parse.com:SerializationException 使用“__type"反序列化 JSON 对象；财产 [英] parse.com: SerializationException deserializing JSON objects with "__type" property

问题描述

我正在开发 Windows 10 UWP 应用，但似乎无法摆脱此错误:在 mscorlib.ni.dll 中发生了‘System.Runtime.Serialization.SerializationException’类型的异常，但未在用户代码中处理"

I'm developing a Windows 10 UWP app and can't seem to get rid of this error: "An exception of type 'System.Runtime.Serialization.SerializationException' occurred in mscorlib.ni.dll but was not handled in user code"

我正在使用 Rest API 从 Parse 上的数据存储检索值并实例化对象.这是我的班级的样子

I'm using the Rest API to retrieve values from a Data Store on Parse and instantiate objects. Here's what my Class looks like

public class ImageTest
{
    public class Image
    {
        public string __type { get; set; }
        public string name { get; set; }
        public string url { get; set; }
    }

    public class Result
    {
        public string createdAt { get; set; }
        public Image image { get; set; }
        public string name { get; set; }
        public string objectId { get; set; }
        public string updatedAt { get; set; }
    }

    public class RootObject
    {
        public List<Result> results { get; set; }
    }
}

这是我的 JSON 输出的样子:

Here's what my JSON output looks like:

{
"results": [
    {
        "createdAt": "2015-11-16T02:04:17.403Z",
        "image": {
            "__type": "File",
            "name": "stark.jpg",
            "url": "http://xyz.parse.com/stark.jpg"
        },
        "name": "Stark",
        "objectId": "2ypGrvkvg0",
        "updatedAt": "2015-11-16T02:04:23.121Z"
    },
    {
        "createdAt": "2015-11-16T02:04:31.409Z",
        "image": {
            "__type": "File",
            "name": "targaryen.jpg",
            "url": "http://xyz.parse.com/targaryen.jpg"
        },
        "name": "Targaryen",
        "objectId": "otgO3scX3k",
        "updatedAt": "2015-11-16T02:04:40.094Z"
    }
]
}

错误信息的详细信息如下:附加信息:元素:image"包含:File"数据协定的数据.解串器不知道映射到此合约的任何类型.将与文件"对应的类型添加到已知类型列表中 - 例如，通过使用 knownTypeAttribute 属性或将其添加到传递给 DataContractSerializer 的已知类型列表中.

The details of the error message are as follows: Additional information: Element ':image' contains data of the ':File' data contract. The deserializer has no knowledge of any type that maps to this contract. Add the type corresponding to 'File' to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding it to the list of known types passed to DataContractSerializer.

推荐答案

您的问题是您正在使用 DataContractJsonSerializer 反序列化你的 JSON，"__type" 是一个保留属性 用于此序列化程序.它用于标识多态类型的派生类型.来自文档:

Your problem is that you are using the DataContractJsonSerializer to deserialize your JSON, and "__type" is a reserved property for this serializer. It is used to identify derived types of polymorphic types. From the docs:

保留类型信息

为了保留类型标识，在将复杂类型序列化为 JSON 时，可以添加类型提示"，反序列化器会识别该提示并采取适当的行动.类型提示"是一个 JSON 键/值对，键名称为__type"(两个下划线后跟单词type").该值是DataContractName:DataContractNamespace"形式的 JSON 字符串(第一个冒号之前的任何内容都是名称)...

To preserve type identity, when serializing complex types to JSON a "type hint" can be added, and the deserializer recognizes the hint and acts appropriately. The "type hint" is a JSON key/value pair with the key name of "__type" (two underscores followed by the word "type"). The value is a JSON string of the form "DataContractName:DataContractNamespace" (anything up to the first colon is the name)...

类型提示与 XML 模式实例标准定义的 xsi:type 属性非常相似，在序列化/反序列化 XML 时使用.

The type hint is very similar to the xsi:type attribute defined by the XML Schema Instance standard and used when serializing/deserializing XML.

名为__type"的数据成员由于与类型提示的潜在冲突而被禁止.

因此您不能手动将此属性添加到您的类中并使其正确翻译.

Thus you cannot manually add this property to your class and have it translate correctly.

然而，您可以利用序列化程序对多态性的处理来"__type"自动，通过定义图像信息的类层次结构，您的Image 类型是预期类型的​​子类.为了清楚起见，让我们将其重命名为 FileImage:

You can, however, leverage the serializer's handling of polymorphism to read and write the "__type" automatically, by defining a class hierarchy of image information in which your Image type is a subclass of the expected type. Let's rename it to FileImage for clarity:

public class ImageTest
{
    [DataContract(Namespace = "")]
    [KnownType(typeof(FileImage))]
    public abstract class ImageBase
    {
    }

    [DataContract(Name = "File", Namespace = "")]
    public sealed class FileImage : ImageBase
    {
        [DataMember(Name = "name")]
        public string name { get; set; }
        [DataMember(Name = "url")]
        public string url { get; set; }
    }

    [DataContract(Namespace = "")]
    public class Result
    {
        [DataMember]
        public string createdAt { get; set; }

        [IgnoreDataMember]
        public FileImage image { get { return imageBase as FileImage; } set { imageBase = value; } }

        [DataMember(Name = "image")] // Need not be public if DataMember is applied.
        ImageBase imageBase { get; set; }

        [DataMember]
        public string name { get; set; }

        [DataMember]
        public string objectId { get; set; }

        [DataMember]
        public string updatedAt { get; set; }
    }

    public class RootObject
    {
        public List<Result> results { get; set; }
    }
}

现在一切正常.

如果稍后您发现服务器正在发送带有附加 "__type" 值和属性数据(例如嵌入的 Base64 图像)的 JSON 数据，您现在可以轻松修改您的数据模型以将附加子类添加到 ImageBase.

If later you find that server is sending JSON data with additional "__type" values and property data (e.g. an embedded Base64 image) you can now easily modify your data model to add additional subclasses to ImageBase.

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