Detect Encoding ANSI

EncodingDetector.cs

public static bool ANSI(string filename)
{
    var file_stream = File.OpenRead(filename);
    if (file_stream == null) return false;

    var stream = new BinaryReader(file_stream);
    var file_size = file_stream.Length;
    byte[] buffer = new byte[file_size];
    stream.Read(buffer, 0, (int)file_size);
    stream.Close();
    if ((file_size >= 3 && buffer[0] == 239 && buffer[1] == 187 && buffer[2] == 191) || (file_size >= 2 && buffer[0] == 254 && buffer[1] == byte.MaxValue) || (file_size >= 2 && buffer[0] == byte.MaxValue && buffer[1] == 254) || (file_size >= 4 && buffer[0] == 0 && buffer[1] == 0 && buffer[2] == 254 && buffer[3] == byte.MaxValue) || (file_size >= 4 && buffer[0] == byte.MaxValue && buffer[1] == 254 && buffer[2] == 0 && buffer[3] == 0))
    {
        return false;
    }
    for (int i = 0; i < file_size; i++)
    {
        byte b = buffer[i];
        if ((b & 0x80) == 0)
        {
            continue;
        }
        int length;
        if ((b & 0xE0) == 192)
        {
            length = 2;
        }
        else if ((b & 0xF0) == 224)
        {
            length = 3;
        }
        else
        {
            if ((b & 0xF8) != 240)
            {
                return true;
            }
            length = 4;
        }
        if (i + length > file_size)
        {
            return true;
        }
        for (int a = 1; a < length; a++)
        {
            i++;
            if ((buffer[i] & 0xC0) != 128)
            {
                return true;
            }
        }
    }
    return false;
}