如何在反序列化期间以编程方式选择构造函数?

问题描述

我想反序列化一个 System.Security.Claims.Claim 对象，通过以下方式序列化:

I would like to deserialize a System.Security.Claims.Claim object serialized in the following way:

{
    "Issuer" : "LOCAL AUTHORITY",
    "OriginalIssuer" : "LOCAL AUTHORITY",
    "Type" : "http://my.org/ws/2015/01/identity/claims/mytype",
    "Value" : "myvalue",
    "ValueType" : "http://www.w3.org/2001/XMLSchema#string"
}

我得到的是一个 JsonSerializationException:

找不到用于类型的构造函数System.Security.Claims.Claim.一个类应该有一个默认值构造函数，一个带参数的构造函数或一个标记的构造函数带有 JsonConstructor 属性.

Unable to find a constructor to use for type System.Security.Claims.Claim. A class should either have a default constructor, one constructor with arguments or a constructor marked with the JsonConstructor attribute.

经过一番调查，我终于明白了上述消息中 one 的含义:JSON 反序列化程序无法找到正确的构造函数 - 在 Claim 的情况下type - 多个带参数的构造函数(尽管存在一个参数与上述属性完全匹配的构造函数).

After some investigation I finally understand the meaning of one in the above message: The JSON deserializer cannot find the right constructor as there are - in the case of the Claim type - multiple constructors with arguments (although there exists a constructor with arguments matching exactly the above properties).

有没有办法告诉反序列化器选择哪个构造函数而不向该 mscorlib 类型添加 JsonConstructor 属性?

Is there a way to tell the deserializer which constructor to choose without adding the JsonConstructor attribute to that mscorlib type?

Daniel Halan 用 Json 补丁解决了这个问题.NET 几年前.这些天有没有办法在不修改 Json.NET 的情况下解决这个问题?

Daniel Halan has solved this issue with a patch to Json.NET a few years ago. Is there a way to solve this without modifying Json.NET these days?

推荐答案

如果无法为目标类添加 [JsonConstructor] 属性(因为你不拥有代码)，那么通常的解决方法是创建一个自定义 JsonConverter 正如@James Thorpe 在评论中所建议的那样.这很简单.您可以将 JSON 加载到 JObject 中，然后从中挑选各个属性来实例化您的 Claim 实例.这是您需要的代码:

If it is not possible to add a [JsonConstructor] attribute to the target class (because you don't own the code), then the usual workaround is to create a custom JsonConverter as was suggested by @James Thorpe in the comments. It is pretty straightforward. You can load the JSON into a JObject, then pick the individual properties out of it to instantiate your Claim instance. Here is the code you would need:

class ClaimConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(System.Security.Claims.Claim));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JObject jo = JObject.Load(reader);
        string type = (string)jo["Type"];
        string value = (string)jo["Value"];
        string valueType = (string)jo["ValueType"];
        string issuer = (string)jo["Issuer"];
        string originalIssuer = (string)jo["OriginalIssuer"];
        return new Claim(type, value, valueType, issuer, originalIssuer);
    }

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

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

要使用转换器，只需将它的一个实例传递给 JsonConvert.DeserializeObject() 方法调用:

To use the converter, simply pass an instance of it to the JsonConvert.DeserializeObject<T>() method call:

Claim claim = JsonConvert.DeserializeObject<Claim>(json, new ClaimConverter());

小提琴:https://dotnetfiddle.net/7LjgGR

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