如何让 ReadJson 返回“默认”行为 - 就好像 CanConvert 返回 false

• 我已经创建了一个实现JsonConverter
• CanConvert总是返回 true。
• In ReadJson我有时想只使用“默认”行为，就好像CanConvert返回错误，我的ReadJson从未被调用过。
• 其他各种帖子建议我做以下一些变化：

existingValue = existingValue ?? serializer
                                     .ContractResolver
                                     .ResolveContract(objectType)
                                     .DefaultCreator();
serializer.Populate(reader, existingValue);

• 然而，这会引发NullReferenceException on .DefaultCreator().
• existingValue总是null
• The ContractResolver从序列化器返回的是我自己的。它扩展了 json.net 的内置CamelCasePropertyNamesContractResolver并简单地重写方法CreateConstructorParameters and CreatePropertyFromConstructorParameter

我如何告诉 json.net - “开玩笑，我不知道如何创建这个东西，如果我告诉你我无法创建它，你会做任何你会做的事情来创建它”

请注意，我已经简化了问题以供讨论。我预计有人会回答“只是有CanCreate return false“事实上，在许多情况下我可以而且应该创建该对象。

从您的问题和评论来看，听起来您在某些情况下希望转换器读取但不写入，而在其他情况下您希望转换器写入但不读取。您通过将功能拆分为两个转换器，然后让每个转换器的CanConvert方法在适当的时候返回 true 或 false。这当然是一种可行的方法，并且似乎对您有用，这很棒。然而，我想提供一个替代解决方案。

除了CanConvert方法、基础JsonConverter提供两个可以覆盖的虚拟布尔属性：CanRead and CanWrite。 （默认情况下两者都返回 true。）这些属性直接控制是否ReadJson and WriteJson由串行器为特定转换器调用。所以，举例来说，如果CanRead返回 false，则ReadJson不会被调用，并且将使用默认的读取行为，即使CanConvert返回 true。这使您可以非常巧妙地设置非对称转换器。例如，您可能会遇到这样的情况：您想要将疯狂的 JSON 格式反序列化为更理智的对象结构，但是当您再次序列化它时，您不想回到疯狂的 JSON 格式 - 您只想要默认序列化。在这种情况下你会覆盖CanWrite在你的转换器中总是返回 false。然后你可以离开执行WriteJson空白或让它抛出一个NotImplementedException;它永远不会被调用。

你的情况听起来比这更复杂一些，但你仍然应该能够操纵CanRead and CanWrite属性来达到您想要的结果。下面是一个人为的例子，展示了我们如何切换ReadJson and WriteJson方法的开启和关闭取决于情况变量。

public class Program
{
    public static void Main(string[] args)
    {
        string json = @"{""keys"":[""foo"",""fizz""],""values"":[""bar"",""bang""]}";

        CustomConverter converter = new CustomConverter();
        JsonSerializerSettings settings = new JsonSerializerSettings();
        settings.Converters.Add(converter);

        // Here we are reading a JSON object containing two arrays into a dictionary
        // (custom read) and then writing out the dictionary JSON (standard write)
        Console.WriteLine("--- Situation 1 (custom read, standard write) ---");
        converter.Behavior = ConverterBehavior.CustomReadStandardWrite;
        json = DeserializeThenSerialize(json, settings);

        // Here we are reading a simple JSON object into a dictionary (standard read)
        // and then writing out a new JSON object containing arrays (custom write)
        Console.WriteLine("--- Situation 2 (standard read, custom write) ---");
        converter.Behavior = ConverterBehavior.StandardReadCustomWrite;
        json = DeserializeThenSerialize(json, settings);
    }

    private static string DeserializeThenSerialize(string json, JsonSerializerSettings settings)
    {
        Console.WriteLine("Deserializing...");
        Console.WriteLine(json);
        var dict = JsonConvert.DeserializeObject<Dictionary<string, string>>(json, settings);
        foreach (var kvp in dict)
        {
            Console.WriteLine(kvp.Key + ": " + kvp.Value);
        }

        Console.WriteLine("Serializing...");
        json = JsonConvert.SerializeObject(dict, settings);
        Console.WriteLine(json);
        Console.WriteLine();

        return json;
    }
}

enum ConverterBehavior { CustomReadStandardWrite, StandardReadCustomWrite }

class CustomConverter : JsonConverter
{
    public ConverterBehavior Behavior { get; set; }

    public override bool CanConvert(Type objectType)
    {
        return typeof(IDictionary<string, string>).IsAssignableFrom(objectType);
    }

    public override bool CanRead
    {
        get { return Behavior == ConverterBehavior.CustomReadStandardWrite; }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        Console.WriteLine("ReadJson was called");

        // Converts a JSON object containing a keys array and a values array
        // into a Dictionary<string, string>
        JObject jo = JObject.Load(reader);
        return jo["keys"].Zip(jo["values"], (k, v) => new JProperty((string)k, v))
                         .ToDictionary(jp => jp.Name, jp => (string)jp.Value);
    }

    public override bool CanWrite
    {
        get { return Behavior == ConverterBehavior.StandardReadCustomWrite; }
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        Console.WriteLine("WriteJson was called");

        // Converts a dictionary to a JSON object containing 
        // a keys array and a values array from the dictionary
        var dict = (Dictionary<string, string>)value;
        JObject jo = new JObject(new JProperty("keys", new JArray(dict.Keys)),
                                 new JProperty("values", new JArray(dict.Values)));
        jo.WriteTo(writer);
    }
}

Output:

--- Situation 1 (custom read, standard write) ---
Deserializing...
{"keys":["foo","fizz"],"values":["bar","bang"]}
ReadJson was called
foo: bar
fizz: bang
Serializing...
{"foo":"bar","fizz":"bang"}

--- Situation 2 (standard read, custom write) ---
Deserializing...
{"foo":"bar","fizz":"bang"}
foo: bar
fizz: bang
Serializing...
WriteJson was called
{"keys":["foo","fizz"],"values":["bar","bang"]}

Fiddle: https://dotnetfiddle.net/BdtSoN https://dotnetfiddle.net/BdtSoN