UTF32Encoding.GetPreamble Method ()

 

Returns a Unicode byte order mark encoded in UTF-32 format, if the UTF32Encoding object is configured to supply one.

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

public override byte[] GetPreamble()

Return Value

Type: System.Byte[]

A byte array containing the Unicode byte order mark, if the UTF32Encoding object is configured to supply one. Otherwise, this method returns a zero-length byte array.

The UTF32Encoding object can provide a preamble, which is an array of bytes that can be prefixed to the sequence of bytes resulting from the encoding process. Prefacing a sequence of encoded bytes with a byte order mark (code points U+0000 U+FEFF) helps the decoder determine the byte order and the transformation format, or UTF. The Unicode byte order mark (BOM) is serialized as follows (in hexadecimal):

  • Big endian byte order: 00 00 FE FF

  • Little endian byte order: FF FE 00 00

You can instantiate a UTF32Encoding object whose GetPreamble method returns a valid BOM in the following ways:

We recommend that you use the BOM, since it provides nearly certain identification of an encoding for files that otherwise have lost reference to the UTF32Encoding object, for example, untagged or improperly tagged web data, or random text files stored when a business did not have international concerns or other data. Often, user problems might be avoided if data is consistently and properly tagged.

For standards that provide an encoding type, a BOM is somewhat redundant. However, it can be used to help a server send the correct encoding header. Alternatively, it can be used as a fallback in case the encoding is otherwise lost.

There are some disadvantages to using a BOM. For example, knowing how to limit the database fields that use a BOM can be difficult. Concatenation of files can be a problem also, for example, when files are merged in such a way that an unnecessary character can end up in the middle of data. In spite of the few disadvantages, however, the use of a BOM is highly recommended.

For more information on byte order and the byte order mark, see The Unicode Standard at the Unicode home page.

System_CAPS_cautionCaution

To ensure that the encoded bytes are decoded properly, you should prefix encoded bytes with a preamble. Note that the GetBytes method does not prepend a BOM to a sequence of encoded bytes; supplying a BOM at the beginning of an appropriate byte stream is the developer's responsibility.

The following code example retrieves and displays the byte order mark for different UTF32Encoding instances.

using System;
using System.Text;

public class SamplesUTF32Encoding
{
   public static void Main()
   {
      // Create instances of UTF32Encoding, with the byte order mark and without.
      UTF32Encoding u32LeNone = new UTF32Encoding();
      UTF32Encoding u32BeNone = new UTF32Encoding( true, false );
      UTF32Encoding u32LeBom  = new UTF32Encoding( false, true );
      UTF32Encoding u32BeBom  = new UTF32Encoding( true, true );

      // Display the preamble for each instance.
      PrintHexBytes( u32LeNone.GetPreamble() );
      PrintHexBytes( u32BeNone.GetPreamble() );
      PrintHexBytes( u32LeBom.GetPreamble() );
      PrintHexBytes( u32BeBom.GetPreamble() );
   }

   public static void PrintHexBytes( byte[] bytes )
   {

      if (( bytes == null ) || ( bytes.Length == 0 ))
         Console.WriteLine( "<none>" );
      else  {
         for ( int i = 0; i < bytes.Length; i++ )
            Console.Write( "{0:X2} ", bytes[i] );
         Console.WriteLine();
      }
   }
}
/*
This example displays the following output.
      FF FE 00 00
      <none>
      FF FE 00 00
      00 00 FE FF
*/

The following example instantiates two UTF32Encoding objects, the first of which does not provide a BOM and the second of which does. It then calls the GetPreamble method to write the BOM to a file before writing a UTF-32-encoded string. As the output from the example shows, the file that saves the bytes from the second encoder has four more bytes that the first.

using System;
using System.IO;
using System.Text;

public class Example
{
   public static void Main()
   {
      String s = "This is a string to write to a file using UTF-32 encoding.";

      // Write a file using the default constructor without a BOM.
      var enc = new UTF32Encoding(! BitConverter.IsLittleEndian, false);
      Byte[] bytes = enc.GetBytes(s);
      WriteToFile(@".\NoPreamble.txt", enc, bytes);

      // Use BOM.
      enc = new UTF32Encoding(! BitConverter.IsLittleEndian, true);
      WriteToFile(@".\Preamble.txt", enc, bytes);
   }

   private static void WriteToFile(String fn, Encoding enc, Byte[] bytes)
   {
      var fs = new FileStream(fn, FileMode.Create);
      Byte[] preamble = enc.GetPreamble();
      fs.Write(preamble, 0, preamble.Length);
      Console.WriteLine("Preamble has {0} bytes", preamble.Length);
      fs.Write(bytes, 0, bytes.Length);
      Console.WriteLine("Wrote {0} bytes to {1}.", fs.Length, fn);
      fs.Close();
      Console.WriteLine();
   }
}
// The example displays the following output:
//       Preamble has 0 bytes
//       Wrote 232 bytes to .\NoPreamble.txt.
//
//       Preamble has 4 bytes
//       Wrote 236 bytes to .\Preamble.txt.

You can also compare the files by using the fc command in a console window, or you can inspect the files in a text editor that includes a Hex View mode. Note that when the file is opened in an editor that supports UTF-32, the BOM is not displayed.

Universal Windows Platform
Available since 10
.NET Framework
Available since 2.0
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