在自定义 JsonConverter 的 ReadJson 方法中处理空对象

I've got a Newtonsoft JSON.NET JsonConverter to help deserialize a property whose type is an abstract class. The gist of it looks like this:

public class PetConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(Animal);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JObject jsonObject = JObject.Load(reader);

        if (jsonObject["Lives"] != null) return jsonObject.ToObject<Cat>(serializer);
        if (jsonObject["StopPhrase"] != null) return jsonObject.ToObject<Parrot>(serializer);

        return null;
    }

    public override bool CanWrite { get { return false; } }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    { throw new NotImplementedException(); }
}

Here are the classes it handles:

public abstract class Animal 
{ }

public class Cat : Animal
{
    public int Lives { get; set; }
}

public class Parrot : Animal
{
    public string StopPhrase { get; set; }
}

public class Person
{
    [JsonConverter(typeof(PetConverter))]
    public Animal Pet { get; set; }
}

当反序列化具有非空Pet的Person时，这可以很好地工作.但是如果Pet为空，那么ReadJson方法在第一行用JsonReaderException中断:

Newtonsoft.Json.dll中出现‘Newtonsoft.Json.JsonReaderException’类型的异常，但未在用户代码中处理

其他信息:从JsonReader读取JObject时出错.当前JsonReader项目不是对象:Null.路径‘Pet’，第1行，位置11.

我查阅了the Custom JsonConverter份文档，但它只是一个writing转换器.我试过以下方法:

if (reader.Value == null) return null; // this inverts the [Test] results

But then I get:

JsonSerializationException: Additional text found in JSON string after finishing deserializing object.

如果该房产有is人居住.

简而言之，处理这种情况的正确方法是什么？

For completeness, here are some unit tests that demonstrate the issue at hand:

[TestFixture]
public class JsonConverterTests
{
    [Test]
    public void Cat_survives_serialization_roundtrip()
    {
        var person = new Person { Pet = new Cat { Lives = 9 } };
        var serialized = JsonConvert.SerializeObject(person);
        var deserialized = JsonConvert.DeserializeObject<Person>(serialized);
        Assert.That(deserialized.Pet, Is.InstanceOf<Cat>());
        Assert.That((deserialized.Pet as Cat).Lives, Is.EqualTo(9));
    }

    [Test]
    public void Parrot_survives_serialization_roundtrip()
    {
        var person = new Person { Pet = new Parrot { StopPhrase = "Lorrie!" } };
        var serialized = JsonConvert.SerializeObject(person);
        var deserialized = JsonConvert.DeserializeObject<Person>(serialized);
        Assert.That(deserialized.Pet, Is.InstanceOf<Parrot>());
        Assert.That((deserialized.Pet as Parrot).StopPhrase, Is.EqualTo("Lorrie!"));
    }

    [Test]
    public void Null_property_does_not_break_converter()
    {
        var person = new Person { Pet = null };
        var serialized = JsonConvert.SerializeObject(person);
        var deserialized = JsonConvert.DeserializeObject<Person>(serialized);
        Assert.That(deserialized.Pet, Is.Null);
    }
}