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IdnMapping Class

Supports the use of non-ASCII characters for Internet domain names. This class cannot be inherited.

Namespace:  System.Globalization
Assembly:  mscorlib (in mscorlib.dll)

Public NotInheritable Class IdnMapping
Dim instance As IdnMapping

An Internet domain name consists of one or more parts, called domain name labels, separated by label separators. For example, the domain name "" consists of the labels, "www", "microsoft", and "com" separated by periods.

The Internationalizing Domain Names in Applications (IDNA) mechanism maps an internationalized domain name that contains Unicode characters outside the US-ASCII character range to an IDNA domain name that consists of Unicode characters solely in the displayable US-ASCII character range. The IDNA mechanism is used to convert only domain names, not data transmitted over the Internet.

The IdnMapping class embodies and extends the IDNA mechanism. The GetAscii method normalizes a domain name, converts the normalized name to a representation that consists of displayable Unicode characters in the US-ASCII code point range (U+0020 to U+007E), and prepends an ASCII Compatible Encoding prefix ("xn--") to each label.

The label separator characters IDEOGRAPHIC FULL STOP (U+3002), FULLWIDTH FULL STOP (U+FF0E), and HALFWIDTH IDEOGRAPHIC FULL STOP (U+FF61) are converted to the label separator FULL STOP (period, U+002E).

For more information, see the RFC 3490 standard, "Internationalizing Domain Names in Applications," at Request for Comments (RFC).

The following code example uses the GetAscii(String, Int32, Int32) method to convert a domain name to a domain name that complies with the IDNA standard. The GetUnicode(String, Int32, Int32) method then converts the standardized domain name back into the original domain name, except that the original label separators are replaced with the standard label separator.

' This example demonstrates the GetAscii and GetUnicode methods. 
' For sake of illustration, this example uses the most complex 
' form of those methods, not the most convenient. 

Imports System
Imports System.Globalization

Class Sample
    Public Shared Sub Main() 

'   Define a domain name consisting of the labels: GREEK SMALL LETTER  
'   THETA (U+03B8); FULLWIDTH FULL STOP (U+FF0E); and "com". 

        Dim name As String = "π。θ.com" 
        Dim international As String 
        Dim nonInternational As String 

        Dim msg1 As String = "the original non-internationalized " & vbCrLf & "domain name:" 
        Dim msg2 As String = "Allow unassigned characters?:     {0}" 
        Dim msg3 As String = "Use non-internationalized rules?: {0}" 
        Dim msg4 As String = "Convert the non-internationalized domain name to international format..." 
        Dim msg5 As String = "Display the encoded domain name:" & vbCrLf & """{0}""" 
        Dim msg6 As String = "the encoded domain name:" 
        Dim msg7 As String = "Convert the internationalized domain name to non-international format..." 
        Dim msg8 As String = "the reconstituted non-internationalized " & vbCrLf & "domain name:" 
        Dim msg9 As String = "Visually compare the code points of the reconstituted string to the " & _
                             "original." & vbCrLf & _
                             "Note that the reconstituted string contains standard label " & _
                             "separators (U+002e)." 
        ' ----------------------------------------------------------------------------
        CodePoints(name, msg1)
        ' ---------------------------------------------------------------------------- 
        Dim idn As New IdnMapping()

        Console.WriteLine(msg2, idn.AllowUnassigned)
        Console.WriteLine(msg3, idn.UseStd3AsciiRules)
        ' ----------------------------------------------------------------------------
        international = idn.GetAscii(name, 0, name.Length)
        Console.WriteLine(msg5, international)
        CodePoints(international, msg6)
        ' ----------------------------------------------------------------------------
        nonInternational = idn.GetUnicode(international, 0, international.Length)
        CodePoints(nonInternational, msg8)
    End Sub 'Main

    ' ---------------------------------------------------------------------------- 
    Shared Sub CodePoints(ByVal value As String, ByVal title As String) 
        Console.WriteLine("Display the Unicode code points of {0}", title)
        Dim c As Char 
        For Each c In  value
            Console.Write("{0:x4} ", Convert.ToInt32(c))
        Next c

    End Sub 'CodePoints
End Class 'Sample

'This code example produces the following results: 

'Display the Unicode code points of the original non-internationalized 
'domain name: 
'03c0 3002 03b8 ff0e 0063 006f 006d 

'Allow unassigned characters?:     False 
'Use non-internationalized rules?: False 

'Convert the non-internationalized domain name to international format... 
'Display the encoded domain name: 

'Display the Unicode code points of the encoded domain name: 
'0078 006e 002d 002d 0031 0078 0061 002e 0078 006e 002d 002d 0074 0078 0061 002e 0063 006f 

'Convert the internationalized domain name to non-international format... 
'Display the Unicode code points of the reconstituted non-internationalized 
'domain name: 
'03c0 002e 03b8 002e 0063 006f 006d 

'Visually compare the code points of the reconstituted string to the original. 
'Note that the reconstituted string contains standard label separators (U+002e). 


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

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