System.Text.Json 将字典序列化为数组

Question

是否可以使用 System.Text.Json 将字典序列化（和反序列化）为数组？

我需要将我的字典序列化为{ "key": "hello", "value": "world" } 而不是{ "hello": "world" }，最好不必在我的类的字典属性上设置属性。

使用 newtonsoft.json 可以这样：

class DictionaryAsArrayResolver : DefaultContractResolver
{
    protected override JsonContract CreateContract(Type objectType)
    {
        if (objectType.GetInterfaces().Any(i => i == typeof(IDictionary) || 
           (i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IDictionary<,>))))
        {
            return base.CreateArrayContract(objectType);
        }

        return base.CreateContract(objectType);
    }
}

Answer 1

您可以使用JsonConverterFactory 来执行此操作，该JsonConverter<T> 为您想要序列化为数组的每个字典类型生成一个特定的JsonConverter<T>。这是一个适用于每个实现IDictionary<TKey, TValue> 的类的转换器：

public class DictionaryConverterFactory : JsonConverterFactory
{
    public override bool CanConvert(Type typeToConvert)
    {
        return typeToConvert.IsClass && typeToConvert.GetDictionaryKeyValueType() != null && typeToConvert.GetConstructor(Type.EmptyTypes) != null;
    }

    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
    {
        var keyValueTypes = typeToConvert.GetDictionaryKeyValueType();
        var converterType = typeof(DictionaryAsArrayConverter<,,>).MakeGenericType(typeToConvert, keyValueTypes.Value.Key, keyValueTypes.Value.Value);
        return (JsonConverter)Activator.CreateInstance(converterType);
    }
}

public class DictionaryAsArrayConverter<TKey, TValue> : DictionaryAsArrayConverter<Dictionary<TKey, TValue>, TKey, TValue>
{
}

public class DictionaryAsArrayConverter<TDictionary, TKey, TValue> : JsonConverter<TDictionary> where TDictionary : class, IDictionary<TKey, TValue>, new()
{
    struct KeyValueDTO
    {
        public TKey Key { get; set; }
        public TValue Value { get; set; }
    }

    public override TDictionary Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var list = JsonSerializer.Deserialize<List<KeyValueDTO>>(ref reader, options);
        if (list == null)
            return null;
        var dictionary = typeToConvert == typeof(Dictionary<TKey, TValue>) ? (TDictionary)(object)new Dictionary<TKey, TValue>(list.Count) : new TDictionary();
        foreach (var pair in list)
            dictionary.Add(pair.Key, pair.Value);
        return dictionary;
    }

    public override void Write(Utf8JsonWriter writer, TDictionary value, JsonSerializerOptions options)
    {
        JsonSerializer.Serialize(writer, value.Select(p => new KeyValueDTO { Key = p.Key, Value = p.Value }), options);
    }
}

public static class TypeExtensions
{
    public static IEnumerable<Type> GetInterfacesAndSelf(this Type type)
    {
        if (type == null)
            throw new ArgumentNullException();
        if (type.IsInterface)
            return new[] { type }.Concat(type.GetInterfaces());
        else
            return type.GetInterfaces();
    }

    public static KeyValuePair<Type, Type>? GetDictionaryKeyValueType(this Type type)
    {
        KeyValuePair<Type, Type>? types = null;
        foreach (var pair in type.GetDictionaryKeyValueTypes())
        {
            if (types == null)
                types = pair;
            else
                return null;
        }
        return types;
    }

    public static IEnumerable<KeyValuePair<Type, Type>> GetDictionaryKeyValueTypes(this Type type)
    {
        foreach (Type intType in type.GetInterfacesAndSelf())
        {
            if (intType.IsGenericType
                && intType.GetGenericTypeDefinition() == typeof(IDictionary<,>))
            {
                var args = intType.GetGenericArguments();
                if (args.Length == 2)
                    yield return new KeyValuePair<Type, Type>(args[0], args[1]);
            }
        }
    }
}

然后在本地添加工厂到JsonSerializerOptions.Converters，如下：

var options = new JsonSerializerOptions
{
    Converters = { new DictionaryConverterFactory() },
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
};

var json = JsonSerializer.Serialize(dictionary, options);

var dictionary2 = JsonSerializer.Deserialize<TDictionary>(json, options);

或全局在 ASP.NET Core 中如 How to set json serializer settings in asp.net core 3? 所示：

services.AddControllers().AddJsonOptions(options =>
{
    options.JsonSerializerOptions.Converters.Add(new DictionaryConverterFactory());
    options.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
});

如果您只想将某些字典类型序列化为数组，也可以直接使用底层的单个转换器DictionaryAsArrayConverter<TKey, TValue>。

注意事项：

• JsonSerializer 目前在序列化KeyValuePair<TKey, TValue> 时不尊重PropertyNamingPolicy（请参阅Issue #1197），因此我不得不引入KeyValueDTO 来获取"key" 和"value" 中所需的大小写你的问题。

• 我没有为非泛型 IDictionary 类型实现转换器。这可以作为答案的延伸。

• 有关转换器工厂模式的更多信息，请参阅How to write custom converters for JSON serialization in .NET : Sample factory pattern converter

• 与DefaultContractResolver 中的DefaultContractResolver 等价的类型 -- JsonClassInfo 和 JsonPropertyInfo -- 是internal。有一个开放的增强功能Equivalent of DefaultContractResolver in System.Text.Json #42001要求公开等效。

演示小提琴here.

Answer 2

如果您想保持简短，可以考虑通过匿名类型进行投影：

var dictionary = new Dictionary<string, string>();
dictionary.Add("hello", "world");
dictionary.Add("how", "are you?");

var o = JsonSerializer.Serialize(dictionary.Select(x => new { key = x.Key, value = x.Value }));
// [{"key":"hello","value":"world"},{"key":"how","value":"are you?"}]

ed：当然，这只是你的自虐感。如果您只想完成工作，请致电.ToList()

JsonSerializer.Serialize(dictionary.ToList());
// [{"Key":"hello","Value":"world"},{"Key":"how","Value":"are you?"}]