JsonSerializerOptions.IgnoreNullValues=true causes JsonConverter override to never be called

[pasisavolainen]

Tried to override null-ignore behavior on some individual cases where it makes sense, but no such luck.

My reasonable expectation is that if a property has converter specified (and converter gets the converter options), then the converter would be called on all and any values that the property is. However the IgnoreNullValue = true causes the converter to never be called and to be unable to make the decision in application.

example:

    internal class KeepNullsConverterExample : JsonConverter<string>
    {
        public override string Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
            => throw new NotImplementedException();

        public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
            => writer.WriteStringValue(value.ToString());
    }
    class TestObject
    {
        [JsonConverter(typeof(KeepNullsConverterExample))]
        public string Foo { get; set; }
    }
    public class OptionItemTests
    {
        [Fact]
        public void NullValueSurvives()
        {
            var items = new TestObject[]
            {
                new TestObject{ Foo = "a" },
                new TestObject{ Foo = null},
            };

            var jsonOptions = new JsonSerializerOptions
            {
                IgnoreNullValues = true,
            };

            var json = JsonSerializer.Serialize(items, jsonOptions);

            var expected = "[{\"Foo\":\"a\"},{\"Foo\":null}]";

            Assert.Equal(expected, json);
        }
    }

[layomia]

We currently don't pass null to user converters, so even if IgnoreNullValues was false in your scenario, the converter's write method would still not be called. For 5.0, we are likely to add a bool property on JsonConverter (as part of work in #1562), say HandleNullValues (defaults to false) for converter's to opt in to handle null tokens. cc @steveharter.

New semantics need to be added for a property not to be ignored when IgnoreNullValues is true. This seems like a good candidate for the Never condition on the new JsonIgnoreCondition property being proposed in #30687. Your class definition could look like this:

class TestObject
{
    [JsonConverter(typeof(KeepNullsConverterExample))]
    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
    public string Foo { get; set; }
}

[layomia]

Closing as duplicate - #34439 tracks providing an opt-in for converters to handle null, and properties can be excluded from being ignored using JsonIgnoreCondition.Never introduced in #30687.