WriteToFileUtf8

public static void WriteToFileUtf8(this MemoryStream stream, String path)
{
    using (var writer = new FileStream(path, FileMode.Create, FileAccess.ReadWrite))
    {
        Encoding enc = new UTF8Encoding(false, false);
        var chars = enc.GetString(stream.ToArray());

        var bytes = enc.GetBytes(chars.ToCharArray());
        writer.Write(bytes, 0, bytes.Length);
    }
}

var path = "C:\\doc.txt";

using (MemoryStream ms = new MemoryStream())

   ms..WriteToFileUtf8(path);
}