How to: Control Serialization of Derived Classes

 

Using the XmlElementAttribute attribute to change the name of an XML element is not the only way to customize object serialization. You can also customize the XML stream by deriving from an existing class and instructing the XmlSerializer instance how to serialize the new class.

For example, given a Book class, you can derive from it and create an ExpandedBook class that has a few more properties. However, you must instruct the XmlSerializer to accept the derived type when serializing or deserializing. This can be done by creating a XmlElementAttribute instance and setting its Type property to the derived class type. Add the XmlElementAttribute to a XmlAttributes instance. Then add the XmlAttributes to a XmlAttributeOverrides instance, specifying the type being overridden and the name of the member that accepts the derived class. This is shown in the following example.

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 Expanded 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 Expanded 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  

XmlSerializer
XmlElementAttribute
XmlAttributes
XmlAttributeOverrides
XML and SOAP Serialization
How to: Serialize an Object
How to: Specify an Alternate Element Name for an XML Stream

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