Char Structure

A String object is a sequential collection of Char structures that represents a string of text. Most Unicode characters can be represented by a single Char object, but a character that is encoded as a base character, surrogate pair, and/or combining character sequence is represented by multiple Char objects. For this reason, a Char structure in a String object is not necessarily equivalent to a single Unicode character.

Multiple 16-bit code units are used to represent single Unicode characters in the following cases:

• Glyphs, which may consist of a single character or of a base character followed by one or more combining characters. For example, the character ä is represented by a Char object whose code unit is U+0061 followed by a Char object whose code unit is U+0308. (The character ä can also be defined by a single Char object that has a code unit of U+00E4.) The following example illustrates that the character ä consists of two Char objects.

  

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  PowerShell

  

  Imports System.IO
  
  Module Example
     Public Sub Main()
        Dim sw As New StreamWriter("chars1.txt")
        Dim chars() As Char = { ChrW(&h0061), ChrW(&h0308) }
        Dim strng As New String(chars)
        sw.WriteLine(strng) 
        sw.Close()
     End Sub 
  End Module 
  ' The example produces the following output: 
  '       ä
  

• Characters outside the Unicode Basic Multilingual Plane (BMP). Unicode supports sixteen planes in addition to the BMP, which represents plane 0. A Unicode code point is represented in UTF-32 by a 21-bit value that includes the plane. For example, U+1D160 represents the MUSICAL SYMBOL EIGHTH NOTE character. Because UTF-16 encoding has only 16 bits, characters outside the BMP are represented by surrogate pairs in UTF-16. The following example illustrates that the UTF-32 equivalent of U+1D160, the MUSICAL SYMBOL EIGHTH NOTE character, is U+D834 U+DD60. U+D834 is the high surrogate; high surrogates range from U+D800 through U+DBFF. U+DD60 is the low surrogate; low surrogates range from U+DC00 through U+DFFF.

  

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  Imports System.IO
  
  Module Example
     Public Sub Main()
        Dim sw As New StreamWriter(".\chars2.txt")
        Dim utf32 As Integer = &h1D160
        Dim surrogate As String = Char.ConvertFromUtf32(utf32)
        sw.WriteLine("U+{0:X6} UTF-32 = {1} ({2}) UTF-16", 
                     utf32, surrogate, ShowCodePoints(surrogate))
        sw.Close()                    
     End Sub 
  
     Private Function ShowCodePoints(value As String) As String 
        Dim retval As String = Nothing 
        For Each ch In value
           retval += String.Format("U+{0:X4} ", Convert.ToUInt16(ch))
        Next 
        Return retval.Trim()
     End Function 
  End Module 
  ' The example produces the following output: 
  '       U+01D160 UTF-32 = ð (U+D834 U+DD60) UTF-16
  

Because a single character can be represented by multiple Char objects, it is not always meaningful to work with individual Char objects. For instance, the following example converts the Unicode code points that represent the Aegean numbers zero through 9 to UTF-16 encoded code units. Because it erroneously equates Char objects with characters, it inaccurately reports that the resulting string has 20 characters.

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PowerShell

Module Example
   Public Sub Main()
      Dim result As String = String.Empty
      For ctr As Integer = &h10107 To &h10110     ' Range of Aegean numbers.
         result += Char.ConvertFromUtf32(ctr)
      Next         
      Console.WriteLine("The string contains {0} characters.", result.Length) 
   End Sub 
End Module 
' The example displays the following output: 
'     The string contains 20 characters.

You can do the following to avoid the assumption that a Char object represents a single character.

• You can work with a String object in its entirety instead of working with its individual characters to represent and analyze linguistic content.

• You can use the StringInfo class to work with text elements instead of individual Char objects. The following example uses the StringInfo object to count the number of text elements in a string that consists of the Aegean numbers zero through nine. Because it considers a surrogate pair a single character, it correctly reports that the string contains ten characters.

  

  VB

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  Imports System.Globalization
  
  Module Example
     Public Sub Main()
        Dim result As String = String.Empty
        For ctr As Integer = &h10107 To &h10110     ' Range of Aegean numbers.
           result += Char.ConvertFromUtf32(ctr)
        Next          
        Dim si As New StringInfo(result)
        Console.WriteLine("The string contains {0} characters.", si.LengthInTextElements) 
     End Sub 
  End Module 
  ' The example displays the following output: 
  '       The string contains 10 characters.
  

• If a string contains a base character that has one or more combining characters, you can call the String.Normalize method to convert the substring to a single UTF-16 encoded code unit. The following example calls the String.Normalize method to convert the base character U+0061 (LATIN SMALL LETTER A) and combining character U+0308 (COMBINING DIAERESIS) to U+00E4 (LATIN SMALL LETTER A WITH DIAERESIS).

  

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  Module Example
     Public Sub Main()
        Dim combining As String = ChrW(&h0061) + ChrW(&h0308)
        ShowString(combining)
  
        Dim normalized As String = combining.Normalize()
        ShowString(normalized)
     End Sub 
  
     Private Sub ShowString(s As String)
        Console.Write("Length of string: {0} (", s.Length)
        For ctr As Integer = 0 To s.Length - 1
           Console.Write("U+{0:X4}", Convert.ToUInt16(s(ctr)))
           If ctr <> s.Length - 1 Then Console.Write(" ")
        Next 
        Console.WriteLine(")")
        Console.WriteLine()
     End Sub 
  End Module 
  ' The example displays the following output: 
  '       Length of string: 2 (U+0061 U+0308) 
  '        
  '       Length of string: 1 (U+00E4)
  

The Char structure provides methods to compare Char objects, convert the value of the current Char object to an object of another type, and determine the Unicode category of a Char object: