XML解析器是否区分了xsi：nil =" true"并省略了元素？ [英] Do XML parsers tell the difference between xsi:nil="true" and omitted elements?

问题描述

XML解析器/反序列化器通常能够区分 nillable elements显式设置为null和省略的可选元素？

Are XML parsers/deserializers in general able to tell the difference between nillable elements explicitly set to null and optional elements that are left out?

假设我们有以下复杂类型：

Assume that we have the following complex type:

<complexType name="NiceType">
  <sequence>
    <element name="niceElem" nillable="true" type="int" minOccurs="0" />
  </sequence>
</complexType>

元素显式设置为null（示例1）：

Element explicitly set to null (example 1):

<niceType>
  <niceElem xsi:nil="true"/> 
</niceType>

省略元素（示例2）：

<niceType>
</niceType>

一般是解析器，例如JAX-B实现或.NET等，例如XML模块WCF，能够分辨出上面的示例1和示例2之间的区别吗？换句话说，您是否能够以可互操作的方式组合两个NULL表示 - 如示例中所示 - 以便传达不同的NULL阴影？

Would parsers in general, such as JAX-B implementations or .NET alikes such as the XML module of WCF, be able to tell the difference between example 1 and example 2 above? In other words, would you in an interoperable manner be able to combine both NULL representations - as in the example - in order to communicate different shades of NULL?

推荐答案

XML解析器（例如 XmlReader ， XmlDocument ， XDocument ）不要特意处理 xsi：nil - 你仍然会看到流/文档中的元素。

XML parsers (e.g. XmlReader, XmlDocument, XDocument) don't treat xsi:nil specially - you still see the element in the stream/document.

XmlSerializer 确实处理 xsi：nil ：在该上下文中它意味着与省略的节点相同;您可以使用 XmlSerializer 使用 XmlSerializerFormatterAttribute DataContract 来使WCF序列化$ c>。

XmlSerializer does handle xsi:nil: within that context it means the same thing as an omitted node; you can make WCF serialize using XmlSerializer by marking your DataContracts using XmlSerializerFormatterAttribute.

DataContractSerializer 确实使用了该属性：但是我不确定它的所有规则是什么使用它们（一个案例是循环引用） - 它更可能省略元素。我不认为您应该将 xsi：nil 传递给 DataContractSerializer ，除非它在这种情况下使用它 - 如 DataContractSerializer 是围绕假设设计的，以提高de /序列化性能。

DataContractSerializer does use the attribute: however I am not sure what all the rules are for it to use them (one case is circular references) - it is much more likely to omit elements. I don't think you should pass xsi:nil to DataContractSerializer unless it uses it in that case - as DataContractSerializer is designed around assumptions to improve de/serialization performance.

来自规范看起来它原本设计为像JavaScript一样工作 null 和 undefined - 其中 null （ xsi：nil ）是一个有效值， undefined （省略）是一个值的完整不存在;特别是复杂类型 - 你可以提供元素但省略它的内容（即使根据模式需要内容）。

From the spec it looks like it was originally designed to work like the JavaScript null and undefined - where null (xsi:nil) is a valid value and undefined (omitted) is the entire non-existence of a value; especially with complex types - you can provide the element but omit it's content (even if the content is required according to the schema).

一般情况下我会避免它。这是非直观的 - 我认为我没有看到使用它的REST / SOAP API（除了专门使用它的InfoPath）;大多数只使用 null = undefined 。 xmlns声明和它的用法也会占用一些额外的宝贵字节。

In general I would avoid it. It's non-intuitive - I don't think I have seen a REST/SOAP API out there that uses it (except InfoPath which uses it exclusively); most just use null = undefined. The xmlns declaration and usage of it also eat a few extra valuable bytes.

奖励标记：如果你使一个元素可选而且它不是'可以为空（例如 xsd：int ）C＃生成器提供< Name>指定的属性 - 你可以添加你的拥有这样的属性。这将允许您区分 xsi：nil 和省略（指定时为nil，null，未指定时省略）。但是，这仅适用于 XmlSerializer 。

Bonus Marks: If you make an element optional and it isn't nullable (e.g. xsd:int) the C# generator provides a <Name>Specified property - you can add your own properties like this. This would allow you to differentiate between xsi:nil and omittance (nil when specified and null, omitted when not specified). However, this only works with XmlSerializer.

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