[question] how to have a abstract class/interface as a property/field without knowing the type before about dahomey.cbor

[mcatanzariti]

The implementation of polymorphism in Dahomey.Cbor is based on a special field in which a special value is associated with an inherited class. The special field/value is called discriminator.

By default the discriminator field is named "_t" and the discriminator values are configured

1. Either by using the attribute CborDiscriminatorAttribute:

[CborDiscriminator("Apple")]
public class Apple : Fruit { ... }

[CborDiscriminator("Orange")]
public class Orange : Fruit { ... }

Moreover , in this option, you need to register the assembly to allow the framework to discover the classed flagged by the CborDiscriminatorAttribute:

CborOptions.Default.Registry.DefaultDiscriminatorConvention.RegisterAssembly(typeof(Fruit ).Assembly);

2. Or by API:

options.Registry.ObjectMappingRegistry.Register<Apple>(om => om.AutoMap().SetDiscriminator("Apple");
options.Registry.ObjectMappingRegistry.Register<Orange>(om => om.AutoMap().SetDiscriminator("Orange");

[TMaddox]

I tried both examples and I also added the [CborDiscriminator("Name")] attribute on all classes implementing the Fruit class, but it still throws:
CborException: A CreatorMapping should be defined for interfaces or abstract classes

@mcatanzariti I am mentioning you by name because I do not have the permission to reopen this issue, but I feel like I should ask here.

[mcatanzariti]

Did you try with the last official version?
The 2 examples I gave you are integrated in the automated tests and they don't fail

[TMaddox]

yes with 1.3.2, I have broken it down to tree classes while testing and found out the following:

    public class Tree
    {
        [CborProperty("age")]
        public readonly int _age;
        
        [CborProperty("fruit")]
        public Fruit _fruit;

        [CborConstructor]
        public Tree(int age, Fruit fruit)
        {
            _age = age;
            _fruit = fruit;
        }
    }

    public abstract class Fruit
    {
        public string id2 = "myid"; // if I remove this it won't throw
    }

    [CborDiscriminator("Apple")]
    public class Apple : Fruit
    {
        [CborProperty("type")]
        public const string type = "I am an apple";
    }

    [CborDiscriminator("Orange")]
    public class Orange : Fruit
    {
        [CborProperty("type")]
        public const string type = "I am an orange";
    }

[TMaddox]

further, serializing the above example will output {"age": 10, "fruit": {}, "id": "123"}, with fruit being empty, I have tried it with const fields and normal ones in combination with a constructor.

[mcatanzariti]

Reproduced

[mcatanzariti]

fix in 1.4.0