FileStream and Encoding

Question

I have a program write save a text file using stdio interface. It swap the 4 MSB with the 4 LSB, except the characters CR and/or LF.

I'm trying to "decode" this stream using a C# program, but I'm unable to get the original bytes.

        StringBuilder sb = new StringBuilder();
        StreamReader sr = new StreamReader("XXX.dat", Encoding.ASCII);
        string sLine;

        while ((sLine = sr.ReadLine()) != null) {
            string s = "";
            byte[] bytes = Encoding.ASCII.GetBytes(sLine);

            for (int i = 0; i < sLine.Length; i++) {
                byte c = bytes[i];
                byte lb = (byte)((c & 0x0F) << 4), hb = (byte)((c & 0xF0) >> 4);
                byte ascii = (byte)((lb) | (hb));

                s += Encoding.ASCII.GetString(new byte[] { ascii });
            }
            sb.AppendLine(s);
        }
        sr.Close();

        return (sb);

I've tried to change encoding in UTF8, but it didn't worked. I've also used a BinaryReader created using the 'sr' StreamReader, but nothing good happend.

     StringBuilder sb = new StringBuilder();
        StreamReader sr = new StreamReader("XXX.shb", Encoding.ASCII);
        BinaryReader br = new BinaryReader(sr.BaseStream);
        string sLine;
        string s = "";

        while (sr.EndOfStream == false) {
            byte[] buffer = br.ReadBytes(1);
            byte c = buffer[0];
            byte lb = (byte)((c & 0x0F) << 4), hb = (byte)((c & 0xF0) >> 4);
            byte ascii = (byte)((lb) | (hb));

            s += Encoding.ASCII.GetString(new byte[] { ascii });
        }
        sr.Close();

        return (sb);

If the file starts with 0xF2 0xF2 ..., I read everything except the expected value. Where is the error? (i.e.: 0xF6 0xF6).

Actually this C code do the job:

            ...
while (fgets(line, 2048, bfd) != NULL) {
    int cLen = strlen(xxx), lLen = strlen(line), i;

    // Decode line
    for (i = 0; i < lLen-1; i++) {
        unsigned char c = (unsigned char)line[i];
        line[i] = ((c & 0xF0) >> 4) | ((c & 0x0F) << 4);
    }

    xxx = realloc(xxx , cLen + lLen + 2);
    xxx = strcat(xxx , line);
    xxx = strcat(xxx , "\n");
}
fclose(bfd);

What wrong in the C# code?

Solution

Got it.

The problem is the BinaryReader construction:

StreamReader sr = new StreamReader("XXX.shb", Encoding.ASCII);
BinaryReader br = new BinaryReader(sr.BaseStream);

I think this construct a BinaryReader based on StreaReader which "translate" characters coming from the file.

Using this code, actually works well:

FileInfo fi = new FileInfo("XXX.shb");
BinaryReader br = new BinaryReader(fi.OpenRead());

I wonder if it is possible to read those kind of data with a Text stream reader line by line, since line endings are preserved during "encoding" phase.