在 Json.net 反序列化中强制构造函数参数值

Question

是否可以强制 Json.net 为构造函数参数传递默认值？

我尝试使用自定义 ContractResolver 覆盖 CreateConstructorParameters 和 CreatePropertyFromConstructorParameter 方法并将返回的 JsonProperty.DefaultValue 属性设置为我想要的值，并且DefaultValueHandling 属性到 DefaultValueHandling.Populate。

通过覆盖 CreateObjectContract 方法，我可以看到匹配的 CreatorParameters 和 Properties 都包含我的默认值。但是，我仍然得到传递给构造函数的 null 值。

我什至尝试将属性上的 Converter 设置为仅 returns 所需默认值的转换器，但仍然没有成功。

ConstractResolver代码如下：

public class DataControllerContractResolver : DefaultContractResolver
{
    ///The desired default value
    private readonly IDataController _dataController;

    private readonly Type _dataControllerType;
    private readonly JsonConverter<IDataController> _converter;

    public DataControllerContractResolver(IDataController dataController)
    {
        _dataController = dataController;
        _dataControllerType = dataController.GetType();
        _converter = SimpleJsonConverter<IDataController>.From((reader, controller, arg3) => _dataController, (writer, controller, arg3) => {});
    }

    protected override IList<JsonProperty> CreateConstructorParameters(ConstructorInfo constructor, JsonPropertyCollection memberProperties)
    {
        var props = base.CreateConstructorParameters(constructor, memberProperties);

        foreach (var prop in memberProperties.Where(p => p.PropertyType.IsAssignableFrom(_dataControllerType)).ToList())
        {
            prop.DefaultValue = _dataController;
            prop.Converter = _converter;
            prop.DefaultValueHandling = DefaultValueHandling.Populate;
            prop.Ignored = false;
        }

        foreach (var prop in props.Where(p => p.PropertyType.IsAssignableFrom(_dataControllerType)).ToList())
        {
            prop.DefaultValue = _dataController;
            prop.Converter = _converter;
            prop.DefaultValueHandling = DefaultValueHandling.Populate;
            prop.Ignored = false;
        }


        return props;
    }

    protected override JsonProperty CreatePropertyFromConstructorParameter(JsonProperty matchingMemberProperty, ParameterInfo parameterInfo)
    {
        var prop = base.CreatePropertyFromConstructorParameter(matchingMemberProperty, parameterInfo);

        if (prop.PropertyType.IsAssignableFrom(_dataControllerType))
        {
            prop.DefaultValue = _dataController;
            prop.Converter = _converter;
            prop.MemberConverter = _converter;
            prop.DefaultValueHandling = DefaultValueHandling.Populate;
            prop.Ignored = false;
        }

        return prop;
    }

    protected override JsonObjectContract CreateObjectContract(Type objectType)
    {
        var contract = base.CreateObjectContract(objectType);
        return contract;
    }
}

Answer 1

如果我覆盖 property.DefaultValue and property.DefaultValueHandling in CreateProperties(Type type, MemberSerialization memberSerialization)，我可以完成这项工作。我没有你所有的 classes（没有 IDataController），所以这里有一个简单的例子：

public class DefaultStringValueContractResolver : DefaultContractResolver
{
    public string DefaultStringValue { get; set; }

    public DefaultStringValueContractResolver(string defaultStringValue)
    {
        this.DefaultStringValue = defaultStringValue;
    }

    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
    {
        var properties = base.CreateProperties(type, memberSerialization);
        // Set all string properties to have a default value of "this is a default value"
        foreach (var property in properties.Where(p => p.PropertyType == typeof(string)))
        {
            property.DefaultValue = DefaultStringValue;
            property.DefaultValueHandling = DefaultValueHandling.Populate;
        }
        return properties;
    }
}

然后：

public class TestClass
{
    const string DefaultStringValue = "This is a default string value";

    public string Property1 { get; set; }

    public string Property2 { get; set; }

    public static void Test()
    {
        var settings = new JsonSerializerSettings { ContractResolver = new DefaultStringValueContractResolver(DefaultStringValue) };
        var test = JsonConvert.DeserializeObject<TestClass>("{}", settings);
        Debug.Assert(test.Property1 == DefaultStringValue && test.Property2 == DefaultStringValue); // No assert
        Debug.WriteLine(JsonConvert.SerializeObject(test)); // Prints {"Property1":"This is a default string value","Property2":"This is a default string value"}
    }
}

需要注意的一件事：如果默认值是引用类型，则包含属性的 class 的所有实例都将具有引用相同默认值实例的默认值。

Answer 2

在找不到针对此问题的内置解决方案后，我全力以赴并添加了默认值检查和 pull-request on Github. JamesNK (the Json.net creator) implemented the solution 略有不同，但它在最新版本中可用。

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