Ensure JsonConverter<T>.Read/WriteAsPropertyName methods are user-callable.

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

@@ -111,6 +111,7 @@ The System.Text.Json library is built-in as part of the shared framework in .NET
     <Compile Include="System\Text\Json\Serialization\Converters\Collection\StackOrQueueConverterWithReflection.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\JsonMetadataServicesConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\Object\ObjectWithParameterizedConstructorConverter.Large.Reflection.cs" />
+    <Compile Include="System\Text\Json\Serialization\Converters\Value\JsonPrimitiveConverter.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" />

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

@@ -74,12 +74,28 @@ internal override bool OnTryRead(ref Utf8JsonReader reader, Type typeToConvert,
             return true;
         }
 
+        public override object ReadAsPropertyName(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            ThrowHelper.ThrowNotSupportedException_DictionaryKeyTypeNotSupported(TypeToConvert, this);
+            return null!;
+        }
+
         internal override object ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
             ThrowHelper.ThrowNotSupportedException_DictionaryKeyTypeNotSupported(TypeToConvert, this);
             return null!;
         }
 
+        public override void WriteAsPropertyName(Utf8JsonWriter writer, object? value, JsonSerializerOptions options)
+        {
+            if (value is null)
+            {
+                ThrowHelper.ThrowArgumentNullException(nameof(value));
+            }
+
+            WriteAsPropertyNameCore(writer, value, options, isWritingExtensionDataProperty: false);
+        }
+
         internal override void WriteAsPropertyNameCore(Utf8JsonWriter writer, object? value, JsonSerializerOptions options, bool isWritingExtensionDataProperty)
         {
             // This converter does not handle nulls.

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

@@ -6,7 +6,7 @@
 
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class BooleanConverter : JsonConverter<bool>
+    internal sealed class BooleanConverter : JsonPrimitiveConverter<bool>
     {
         public override bool Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
@@ -20,6 +20,7 @@ public override void Write(Utf8JsonWriter writer, bool value, JsonSerializerOpti
 
         internal override bool ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             ReadOnlySpan<byte> propertyName = reader.GetSpan();
             if (!(Utf8Parser.TryParse(propertyName, out bool value, out int bytesConsumed)
                   && propertyName.Length == bytesConsumed))

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class ByteConverter : JsonConverter<byte>
+    internal sealed class ByteConverter : JsonPrimitiveConverter<byte>
     {
         public ByteConverter()
         {
@@ -22,6 +24,7 @@ public override void Write(Utf8JsonWriter writer, byte value, JsonSerializerOpti
 
         internal override byte ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetByteWithQuotes();
         }
 

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

@@ -6,7 +6,7 @@
 
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class CharConverter : JsonConverter<char>
+    internal sealed class CharConverter : JsonPrimitiveConverter<char>
     {
         private const int MaxEscapedCharacterLength = JsonConstants.MaxExpansionFactorWhileEscaping;
 
@@ -40,7 +40,10 @@ public override void Write(Utf8JsonWriter writer, char value, JsonSerializerOpti
         }
 
         internal override char ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
-            => Read(ref reader, typeToConvert, options);
+        {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
+            return Read(ref reader, typeToConvert, options);
+        }
 
         internal override void WriteAsPropertyNameCore(Utf8JsonWriter writer, char value, JsonSerializerOptions options, bool isWritingExtensionDataProperty)
         {

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

@@ -7,7 +7,7 @@
 
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class DateOnlyConverter : JsonConverter<DateOnly>
+    internal sealed class DateOnlyConverter : JsonPrimitiveConverter<DateOnly>
     {
         public const int FormatLength = 10; // YYYY-MM-DD
         public const int MaxEscapedFormatLength = FormatLength * JsonConstants.MaxExpansionFactorWhileEscaping;
@@ -24,10 +24,11 @@ public override DateOnly Read(ref Utf8JsonReader reader, Type typeToConvert, Jso
 
         internal override DateOnly ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return ReadCore(ref reader);
         }
 
-        private DateOnly ReadCore(ref Utf8JsonReader reader)
+        private static DateOnly ReadCore(ref Utf8JsonReader reader)
         {
             if (!JsonHelpers.IsInRangeInclusive(reader.ValueLength, FormatLength, MaxEscapedFormatLength))
             {

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class DateTimeConverter : JsonConverter<DateTime>
+    internal sealed class DateTimeConverter : JsonPrimitiveConverter<DateTime>
     {
         public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
@@ -17,6 +19,7 @@ public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializer
 
         internal override DateTime ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetDateTimeNoValidation();
         }
 

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class DateTimeOffsetConverter : JsonConverter<DateTimeOffset>
+    internal sealed class DateTimeOffsetConverter : JsonPrimitiveConverter<DateTimeOffset>
     {
         public override DateTimeOffset Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
@@ -17,6 +19,7 @@ public override void Write(Utf8JsonWriter writer, DateTimeOffset value, JsonSeri
 
         internal override DateTimeOffset ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetDateTimeOffsetNoValidation();
         }
 

