XmlAttributes.XmlDefaultValue Properti

Definisi

Ruang nama:

System.Xml.Serialization

Rakitan:

netstandard.dll, System.Xml.XmlSerializer.dll

Sumber:

XmlAttributes.cs

Mendapatkan atau mengatur nilai default elemen atau atribut XML.

public:
 property System::Object ^ XmlDefaultValue { System::Object ^ get(); void set(System::Object ^ value); };

public object? XmlDefaultValue { get; set; }

member this.XmlDefaultValue : obj with get, set

Public Property XmlDefaultValue As Object

Nilai Properti

Object

Yang Object mewakili nilai default elemen atau atribut XML.

Contoh

Contoh berikut menunjukkan kelas bernama Pet yang berisi bidang yang memiliki nilai default yang diatur ke "Anjing". Namun, contohnya juga membuat XmlAttributes objek, dan mengatur propertinya XmlDefaultValue ke nilai default baru ("Cat"). Ini mengambil alih nilai default asli. Dengan demikian, jika nilai bidang diatur ke "Cat", XmlSerializer memperlakukannya sebagai nilai default, dan tidak menserialisasikannya. Jika diatur ke nilai lain, nilai serialisasi XmlSerializer .

using System;
using System.IO;
using System.Xml;
using System.Xml.Serialization;
using System.ComponentModel;

// This is the class that will be serialized.
public class Pet
{
   // The default value for the Animal field is "Dog".
   [DefaultValueAttribute("Dog")]
   public string Animal ;
}

public class Run
{
   public static void Main()
   {
      Run test = new Run();
      test.SerializeObject("OverrideDefaultValue.xml");
      test.DeserializeObject("OverrideDefaultValue.xml");
   }

   // Return an XmlSerializer used for overriding.
   public XmlSerializer CreateOverrider()
   {
      // Create the XmlAttributeOverrides and XmlAttributes objects.
      XmlAttributeOverrides xOver = new XmlAttributeOverrides();
      XmlAttributes xAttrs = new XmlAttributes();

      // Add an override for the default value of the GroupName.
      Object defaultAnimal= "Cat";
      xAttrs.XmlDefaultValue = defaultAnimal;

      // Add all the XmlAttributes to the XmlAttributeOverrides object.
      xOver.Add(typeof(Pet), "Animal", xAttrs);

      // Create the XmlSerializer and return it.
      return new XmlSerializer(typeof(Pet), xOver);
   }

   public void SerializeObject(string filename)
   {
      // Create an instance of the XmlSerializer class.
      XmlSerializer mySerializer = CreateOverrider();

      // Writing the file requires a TextWriter.
      TextWriter writer = new StreamWriter(filename);

      // Create an instance of the class that will be serialized.
      Pet myPet = new Pet();

      /* Set the Animal property. If you set it to the default value,
         which is "Cat" (the value assigned to the XmlDefaultValue
         of the XmlAttributes object), no value will be serialized.
         If you change the value to any other value (including "Dog"),
         the value will be serialized.
      */
      // The default value "Cat" will be assigned (nothing serialized).
      myPet.Animal= "";
      // Uncommenting the next line also results in the default
      // value because Cat is the default value (not serialized).
      //  myPet.Animal = "Cat";

      // Uncomment the next line to see the value serialized:
      // myPet.Animal = "fish";
      // This will also be serialized because Dog is not the
      // default anymore.
      //  myPet.Animal = "Dog";
      // Serialize the class, and close the TextWriter.
      mySerializer.Serialize(writer, myPet);
      writer.Close();
   }

   public void DeserializeObject(string filename)
   {
      XmlSerializer mySerializer = CreateOverrider();
      FileStream fs = new FileStream(filename, FileMode.Open);
      Pet myPet= (Pet)mySerializer.Deserialize(fs);
      Console.WriteLine(myPet.Animal);
   }
}

Imports System.IO
Imports System.Xml
Imports System.Xml.Serialization
Imports System.ComponentModel


' This is the class that will be serialized. 
Public Class Pet
    ' The default value for the Animal field is "Dog". 
    <DefaultValueAttribute("Dog")> Public Animal As String 
End Class


Public Class Run
    
    Public Shared Sub Main()
        Dim test As New Run()
        test.SerializeObject("OverrideDefaultValue.xml")
        test.DeserializeObject("OverrideDefaultValue.xml")
    End Sub
    
