[System.Text.Json] Conditionally ignore an open-generic type at the type level

[RenderMichael]

We have a DTO to interface with an API where null and "missing property" are both meaningful while having different semantics.

We have created a type to represent this, Optional<T>, and we want to be able to create arbitrary properties with it without having to worry about JSON configuration.

Implementation of Optional<T>.

public readonly struct Optional<T>
{
    // Basically a re-implementation of Nullable<T>
    private readonly T? _value;
    public bool HasValue { get; }
    public T? Value => HasValue ? _value : throw new InvalidOperationException($"Optional has no value. Check {nameof(HasValue)} first.");

    public Optional(T? value)
    {
        _value = value;
        HasValue = true;
    }

    public static Optional<T> None => default;

    public static implicit operator Optional<T>(T? value) => new(value);
}

Sample usage:

public sealed class SetViewportOptions : CommandOptions
{
    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
    public Optional<Viewport?> Viewport { get; set; }

    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
    public Optional<double?> DevicePixelRatio { get; set; }
}

public readonly record struct Viewport(long Width, long Height);

Functionally, we want to add [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)] directly on the Optional<T> type; that however is not supported. We have to add that attribute to each property.

A workaround we have considered is applying modifier to the serialization context with WithAddedModifier. I have not figured out if we can make it work here (likely yes, with the JsonPropertyInfo.ShouldSerialize method), but there are two more fundamental issues: we lose our strongly typed serialization context, and we cannot hand our type off to consumers to use out of the box. Both of these are non-starters.

We have a mitigation which is admittedly not AOT-friendly (important for us): an OptionalConverterFactory that creates converters that throw when Writeing a default(Optional<T>), with a reminder to add WhenWritingDefault on the property.

We have tried to make the Write method ignore the property entirely, but the closest we get is invalid json, eg. { "viewport": }. Is there something we can to do un-write the property name there?

Or, more broadly, is there some solution out there to make Optional<T> work as expected without any other configuration?