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XmlWriter.LookupPrefix Method

When overridden in a derived class, returns the closest prefix defined in the current namespace scope for the namespace URI.

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

Public MustOverride Function LookupPrefix ( _
	ns As String _
) As String


Type: System.String

The namespace URI whose prefix you want to find.

Return Value

Type: System.String
The matching prefix or Nothing if no matching namespace URI is found in the current scope.


ns is either Nothing or String.Empty.


An XmlWriter method was called before a previous asynchronous operation finished. In this case, InvalidOperationException is thrown with the message “An asynchronous operation is already in progress.”

The following example writes out a book.

Option Strict
Option Explicit

Imports System
Imports System.IO
Imports System.Xml

Public Class Sample
    Private Shared m_Document As String = "sampledata.xml" 

    Public Shared Sub Main()
        Dim writer As XmlWriter = Nothing 


        Dim settings As XmlWriterSettings = new XmlWriterSettings()
        settings.Indent = true
        writer = XmlWriter.Create (m_Document, settings)

        writer.WriteComment("sample XML fragment")

        ' Write an element (this one is the root).

        ' Write the namespace declaration.
        writer.WriteAttributeString("xmlns", "bk", Nothing, "urn:samples")

        ' Write the genre attribute.
        writer.WriteAttributeString("genre", "novel")

        ' Write the title.
        writer.WriteString("The Handmaid's Tale")

        ' Write the price.
        writer.WriteElementString("price", "19.95")

        ' Lookup the prefix and write the ISBN element. 
        Dim prefix As String = writer.LookupPrefix("urn:samples")
        writer.WriteStartElement(prefix, "ISBN", "urn:samples")

        ' Write the style element (shows a different way to handle prefixes).
        writer.WriteElementString("style", "urn:samples", "hardcover")

        ' Write the close tag for the root element.

        ' Write the XML to file and close the writer.

            If Not (writer Is Nothing) Then
            End If 
        End Try 

    End Sub 'Main 
End Class 'Sample

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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