utf8JsonReader.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Write.ByteArray.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Write.HandleMetadata.cs" />
-    <Compile Include="System\Text\Json\Serialization\JsonSerializer.Write.Helpers.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Write.Stream.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Write.String.cs" />
     <Compile Include="System\Text\Json\Serialization\JsonSerializer.Write.Utf8JsonWriter.cs" />

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

@@ -129,12 +129,6 @@ internal virtual JsonTypeInfo CreateCustomJsonTypeInfo(JsonSerializerOptions opt
         /// </summary>
         internal bool IsInternalConverterForNumberType { get; init; }
 
-        /// <summary>
-        /// Loosely-typed ReadCore() that forwards to strongly-typed ReadCore().
-        /// </summary>
-        internal abstract object? ReadCoreAsObject(ref Utf8JsonReader reader, JsonSerializerOptions options, scoped ref ReadStack state);
-
-
         internal static bool ShouldFlush(Utf8JsonWriter writer, ref WriteStack state)
         {
             // If surpassed flush threshold then return false which will flush stream.
@@ -149,11 +143,6 @@ internal static bool ShouldFlush(Utf8JsonWriter writer, ref WriteStack state)
 
         internal abstract bool TryWriteAsObject(Utf8JsonWriter writer, object? value, JsonSerializerOptions options, ref WriteStack state);
 
-        /// <summary>
-        /// Loosely-typed WriteCore() that forwards to strongly-typed WriteCore().
-        /// </summary>
-        internal abstract bool WriteCoreAsObject(Utf8JsonWriter writer, object? value, JsonSerializerOptions options, ref WriteStack state);
-
         /// <summary>
         /// Loosely-typed WriteToPropertyName() that forwards to strongly-typed WriteToPropertyName().
         /// </summary>

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

@@ -61,16 +61,6 @@ internal JsonConverter GetConverterInternal(Type typeToConvert, JsonSerializerOp
             return converter;
         }
 
-        internal sealed override object ReadCoreAsObject(
-            ref Utf8JsonReader reader,
-            JsonSerializerOptions options,
-            scoped ref ReadStack state)
-        {
-            Debug.Fail("We should never get here.");
-
-            throw new InvalidOperationException();
-        }
-
         internal sealed override bool OnTryReadAsObject(
             ref Utf8JsonReader reader,
             JsonSerializerOptions options,
@@ -106,17 +96,6 @@ internal JsonConverter GetConverterInternal(Type typeToConvert, JsonSerializerOp
 
         internal sealed override Type TypeToConvert => null!;
 
-        internal sealed override bool WriteCoreAsObject(
-            Utf8JsonWriter writer,
-            object? value,
-            JsonSerializerOptions options,
-            ref WriteStack state)
-        {
-            Debug.Fail("We should never get here.");
-
-            throw new InvalidOperationException();
-        }
-
         internal sealed override void WriteAsPropertyNameCoreAsObject(
             Utf8JsonWriter writer, object value,
             JsonSerializerOptions options,

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.ReadCore.cs

@@ -5,18 +5,10 @@ namespace System.Text.Json.Serialization
 {
     public partial class JsonConverter<T>
     {
-        internal sealed override object? ReadCoreAsObject(
-            ref Utf8JsonReader reader,
-            JsonSerializerOptions options,
-            scoped ref ReadStack state)
-        {
-            return ReadCore(ref reader, options, ref state);
-        }
-
         internal T? ReadCore(
             ref Utf8JsonReader reader,
             JsonSerializerOptions options,
-            scoped ref ReadStack state)
+            ref ReadStack state)
         {
             try
             {

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.WriteCore.cs

@@ -5,37 +5,6 @@ namespace System.Text.Json.Serialization
 {
     public partial class JsonConverter<T>
     {
-        internal sealed override bool WriteCoreAsObject(
-            Utf8JsonWriter writer,
-            object? value,
-            JsonSerializerOptions options,
-            ref WriteStack state)
-        {
-            if (
-#if NETCOREAPP
-                // Treated as a constant by recent versions of the JIT.
-                typeof(T).IsValueType)
-#else
-                IsValueType)
-#endif
-            {
-                // Value types can never have a null except for Nullable<T>.
-                if (default(T) is not null && value is null)
-                {
-                    ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(TypeToConvert);
-                }
-
-                // Root object is a boxed value type, we need to push it to the reference stack before it gets unboxed here.
-                if (options.ReferenceHandlingStrategy == ReferenceHandlingStrategy.IgnoreCycles && value != null)
-                {
-                    state.ReferenceResolver.PushReferenceForCycleDetection(value);
-                }
-            }
-
-            T actualValue = (T)value!;
-            return WriteCore(writer, actualValue, options, ref state);
-        }
-
         internal bool WriteCore(
             Utf8JsonWriter writer,
             in T value,

