Procedura: controllare la serializzazione delle classi derivate
L'utilizzo dell'attributo XmlElementAttribute per modificare il nome di un elemento XML non rappresenta l'unico modo per personalizzare la serializzazione dell'oggetto. È anche possibile personalizzare il flusso XML derivando da una classe esistente e istruendo l'istanza XmlSerializer su come serializzare la nuova classe.
Ad esempio, in presenza di una classe Book, è possibile derivare da essa e creare una classe ExpandedBook che disponga di alcune proprietà in più. Tuttavia, è necessario istruire XmlSerializer ad accettare il tipo derivato durante la serializzazione o la deserializzazione. È possibile ottenere questo risultato creando un'istanza di XmlElementAttribute e impostandone la proprietà Type sul tipo di classe derivato. Aggiungere XmlElementAttribute a un'istanza di XmlAttributes. Quindi aggiungere XmlAttributes a un'istanza di XmlAttributeOverrides, specificando il tipo da sottoporre a override e il nome del membro che accetta la classe derivata, come illustrato nell'esempio riportato di seguito.
Esempio
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
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 Expanded 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 Expanded 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);
}
}
}
Vedere anche
Attività
Procedura: serializzare un oggetto
Procedura: specificare un nome di elemento alternativo per un flusso XML
Riferimento
XmlSerializer
XmlElementAttribute
XmlAttributes
XmlAttributeOverrides
Altre risorse
Data di compilazione: 2010-03-10