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Vorgehensweise: Steuern der Serialisierung abgeleiteter Klassen

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Neben der Verwendung des XmlElementAttribute-Attributs zum Ändern eines XML-Elementnamens lässt sich die Objektserialisierung auch auf andere Weise anpassen. Sie können den XML-Stream ebenfalls anpassen, indem Sie von einer vorhandenen Klasse ableiten und die XmlSerializer-Instanz anweisen, wie die neue Klasse serialisiert werden soll.

Beispielsweise können Sie im Fall einer Book-Klasse von dieser Klasse ableiten und eine ExpandedBook-Klasse erstellen, die über zusätzliche Eigenschaften verfügt. Sie müssen XmlSerializer jedoch anweisen, den abgeleiteten Typ beim Serialisieren oder Deserialisieren zu akzeptieren. Erstellen Sie hierzu eine XmlElementAttribute-Instanz, und legen Sie deren Typ-Eigenschaft auf den Typ der abgeleiteten Klasse fest. Fügen Sie XmlElementAttribute zu einer XmlAttributes-Instanz hinzu. Fügen Sie dann XmlAttributes zu einer XmlAttributeOverrides-Instanz hinzu, und geben Sie dabei sowohl den Typ des zu überschreibenden Objekts als auch den Namen des Members an, der die abgeleitete Klasse akzeptiert. Dies wird im folgenden Beispiel gezeigt.

Beispiel

Public Class Orders  
    public Books() As Book  
End Class
  
Public Class Book  
    public ISBN As String
End Class  
  
Public Class ExpandedBook  
    Inherits Book  
    public NewEdition As Boolean  
End Class  
  
Public Class Run  
    Shared Sub Main()  
        Dim t As Run = New Run()  
        t.SerializeObject("Book.xml")  
        t.DeserializeObject("Book.xml")  
    End Sub  
  
    Public Sub SerializeObject(filename As String)  
        ' Each overridden field, property, or type requires
        ' an XmlAttributes instance.
        Dim attrs As XmlAttributes = New XmlAttributes()  
  
        ' Creates an XmlElementAttribute instance to override the
        ' field that returns Book objects. The overridden field  
        ' returns ExpandedBook objects instead.  
        Dim attr As XmlElementAttribute = _  
        New XmlElementAttribute()  
        attr.ElementName = "NewBook"  
        attr.Type = GetType(ExpandedBook)  
  
        ' Adds the element to the collection of elements.  
        attrs.XmlElements.Add(attr)  
  
        ' Creates the XmlAttributeOverrides.  
        Dim attrOverrides As XmlAttributeOverrides = _  
        New XmlAttributeOverrides()  
  
        ' Adds the type of the class that contains the overridden
        ' member, as well as the XmlAttributes instance to override it
        ' with, to the XmlAttributeOverrides instance.
        attrOverrides.Add(GetType(Orders), "Books", attrs)  
  
        ' Creates the XmlSerializer using the XmlAttributeOverrides.  
        Dim s As XmlSerializer  = _  
        New XmlSerializer(GetType(Orders), attrOverrides)  
  
        ' Writing the file requires a TextWriter instance.  
        Dim writer As TextWriter = New StreamWriter(filename)  
  
        ' Creates the object to be serialized.  
        Dim myOrders As Orders = New Orders()  
  
        ' Creates an object of the derived type.  
        Dim b As ExpandedBook = New ExpandedBook()  
        b.ISBN= "123456789"  
        b.NewEdition = True  
        myOrders.Books = New ExpandedBook(){b}  
  
        ' Serializes the object.  
        s.Serialize(writer,myOrders)  
        writer.Close()  
    End Sub  
  
    Public Sub DeserializeObject(filename As String)  
        Dim attrOverrides As XmlAttributeOverrides = _  
        New XmlAttributeOverrides()  
        Dim attrs As XmlAttributes = New XmlAttributes()  
  
        ' Creates an XmlElementAttribute to override the
        ' field that returns Book objects. The overridden field  
        ' returns ExpandedBook objects instead.
        Dim attr As XmlElementAttribute = _  
        New XmlElementAttribute()  
        attr.ElementName = "NewBook"  
        attr.Type = GetType(ExpandedBook)  
  
        ' Adds the XmlElementAttribute to the collection of objects.  
        attrs.XmlElements.Add(attr)  
  
        attrOverrides.Add(GetType(Orders), "Books", attrs)  
  
