使用DataContractSerializer和CollectionDataContract对对象引用进行反序列化

Question

一旦涉及对象引用，我似乎无法弄清楚如何正确使用CollectionDataContract属性。非常感谢任何帮助。警告:代码示例可能看起来有点长，但我还没有找到一种更短的解释方法，所以请耐心等待。(更新:今天我知道了foobar是邪恶的。我纠正了，所以我用一个更有意义的例子取代了所有对"foo"one_answers"bar"的引用。)

考虑一个假想的测验应用程序的简单抽认卡界面:

interface IFlashcard
{
    string Question {get; set;}
    string Answer {get; set;}
}

由Flashcard和FlippedFlashcard类实现。FlippedFlashcard的实例应该保持对Flashcard相应实例的引用，以便反转Question和Answer，因为一些卡可能在"危险模式"下工作:

[DataContract]
class Flashcard : IFlashcard
{
    [DataMember]
    public string Question { get; set; }
    [DataMember]
    public string Answer { get; set; }
}
class FlippedFlashcard : IFlashcard
{
    [DataMember]
    public Flashcard OriginalFlashcard;
    [DataMember]
    public string Question
    {
        get { return OriginalFlashcard.Answer; }
        set { OriginalFlashcard.Answer = value; }
    }
    [DataMember]
    public string Answer
    {
        get { return OriginalFlashcard.Question; }
        set { OriginalFlashcard.Question = value; }
    }
}

让我们定义一个符合IFlashcard的可序列化的对象集合…

[CollectionDataContract (ItemName = "Flashcard")]
class DeckOfFlashcards : List<IFlashcard>
{
}

…用一些抽认卡填满它:

Flashcard f1 = new Flashcard() { Question = "What do you get when you multiply 6 by 7?", Answer = "42" };
FlippedFlashcard f2 = new FlippedFlashcard { OriginalFlashcard = f1 }; 
DeckOfFlashcards myDeck = new DeckOfFlashcards();
myDeck.Add(f1);
myDeck.Add(f2);
f1.Question = "What is the answer to life, the universe and everything?";
Console.WriteLine(f2.Answer); // >> What is the answer to life, the universe and everything?

直到现在，一切都按照我想要的那样工作。当我将f1中的Question从"将6乘以7得到什么?"改为"生命，宇宙和一切的答案是什么?"时，f2中的Answer正确地反映了这一变化:

f1.Question = "What is the answer to life, the universe and everything?";
Console.WriteLine(f2.Answer); // >> What is the answer to life, the universe and everything?

我想序列化这个集合，并保持Flashcard和FlippedFlashcard实例之间的引用。下面是我的尝试:

List<Type> types = new List<Type> { typeof(Flashcard), typeof(FlippedFlashcard) };
DataContractSerializer dcs = new DataContractSerializer(typeof(DeckOfFlashcards), types, 100, false, true, null);
XmlWriterSettings settings = new XmlWriterSettings();
settings.Indent = true;
using (XmlWriter writer = XmlWriter.Create("Flashcards.xml", settings))
    dcs.WriteObject(writer, myDeck); // >> Question, Answer and OriginalFlashcard from f2 are serialized with "i:nil="true"

不幸的是，结果Xml文件将f2中的Question, Answer和OriginalFlashcard列为空:

<?xml version="1.0" encoding="utf-8"?>
<DeckOfFlashcards xmlns:i="http://www.w3.org/2001/XMLSchema-instance" z:Id="1" z:Size="2" xmlns:z="http://schemas.microsoft.com/2003/10/Serialization/" xmlns="http://schemas.datacontract.org/2004/07/Foobar">
  <Flashcard z:Id="2" i:type="Flashcard">
    <Answer z:Id="3">42</Answer>
    <Question z:Id="4">What is the answer to life, the universe and everything?</Question>
  </Flashcard>
  <Flashcard z:Id="5" i:type="FlippedFlashcard">
    <Answer z:Ref="4" i:nil="true" />
    <OriginalFlashcard z:Ref="2" i:nil="true" />
    <Question z:Ref="3" i:nil="true" />
  </Flashcard>
</DeckOfFlashcards>

当我试图从文件中反序列化集合时…

DeckOfFlashcards myOtherDeck;
using (var reader = XmlReader.Create("Flashcards.xml"))
    myOtherDeck = (DeckOfFlashcards)dcs.ReadObject(reader); // >> fires NullReferenceException at "set { OriginalFoo.Bar2 = value; }" with value "What is the answer to life, the universe and everything?"

…我得到一个NullReferenceException，可能是当FlippedFlashcard的Xml被击中。

有人知道吗?

Answer 1

明白了!

为了防止NullPointerException，有必要检查[DataContract]和[DataMember]属性。interface IFlashcard完全不需要合约:

interface IFlashcard
{
    string Question { get; set;  }
    string Answer { get; set; }
}

class Flashcard的代码保持不变，就像我在描述中写的那样。唯一真正必要的更改是从class FlippedFlashcard中的Question和Answer中删除 [DataMember]属性，如下所示:

[DataContract]
class FlippedFlashcard : IFlashcard
{
    [DataMember]
    public Flashcard OriginalFlashcard { get; set; }
    public string Question
    {
        get { return OriginalFlashcard.Answer; }
        set { OriginalFlashcard.Answer = value; }
    }
    public string Answer
    {
        get { return OriginalFlashcard.Question; }
        set { OriginalFlashcard.Question = value; }
    }
}

现在一切正常。: -)