C# UTF-8 with BOM 編碼

var utf8WithBom = new UTF8Encoding(true);
    using (var ms = new MemoryStream())
    {
        using (var swr = new StreamWriter(ms, utf8WithBom))
        {
            swr.Write("abc");
        }
        File.WriteAllBytes(@"X:\abc.txt", ms.ToArray());
    }