How to convert utf8 string to utf8 byte array?

How can I convert string to utf8 byte array, I have this sample code:

This works ok:

StreamWriter file = new StreamWriter(file1, false, Encoding.UTF8);
file.WriteLine(utf8string);
file.Close();

This works wrong, file is in ASCII:

byte[] bytes = System.Text.UTF8Encoding.UTF8.GetBytes(utf8string);
FileStream fs = new FileStream(file2, FileMode.CreateNew);
fs.Write(bytes, 0, bytes.Length);
fs.Close();

I would like to get byte array what returned by this function:

System.IO.File.ReadAllBytes(path_to_file)

because this works ok:

byte[] datab = File.ReadAllBytes(file1);
FileStream fs2 = new FileStream(file3, FileMode.CreateNew);
fs2.Write(datab, 0, datab.Length);
fs2.Close();