如果空的枚举列表在xmlattribute中，则C#XmlSerializer会在反序列化空枚举列表时失败 [英] C# XmlSerializer fails on deserialization of an empty enum list if it's in an xmlattribute

问题描述

这是对我未来自我的记录，也是对他人有益的记录. 所描述的行为并不明显...

This is a note to my future self, and for the benefit of others. The described behavoir is NOT obvious...

我有一些C#:

public enum Choices 
{
  One,
  Two,
  Three,
}

public class Element 
{
  List<Choices> _allowedChoices;
  [XmlAttribute]
  public List<Choices> AllowedChoices
  {
    get {return _allowedChoices ?? ( _allowedChoices = new List<Choices>() );}
    set { _allowedChoices = value; }
  }
}

[Test]
public void testing_empty_enum_list_serialization()
{
    var ser = new XmlSerializer(typeof (Element));
    using (var sw = new StringWriter())
    {
        ser.Serialize(sw, new Element
        {
            AllowedChoices = {},
        });
        var text = sw.ToString();
        Console.WriteLine(text);
        using (var sr = new StringReader(text))
        {
            var deserialized  = (Element) ser.Deserialize(sr);
        }
    }
}

如果我使用XmlSerializer将其序列化为xml，则会得到:

If I use the XmlSerializer to serialize this to xml I get:

(请注意最后的空AllowedChoices属性)

( Note the empty AllowedChoices attribute at the end )

<?xml version="1.0" encoding="utf-16"?>
<Element xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" AllowedChoices="" />

如果我随后使用XmlSerializer对这个xml反序列化，则类似于:

If I then use the XmlSerializer to deserialize this xml I something like:

System.InvalidOperationException : There is an error in XML document (2, 109).
  ----> System.InvalidOperationException : Instance validation error: '' is not a valid value for Choices.

这是一个空的列表枚举，可以无错误地序列化，是的，请不要反序列化！

This is an empty list of enums that serializes without error, Y U NO deserialize!?

推荐答案

我发现了以下相关问题，这些问题由于预期的原因而失败，即枚举不具有空默认值，这是一个提示...

I've found these related questions, which fail for expected reasons, namely that an enum does not have a null default value, which is a clue...

反序列化枚举

XmlSerializer枚举反序列化在(不存在)空白上失败

这是解决方案:

如果AllowedChoices的实现是自动属性，并且未在构造函数中初始化(即当反序列化到达该属性时为空)，则它将按预期工作，并且不会在反序列化时爆炸.

If the implementation of AllowedChoices is an auto-property and is not initialized in the constructor (i.e. it's null when deserialization gets to that property) it works as expected, and does not bomb on deserialization.

我对源代码拥有完全的控制权，所以我要务实的是，使用[XmlEnum(")]属性将None值添加到我的Choices枚举中，如果唯一的值是空白，则将列表视为空而不是 not 进行列表的自动初始化.

I have full control over the source, so I'm going to be pragmatic and add an None value to my Choices enum with a [XmlEnum("")] attribute, and treat the list as empty if that's the only value in the list instead of not doing auto-initialization of the list.

请参见 http://tech.pro/blog/1370/why-collections-should-ever-never-null-null (为什么我想要那样).

See http://tech.pro/blog/1370/why-collections-should-never-be-null for why I want that.

奖金提示:

如果要创建枚举别名的空字符串，请执行以下操作:

If you want to create an empty-string of an enum alias do it like this:

public enum Choices
{
    [XmlEnum("default")]
    Default = 0,

    [XmlEnum("")]
    DefaultAlias = Default,
}

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