Attribute based, polymorphic support for System.Text.Json (and optionally Newtonsoft.Json). It supports both serialization and deserialization and is reasonably fast.

Package name	Description	Badge
PolyJson	Basic types and System.Text.Json support
PolyJson.NewtonsoftJson	Optional Newtonsoft.Json support

Decorate the base class with the PolyJsonConverter attribute and register the subtypes:

[PolyJsonConverter("_t")]
//[Newtonsoft.Json.JsonConverter(typeof(PolyJsonNewtonsoftJsonConverter))] // Optional
[PolyJsonConverter.SubType(typeof(Dog), "dog")]
[PolyJsonConverter.SubType(typeof(Cat), "cat")]
public abstract class Animal
{
    [JsonPropertyName("_t")]
    //[Newtonsoft.Json.JsonProperty("_t")] // Optional
    public string Discriminator => DiscriminatorValue.Get(GetType());
    public int Id { get; set; }
}

The PolyJsonConverter specifies the discriminator field, in this case _t. And all possible sub types are registered with their discriminator value. The Newtonsoft.Json support is optional, but uses the same configured subtypes as that configured for System.Text.Json. A sub type does not have any attributes and can for example be:

public class Dog : Animal
{
    public bool CanBark { get; set; }
}