当值可以是对象或空数组时，反序列化JSON [英] Deserialize JSON when a value can be an object or an empty array

问题描述

我正在使用VK API.有时服务器可以返回空数组而不是对象，例如:

I`m working with VK API. Sometimes server can return empty array instead of object, for example:

personal: [] //when it is empty

或

personal: {
religion: 'Нет',
smoking: 1,
alcohol: 4
} //when not empty.

我将使用JsonConvert.DeserializeObject反序列化大多数json，并使用

I`m deserializing most of json with JsonConvert.DeserializeObject, and this part of json with

MainObject = ((MainObject["response"].GetObject())["user"].GetObject())["personal"].GetObject();
try
{
Convert.ToByte(MainObject["political"].GetNumber();
} 
catch {}

但是，当它处理大量肽段时，它会使应用程序运行缓慢.刚才我意识到，这里还有其他一些字段，当它们为空时，它们可能会返回数组.我只是不知道如何快速而清晰地做到这一点.有什么建议吗?

But it makes app works slowly when it`s handling a lot of exeptions. And just now i realised that here are some more fields that might return array when empty. I just have no ideas how to make it fastly and clearly. Any suggestions?

我的反序列化类(当字段为空时不起作用)

My deserializing class (doen`t work when field is empty):

     public class User
            {
//some other fields...
                public Personal personal { get; set; }
//some other fields...
             }
    public class Personal
            {
                public byte political { get; set; }
                public string[] langs { get; set; }
                public string religion { get; set; }
                public string inspired_by { get; set; }
                public byte people_main { get; set; }
                public byte life_main { get; set; }
                public byte smoking { get; set; }
                public byte alcohol { get; set; }
            }

另一个想法(不为空时不起作用):

Another idea (doesn`t work when not empty):

public List<Personal> personal { get; set; }

推荐答案

您可以创建

You could make a JsonConverter like the following, that looks for either an object of a specified type, or an empty array. If an object, it deserializes that object. If an empty array, it returns null:

public class JsonSingleOrEmptyArrayConverter<T> : JsonConverter where T : class
{
    public override bool CanConvert(Type objectType)
    {
        return typeof(T).IsAssignableFrom(objectType);
    }

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

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var contract = serializer.ContractResolver.ResolveContract(objectType);
        if (!(contract is Newtonsoft.Json.Serialization.JsonObjectContract || contract is Newtonsoft.Json.Serialization.JsonDictionaryContract))
        {
            throw new JsonSerializationException(string.Format("Unsupported objectType {0} at {1}.", objectType, reader.Path));
        }

        switch (reader.SkipComments().TokenType)
        {
            case JsonToken.StartArray:
                {
                    int count = 0;
                    while (reader.Read())
                    {
                        switch (reader.TokenType)
                        {
                            case JsonToken.Comment:
                                break;
                            case JsonToken.EndArray:
                                return existingValue;
                            default:
                                {
                                    count++;
                                    if (count > 1)
                                        throw new JsonSerializationException(string.Format("Too many objects at path {0}.", reader.Path));
                                    existingValue = existingValue ?? contract.DefaultCreator();
                                    serializer.Populate(reader, existingValue);
                                }
                                break;
                        }
                    }
                    // Should not come here.
                    throw new JsonSerializationException(string.Format("Unclosed array at path {0}.", reader.Path));
                }

            case JsonToken.Null:
                return null;

            case JsonToken.StartObject:
                existingValue = existingValue ?? contract.DefaultCreator();
                serializer.Populate(reader, existingValue);
                return existingValue;

            default:
                throw new InvalidOperationException("Unexpected token type " + reader.TokenType.ToString());
        }
    }

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

public static partial class JsonExtensions
{
    public static JsonReader SkipComments(this JsonReader reader)
    {
        while (reader.TokenType == JsonToken.Comment && reader.Read())
            ;
        return reader;
    }
}

然后像这样使用它:

public class User
{
    //some other fields...
    [JsonConverter(typeof(JsonSingleOrEmptyArrayConverter<Personal>))]
    public Personal personal { get; set; }
    //some other fields...
}

您现在应该可以将用户反序列化到您的User类中.

You should now be able to deserialize a user into your User class.

注意:

• 可以通过属性或在 JsonSerializerSettings.Converters中应用转换器.

• The converter can be applied via attributes or in JsonSerializerSettings.Converters.

该转换器并非旨在与诸如字符串之类的简单类型一起使用，而是为映射到JSON对象的类而设计的.那是因为它使用 JsonSerializer.Populate() 来避免读取过程中的无限递归.

The converter isn't designed to work with simple types such as strings, it's designed for classes that map to a JSON object. That's because it uses JsonSerializer.Populate() to avoid an infinite recursion during reading.

工作示例.Net小提琴此处和此处.

Working sample .Net fiddles here and here.

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