streamwriter write byte array
using (FileStream fsStream = new FileStream("Bytes.data", FileMode.Create))
using (BinaryWriter writer = new BinaryWriter(fsStream, Encoding.UTF8))
{
// Writing the strings.
writer.Write("The");
writer.Write(" strings");
writer.Write(" I");
writer.Write(" want");
writer.Write(".");
// Writing your bytes afterwards.
writer.Write(new byte[]
{
0xff,
0xfe
});
}