SystemTextJsonOutputFormatter calls the serializer with unexpected Type

[akurone]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

hi there,
after a short talk with lovely people on runtime team (#dotnet/runtime/issues/77532); i think the issue below needs to be resolved here:
i want to utilize the new json polymorphism features:

[JsonPolymorphic]
[JsonDerivedType(typeof(TheDerivedType), nameof(TheDerivedType))]
public class TheBaseType { public int Prop1 { get; set; } = 1; }

public class TheDerivedType: TheBaseType { public int Prop2 { get; set; } = 2; }

//and in the controller:
[HttpGet] public TheBaseType A() => new TheDerivedType();
// call to this endpoint produces:
// {"prop2":2,"prop1":1}

what i expect to get is:

{"$type":"TheDerivedType","prop2":2,"prop1":1}

but as discussed in #41399 SystemTextJsonOutputFormatter calls the serializer in a very specific way. this call practically hides all the new metadata from the serializer and produces json without discriminator info. without discriminator, the receiving party fails to deserialize this json.

Expected Behavior

SystemTextJsonOutputFormatter and other json returning types (like JO result etc.) rendering with STJ should respect the polymorphic serialization metadata provided.

current:

{"prop2":2,"prop1":1}

expected:

{"$type":"TheDerivedType","prop2":2,"prop1":1}

Steps To Reproduce

clone the repo and make the get reqs below:

• test/a
• test/b

Exceptions (if any)

No response

.NET Version

7.0.100-rc.2.22477.23

Anything else?

No response

Thanks for contacting us.
We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. Because it's not immediately obvious that this is a bug in our framework, we would like to keep this around to collect more feedback, which can later help us determine the impact of it. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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[marcelbarner]

I had the same problem and I created a repo/package to solve it.

Repo: marcelbarner/AspNetCore.Json.SinglePolymorphicResponse

Package: AspNetCore.Json.SinglePolymorphicResponse

[stephenwelsh]

This looks incorrect to me, different output depending if I use the Generic form or not:

var test = new TestA{ Id="123", Name="TestA"}
var nonGenericCall = JsonSerializer.Serialize(test, test.GetType());
var genericCall = JsonSerializer.Serialize<TestBase>(test);

[JsonDerivedType(typeof(TestBase), typeDiscriminator: nameof(TestBase))]
[JsonDerivedType(typeof(TestA), typeDiscriminator: nameof(TestA))]
public class TestBase
{
    public string Id { get; set; }
}
public class TestA : TestBase
{
    public string Name { get; set; }
}

I get the following:

nonGenericCall = "{"id"="123","name"="TestA"}"
genericCall = "{$type="TestA", "id"="123", "name"="TestA"}"

*** Update ***

Looks like my bad, specifying base class as type argument works:
var nonGenericCallWithBase = JsonSerializer.Serialize(test, typeof(TestBase));
nonGenericCallWithBase = "{$type="TestA", "id"="123", "name"="TestA"}"

[stephenwelsh]

In my scenario (bi directional polymorphic events using SignalR with .Net client) it is not possible to specify a base class and normally a custom JsonConverter is used to manually add the type discriminators.
Looking to use the new Net7 support for polymorphic types exposed this differentiated behaviour when in some SignalR calling directions the base type is not known.

After further research I found that duplicating the JsonDerivedType attribute on the corresponding Derived class enables the non generic call to JsonSerializer.Serialize to add the type discriminator.

This restriction and work around may be worth adding to the Serialize properties of derived classes documentation.

var test = new TestA{ Id="123", Name="TestA"}
var nonGenericCall = JsonSerializer.Serialize(test, test.GetType());
var genericCall = JsonSerializer.Serialize<TestBase>(test);

[JsonDerivedType(typeof(TestBase), typeDiscriminator: nameof(TestBase))]
[JsonDerivedType(typeof(TestA), typeDiscriminator: nameof(TestA))]
public class TestBase
{
    public string Id { get; set; }
}
[JsonDerivedType(typeof(TestA), typeDiscriminator: nameof(TestA))]
public class TestA : TestBase
{
    public string Name { get; set; }
}

I now get the desired output as follows:

nonGenericCall = "{$type="TestA", "id"="123", "name"="TestA"}"
genericCall = "{$type="TestA", "id"="123", "name"="TestA", $type="TestA"}"

[brunolins16]

This restriction and work around may be worth adding to the Serialize properties of derived classes documentation.

cc @dotnet/area-system-text-json

[eiriktsarpalis]

After further research I found that duplicating the JsonDerivedType attribute on the corresponding Derived class enables the non generic call to JsonSerializer.Serialize to add the type discriminator.

This restriction and work around may be worth adding to the Serialize properties of derived classes documentation.

cc @dotnet/area-system-text-json

I don't think this will be necessary once #46008 has been merged, no?