XML 序列化参考 - 重复

class Author {
  public String id;
  public String name;
}
class Book {
  public String id;
  public Author author;
  public String title;
}

<store>
  <authors>
   <author id="PK">
     <name>Philip Kindred</name>
    </author>
  </authors>
  <books>
    <book id="u1">
      <author>PK</author> <!--  use only ID -->
      <title>Ubik</title>
    </book>
  </books>
</store>

您面临着在 XML 中表示聚合而不是组合关系的问题。当您保留父子关系（即组合）时，XML 序列化非常简单。在这种情况下，一本书有一个（或更多）作者，但不拥有它，因为一个作者可以是许多其他书籍的作者。

在这种情况下，您可以执行类似于在数据库中执行的操作，即具有两个单独的条目并通过外键表示关系。请参阅以下示例：

[Serializable]
public class Author
{
    [XmlAttribute("id")]
    public String Id { get; set; }
    [XmlElement("name")]
    public String Name { get; set; }
}
[Serializable]
public class Book
{
    private Author _author;
    [XmlIgnore]
    public Author Author
    {
        get { return _author; }
        set
        {
            _author = value;
            AuthorId = _author != null ? _author.Id : null;
        }
    }
    [XmlAttribute("id")]
    public String Id { get; set; }
    [XmlElement("author")]
    public String AuthorId { get; set; }
     [XmlElement("title")]
    public String Title { get; set; }
}
[Serializable]
public class Store
{
    [XmlArray("authors")]
    [XmlArrayItem("author", Type = typeof(Author))]
    public List<Author> Authors { get; set; }
    [XmlArray("books")]
    [XmlArrayItem("book", Type = typeof(Book))]
    public List<Book> Books { get; set; }
    public Store()
    {
        Books = new List<Book>();
        Authors = new List<Author>();
    }
}

internal class Program
{
    private static void Main(string[] args)
    {
        // Create some authors
        var authors = new List<Author>
        {
            new Author{Id="PK", Name="Philip Kindred"},
            new Author{Id="WS", Name="William Shakespeare"},
        };
        // Create some books linked to the authors
        var books = new List<Book>
        {
            new Book{Id = "U1", Author = authors[0], Title = "Do Androids Dream of Electric Sheep?"},
            new Book{Id = "U2", Author = authors[1], Title = "Romeo and Juliet"}
        };
        var store = new Store {Authors = authors, Books = books};
        var success = Serialiser.SerialiseToXml(store, "store.xml");
        // Deserialize the data from XML
        var store2 = Serialiser.DeserialseFromXml<Store>("store.xml");
        // Resolve the actual Author instances from the saved IDs (foreign key equivalent in databases)
        foreach (var book in store2.Books)
            book.Author = store2.Authors.FirstOrDefault(author => author.Id == book.AuthorId);
        // Now variable 'store' and 'store2' have the same equivalent data
    }
}
// Helper class to serialize and deserialize the data to XML file
public static class Serialiser
{
    public static bool SerialiseToXml(object obj, string filename)
    {
        try
        {
            var ws = new XmlWriterSettings
            {
                NewLineHandling = NewLineHandling.Entitize,
                NewLineChars = Environment.NewLine,
                Indent = true,
                NewLineOnAttributes = false
            };
            var xs = new XmlSerializer(obj.GetType());
            using (var writer = XmlWriter.Create(filename, ws))
                xs.Serialize(writer, obj);
            return true;
        }
        catch(Exception ex)
        {
            return false;
        }
    }
    public static T DeserialseFromXml<T>(string filename) where T : new()
    {
        var typeofT = typeof(T);
        try
        {
            var xs = new XmlSerializer(typeofT);
            using (var reader = XmlReader.Create(filename))
                return (T)xs.Deserialize(reader);
        }
        catch(Exception ex)
        {
            return default(T);
        }
    }
}

"商店.xml"将如下所示：

<?xml version="1.0" encoding="utf-8"?>
<Store xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<authors>
    <author id="PK">
    <name>Philip Kindred</name>
    </author>
    <author id="WS">
    <name>William Shakespeare</name>
    </author>
</authors>
<books>
    <book id="U1">
    <author>PK</author>
    <title>Do Androids Dream of Electric Sheep?</title>
    </book>
    <book id="U2">
    <author>WS</author>
    <title>Romeo and Juliet</title>
    </book>
</books>
</Store>

