UnicodeEncoding.GetPreamble Method

public:
 override cli::array <System::Byte> ^ GetPreamble();

public override byte[] GetPreamble ();

override this.GetPreamble : unit -> byte[]

Public Overrides Function GetPreamble () As Byte()

Returns

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

Examples

The following example demonstrates how to use the GetPreamble method to retrieve the Unicode byte order mark in big endian or little endian byte order for an instance of a UnicodeEncoding.

using namespace System;
using namespace System::Text;
using namespace System::Collections;
int main()
{
   array<Byte>^byteOrderMark;
   byteOrderMark = Encoding::Unicode->GetPreamble();
   Console::WriteLine( "Default (little-endian) Unicode Preamble:" );
   IEnumerator^ myEnum = byteOrderMark->GetEnumerator();
   while ( myEnum->MoveNext() )
   {
      Byte b = safe_cast<Byte>(myEnum->Current);
      Console::Write( "[{0}]", b );
   }

   Console::WriteLine( "\n" );
   UnicodeEncoding^ bigEndianUnicode = gcnew UnicodeEncoding( true,true );
   byteOrderMark = bigEndianUnicode->GetPreamble();
   Console::WriteLine( "Big-endian Unicode Preamble:" );
   myEnum = byteOrderMark->GetEnumerator();
   while ( myEnum->MoveNext() )
   {
      Byte b = safe_cast<Byte>(myEnum->Current);
      Console::Write( "[{0}]", b );
   }
}

using System;
using System.Text;

class UnicodeEncodingExample {
    public static void Main() {
        Byte[] byteOrderMark;
        
        byteOrderMark = Encoding.Unicode.GetPreamble();
        Console.WriteLine("Default (little-endian) Unicode Preamble:");
        foreach (Byte b in byteOrderMark) {
            Console.Write("[{0}]", b);
        }
        Console.WriteLine("\n");

        UnicodeEncoding bigEndianUnicode = new UnicodeEncoding(true, true);
        byteOrderMark = bigEndianUnicode.GetPreamble();
        Console.WriteLine("Big-endian Unicode Preamble:");
        foreach (Byte b in byteOrderMark) {
            Console.Write("[{0}]", b);
        }
    }
}

Imports System.Text

Class UnicodeEncodingExample
    
    Public Shared Sub Main()
        Dim byteOrderMark() As Byte
        Dim b As Byte
        
        byteOrderMark = Encoding.Unicode.GetPreamble()
        Console.WriteLine("Default (little-endian) Unicode Preamble:")
        For Each b In  byteOrderMark
            Console.Write("[{0}]", b)
        Next b
        Console.WriteLine(ControlChars.NewLine)
        
        Dim bigEndianUnicode As New UnicodeEncoding(True, True)
        byteOrderMark = bigEndianUnicode.GetPreamble()
        Console.WriteLine("Big-endian Unicode Preamble:")
        For Each b In  byteOrderMark
            Console.Write("[{0}]", b)
        Next b
    End Sub
End Class

The following example instantiates two UnicodeEncoding 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 Unicode-encoded string. As the console output from the example shows, the file that saves the bytes from the second encoder has three more bytes than 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-16 encoding.";

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

      // Use BOM.
      enc = new UnicodeEncoding(! 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 116 bytes to .\NoPreamble.txt.
//
//       Preamble has 2 bytes
//       Wrote 118 bytes to .\Preamble.txt.

Imports System.IO
Imports System.Text

Module Example
   Public Sub Main()
      Dim s As String = "This is a string to write to a file using UTF-16 encoding."
      
      ' Write a file using the default constructor without a BOM.
      Dim enc As New UnicodeEncoding(Not BitConverter.IsLittleEndian, False)
      Dim bytes() As Byte = enc.GetBytes(s)
      WriteToFile("NoPreamble.txt", enc, bytes)

      ' Use BOM.
      enc = New UnicodeEncoding(Not BitConverter.IsLittleEndian, True)
      WriteToFile("Preamble.txt", enc, bytes)
   End Sub

   Private Sub WriteToFile(fn As String, enc As Encoding, bytes As Byte())
      Dim fs As New FileStream(fn, FileMode.Create)
      Dim preamble() As Byte = 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()
   End Sub
End Module
' The example displays the following output:
'       Preamble has 0 bytes
'       Wrote 116 bytes to .\NoPreamble.txt.
'
'       Preamble has 2 bytes
'       Wrote 118 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-16 encoding, the BOM is not displayed.

Remarks

The UnicodeEncoding object can provide a preamble, which is a byte array 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 point 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: FE FF

• Little endian byte order: FF FE

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

• By retrieving the UnicodeEncoding object returned by the Encoding.Unicode or Encoding.BigEndianUnicode property.

• By calling the parameterless UnicodeEncoding() constructor to instantiate a UnicodeEncoding object.

• By supplying true as the value of the byteOrderMark argument to the UnicodeEncoding(Boolean, Boolean) or UnicodeEncoding(Boolean, Boolean, Boolean) constructors.

We recommended that you use the BOM, since it provides nearly certain identification of an encoding for files that otherwise have lost a reference to their encoding, such as untagged or improperly tagged web data or random text files stored when a business did not have international concerns. 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.

Applies to