System.Text.Json 将字典序列化为数组

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

代替{ "hello": "world" }我需要将我的字典序列化为{ "key": "hello", "value": "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);
    }
}

您可以使用JsonConverterFactory https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonconverterfactory制造特定的JsonConverter<T> https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonconverter-1对于要序列化为数组的每个字典类型。这是一个这样的转换器，适用于实现的每个类IDictionary<TKey, TValue>:

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]);
            }
        }
    }
}

然后将工厂添加到JsonSerializerOptions.Converters https://learn.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializeroptions.converters?System_Text_Json_JsonSerializerOptions_Converters本地如下：

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

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

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

或者在 ASP.NET Core 中全局使用，如下所示如何在asp.net core 3中设置json序列化器设置？ https://stackoverflow.com/q/58392039/3744182:

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

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

Notes:

• JsonSerializer目前不尊重PropertyNamingPolicy https://learn.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializeroptions.propertynamingpolicy序列化时KeyValuePair<TKey, TValue> (see 问题 #1197 https://github.com/dotnet/runtime/issues/1197）所以我不得不介绍一个KeyValueDTO得到外壳"key" and "value"按照你的问题的要求。

• 我没有实现非通用转换器IDictionary类型。这可以作为答案的延伸来完成。

• 有关转换器工厂模式的更多信息，请参阅如何在 .NET 中编写用于 JSON 序列化的自定义转换器：示例工厂模式转换器 https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to#sample-factory-pattern-converter

• 类型相当于DefaultContractResolver in System.Text.Json -- JsonClassInfo https://github.com/dotnet/corefx/blob/master/src/System.Text.Json/src/System/Text/Json/Serialization/JsonClassInfo.cs and JsonPropertyInfo https://github.com/dotnet/corefx/blob/master/src/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfo.cs -- are internal。有一个开放的增强功能System.Text.Json 中的 DefaultContractResolver 的等效项 #42001 https://github.com/dotnet/corefx/issues/42001要求公共同等的。

演示小提琴here https://dotnetfiddle.net/0ApBYW.