    
    ' Return an XmlSerializer used for overriding. 
    Public Function CreateOverrider() As XmlSerializer
        ' Create the XmlAttributeOverrides and XmlAttributes objects. 
        Dim xOver As New XmlAttributeOverrides()
        Dim xAttrs As New XmlAttributes()
        
        ' Add an override for the default value of the GroupName. 
        Dim defaultName As Object = "Cat"
        xAttrs.XmlDefaultValue = defaultName
        
        ' Add all the XmlAttributes to the XmlAttributeOverrides object. 
        xOver.Add(GetType(Pet), "Animal", xAttrs)
        
        ' Create the XmlSerializer and return it.
        Return New XmlSerializer(GetType(Pet), xOver)
    End Function
    
    
    Public Sub SerializeObject(ByVal filename As String)
       ' Create an instance of the XmlSerializer class.
       Dim mySerializer As XmlSerializer = CreateOverrider()
        
       ' Writing the file requires a TextWriter.
       Dim writer As New StreamWriter(filename)
        
       ' Create an instance of the class that will be serialized. 
       Dim myPet As New Pet()
        
       ' Set the Animal property. If you set it to the default value,
       ' which is "Cat" (the value assigned to the XmlDefaultValue
       ' of the XmlAttributes object), no value will be serialized.
       ' If you change the value to any other value (including "Dog"),
       ' the value will be serialized.

      ' The default value "Cat" will be assigned (nothing serialized).
      myPet.Animal = ""
      ' Uncommenting the next line also results in the default 
      ' value because Cat is the default value (not serialized).
      '  myPet.Animal = "Cat"; 
      
      ' Uncomment the next line to see the value serialized:
      ' myPet.Animal = "fish";
      ' This will also be serialized because Dog is not the 
      ' default anymore.
      '  myPet.Animal = "Dog";
       ' Serialize the class, and close the TextWriter.
        mySerializer.Serialize(writer, myPet)
        writer.Close()
    End Sub
    
    
    Public Sub DeserializeObject(ByVal filename As String)
        Dim mySerializer As XmlSerializer = CreateOverrider()
        Dim fs As New FileStream(filename, FileMode.Open)
        Dim myPet As Pet = CType(mySerializer.Deserialize(fs), Pet)
        Console.WriteLine(myPet.Animal)
    End Sub
End Class

Keterangan

Anda dapat menentukan nilai default elemen XML atau atribut XML dengan menerapkan DefaultValueAttribute ke anggota. Untuk memeriksa hasil penerapan nilai, kompilasi aplikasi ke dll atau dapat dieksekusi, dan teruskan file yang dihasilkan sebagai argumen ke alat Definisi Skema XML (XSD.exe). Dalam dokumen skema XML, nilai default ditetapkan ke default atribut . Dalam contoh berikut, default untuk <elemen Hewan> adalah "Anjing."

<?xml version="1.0"?>
 <schema attributeFormDefault="qualified"
 elementFormDefault="qualified" targetNamespace=""
 xmlns="http://www.w3.org/2000/10/XMLSchema">
   <element name="Pets" nullable="true" type="Pets"/>
   <complexType name="Pets">
     <sequence>
       <element default="Dogs" name="Animal" nullable="true"
        type="string" minOccurs="0"/>
     </sequence>
   </complexType>
 </schema>

Untuk mengambil alih nilai default, buat Object dan tetapkan ke XmlDefaultValue.

Jika nilai yang ditetapkan ke bidang atau properti sama dengan nilai default untuk bidang atau properti tersebut XmlSerializer , nilai tidak diserialisasikan ke xml-instance. Ini karena nilai yang ditetapkan dapat dipulihkan dari skema XML. Dengan kata lain, mengatur bidang atau properti ke nilai defaultnya sendiri setara dengan tidak mengaturnya sama sekali. Demikian juga, jika tidak ada nilai yang diatur untuk bidang atau properti, XmlSerializer menggunakan nilai default yang ditemukan dalam skema. Dalam kedua kasus, (mengatur properti ke defaultnya, atau tidak mengaturnya sama sekali), instans XML-document tidak berisi nilai apa pun untuk properti .

Anda dapat menggunakan konstruktor kelas alih-alih skema untuk menetapkan nilai default. Jika Anda menggunakan Xsd.exe untuk menghasilkan kelas dari skema, Anda dapat mengomentari atau menghapus semua atribut [System.ComponentModel.DefaultValueAttribute("myFieldName")] dari file kelas.