JsonSerializer throws when serializing null value in a value-type nullable union case

[DeagleGross]

Description

I've tried union as return type in aspnetcore

public union NullableCaseUnion(int?, string);

app.MapGet("/null",   () => new NullableCaseUnion((int?)null));

and got the following serialization error (full stacktrace below):

System.Text.Json.JsonException : The JSON value could not be converted to System.Int32. Path: $.

I think JsonUnionConverter<TUnion>.OnTryWrite misses a check for caseValue being null in case of valueType. Read side of the converter works well, since it short-curcuits the return checking the tokenType:

if (reader.TokenType is JsonTokenType.Null) {
    Type? nullableCaseType = typeInfo.UnionNullableCaseType;
    value = constructor(nullableCaseType, null);
    return true;
}

Reproduction Steps

string json = await Serializer.SerializeWrapper<ValueTypeNullablePairUnion>(new ValueTypeNullablePairUnion((int?)null));
            ValueTypeNullablePairUnion roundTripped = await Serializer.DeserializeWrapper<ValueTypeNullablePairUnion>(json);
            Assert.Null(roundTripped.Value);

Expected behavior

Serialization rountrip on union union ValueTypeNullablePairUnion(int, int?) works without exception

Actual behavior

throws System.Text.Json.JsonException : The JSON value could not be converted to System.Int32. Path: $.

Serialization error stacktrace

System.Text.Json.JsonException : The JSON value could not be converted to System.Int32. Path: $.

   at System.Text.Json.ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(Type propertyType)
        ThrowHelper.Serialization.cs:line 52
   at System.Text.Json.JsonSerializer.UnboxOnWrite[T](Object value)
        JsonSerializer.Helpers.cs:line 139
   at System.Text.Json.Serialization.JsonConverter`1.TryWriteAsObject(
            Utf8JsonWriter writer, Object value, JsonSerializerOptions options, WriteStack& state)
        JsonConverterOfT.cs:line 112
   at System.Text.Json.Serialization.Converters.JsonUnionConverter`1.OnTryWrite(
            Utf8JsonWriter writer, TUnion value, JsonSerializerOptions options, WriteStack& state)
        JsonUnionConverter.cs:line 195                         ← bug origin
   at System.Text.Json.Serialization.JsonConverter`1.TryWrite(
            Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
        JsonConverterOfT.cs:line 425
   at System.Text.Json.Serialization.JsonConverter`1.WriteCore(
            Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
        JsonConverterOfT.WriteCore.cs:line 48
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.Serialize(
            Utf8JsonWriter writer, T& rootValue, Object rootValueBoxed)
        JsonTypeInfoOfT.WriteHelpers.cs:line 56
   at System.Text.Json.JsonSerializer.WriteString[TValue](TValue& value, JsonTypeInfo`1 jsonTypeInfo)
        JsonSerializer.Write.String.cs:line 154
   at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)
        JsonSerializer.Write.String.cs:line 33
   at System.Text.Json.Serialization.Tests.JsonSerializerWrapper.StringSerializerWrapper.SerializeWrapper[T](
            T value, JsonSerializerOptions options)
        JsonSerializerWrapper.Reflection.cs:line 119
   at System.Text.Json.Serialization.Tests.UnionTests
            .<UnionWithValueTypeNullableCase_SerializesNullAsJsonNull>d__53.MoveNext()
        UnionTests.cs:line 571
   --- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

Related #127299

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