XmlAttributes.XmlDefaultValue Property


Gets or sets the default value of an XML element or attribute.

Namespace:   System.Xml.Serialization
Assembly:  System.Xml (in System.Xml.dll)

Public Property XmlDefaultValue As Object

Property Value

Type: System.Object

An Object that represents the default value of an XML element or attribute.

You can specify the default value of an XML element or XML attribute by applying a DefaultValueAttribute to a member. To examine the result of applying the value, compile the application into a DLL or executable, and pass the resulting file as an argument to the XML Schema Definition tool (XSD.exe). In the XML schema document, a default value is assigned to the default attribute. In the following example, the default for the <Animal> element is "Dogs."

<?xml version="1.0"?>
 <schema attributeFormDefault="qualified" 
 elementFormDefault="qualified" targetNamespace="" 
   <element name="Pets" nullable="true" type="Pets"/>
   <complexType name="Pets">
       <element default="Dogs" name="Animal" nullable="true" 
        type="string" minOccurs="0"/>

To override the default value, create an Object and assign it to the XmlDefaultValue.

If the value assigned to a field or property is equal to the default value for that field or property, the XmlSerializer does not serialize the value to the XML-instance. This is because the assigned value can be recovered from the XML schema. In other words, setting a field or property to its own default value is equivalent of not setting it at all. Likewise, if no value is set for the field or property, the XmlSerializer uses the default value found in the schema. In both cases, (setting the property to its default, or not setting it at all), the XML-document instance does not contain any value for the property.

You can use the class constructors instead of the schema to assign the default values. If you are using Xsd.exe to generate a classes from schemas, you can comment out or remove all of the [System.ComponentModel.DefaultValueAttribute("myFieldName")] attributes from the class files.

The following example shows a class named Pet that contains a field that has a default value set to "Dog". However, the example also creates an XmlAttributes object, and sets its XmlDefaultValue property to a new default value ("Cat"). This overrides the original default value. Thus, if the field value is set to "Cat", the XmlSerializer treats it as the default value, and not serialize it. If it is set to any other value, the XmlSerializer serializes the value.

Imports System
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()
    End Sub 'Main

    ' 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)
    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)
    End Sub
End Class

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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