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XmlNamespaceDeclarationsAttribute Class

Specifies that the target property, parameter, return value, or class member contains prefixes associated with namespaces that are used within an XML document.

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

[AttributeUsageAttribute(AttributeTargets.Property|AttributeTargets.Field|AttributeTargets.Parameter|AttributeTargets.ReturnValue, AllowMultiple = false)]
public class XmlNamespaceDeclarationsAttribute : Attribute

The XmlNamespaceDeclarationsAttribute attribute can only be applied once in a class to a field or property that returns an XmlSerializerNamespaces object.

The XmlNamespaceDeclarationsAttribute allows you to store the prefixes, and the associated namespaces, used in an XML document. For example, one common usage of the attribute is to store XPath data, as it is defined by the World Wide Web Consortium ( document named "XML Language (XPath) Version 1.0". In brief, an XPath is a string that contains many namespace prefixes and local names, along with some other syntax.

The XPath language allows for the association of a prefix with a path, and using the prefix within the XML document. For example, the following XML document named "select" contains a prefix ("cal") associated with a specific URI ( The element contains an attribute named "path" that contains the XPath.

 <select xmlns:cal ="" path="cal:appointments/@startTime" />

The schema for this might be:

 <element name="select">
          <attribute name="path" />

Without the XmlNamespaceDeclarationsAttribute, the association between the prefix and the namespace is lost.

To retain the association between the prefix and the namespace URI, add a member that returns an XmlSerializerNamespaces object and apply the XmlNamespaceDeclarationsAttribute attribute to the member, as shown in the following C# and Visual Basic code:

 // C#
 public class Select {
   [XmlAttribute] public string path;
   [XmlNamespaceDeclarations] public XmlSerializerNamespaces xmlns;
 ' Visual Basic
 Public Class Select
    <XmlAttribute> Public path As String
    <XmlNamespaceDeclarations> Public xmlns As XmlSerializerNamespaces
 End Class

When serialized, the schema for the generated XML document contains the XML Schema definition (XSD) element named appinfo. The element further contains a metadata element named keepNamespaceDeclarations, set to the name of the member that contains the namespace declarations. The following XML fragment shows the schema:

 <xs:element name="select">
          <xs:attribute name="path" />

On deserialization, the xmlns field contains an XmlSerializerNamespaces object that contains all namespace prefix definitions.

On serialization, the user can add prefix-namespace pairs to the XmlSerializerNamespaces object using the Add method. This is shown in the following C# and Visual Basic code:

 // C#
 using System;
 using System.IO;
 using System.Xml.Serialization;
 public class Select {
    public string xpath;
    public XmlSerializerNamespaces xmlns;
 public class Test {
    public static void Main(string[] args) {
       Select mySelect = new Select();
       mySelect.xpath = "myNS:ref/@common:y";
       mySelect.xmlns = new XmlSerializerNamespaces();
       mySelect.xmlns.Add("MyNS", "");
       mySelect.xmlns.Add("common", "");
       XmlSerializer ser = new XmlSerializer(typeof(Select));
       ser.Serialize(Console.Out, mySelect);
 // Output:
 // <?xml version="1.0" encoding="IBM437"?>
 // <select xmlns:xsd="" xmlns:xsi="" 
 // xmlns:common="" xmlns:MyNS="" xpath="myNS:ref/@common:y" />
 ' Visual Basic
 Imports System
 Imports System.IO
 Imports System.Xml.Serialization
 <XmlRoot("select")> _
 Public Class SelectPath
    <XmlAttribute> _
    Public xpath As String 
    <XmlNamespaceDeclarations> _
    public xmlns As XmlSerializerNamespaces 
 End Class
 Public Class Test 
    Public Shared Sub Main() 
       Dim mySelect As SelectPath = New SelectPath()
       mySelect.xpath = "myNS:ref/@common:y"
       mySelect.xmlns = New XmlSerializerNamespaces()
       mySelect.xmlns.Add("MyNS", "")
       mySelect.xmlns.Add("common", "")
       Dim ser As XmlSerializer = New XmlSerializer(mySelect.GetType)
       ser.Serialize(Console.Out, mySelect)
    End Sub
 End Class
 ' <?xml version="1.0" encoding="IBM437"?>
 ' <select xmlns:xsd="" xmlns:xsi="" 
 ' xmlns:common="" xmlns:MyNS="" xpath="myNS:ref/@common:y" />

Also note that the member to which the attribute is applied contains only the prefix-namespace pairs that belong to the XML element defined by the class. For example, in the following XML document, only the prefix pair "cal" is captured, but not the "x" prefix. To get that data, add a member with the XmlNamespaceDeclarationsAttribute to the class that represents the root element.

 <?xml version="1.0"?>
 <x:root xmlns:x="">
   <x:select xmlns:cal="" path="cal:appointments/@cal:startTime" />


Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0