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs

@@ -54,6 +54,23 @@ private static JsonTypeInfo GetTypeInfo(JsonSerializerContext context, Type inpu
             return info;
         }
 
+        private static void ValidateInputType(object? value, Type inputType)
+        {
+            if (inputType is null)
+            {
+                ThrowHelper.ThrowArgumentNullException(nameof(inputType));
+            }
+
+            if (value is not null)
+            {
+                Type runtimeType = value.GetType();
+                if (!inputType.IsAssignableFrom(runtimeType))
+                {
+                    ThrowHelper.ThrowArgumentException_DeserializeWrongType(inputType, value);
+                }
+            }
+        }
+
         internal static bool IsValidNumberHandlingValue(JsonNumberHandling handling) =>
             JsonHelpers.IsInRangeInclusive((int)handling, 0,
                 (int)(

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.Document.cs

@@ -35,8 +35,9 @@ public static partial class JsonSerializer
                 ThrowHelper.ThrowArgumentNullException(nameof(document));
             }
 
-            JsonTypeInfo jsonTypeInfo = GetTypeInfo(options, typeof(TValue));
-            return ReadDocument<TValue>(document, jsonTypeInfo);
+            JsonTypeInfo<TValue> jsonTypeInfo = GetTypeInfo<TValue>(options);
+            ReadOnlySpan<byte> utf8Json = document.GetRootRawValue().Span;
+            return ReadFromSpan(utf8Json, jsonTypeInfo);
         }
 
         /// <summary>
@@ -70,7 +71,8 @@ public static partial class JsonSerializer
             }
 
             JsonTypeInfo jsonTypeInfo = GetTypeInfo(options, returnType);
-            return ReadDocument<object?>(document, jsonTypeInfo);
+            ReadOnlySpan<byte> utf8Json = document.GetRootRawValue().Span;
+            return ReadFromSpanAsObject(utf8Json, jsonTypeInfo);
         }
 
         /// <summary>
@@ -106,7 +108,8 @@ public static partial class JsonSerializer
             }
 
             jsonTypeInfo.EnsureConfigured();
-            return ReadDocument<TValue>(document, jsonTypeInfo);
+            ReadOnlySpan<byte> utf8Json = document.GetRootRawValue().Span;
+            return ReadFromSpan(utf8Json, jsonTypeInfo);
         }
 
         /// <summary>
@@ -161,13 +164,8 @@ public static partial class JsonSerializer
             }
 
             JsonTypeInfo jsonTypeInfo = GetTypeInfo(context, returnType);
-            return ReadDocument<object?>(document, jsonTypeInfo);
-        }
-
-        private static TValue? ReadDocument<TValue>(JsonDocument document, JsonTypeInfo jsonTypeInfo)
-        {
             ReadOnlySpan<byte> utf8Json = document.GetRootRawValue().Span;
-            return ReadFromSpan<TValue>(utf8Json, jsonTypeInfo);
+            return ReadFromSpanAsObject(utf8Json, jsonTypeInfo);
         }
     }
 }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.Element.cs

@@ -27,8 +27,9 @@ public static partial class JsonSerializer
         [RequiresDynamicCode(SerializationRequiresDynamicCodeMessage)]
         public static TValue? Deserialize<TValue>(this JsonElement element, JsonSerializerOptions? options = null)
         {
-            JsonTypeInfo jsonTypeInfo = GetTypeInfo(options, typeof(TValue));
-            return ReadUsingMetadata<TValue>(element, jsonTypeInfo);
+            JsonTypeInfo<TValue> jsonTypeInfo = GetTypeInfo<TValue>(options);
+            ReadOnlySpan<byte> utf8Json = element.GetRawValue().Span;
+            return ReadFromSpan(utf8Json, jsonTypeInfo);
         }
 
         /// <summary>
@@ -58,7 +59,8 @@ public static partial class JsonSerializer
             }
 