C# 答案

如果您有一些较大的容器类可以管理对象之间的交叉引用，则可以执行此操作。 在您的情况下，您似乎有一个可以达到此目的的Store对象。 Store按姓名维护书籍和作者的词典;Book会记住其作者的id，并根据需要从商店中获取实际Author。 当然，这需要Author和Book都知道它们所在的商店。

示例实现可能如下所示：

public class Author
{
    string id;
    Store store;
    [XmlIgnore]
    public Store Store {
        get {
            return store;
        }
        set {
            if (store != null && id != null)
                store.Authors.Remove(id);
            this.store = value;
            if (store != null && id != null)
                store.Authors[id] = this;
        }
    }
    [XmlAttribute("id")]
    public string Id
    {
        get
        {
            return id;
        }
        set
        {
            if (store != null && id != null)
                store.Authors.Remove(id);
            this.id = value;
            if (store != null && id != null)
                store.Authors[id] = this;
        }
    }
    [XmlElement("name")]
    public string Name { get; set; }
}
public class Book
{
    string authorId;
    string id;
    Store store;
    [XmlIgnore]
    public Store Store
    {
        get
        {
            return store;
        }
        set
        {
            if (store != null && id != null)
                store.Books.Remove(id);
            this.store = value;
            if (store != null && id != null)
                store.Books[id] = this;
        }
    }
    [XmlAttribute("id")]
    public string Id
    {
        get
        {
            return id;
        }
        set
        {
            if (store != null && id != null)
                store.Books.Remove(id);
            this.id = value;
            if (store != null && id != null)
                store.Books[id] = this;
        }
    }
    [XmlElement("author")]
    public string AuthorID
    {
        get
        {
            return authorId;
        }
        set
        {
            authorId = value;
        }
    }
    [XmlIgnore]
    public Author Author
    {
        get
        {
            if (store == null)
                return null;
            if (AuthorID == null)
                return null;
            return store.Authors[AuthorID];
        }
        set
        {
            if (value == Author)
                return;
            if (value == null)
            {
                authorId = null;
            }
            else
            {
                if (value.Id == null)
                    throw new ArgumentException();
                authorId = value.Id;
            }
            AssertCorrectAuthor(value);
        }
    }
    [Conditional("DEBUG")]
    private void AssertCorrectAuthor(Author author)
    {
        if (store != null)
            Debug.Assert(author == Author);
    }
    [XmlElement("title")]
    public string Title { get; set; }
}
[XmlRoot("store")]
public class Store
{
    readonly Dictionary<string, Book> books = new Dictionary<string, Book>();
    readonly Dictionary<string, Author> authors = new Dictionary<string, Author>();
    [XmlIgnore]
    public IDictionary<string, Book> Books
    {
        get
        {
            return books;
        }
    }
    [XmlIgnore]
    public IDictionary<string, Author> Authors
    {
        get
        {
            return authors;
        }
    }
    [Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    [XmlArray("authors")]
    [XmlArrayItem("author")]
    public Author[] AuthorList // proxy array for serialization.
    {
        get
        {
            return Authors.Values.ToArray();
        }
        set
        {
            foreach (var author in authors.Values)
            {
                author.Store = null;
            }
            Authors.Clear();
            if (value == null)
                return;
            foreach (var author in value)
            {
                author.Store = this;
            }
        }
    }
    [Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    [XmlArray("books")]
    [XmlArrayItem("book")]
    public Book[] BookList // proxy array for serialization.
    {
        get
        {
            return Books.Values.ToArray();
        }
        set
        {
            foreach (var book in Books.Values)
            {
                book.Store = null;
            }
            Books.Clear();
            if (value == null)
                return;
            foreach (var book in value)
            {
                book.Store = this;
            }
        }
    }
}

并且，要测试：

public static class TestStore
{
    public static void Test()
    {
        string xml = @"<?xml version=""1.0"" encoding=""UTF-8""?>
<store>
  <authors>
   <author id=""PK"">
     <name>Philip Kindred</name>
    </author>
  </authors>
  <books>
    <book id=""u1"">
      <author>PK</author> <!--  use only ID -->
      <title>Ubik</title>
    </book>
    <book id=""t1"">
      <author>PK</author> <!--  use only ID -->
      <title>The Transmigration of Timothy Archer</title>
    </book>
  </books>
</store>
";
        var store = xml.LoadFromXML<Store>();
        Debug.Assert(store.BookList[0].Author == store.AuthorList[0]); // no assert
        Debug.Assert(store.BookList[1].Author == store.AuthorList[0]); // no assert; verify that all books use the same instance of the `Author` class.
    }
}