Serialize interface as JsonResult

Question

I'm trying to write a .NET Core API that returns a list of objects. The following class is being used for sending responses:

class AnimalResponse {
    public IAnimal Animal { get; set; }
}

There is an empty interface:

interface IAnimal {

}

There are also 2 classes that implement this interface:

class Cat : IAnimal {
    public string CatProperty { get; set; }
}

class Dog : IAnimal { 
    public string DogProperty { get; set; }
}

As you can see, the Animal property of AnimalResponse class can contain either a Cat or a Dog class object. This is how I send response:

var response = new AnimalResponse() {
    Animal = new Cat() { 
         CatProperty = "Cat Property Value"
    }
};
return JsonResult(response);

For some reason CatProperty gets missing from an API response after serialization. API returns the following json: {Animal:{}}.

So, how do I make it include all the class-specific properties?

Note: I'm using Microsoft.AspNetCore.Mvc.JsonResult, not Newtonsoft.Json.

Nope, it looks like it's impossible to achieve this behavior. — Hirasawa Yui, Aug 19 '21 at 08:28

score 2 · Answer 1 · answered Apr 06 '21 at 20:13

2

1 install NewtonsoftJson

Install-Package Microsoft.AspNetCore.Mvc.NewtonsoftJson

2 Configure Services

services.AddControllersWithViews().AddNewtonsoftJson();

answered Apr 06 '21 at 20:13

user1894843

21
2

score 1 · Accepted Answer · answered Aug 19 '21 at 13:23

Serialization of derived types works only if the variable type is object.

In other words, everything will work if you redefine your AnimalResponse:

class AnimalResponse {
    public object Animal { get; set; }
}

You lose a bit of type safety, but you can get around that like so:

class AnimalResponse {
    public AnimalResponse(IAnimal animal) { Animal = animal; }
    public object Animal { get; }
}

This works because System.Text.Json has a special case for object.

(There are a few articles out there that explain why and how to work with this, but unfortunately, it was a while ago since I found the one that actually made it clear for me.)

Michaelsoft · Answer 3 · 2020-05-04T14:59:23.460

0

If you read also here Why are interfaces not [Serializable]? interfaces are not serializable because they define a contract and not a data.

They are a way to define a behaviour for the class.

By the way you should be able to make it serializable by implementing the ISerializable interface.

EDIT: Consider using a base Class instead of an Interface if you only need to serialize data. If you need to have both maybe a better solution is to have a base Class and an Interface and implement both.

UPDATE: Base class must implement the ISerializable interface.

edited May 04 '20 at 14:59

answered May 04 '20 at 14:26

Michaelsoft

777
5
18

The issue is still there after changing an interface to a class. – Hirasawa Yui May 04 '20 at 14:36
Make the class implement the ISerializable interface too. Sorry for the misunderstanding, an object that should be serialized must implement ISerializable, then in my opinion it's better that it is a class to implement it and not an interface, it could work too but IMHO it's going against the objectives of interfaces. – Michaelsoft May 04 '20 at 14:42

Serialize interface as JsonResult

3 Answers3