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class DecimalConverter : JsonConverter<decimal>
+    internal sealed class DecimalConverter : JsonPrimitiveConverter<decimal>
     {
         public DecimalConverter()
         {
@@ -22,6 +24,7 @@ public override void Write(Utf8JsonWriter writer, decimal value, JsonSerializerO
 
         internal override decimal ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetDecimalWithQuotes();
         }
 

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class DoubleConverter : JsonConverter<double>
+    internal sealed class DoubleConverter : JsonPrimitiveConverter<double>
     {
         public DoubleConverter()
         {
@@ -22,6 +24,7 @@ public override void Write(Utf8JsonWriter writer, double value, JsonSerializerOp
 
         internal override double ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetDoubleWithQuotes();
         }
 

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

@@ -10,7 +10,7 @@
 
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class EnumConverter<T> : JsonConverter<T>
+    internal sealed class EnumConverter<T> : JsonPrimitiveConverter<T>
         where T : struct, Enum
     {
         private static readonly TypeCode s_enumTypeCode = Type.GetTypeCode(typeof(T));

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class GuidConverter : JsonConverter<Guid>
+    internal sealed class GuidConverter : JsonPrimitiveConverter<Guid>
     {
         public override Guid Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
@@ -17,6 +19,7 @@ public override void Write(Utf8JsonWriter writer, Guid value, JsonSerializerOpti
 
         internal override Guid ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetGuidNoValidation();
         }
 

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/Int16Converter.cs

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class Int16Converter : JsonConverter<short>
+    internal sealed class Int16Converter : JsonPrimitiveConverter<short>
     {
         public Int16Converter()
         {
@@ -23,6 +25,7 @@ public override void Write(Utf8JsonWriter writer, short value, JsonSerializerOpt
 
         internal override short ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetInt16WithQuotes();
         }
 

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/Int32Converter.cs

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class Int32Converter : JsonConverter<int>
+    internal sealed class Int32Converter : JsonPrimitiveConverter<int>
     {
         public Int32Converter()
         {
@@ -23,6 +25,7 @@ public override void Write(Utf8JsonWriter writer, int value, JsonSerializerOptio
 
         internal override int ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetInt32WithQuotes();
         }
 

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/Int64Converter.cs

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class Int64Converter : JsonConverter<long>
+    internal sealed class Int64Converter : JsonPrimitiveConverter<long>
     {
         public Int64Converter()
         {
@@ -22,6 +24,7 @@ public override void Write(Utf8JsonWriter writer, long value, JsonSerializerOpti
 
         internal override long ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetInt64WithQuotes();
         }
 

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

@@ -0,0 +1,31 @@
+// 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.Converters
+{
+    /// <summary>
+    /// Inherited by built-in converters serializing types as JSON primitives that support property name serialization.
+    /// </summary>
+    internal abstract class JsonPrimitiveConverter<T> : JsonConverter<T>
+    {
+        public sealed override void WriteAsPropertyName(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
+        {
+            if (value is null)
+            {
+                ThrowHelper.ThrowArgumentNullException(nameof(value));
+            }
+
+            WriteAsPropertyNameCore(writer, value, options, isWritingExtensionDataProperty: false);
+        }
+
+        public sealed override T ReadAsPropertyName(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            if (reader.TokenType != JsonTokenType.PropertyName)
+            {
+                ThrowHelper.ThrowInvalidOperationException_ExpectedPropertyName(reader.TokenType);
+            }
+
+            return ReadAsPropertyNameCore(ref reader, typeToConvert, options);
+        }
+    }
+}

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class SByteConverter : JsonConverter<sbyte>
+    internal sealed class SByteConverter : JsonPrimitiveConverter<sbyte>
     {
         public SByteConverter()
         {
@@ -22,6 +24,7 @@ public override void Write(Utf8JsonWriter writer, sbyte value, JsonSerializerOpt
 
         internal override sbyte ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetSByteWithQuotes();
         }
 

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class SingleConverter : JsonConverter<float>
+    internal sealed class SingleConverter : JsonPrimitiveConverter<float>
     {
 
         public SingleConverter()
@@ -23,6 +25,7 @@ public override void Write(Utf8JsonWriter writer, float value, JsonSerializerOpt
 
         internal override float ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetSingleWithQuotes();
         }
 

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

@@ -1,9 +1,11 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Text.Json.Serialization.Converters
 {
-    internal sealed class StringConverter : JsonConverter<string>
+    internal sealed class StringConverter : JsonPrimitiveConverter<string>
     {
         public override string? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
@@ -25,6 +27,7 @@ public override void Write(Utf8JsonWriter writer, string? value, JsonSerializerO
 
         internal override string ReadAsPropertyNameCore(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);
             return reader.GetString()!;
         }