Exception when serializing polymorphic c# class with method hiding to json, ASP.NET core API

Question

To get deep into the problem, here is a console app:

class Program
{
    static void Main()
    {
        Console.WriteLine(JsonSerializer.Serialize(
            new PolyMorphicClass { Data = new SomeData { N = 2} }));
    }
    class BaseClass
    {
        public virtual object Data { get; set; }
    }
    class PolyMorphicClass : BaseClass
    {
        public new SomeData Data { get; set; }
    }
    class SomeData
    {
        public int N { get; set; }
    }
}

This code throw an invalid operation exception with this message:

The JSON property name for 'ConsoleApp_for_test.Program+PolyMorphicClass.Data' collides with another property.

I found that if I initializing BaseClass instead, like below, it works

static void Main()
{
    Console.WriteLine(JsonSerializer.Serialize(
           new BaseClass { Data = new SomeData { N = 2} }));
}

My actual problem is: in a WebAPI where PolymorphicClass is the response type of a controller action that is being serialized to json, and this same exception happens.

extra note: In the API I use this polymorphic behavior to make the response consistent across endpoints ie similar data type.

My questions are: is it ok to use the BaseClass instead of the polymorphicClass like I said above in the context of initializing API response? Is there other solutions to serialize this? Can someone explain why the exception is happening?

Answer 1

You can't change the return type with the new keyword, all it does is hide it and requires the use of the same signature.

You could fix this in a couple of ways.

Using Generics

Replacing object with a generic type would allow for you to define PolyMorphicClass with a specific type for Data , which I believe is similar to what you're trying to do here.

class BaseClass<T>
{
    public virtual T Data { get; set; }
}

class PolyMorphicClass : BaseClass<SomeData>
{

}

Provide implementations for the property

Properties are essentially 2 methods (a getter and a setter) and you use some default ones with { get; set; } { get; set; } { get; set; } . These defaults get and set, respectively, a private member underneath the hood.

virtual properties are basically saying "You should override my getter and setter". Just specify an underlying member with the type SomeData to get and set. Here's a basic example.

class BaseClass
{
    public virtual object Data { get; set; }
}
class PolyMorphicClass : BaseClass
{
    private SomeData data { get; set; }

    public override object Data
    {
        get
        {
            return data;
        }
        set
        {
            data = (SomeData) value;
        }
    }
}

Note that if you deserialize some JSON that can't be casted to SomeData you'll run into a runtime exception of System.InvalidCastException , so you may want to add some additional type checking in your setter.