如何让 ReadJson 返回"default"行为 - 好像 CanConvert 返回了 false

Question

• 我创建了JsonConverter的实现
• CanConvert总是返回true。
• 在ReadJson中，我有时只想使用"默认"行为，就好像CanConvert返回false，而我的ReadJson从未被调用。
• 其他各种帖子都建议我做以下一些变化：

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

• 但是，这会在.DefaultCreator()上抛出NullReferenceException。
• existingValue总是null
• 从序列化器返回的ContractResolver是我自己的。它扩展了JSON.NET的内置CamelCasePropertyNamesContractResolver，只需覆盖方法CreateConstructorParameters和CreatePropertyFromConstructorParameter

我如何告诉json.net-"开玩笑，我不知道如何创建这个东西，尽一切努力创建它，如果我告诉你我无法创建它"

请注意，我简化了讨论的问题。我希望有人会回答"只有CanCreate返回false"实际上，在我可以并且应该创建对象的许多情况下。

Answer 1

从您的问题和评论中，听起来您有某些情况，您希望转换器读取但不编写，而其他情况则是您想要编写但不阅读的情况。您已经通过将功能分为两个转换器，然后让每个转换器的CanConvert方法在适当时间返回true或false来解决问题。这当然是一种可行的方法，似乎对您有用，这很棒。但是，我想提供另一种解决方案。

除了CanConvert方法外，基本JsonConverter还提供两个虚拟布尔属性，您可以覆盖：CanRead和CanWrite。（默认情况下均返回True。）这些属性直接控制ReadJson和WriteJson是否由序列化器调用特定转换器。因此，例如，如果CanRead返回false，则不会调用ReadJson，即使CanConvert返回true，也将使用默认的读取行为。这使您可以非常整齐地设置一个不对称的转换器。例如，您可能会有一种情况，您希望将疯狂的JSON格式化为更明智的对象结构，但是当您再次序列化时，您不想回到疯狂的JSON格式 - 您只是想要默认序列化。在这种情况下，您将覆盖转换器中的CanWrite以始终返回false。然后，您可以将WriteJson的实现留为空白，也可以将其投掷NotImplementedException;它永远不会被称为。

您的情况听起来比这更复杂，但是您仍然应该能够操纵CanRead和CanWrite属性以实现所需的结果。以下是一个人为的示例，它显示了我们如何根据情境变量切换ReadJson和WriteJson方法。

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);
    }
}

输出：

--- 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"]}

小提琴：https：//dotnetfiddle.net/bdtson