C# Encoding.CreateTranscodingStream does not output file with desired target encoding

Question

I have the following piece of code:

using (Stream inputFileStream = File.OpenRead("C:\\Users\\User\\Downloads\\test.txt"))
{
    using (Stream transcodingStream = Encoding.CreateTranscodingStream(inputFileStream, Encoding.GetEncoding(500), new UnicodeEncoding(bigEndian: true, byteOrderMark: true)))
    {
        using (Stream outputStream = File.OpenWrite("C:\\Users\\User\\Downloads\\test.txt"))
        {
            await transcodingStream.CopyToAsync(outputStream, cancellationToken);
        }
    }
}

My file before transcoding has the following first 16 bytes and is of Ebcdic type encoding (code page 500):

F5 F1 F1 F0 F2 C2 D4 E6 40 40 40 40 40 40 F1 F1 = 51102BMW 11

After performing the transcoding to Unicode with Big-Endiant and Byte Order Markings, I expect the file to begin with:

FF FE

However, I get:

00 35 00 31 00 31 00 30 00 32 00 42 00 4D 00 57 = �5�1�1�0�2�B�M�W

Where am I going wrong with this?

Answer 1

It seems like the transcoding stream does not care about maintaining the BOM for target encoding and it's something you have to manage yourself.

I've implemented the following solution: targetEncoding is of type Encoding

outputStream.Seek(0, SeekOrigin.Begin);
await outputStream.WriteAsync(targetEncoding.Preamble.ToArray(), 0, targetEncoding.Preamble.Length, cancellationToken);