        ' Creates the XmlSerializer using the XmlAttributeOverrides.  
        Dim s As XmlSerializer = _  
        New XmlSerializer(GetType(Orders), attrOverrides)  
  
        Dim fs As FileStream = New FileStream(filename, FileMode.Open)  
        Dim myOrders As Orders = CType( s.Deserialize(fs), Orders)  
        Console.WriteLine("ExpandedBook:")  
  
        ' The difference between deserializing the overridden
        ' XML document and serializing it is this: To read the derived
        ' object values, you must declare an object of the derived type
        ' and cast the returned object to it.
        Dim expanded As ExpandedBook
        Dim b As Book  
        for each b  in myOrders.Books  
            expanded = CType(b, ExpandedBook)  
            Console.WriteLine(expanded.ISBN)  
            Console.WriteLine(expanded.NewEdition)  
        Next  
    End Sub  
End Class  

public class Orders  
{  
    public Book[] Books;  
}
  
public class Book  
{  
    public string ISBN;  
}  
  
public class ExpandedBook:Book  
{  
    public bool NewEdition;  
}  
  
public class Run  
{  
    public void SerializeObject(string filename)  
    {  
        // Each overridden field, property, or type requires
        // an XmlAttributes instance.  
        XmlAttributes attrs = new XmlAttributes();  
  
        // Creates an XmlElementAttribute instance to override the
        // field that returns Book objects. The overridden field  
        // returns ExpandedBook objects instead.  
        XmlElementAttribute attr = new XmlElementAttribute();  
        attr.ElementName = "NewBook";  
        attr.Type = typeof(ExpandedBook);  
  
        // Adds the element to the collection of elements.  
        attrs.XmlElements.Add(attr);  
  
        // Creates the XmlAttributeOverrides instance.  
        XmlAttributeOverrides attrOverrides = new XmlAttributeOverrides();  
  
        // Adds the type of the class that contains the overridden
        // member, as well as the XmlAttributes instance to override it
        // with, to the XmlAttributeOverrides.  
        attrOverrides.Add(typeof(Orders), "Books", attrs);  
  
        // Creates the XmlSerializer using the XmlAttributeOverrides.  
        XmlSerializer s =
        new XmlSerializer(typeof(Orders), attrOverrides);  
  
        // Writing the file requires a TextWriter instance.  
        TextWriter writer = new StreamWriter(filename);  
  
        // Creates the object to be serialized.  
        Orders myOrders = new Orders();  
  
        // Creates an object of the derived type.  
        ExpandedBook b = new ExpandedBook();  
        b.ISBN= "123456789";  
        b.NewEdition = true;  
        myOrders.Books = new ExpandedBook[]{b};  
  
        // Serializes the object.  
        s.Serialize(writer,myOrders);  
        writer.Close();  
    }  
  
    public void DeserializeObject(string filename)  
    {  
        XmlAttributeOverrides attrOverrides =
            new XmlAttributeOverrides();  
        XmlAttributes attrs = new XmlAttributes();  
  
        // Creates an XmlElementAttribute to override the
        // field that returns Book objects. The overridden field  
        // returns ExpandedBook objects instead.  
        XmlElementAttribute attr = new XmlElementAttribute();  
        attr.ElementName = "NewBook";  
        attr.Type = typeof(ExpandedBook);  
  
        // Adds the XmlElementAttribute to the collection of objects.  
        attrs.XmlElements.Add(attr);  
  
        attrOverrides.Add(typeof(Orders), "Books", attrs);  
  
        // Creates the XmlSerializer using the XmlAttributeOverrides.  
        XmlSerializer s =
        new XmlSerializer(typeof(Orders), attrOverrides);  
  
        FileStream fs = new FileStream(filename, FileMode.Open);  
        Orders myOrders = (Orders) s.Deserialize(fs);  
        Console.WriteLine("ExpandedBook:");  
  
        // The difference between deserializing the overridden
        // XML document and serializing it is this: To read the derived
        // object values, you must declare an object of the derived type
        // and cast the returned object to it.  
        ExpandedBook expanded;  
        foreach(Book b in myOrders.Books)
        {  
            expanded = (ExpandedBook)b;  
            Console.WriteLine(  
            expanded.ISBN + "\n" +
            expanded.NewEdition);  
        }  
    }  
}

Siehe auch