如何在自定义 System.Text.Json JsonConverter 中使用默认序列化？

Question

我正在写一个custom System.Text.Json.JsonConverter to upgrade an old data model to a new version. I have overridden Read() and implemented the necessary postprocessing. However, I don't need to do anything custom at all in the Write()方法。如果根本没有转换器，我如何自动生成默认序列化？显然我可以使用不同的 JsonSerializerOptions 来进行反序列化和序列化，但是我的框架并没有直接为每个选项提供不同的选项。

下面是一个简化的例子。假设我以前有以下数据模型：

public record Person(string Name);

我已经升级到

public record Person(string FirstName, string LastName);

我写了一个转换器如下：

public sealed class PersonConverter : JsonConverter<Person>
{
    record PersonDTO(string FirstName, string LastName, string Name); // A DTO with both the old and new properties.

    public override Person Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var dto = JsonSerializer.Deserialize<PersonDTO>(ref reader, options);
        var oldNames = dto?.Name?.Split(' ', StringSplitOptions.RemoveEmptyEntries) ?? Enumerable.Empty<string>();
        return new Person(dto.FirstName ?? oldNames.FirstOrDefault(), dto.LastName ?? oldNames.LastOrDefault());
    }

    public override void Write(Utf8JsonWriter writer, Person person, JsonSerializerOptions options)
        => // What do I do here? I want to preserve other options such as options.PropertyNamingPolicy, which are lost by the following call
        JsonSerializer.Serialize(writer, person);
}

与

往返

var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    Converters = { new PersonConverter() },
};
var person = JsonSerializer.Deserialize<Person>(json, options);
var json2 = JsonSerializer.Serialize(person, options);

那么结果就是{"FirstName":"FirstName","LastName":"LastName"}——也就是序列化时的驼峰壳丢失了。但是如果我通过递归调用

    public override void Write(Utf8JsonWriter writer, Person person, JsonSerializerOptions options)
        => // What do I do here? I want to preserve other options such as options.PropertyNamingPolicy, which are lost by the following call
        JsonSerializer.Serialize(writer, person, options);

然后序列化因堆栈溢出而失败。

如何获得忽略自定义转换器的精确默认序列化？没有等同于 Json.NET 的 JsonConverter.CanWrite 属性.

演示 fiddle here.

Answer 1

如 docs 中所述，转换器的选择优先级如下：

[JsonConverter] applied to a property.

A converter added to the Converters collection.

[JsonConverter] applied to a custom value type or POCO.

每个案例都需要单独处理。

如果您将 [JsonConverter] 应用于属性.，则只需调用 JsonSerializer.Serialize(writer, person, options); 即可生成默认值序列化。
如果您将 A 转换器添加到 Converters 集合中。，则在 Write()（或 Read()) 方法，可以使用JsonSerializerOptions copy constructor, remove the converter from the copy's Converters列表复制传入的options，将修改后的副本传入JsonSerializer.Serialize<T>(Utf8JsonWriter, T, JsonSerializerOptions);

这在 .NET Core 中无法轻松完成 3.x，因为该版本中不存在复制构造函数。临时修改传入选项的 Converters 集合以删除转换器不是线程安全的，因此不推荐。相反，需要创建新选项并手动复制每个属性以及 Converters 集合，跳过 converterType.
类型的转换
如果您将 [JsonConverter] 应用于自定义值类型或 POCO。 似乎没有生成默认序列化的方法.

因为在问题中，转换器被添加到 Converters 列表中，以下修改版本正确生成默认序列化：

public sealed class PersonConverter : DefaultConverterFactory<Person>
{
    record PersonDTO(string FirstName, string LastName, string Name); // A DTO with both the old and new properties.

    protected override Person Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions modifiedOptions)
    {
        var dto = JsonSerializer.Deserialize<PersonDTO>(ref reader, modifiedOptions);
        var oldNames = dto?.Name?.Split(' ', StringSplitOptions.RemoveEmptyEntries) ?? Enumerable.Empty<string>();
        return new Person(dto.FirstName ?? oldNames.FirstOrDefault(), dto.LastName ?? oldNames.LastOrDefault());
    }
}

public abstract class DefaultConverterFactory<T> : JsonConverterFactory
{
    class DefaultConverter : JsonConverter<T>
    {
        readonly JsonSerializerOptions modifiedOptions;
        readonly DefaultConverterFactory<T> factory;

        public DefaultConverter(JsonSerializerOptions options, DefaultConverterFactory<T> factory)
        {
            this.factory = factory;
            this.modifiedOptions = options.CopyAndRemoveConverter(factory.GetType());
        }

        public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options) => factory.Write(writer, value, modifiedOptions);

        public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => factory.Read(ref reader, typeToConvert, modifiedOptions);
    }

    protected virtual T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions modifiedOptions)
        => (T)JsonSerializer.Deserialize(ref reader, typeToConvert, modifiedOptions);

    protected virtual void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions modifiedOptions) 
        => JsonSerializer.Serialize(writer, value, modifiedOptions);

    public override bool CanConvert(Type typeToConvert) => typeof(T) == typeToConvert;

    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options) => new DefaultConverter(options, this);
}

public static class JsonSerializerExtensions
{
    public static JsonSerializerOptions CopyAndRemoveConverter(this JsonSerializerOptions options, Type converterType)
    {
        var copy = new JsonSerializerOptions(options);
        for (var i = copy.Converters.Count - 1; i >= 0; i--)
            if (copy.Converters[i].GetType() == converterType)
                copy.Converters.RemoveAt(i);
        return copy;
    }
}

备注：

我使用转换器工厂而不是转换器作为 PersonConverter 的基础 class 因为它允许我方便地在制造的转换器中缓存复制的选项。
如果您尝试将 DefaultConverterFactory<T> 应用于自定义值类型或 POCO，例如
```
[JsonConverter(typeof(PersonConverter))] public record Person(string FirstName, string LastName);
```
会发生严重的堆栈溢出。

演示 fiddle here.

如何在自定义 System.Text.Json JsonConverter 中使用默认序列化？

How to use default serialization in a custom System.Text.Json JsonConverter?

c#

serialization

json

.net-5

system.text.json