Properties defined in inherited interface sometimes fail to serialize

[insolublemaize]

Describe the bug

Suppose you have ICar which inherits from IVehicle. When ICar is used as the return type from a controller action, it is serialized correctly.

But if a class CarWrapper has a property of type ICar, when CarWrapper is used as the return type from a controller action, the properties defined in IVehicle fail to serialize.

To Reproduce

An example can be found here https://github.com/insolublemaize/aspnetcore-serialization-bug. Run the project and go to /vehicle/GetCar and /vehicle/GetCarWrapper to see the difference between the serialized car objects.

The relevant section of code is:

public interface IVehicle
{
    public int Speed { get; set; }
}

public interface ICar : IVehicle
{
    public int NumberOfWheels { get; set; }
}

public class Car : ICar
{
    public int NumberOfWheels { get; set; }
    public int Speed { get; set; }
}

public class CarWrapper
{
    public ICar Car { get; set; }
}

/* Returns
{
    "numberOfWheels": 0,
    "speed": 0
}
*/
[HttpGet(nameof(GetCar))]
public ICar GetCar()
{
    return new Car();
}

/* Returns (notice that "speed" is missing)
{
    "car": 
    {
        "numberOfWheels": 0
    }
}
*/
[HttpGet(nameof(GetCarWrapper))]
public CarWrapper GetCarWrapper()
{
    return new CarWrapper
    {
        Car = new Car()
    };
}

Further technical details

• ASP.NET Core version: 5.0.8
• IDE: Microsoft Visual Studio Enterprise 2019 Version 16.10.1
• dotnet --info:

.NET SDK (reflecting any global.json):
 Version:   6.0.100-preview.6.21355.2
 Commit:    7f8e0d76c0

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19042
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.100-preview.6.21355.2\

Host (useful for support):
  Version: 6.0.0-preview.6.21352.12
  Commit:  770d630b28

.NET SDKs installed:
  1.0.4 [C:\Program Files\dotnet\sdk]
  2.1.701 [C:\Program Files\dotnet\sdk]
  3.1.411 [C:\Program Files\dotnet\sdk]
  5.0.104 [C:\Program Files\dotnet\sdk]
  5.0.301 [C:\Program Files\dotnet\sdk]
  5.0.302 [C:\Program Files\dotnet\sdk]
  6.0.100-preview.6.21355.2 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 6.0.0-preview.6.21355.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 1.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 6.0.0-preview.6.21352.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 6.0.0-preview.6.21353.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

[davidfowl]

This is due to the JSON serializer not supporting polymorphic serialization. This is supported in .NET 6 with additional configuration #30083 (comment).

cc @eiriktsarpalis

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[eiriktsarpalis]

Note that polymorphism support has been cut from .NET 6 and the feature will likely be delivered in .NET 7. In the meantime, using a custom converter that implements polymorphism is the only workaround.

Duplicate of #30083.

[davidfowl]

In the meantime, using a custom converter that implements polymorphism is the only workaround.

Do we have a sample that we can document and point people at?

[eiriktsarpalis]

There are a few workarounds floating around in the wild, although they typically come with too many gotchas and/or security issues for us to give an implicit seal of approval. The simplest way to do polymorphism drawing from the sample above is probably the following:

public class VehicleConverter : JsonConverter<IVehicle>
{
    public override IVehicle Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => throw new NotSupportedException("Polymorphic deserialization not supported in this model.");

    public override void Write(Utf8JsonWriter writer, IVehicle vehicle, JsonSerializerOptions options)
    {
        switch(vehicle)
        {
            case ICar car:
                writer.WriteStartObject();
                writer.WriteNumber(nameof(ICar.NumberOfWheels), car.NumberOfWheels);
                writer.WriteNumber(nameof(ICar.Speed), car.Speed);
                writer.WriteEndObject();
                break;

            case IScooter scooter:
                writer.WriteStartObject();
                writer.WriteString(nameof(IScooter.Color), scooter.Color);
                writer.WriteNumber(nameof(IScooter.Speed), scooter.Speed);
                writer.WriteEndObject();
                break;

            default:
                writer.WriteStartObject();
                writer.WriteNumber(nameof(IVehicle.Speed), vehicle.Speed);
                writer.WriteEndObject();
                break;
        };
    }
}