             JsonTypeInfo jsonTypeInfo = GetTypeInfo(options, returnType);
-            return ReadUsingMetadata<object?>(element, jsonTypeInfo);
+            ReadOnlySpan<byte> utf8Json = element.GetRawValue().Span;
+            return ReadFromSpanAsObject(utf8Json, jsonTypeInfo);
         }
 
         /// <summary>
@@ -86,7 +88,8 @@ public static partial class JsonSerializer
             }
 
             jsonTypeInfo.EnsureConfigured();
-            return ReadUsingMetadata<TValue>(element, jsonTypeInfo);
+            ReadOnlySpan<byte> utf8Json = element.GetRawValue().Span;
+            return ReadFromSpan(utf8Json, jsonTypeInfo);
         }
 
         /// <summary>
@@ -133,13 +136,8 @@ public static partial class JsonSerializer
             }
 
             JsonTypeInfo jsonTypeInfo = GetTypeInfo(context, returnType);
-            return ReadUsingMetadata<object?>(element, jsonTypeInfo);
-        }
-
-        private static TValue? ReadUsingMetadata<TValue>(JsonElement element, JsonTypeInfo jsonTypeInfo)
-        {
             ReadOnlySpan<byte> utf8Json = element.GetRawValue().Span;
-            return ReadFromSpan<TValue>(utf8Json, jsonTypeInfo);
+            return ReadFromSpanAsObject(utf8Json, jsonTypeInfo);
         }
     }
 }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.Node.cs

@@ -29,8 +29,8 @@ public static partial class JsonSerializer
         [RequiresDynamicCode(SerializationRequiresDynamicCodeMessage)]
         public static TValue? Deserialize<TValue>(this JsonNode? node, JsonSerializerOptions? options = null)
         {
-            JsonTypeInfo jsonTypeInfo = GetTypeInfo(options, typeof(TValue));
-            return ReadNode<TValue>(node, jsonTypeInfo);
+            JsonTypeInfo<TValue> jsonTypeInfo = GetTypeInfo<TValue>(options);
+            return ReadFromNode(node, jsonTypeInfo);
         }
 
         /// <summary>
@@ -57,7 +57,7 @@ public static partial class JsonSerializer
             }
 
             JsonTypeInfo jsonTypeInfo = GetTypeInfo(options, returnType);
-            return ReadNode<object?>(node, jsonTypeInfo);
+            return ReadFromNodeAsObject(node, jsonTypeInfo);
         }
 
         /// <summary>
@@ -85,7 +85,7 @@ public static partial class JsonSerializer
             }
 
             jsonTypeInfo.EnsureConfigured();
-            return ReadNode<TValue>(node, jsonTypeInfo);
+            return ReadFromNode(node, jsonTypeInfo);
         }
 
         /// <summary>
@@ -132,10 +132,10 @@ public static partial class JsonSerializer
             }
 
             JsonTypeInfo jsonTypeInfo = GetTypeInfo(context, returnType);
-            return ReadNode<object?>(node, jsonTypeInfo);
+            return ReadFromNodeAsObject(node, jsonTypeInfo);
         }
 
-        private static TValue? ReadNode<TValue>(JsonNode? node, JsonTypeInfo jsonTypeInfo)
+        private static TValue? ReadFromNode<TValue>(JsonNode? node, JsonTypeInfo<TValue> jsonTypeInfo)
         {
             JsonSerializerOptions options = jsonTypeInfo.Options;
 
@@ -153,7 +153,28 @@ public static partial class JsonSerializer
                 }
             }
 
-            return ReadFromSpan<TValue>(output.WrittenMemory.Span, jsonTypeInfo);
+            return ReadFromSpan(output.WrittenMemory.Span, jsonTypeInfo);
+        }
+
+        private static object? ReadFromNodeAsObject(JsonNode? node, JsonTypeInfo jsonTypeInfo)
+        {
+            JsonSerializerOptions options = jsonTypeInfo.Options;
+
+            // For performance, share the same buffer across serialization and deserialization.
+            using var output = new PooledByteBufferWriter(options.DefaultBufferSize);
+            using (var writer = new Utf8JsonWriter(output, options.GetWriterOptions()))
+            {
+                if (node is null)
+                {
+                    writer.WriteNullValue();
+                }
+                else
+                {
+                    node.WriteTo(writer, options);
+                }
+            }
+
+            return ReadFromSpanAsObject(output.WrittenMemory.Span, jsonTypeInfo);
         }
     }
 }