[API Proposal]: attribute for polymorphic json serialisation (not deserialisation)

[AartBluestoke]

Background and motivation

https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-polymorphism
says "You can get polymorphic serialization for lower-level objects if you define them as type object."
and provides an example of serializing a derived WeatherForecast via:

"instead of

public class WeatherForecastWithPrevious
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureCelsius { get; set; }
    public string? Summary { get; set; }
    public WeatherForecast? PreviousForecast { get; set; }
}

you should use 'object' as the type of the child:

public class WeatherForecastWithPreviousAsObject
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureCelsius { get; set; }
    public string? Summary { get; set; }
    public object? PreviousForecast { get; set; } //Polymorphic serialisation of some type of forecast
}

"

The above code doesn't produce an object capable of being deserialised. If we are making people write custom deseralisers for this event, can we at least make the serialisation easy?

API Proposal

a new configuration option for jsonAttribute or jsonserializeroptions to allow for serialisation of objects as the type they actually are, not the type they are referenced as.

API Usage

public class WeatherForecastWithPrevious
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureCelsius { get; set; }
    public string? Summary { get; set; }
    [JsonAttribute(PolymorphicSerialisation=True)]
    public WeatherForecast? PreviousForecast { get; set; }
}

Alternative Designs

recommend custom serialisers in this situation, not just custom deserialisers.

Risks

• a lack of action here keeps the recommendation to have objects as property types if they might be both inhereted and serialised. This is not inline with good practice.

• poor action here will lead people to implement polymorphic serialisation without polymorphic de-serialisation, with silent data loss. (polymorphic deserialisation into object isn't a supported

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-polymorphism
says "You can get polymorphic serialization for lower-level objects if you define them as type object."
and provides an example of serializing a derived WeatherForecast via:

"instead of

public class WeatherForecastWithPrevious
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureCelsius { get; set; }
    public string? Summary { get; set; }
    public WeatherForecast? PreviousForecast { get; set; }
}

you should use 'object' as the type of the child:

public class WeatherForecastWithPreviousAsObject
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureCelsius { get; set; }
    public string? Summary { get; set; }
    public object? PreviousForecast { get; set; } //Polymorphic serialisation of some type of forecast
}

"

The above code doesn't produce an object capable of being deserialised. If we are making people write custom deseralisers for this event, can we at least make the serialisation easy?

API Proposal

a new configuration option for jsonAttribute or jsonserializeroptions to allow for serialisation of objects as the type they actually are, not the type they are referenced as.

API Usage

public class WeatherForecastWithPrevious
{
    public DateTimeOffset Date { get; set; }
    public int TemperatureCelsius { get; set; }
    public string? Summary { get; set; }
    [JsonAttribute(PolymorphicSerialisation=True)]
    public WeatherForecast? PreviousForecast { get; set; }
}

Alternative Designs

recommend custom serialisers in this situation, not just custom deserialisers.

Risks

• a lack of action here has recommendations for a reasonably common situation not inline with reasonable practice (let alone best practice)

• poor action here will lead people to implement polymorphic serialisation without polymorphic de-serialisation, with silent data loss. (polymorphic deserialisation into object isn't a supported

Author:	AartBluestoke
Assignees:	-
Labels:	api-suggestion, area-System.Text.Json, untriaged
Milestone:	-

[Symbai]

There has been a long existing issue about that #63747 and the implementation has been completed 6 days ago #71346, the article you linked to is older.

[eiriktsarpalis]

Closing as duplicate of #63747. Note that the shipped API requires explicitly opting in the supported derived types.