使用JsonDerivedType属性序列化和反序列化.NET 7中的多态对象

Question

JSON.NET (由Newtonsoft)支持复杂对象的序列化和反序列化。

我对使用System.Text.Json而不是JSON.NET很好奇，我在网上找不到一个很好的教程。

.NET 7预览支持反序列化多态对象。下面是一个使用.NET 7预览和C# 11的示例：

// _To run it you will need net7 preview and c# 11_

using System.Text.Json.Serialization;

var testPolymorphism = new TestPolymorphysm()
{
    Animals = new List<Animal>()
    {
        new Fish() {
             Id = "fish1",
             Name = "GoldFish",
             Action = new ActionSwim() { DistanceSwam = 10 }
        },
        new Dog() {
             Id = "dog1",
             Name = "Tom",
             Action = new ActionRun() { DistanceRan = 50  }
        }
    }
};

// serialize
var jsonSerialized = System.Text.Json.JsonSerializer.Serialize(testPolymorphism);
Console.WriteLine(jsonSerialized);

// deserialize
var clonePolymorphysm = System.Text.Json.JsonSerializer.Deserialize<TestPolymorphysm>(jsonSerialized);
Console.WriteLine(clonePolymorphysm);


// === MODELS ===

class TestPolymorphysm
{
    public List<Animal> Animals { get; set; } = new();
}

[JsonDerivedType(derivedType: typeof(Dog), typeDiscriminator: "foo1")]
[JsonDerivedType(derivedType: typeof(Fish), typeDiscriminator: "foo2")]
abstract class Animal
{
    public required string Id { get; set; }
    public required string Name { get; set; }
}

class Dog : Animal
{
    public required IAction Action { get; set; }
    public AnimalType ExtensionType => AnimalType.Dog;
}

class Fish : Animal
{
    public required IAction Action { get; set; }
    public AnimalType ExtensionType => AnimalType.Fish;
}

[JsonDerivedType(derivedType: typeof(ActionSwim), typeDiscriminator: "foo3")]
[JsonDerivedType(derivedType: typeof(ActionRun), typeDiscriminator: "foo4")]
interface IAction { }
class ActionSwim : IAction
{
    public required int DistanceSwam { get; set; }
}

class ActionRun : IAction
{
    public required int DistanceRan { get; set; }
}

public enum AnimalType
{
    Fish,
    Dog
}

无论如何，多亏了JsonDerivedType属性，这段代码才能工作，但我不知道它为什么工作。为什么如果我删除typeDiscriminators foo1、foo2、foo3和foo4，它就不能工作？在我使用它之前，我要确保我理解它的工作原理。

Answer 1

对不起，我没有注意序列化对象。它包含："$type": "foo1",，"$type": "foo2",等。

这就是反序列化器知道如何反序列化对象的原因。